Delineate Protection Area
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The delineation process is a fundamental
step in developing a Wellhead Protection Plan in that it identifies the area
surrounding the well, wellfield or spring that is at the surface directly above the
portion of the aquifer that supplies groundwater to the well, wellfield or spring. The
delineation allows for the recognition of the area where protection strategies will have
the greatest impact on groundwater quality and allows for more efficient use of limited
resources. The Wellhead (citizen) Advisory Committee and the Wellhead Rules and Guidance
Committee developed an approach to delineation that considered both protection of the
source of their drinking water and the community's available resources.
All delineation techniques acceptable in the Oregon program require
consideration of the pumping rate for the well(s). The Oregon program requires the use of
an adjusted pump rate of 125 percent of the average daily use over the 3-month period of
highest water usage during the year in order to accommodate potential growth in the
community. The delineation process frequently results in the identification of the well,
wellfield or spring's entire zone of contribution or capture zone (Figure
3-1), an area that ultimately supplies water to the source, but may, because of size,
pose significant difficulties with respect to management. Accordingly, Oregon requires
only that portion of the capture zone that will supply the water over a 10-year period.
That portion of the capture zone that comprises a 10-year time-of-travel (TOT) for
groundwater is referred to as the wellhead protection area (WHPA). Delineations must be
accomplished by an Oregon registered geologist, engineering geologist, or other licensed
professional with demonstrated experience in hydrogeology.
The minimum delineation effort that a system must make in order to
obtain a state-certified program is dependent upon the population (Figure
3-2). For communities with populations less than or equal to 500 and obtaining their
groundwater through use of a well or wells, delineation can be accomplished through the
calculated fixed radius method. This technique uses minimum site-specific data and does
not consider important aquifer characteristics or groundwater flow. Systems with
significant potential threats to groundwater, evaluated through an optional Pre-Plan
Assessment, should choose a more technically defensible method for their delineation.
All other systems, e.g., spring-fed or wells serving more than 500
population, must develop a conceptual model of the groundwater system. This model
identifies critical characteristics of the groundwater system and provides for a
delineation that is more representative of the actual groundwater conditions. For systems
serving populations from 500 to 50,000, the minimum method is combined analytical methods
and hydrogeologic mapping. Systems serving 3,300 or more must collect site-specific data
(e.g., conduct an aquifer test) to be utilized in the process.
Systems serving more than 50,000 will utilize numerical or comparable
analytical methods in the delineation process. These methods are able to handle
significant heterogeneities within the aquifer, complex boundaries and variable gradient
directions to a much greater degree than analytical methods. These models are also capable
of being calibrated to the specific site.
Systems are encouraged to conduct susceptibility analyses within the
WHPA. The determination of the WHPA generally considers only travel within the aquifer;
i.e., it does not address the travel time from the surface to the aquifer. A
susceptibility analysis considers factors related to water movement from the surface to
the aquifer and the probability that a given contaminant will migrate to the aquifer. This
allows the system to prioritize various parts of the WHPA in terms of the susceptibility
and to tailor management strategies to specific activities on the surface.
Communities can reduce the overall expense of the delineation process by
conducting parts of the investigation themselves, including the following:
The Oregon Health Division (OHD) will work with the water system/community to complete the delineation process. This includes early discussions regarding the general nature of the system, what the community can do up front, assistance in preparing a Request for Proposals from the consulting community, providing technical input during the delineation and approval of the final delineation product.
Additionally, communities that have groundwater sources close together may wish
to pool their efforts, obtaining one consultant to delineate their separate wells.
Providing the wells are within the same aquifer and same general hydrologic setting, this
combined effort may prove to be less costly than each community developing their own
independent delineation. It should be noted, that this effort will be successful only if
the consultant is able to use a single conceptual model to describe the hydrogeologic
setting of both communities.
Delineation of a wellhead protection area (WHPA) for the well(s) in question is
a fundamental aspect of a Wellhead Protection Plan. The U.S. Environmental Protection
Agency (EPA) has defined the WHPA as "...the surface and subsurface area
surrounding a water well or wellfield, supplying a public water system, through which
contaminants are reasonably likely to move toward and reach such water well or
wellfield." A contaminant released within the WHPA may ultimately reach the water
that is being pumped from the well. Therefore, it is within the boundaries of the WHPA
that management activities, designed to eliminate or reduce the threat of groundwater
contamination from surface or near surface activities, would be concentrated.
Proposed WHPA delineation techniques vary from drawing a circle of arbitrary
radius around the wellhead to more sophisticated computer assisted models that account for
site-specific characteristics of the aquifer and of groundwater flow (EPA, 1987). Clearly,
the utility of the resulting WHPA increases as the level of delineation sophistication
increases. However, so does the cost and level of required technical expertise. From a
cost-benefit perspective, however, it is in the best interest of the public water supply
to invest now in a proactive program of groundwater protection. Experience clearly
indicates that the cost and hardships associated with remediating or perhaps losing a
groundwater source as a result of contamination far exceeds the cost of implementing a
Participation in Oregon's wellhead protection program is voluntary; however, in
order to protect their drinking water resource for present and future generations, all
public water systems in Oregon should seriously consider developing a WHP program. In
order for a water system to have a state-certified program, the system must address all
the elements described in Oregon Wellhead Protection Program Guidance Manual.
Flexibility is allowed in structuring the WHP program, however, so that the individual
water system's characteristics and needs can be accommodated. The State will provide
guidance, technical assistance and will review for approval the delineation of each
system's WHPA. In compliance with ORS 672.505 to 672.705, the delineation will have to be
accomplished by a registered geologist, a registered certified specialty geologist (e.g.,
a registered engineering geologist), or other registered professional with demonstrable
experience in hydrogeology.
NOTE: At present, Oregon's registration requirements for
geologists, engineering geologists, or other professionals do not specifically address
hydrogeology, e.g., an individual does not have to be proficient in hydrogeology to become
a registered geologist in the state. Therefore, prior to contracting with an individual to
perform the delineation, the water system should ascertain that the individual being
considered actually has experience in the field of hydrogeology.
What Can Water Systems Do Now?
Water systems can facilitate the delineation process as well as become more
informed about their drinking water resource if they play an active role throughout the
exercise. It is clear that the water system has a more complete knowledge than anyone else
of the history of the system and the occurrence of other wells, irrigation practices,
etc., that might affect the delineation. In addition to this expertise, the water system
can reduce costs to the community by participating in the data collection process.
Well Inventory - An important part of the delineation is the
development of a conceptual model of how groundwater occurs and moves in the area. This
will need to be accomplished by your consultant. The data your consultant will use will
include well reports from the area (see Appendix A).
Compilation of the well reports may be time-consuming and expensive if accomplished by the
consultant. It may be less expensive for the water system to obtain the well reports from
the local watermaster's office.
In addition to compiling the well reports, the consultant will need to have them
plotted on a map to show their distribution. Again, it will likely be less expensive for
the water system to accomplish this. It is also probable that with the water system's
knowledge of the local area, a more complete map will result. The City of Boardman (Morrow
County, Oregon) has developed a guide for preparation of a well report inventory. With the
City's permission, OHD has made copies and will distribute those to interested water
systems. Systems should communicate with OHD, or their consultant, prior to initiating the
survey in order to more efficiently design their efforts.
Even after the delineation exercise, the City of Boardman continues to use their
well report inventory in day-to-day issues regarding their system. The inventory has
proven to be a very useful management tool for the City.
Aquifer Test - Additional data that a water system can begin to
collect include static water levels for their wells and an aquifer test for their
production wells. Procedures and recommendations for aquifer tests are described in
Appendix A. Under any condition, aquifer tests are costly and under no circumstance should
a water system conduct one without first contacting OHD or the Water Resources Department
or their consultant for technical assistance. It is important to note that the 4-hour pump
test requirement for groundwater right holders required by the Water Resources Department
generally is insufficient to obtain representative aquifer characteristics.
Public Education - Lastly, water systems can work with the local
community to initiate a public education program. This program should be designed to raise
the level of awareness of the citizens of the community regarding groundwater and its
vulnerability. Water systems can contact the Department of Environmental Quality or OHD
regarding educational materials and programs that might be available to the community.
Request for Proposal
Water systems or communities who are beginning the delineation step in wellhead
protection plan development will work with the Drinking Water Program at OHD. OHD will
review the delineation requirements with the water system or community and will provide
technical assistance in preparing a "Request for Proposal" (RFP) to be
distributed to the consulting community.
Prior to and during the delineation process, OHD will provide input to the
system and their consultant as needed. The final delineation and supporting documentation
must be submitted to OHD for review and approval. OHD may enlist assistance from other
agencies, e.g., Water Resources Department, Oregon Department of Geology and Mineral
Industries and the Department of Environmental Quality as needed in the review of the
Delineation Method Requirements:
Figure 3-1 illustrates the distinction between three areas within the aquifer that can be recognized with respect to a pumping well: the zone of influence (ZOI), the zone of contribution (ZOC) and the WHPA. The ZOI, or the drawdown area, is that part of the aquifer where the hydraulic head is lowered as a direct result of the well discharge. For purposes of identification, it is defined here as the area where the drawdown is greater than 0.05 feet (0.6 inches). The ZOC is that part of the aquifer that will supply water to the well in the future. The WHPA is a subset of the ZOC: that part of the ZOC that will supply water to the well for the next 10 years. It is the WHPA, where groundwater protection policies are implemented, that delineation identifies.
In designing the state's delineation requirements, the citizen's advisory
committees sought a balance between factors of potential risk, aquifer sensitivity, and
available expertise and funds in designing the state's delineation requirements. The
delineation requirements also reflect input from the consulting community through
demonstration projects at Boardman, Klamath Falls and Springfield, as well as comments
solicited by the committee through consultant review of earlier drafts of this document.
In subsequent sections, these will be referred to as the "wellhead demonstration
The result of these efforts is a flow chart (Figure 3-2)
that bases the delineation method primarily on the population served by the water system.
The delineation techniques in the flow chart are discussed in greater detail below. In
order of increasing site-specific character, they are: (1) the calculated fixed radius
method, (2) combined application of analytical techniques and hydrogeologic mapping, and
(3) numerical methods. It must be emphasized that, in most cases, the delineation
technique identified in Figure 3-2 is to be considered a minimum
only. A water system should inventory the data available to them (see below), and if the
data allow, a more site specific delineation method should be used. In addition, the
potential risk of contamination of the system's groundwater source should also influence
the community's decision on the level of delineation used. Systems may wish to complete
the Pre-Plan Assessment option described in Step 1 of this document in order to perform a
preliminary evaluation of the number and types of potential contaminants that occur in the
vicinity of their well(s) or spring(s).
NOTE: An important finding of the wellhead demonstration projects
was that a more site-specific delineation, though costing more initially, resulted in
significant savings later. The savings accrued because the site-specific delineation was
more legally defensible and often encompassed a smaller area leading to reduced management
costs later in the process.
An important concept to remember, both during and after the delineation
exercise, is that the aquifer from which a well derives its water is a three dimensional
body that may have considerable regional extent, and from which other water systems, as
well as numerous private individuals, derive their water. A truly comprehensive
groundwater protection plan should address the entire aquifer rather than targeting a
limited portion that contributes water to a well over the next 10 years (i.e., the WHPA).
In practice, however, such a comprehensive approach would be difficult to implement and as
a result, the delineation of the individual well's WHPA is utilized.
The sections below follow the order of the flow chart in Figure
3-2. Note that various water systems will complete their delineation requirements at
different parts of the flow chart. It is not necessary that all systems complete all the
phases that are indicated in Figure 3-2. Very little discussion or recommendations are
given below regarding the specific model(s) to be used in the more complex situations
where analytical or numerical methods are employed. It is accepted that the professionals
performing those delineations will be well versed in the available techniques.
More discussion is provided with respect to the Calculated Fixed Radius method
because of its limited ability to incorporate even the most basic hydrogeologic data into
the model. As a result, recommendations on how to treat some of these more common
situations are provided. In addition, some effort is expended on providing minimum
expectations regarding the data that will be incorporated into the conceptual model(s). It
is hoped that this is viewed as a mechanism to facilitate review by the state as opposed
to the state dictating procedures to be followed by the consulting community.
Sources of Information
In addition to the discussion below, the reader may wish to become familiar with
the EPA guidance documents that address delineation of wellhead protection areas. These
Copies of these documents can be obtained free of charge by
calling the National Center for Environmental Publications and Information at (513)
Parameters Common to All Delineation Methods
Adjusted Pump Rate (Qa) - All WHPA delineation methods
acceptable for use in Oregon require incorporating a pump rate in the equations. In order
to provide for protection of the groundwater source for potentially expanded use in the
future, the minimum pump rate used in the models should be adjusted as follows: 125
percent of the average daily use calculated over the three months that are traditionally
the high-demand period for the system. If the average daily use is not available, the
adjusted pump rate should be determined by one of the following: 125 percent of the
average pump rate based on a comparable size community in the region, the design capacity
of the installed pump, or 90 percent of the safe yield of the well (see
Appendix A), whichever is the smallest. As an example, consider a community
using a well with a safe yield of 600 gpm. No daily use data is available. The design
capacity of the pump in the well is 500 gpm. The estimated usage based on a community with
similar residential and industrial characteristics is 390 gpm. A value of 487 gpm (1.25 ´ 390) would be used in the delineation
exercise. If no other data is available to a water system regarding daily water usage, the
system may use a figure derived by multiplying the population served by 125 gallons/day.
The determination of Qa is not, of course, a hard and fast
rule. It is in the water system's best interest to delineate a capture zone that will
accommodate growth in the future. If for example, the water system's master plan calls for
the installation of a larger pump in the future to accommodate projected growth, the
delineation should reflect that increased demand.
Time of Travel (TOT) - As discussed above, only a portion
of the aquifer is delineated by the WHPA methods. For management purposes, the WHPA is
generally limited in size by a criterion that is designed to achieve a predetermined
protection level. Examples of these criteria are: (1) drawdown of the water table or
potentiometric surface imposed by pumping of the well; (2) the aquifer's assimilative
capacity (i.e., the distance groundwater must travel through the aquifer to mitigate the
contaminant through dilution or breakdown); and (3) the time of travel, a factor related
to the time required to respond to the development of a contamination threat at the WHPA
In all of the Oregon approved techniques the TOT parameter is utilized
to constrain the WHPA. The TOT criteria effectively determines the radius of the
calculated fixed radius method and the upgradient distance delineated within the
analytical and numerical models. For most delineation techniques, a minimum TOT threshold
value of 10 years was chosen based on State review of the time required to remediate
and/or develop a new source should a significant contamination event take place at, or
arrive at, the WHPA boundary. The 10-year TOT should be regarded as a minimum. Longer TOT
thresholds are recommended in those cases where the understanding of the groundwater
system is more limited (see below) or significant threats to groundwater quality occur.
The water system will probably also wish to determine additional zones
within the WHPA based on shorter travel times. After reviewing WHP plans from other
states, Oregon recommends the following TOT zones be delineated within the WHPA:
Figure 3-3: Components of the Calculated FixedRadius Equation
Calculated Fixed Radius
Water Systems Affected - For water systems serving a
population of 500 or less, and deriving their drinking water from a well or wellfield, a
calculated fixed radius (CFR) is an acceptable delineation technique. Water systems should
understand that this technique uses only minimal site specific data and does not account
for some important aquifer characteristics (e.g., permeability or hydraulic gradient,
which control how fast and in which direction groundwater is flowing). As a result of the
limitations of using the (CFR) a TOT of 15 years is used for this technique (Figure 3-2). If the pre-Plan assessment indicates a significant
number of potential contaminant sources in the vicinity of their well(s), a more
site-specific technique should be considered.
Doing the Calculation - The CFR technique is shown
diagrammatically in Figure 3-3. The technique assumes a static system, i.e., no regional
groundwater flow, and that all of the water released to the well comes from storage within
the aquifer proximal to the well. The technique determines the volume of the aquifer that
is needed to supply the demand from the well over the TOT period. Because of the above
assumptions, the volume of the aquifer supplying the water is in the shape of a cylinder,
the radius of which is determined by the demand.
The water demand over the TOT is calculated by multiplying the adjusted
pump rate Qa (in cubic feet per year) by the TOT (15 years). To determine the
volume of the aquifer needed to supply the demand, we multiply the area of the circle
(3.14r2) times the thickness of the water bearing zone (I). However, only part
of that aquifer volume contains water that can drain to a well. That part is determined by
multiplying the equation for the volume of a cylinder by the effective porosity (n), that
fraction of the aquifer that consists of interconnected pore spaces. We then set the
volume expression equal to the demand and solve for "r", the radius of the
cylinder (see equation in Figure 3-3 and Calculated Fixed Radius example in Appendix A).
Appropriate values are substituted into the equation and the value of
"r" is determined. The adjusted pump rate (Qa) is determined as
described above. The effective porosity can be estimated using Table 1 (EPA, 1994). The
value of "I" should be obtained as appropriate in one of the following ways:
After the value of "r" is determined by the equation in Figure
3-3 (see also the Calculated Fixed Radius example in Appendix A),
a circle with a radius of "r" around the wellhead constitutes the WHPA for that
well. This is the area at the surface that directly overlies the cylinder determined
above. It is this area where released contaminants might percolate downward to the part of
the aquifer supplying the well. It is in this area that protection procedures will be
For examples of determining the CFR, and for special cases
where the circles overlap, intersect streams, or when the technique is applied to a
wellfield (see Appendix A).
Conceptual Model Development
What is a Conceptual Model? A conceptual hydrogeologic
model is a three-dimensional portrayal of the groundwater system in the study area. Within
the model, the distribution and geometries of the hydrogeologic units, their hydraulic
properties, including variations in hydraulic head, the direction of groundwater flow and
the location(s) of hydrogeologic boundaries, and areas of recharge and discharge are
The conceptual model provides the framework for decision making
regarding the groundwater system in an area. It provides a vehicle for determining those
hydrogeologic features that are especially important in controlling groundwater flow, It
also provides for the testing of assumptions and recognizing where more data are needed. A
well-conceived conceptual model is fundamental to developing a WHPA delineation that
accurately reflects the groundwater system in the area. The components of conceptual
models are discussed in Appendix A.
Water Systems Affected - As a general rule, as the
population in a community increases, the potential risks to water quality also increase
(population density increases and more industry and businesses are required to serve the
community). Because of the greater risk, more detailed information is necessary to provide
adequate protection of drinking water resources. Therefore, public water systems that have
>500 population must accomplish their delineation using techniques that utilize more
Further, the calculated fixed radius method cannot be used for water systems that derive their drinking water from springs. The discharge of a spring cannot be simply related to circle around that spring. As a result, these water systems must obtain more site-specific data in order to delineate the source of their drinking water. A requirement of all systems that are at this point on the flow chart is the development of a conceptual hydrogeologic model (Figure 3-2).
The flow chart in Figure 3-2 indicates two
levels of conceptual model development. The first, required of systems with a population
of 500 to 3,299, is developed from existing data, often regional in character. By
regional, we mean that aquifer characteristics may have been averaged over a large area
and that the hydraulic gradient has been determined over an area of which we are concerned
with only a small part. With such an approach, local variations are masked. For a
particular WHPA, however, local variations from the norm may be the controlling factors
for groundwater flow. For smaller systems, with presumably lower risks to groundwater, the
regional approach is considered adequate for the delineation exercise. If the pre-Plan
assessment indicates a high risk, the system should consider collecting more site-specific
data (see below).
For systems with populations of 3,300 or greater, more site-specific
information is required in developing the conceptual model. In the site-specific approach,
data are collected through aquifer tests, direct measurement of static water levels and
the mapping of spatial variations in aquifer characteristics.
Communities that are located in proximity to one another may wish to pursue the development of a common conceptual model through one consultant. What constitutes proximity varies across the state; however, if communities are within several miles of one another, they should consider the possibility that the data may allow a single conceptual model to be developed that will apply to both areas. As an example, consider two communities separated by three miles. These communities have wells that are within the same aquifer and there is no significant difference in the gradient nor are there any hydrogeologic boundaries that separate them. The application of the analytical models discussed below would be able to delineate the separate wells, using specific pumping characteristics, during the same model run. Such a multiple delineation approach, if appropriate, would certainly be less costly to the communities than if each community approached the conceptual model/delineation step independently.
Porous Media Assumption
What is Being Assumed? For the analytical models discussed
below, it is assumed that the openings within the aquifer through which the water moves
are such that water movement is directly down-gradient, perpendicular to the hydraulic
head contours. This assumption is generally valid in an aquifer where the open spaces
consist of pore spaces (voids that occur between individual particles that comprise the
aquifer). It may not be valid when the open spaces comprise discrete fractures that occur
within the rock material that makes up the aquifer.
In a typical porous material, such as sediment, e.g., sand and gravel,
the openings are primary, that is they represent the spaces between grains that were
formed when the sediment was originally deposited. Consequently, they are numerous and
random in occurrence. As a result, the concentration and orientation of the open spaces
tends to be isotropic (uniform in all directions) within the aquifer. Groundwater flow is
controlled primarily by gradient direction in porous media (Figure 3-4a).
The Impact of Fractures - Fractures are secondary
features. They are generated after the aquifer formed, often as a result of stresses
applied to the aquifer. Fractures tend to develop in a specific orientation with respect
to the direction of applied stress. As a result, the fractures may not be random in their
orientation and the aquifer's secondary porosity may be anisotropic (not uniform in all
directions). Groundwater in an anisotropic medium will be driven in a down-gradient
direction; however, it may be forced to move along the fractures which often are at some
angle from the gradient direction (Figure 3-4b).
Figure 3-4: Effect of Fracture Anisotropy on The Orientation of The Zone of Contribution to A Pumping Well
NOTE: A porous medium can still be anisotropic with
respect to other characteristics, e.g., hydraulic conductivity and thickness.
Fortunately, there are a number of settings in which fractured rock
behaves as a porous medium with respect to transmitting groundwater. At this point in the
flow chart (Figure 3-2), an analysis must be made to determine
whether the aquifer can be treated as isotropic with respect to its pore space
As reported by Long and others (1982), fractured rocks behave like
porous media when: (1) fracture density is high, (2) fracture orientation is not uniform,
(3) fracture openings are relatively uniform, and (4) the volume of aquifer concerned is
large. Recommendations for evaluating the porous media assumption are provided in the
Evaluation of the Porous Media Assumption section in Appendix A.
The Impact of Heterogeneities in an Alluvial Aquifer -
Even alluvial aquifers are not always uniform in character. Highly permeable channel
de-posits may occur at depth within the deposit. These channels are rarely in a straight
line; they meander back and forth across what was the valley floor when they formed. If
these channels are surrounded by low permeability sediments such as silts and clays or
bedrock, groundwater will preferentially flow along the channel even if this direction is
not immediately down gradient. If the conceptual model identifies significant channel
deposits, their impact on groundwater flow in the vicinity of the well(s) should be
The Impact of Heterogeneities Within Layered Volcanic Rocks
- Layered volcanic rocks (e.g., basalts, andesites, etc.) may also exhibit preferential
flow directions for groundwater. A typical volcanic section will consist of a sequence of
erupted rocks (e.g., lava flows, ash deposits, mudflows, etc). Owing to the character of
these rocks and the environment in which they occur, groundwater may not behave as if it
was in a porous medium. Lava flows typically have rather dense interiors and highly porous
and permeable tops and bottoms. The permeable tops and bottoms originate primarily from
the highly porous, i.e., vesicular, nature of these zones within the lava. Therefore, a
section of lava flows commonly has very low vertical hydraulic conductivity overall, and
high horizontal hydraulic conductivity in these vesicular zones. Groundwater
preferentially moves within these interflow zones, even if these zones are not
perpendicular to the gradient. For example, a vertical gradient may actually produce
significant horizontal flow within an interflow zone.
Additional complications may arise from the fact that in
an area where volcanic rocks have been erupted, significant discontinuities may occur on
the scale of a delineated WHPA. These result from the fact that the surface upon which the
lava erupts may not be horizontal. It is common in a typical volcanic sequence that
sufficient time between eruptions occurs, allowing the redevelopment of drainage systems
and canyon development. Subsequent eruptions may be confined largely to those drainages.
These intracanyon flows, often underlain by stream deposits are linear in character as
opposed to the sheet-like form that is often associated with lava flows. Groundwater will
preferentially move along these intracanyon flows in the subsurface. Features such as lava
tubes will further complicate groundwater flow.
Analytical Techniques with Hydrogeologic
Analytical Techniques - Analytical methods make use of
equations that define groundwater flow and contaminant transport and are frequently used
in areas with a sloping water table. The analytical equations require that various
hydrogeologic parameters be known or can be reliably estimated. When these parameters are
substituted into the equations, the resulting solutions provide information regarding the
dimensions of the ZOC with respect to the down-gradient divide and the width of the ZOC in
the up-gradient direction.
Most analytical models assume that the aquifer is uniform in character,
the gradient is constant throughout the modeled area and groundwater flow in the aquifer
is two-dimensional in a horizontal plane (vertical flow is not considered). If the
conceptual model indicates that any of these assumptions is significantly wrong, the use
of a more sophisticated model should be considered.
The WHPA Software - The USEPA, through contractors, has developed
software designed to facilitate the determination of wellhead protection areas in
hydrogeologic settings that include the above assumptions.
NOTE: There are many programs available commercially and
the selection of which one to use will generally be made by the consultant.
Here we briefly review EPA's software as a basis for discussion of
EPA's programs are inexpensive, easy to use and are well documented.
Used in the context of the conceptual model, and within individual program limitations
they will provide adequate delineations of wellhead protection areas. Graphic output is
limited with the programs, however most of the programs will produce hard copies of the
delineation that can be scaled automatically to most common maps. The option to save the
model results in an ASCII file that may be used as input to the ARC/INFO proprietary GIS
developed by the Environmental Systems Research Institute (ESRI) also exists for most of
The software package known collectively as WHPA code has the most
widespread distribution of EPA's delineation programs. This package consists of four
programs: MWCAP, RESSQC, GPTRAC, and MONTEC. All the models are based on an analytical
approach and use a particle-tracking routine to delineate the capture zones (Blandford and
Huyakorn, 1991). All models assume steady-state groundwater flow. Data input requirements
for the techniques are summarized in Table 3-1.
MWCAP. The MWCAP model will produce steady-state, time-related
and hybrid capture zones for single or multiple wells within confined or unconfined
aquifers. It can accommodate aquifer boundaries (impermeable and stream) but does not
consider well interference. Boundaries are modeled as straight lines and fully
penetrating. For stream boundaries where the stream is not fully penetrating or if the
presence of a clogging layer inhibits the hydraulic connection between the stream and the
aquifer, the predicted WHPA will be smaller than the actual capture zone. Forward
particle-tracking is an option allowing the path of a given constituent to be determined
within the flow field.
RESSQC. RESSQC assumes steady-state groundwater flow conditions and can model multiple discharging and recharge wells. Well interference is determined. The aquifer may be confined or unconfined if the drawdown to initial saturated thickness is less than approximately 0.1 (Blandford and Huyakorn, 1991). The program can also calculate contaminant fronts that migrate away from recharge wells. Boundaries are not addressed automatically and must be modeled through image well placement which must be input by the modeler. Forward and reverse particle-tracking is an option.
Table 3-1: Input Requirements for WHPA Models1
Study Area Limits
Maximum Step Length
Number of Pumping Wells
Number of Recharge Wells
Pumping/Injection Rates (Q)
Aquifer Transmissivity (T)
Aquifer Porosity (n)
Aquifer Thickness (b)
Angle of Ambient Flow
Hydraulic Gradient (i)
Areal Recharge Rate
Confining Layer Hydraulic Conductivity
Confining Layer Thickness
Boundary Condition Type
Perpendicular Distance from Well to Boundary
Orientation of Boundary
Capture Zone Type3
No. of Patholines Used to Delineate Capture Zone
Capture Zone Time
Rectangular Grid Parameter
No. Forward/Reverse Pathlines
Nodal Head Values
No. of Heterogeneous Aquifer Zones
Heterogeneous Aquifer Properties
1 From Blandford and Huyakorn, 1991.
2 Confined, unconfined or leaky confined.
3 Time-related, hybrid or steady-state.
GPTRAC. GPTRAC contains both semi-analytical and numerical
options. The semi-analytical method is similar to MWCAP and RESSQC in that it assumes a
uniform aquifer. Consideration of simple straight line fully penetrating boundaries are
options in the routine. It has greater flexibility in that unconfined, semi-confined and
confined aquifers can be dealt with directly. Discharging and recharge wells can be
modeled and well interference is accounted for. Forward and reverse particle tracking is
available. Areal recharge is an option for unconfined aquifers. Multiple straight line
boundaries can be considered.
A word of caution is appropriate for using the semi-analytical option of
GPTRAC for an unconfined aquifer. The user is prompted for the radius of influence of the
pumping well. The resulting delineation is very sensitive to the value that is used for
this variable. It is recommended that unless the user has specific information from either
drawdown measurements or calculations regarding the zone of influence of the well, that
this routine not be used for determining the WHPA for a well in an unconfined aquifer. The
user should use MWCAP, if there are no interfering wells, or RESSQC instead.
The numerical option of GPTRAC can perform the tasks associated with the
semi-analytical mode as well as allow for variations in aquifer characteristics, e.g.,
transmissivity, and changes in gradient magnitude and direction. Further, the program
allows for anisotropic character in terms of the transmissivity (i.e., Tx not
equal Ty). The program can be used as a post-processor for numerical programs
such as MODFLOW in that GPTRAC will read a head file and calculate flow paths accordingly.
The head file can be generated from field data or from published contour maps by
interpolating heads on a grid and using the program HEDCON in the WHPA package to generate
a file that can be utilized by GPTRAC's numerical option.
MONTEC. The MONTEC routine is similar to MWCAP with the exception
that it is based on a stochastic approach, i.e., evaluates the impact of known variations
in parameters (e.g., hydraulic conductivity permeability (K), gradient (i), pump rate (Q),
etc.) on the delineated capture zone. Input to the program is similar to MWCAP; however,
the modeler can enter a range of possible values for parameters of choice during the
input. The resulting output consists of a series of capture zones that are presented along
with the level of confidence that the actual capture zone is within the delineated area.
The program is limited to a single pumping well in a confined or semi-confined aquifer.
Uniform Flow Equation - Though not part of the WHPA
package, the Uniform Flow analytical model (Todd, 1980) is advocated in several of EPA's
documents (EPA, 1987; 1993). The results of this technique can be fit to known hydraulic
head distribution and therefore account for variable gradient direction (Bradbury et al.,
1991). Values for "K", "b", "i", and "Q" (see Table 3-1) are substituted into appropriate equations yielding
the down-gradient stagnation point and the width of the capture zone at any point
"X" along the flow path that intersects the well.
Analytic Element Technique - A disadvantage of the
analytical techniques described above is that they cannot be calibrated to existing
conditions. As a result, uncertainty regarding the application of the model to the
groundwater flow system remains. The analytic element method (Strack et al., 1994; Wuolo
et al., 1995) overcomes some of this concern while maintaining the relative simplicity of
the analytical model development.
The EPA has recently distributed the program CZAEM (Strack et al., 1994) which is designed to generate capture zones using the analytic element technique. The method generates a uniform flow field based on limited head and conductivity data. Using that flow field the program will predict head values within the area that can be checked against observations to see if the assumptions inherent in the model are appropriate. The program will also predict changes in head values as a function of model elements (e.g., a well) within the field. Comparing these to observed head changes provides for a means of calibration of the model. The EPA program WhAEM (Haitjema et al., 1994) links CZAEM with a Geographic Analytic Element Preprocessor (GAEP) to facilitate the analysis.
The program CAPZONE (Bair et al., 1991) also provides for calibration of
an analytical technique by comparing computed and observed drawdown within a stressed
aquifer. The program computes drawdown at rectangular grid points across the area of
concern. These drawdowns are compared with observed head values and adjustments to the
conceptual model are made if necessary. After calibration is complete, the drawdowns are
superposed on the regional head distribution and the resulting hydraulic head map is used
as the basis of groundwater flowpath determination.
Hydrogeologic Mapping - Hydrogeologic mapping is performed
in conjunction with the application of the analytical technique. With the equations alone,
there is no provision for determining the up-gradient boundary of the ZOC. Application of
Darcy's law using the TOT value is commonly done to terminate the up-gradient position of
the WHPA. There may be, however, geological reasons for terminating the ZOC at positions
closer to the wellhead than the WHPA boundary, e.g., a lithologic boundary, groundwater
divide or a stream boundary (Figure 3-5). Such features are
recognized in the process of hydrogeologic mapping and are incorporated into the
The choice of analytical technique used in the delineation exercise is,
of course, up to the modeler. In Table 3-1, we provide a table
that lists the required input for the WHPA codes RESSQC, MWCAP, GPTRAC Semi-Analytical and
GPTRAC numerical options. This table is presented for the purpose of illustrating extent
of flexibility that exists within these analytical techniques and should not be construed
as either endorsing or requiring these specific procedures.
Figure 3-5: Wellhead Protection Delineation Using Hydrogeologic Boundaries
Delineation Considerations - There are several decision
points encountered by the modeler during the construction of the model. These are often
critical in that they may result in very different delineations depending on the decision.
It is of importance that the decision made be the one that most accurately reflects the
aquifer and well in question. Listed below are several of these decision points with some
directions indicated in order to resolve the questions that arise:
Confined Versus Unconfined Aquifer. In many analytical
techniques, the modeler must specify directly or indirectly whether the aquifer of concern
is confined or unconfined. Further, in developing management strategies appropriate for a
given area, it is very useful to know the level of natural protection (i.e., confinement)
that characterizes the aquifer (Kreitler and Senger, 1991).
For purposes of classification, this document will adopt the Water
Resources Department's definition of "artesian aquifer" [OAR 690-200-050(8)] for
that of a confined aquifer. Using that definition, a confined aquifer is one in which
groundwater is under sufficient head to rise above the level at which it was first
encountered. Clearly, this definition does not make the distinction between various levels
of confinement, a characteristic that would have to be addressed in some of the analytical
An unconfined aquifer is defined as an aquifer in which the upper
surface, the water table, is at atmospheric pressure. The systems will be expected to have
accomplished an extensive evaluation of existing information in making the distinction of
confined or unconfined conditions.
It is important that the distinction between confined and unconfined be
supported by the conceptual model and not be based on a single well report. It is not
uncommon for a driller to bore through a finer-grained portion of the aquifer and, because
of low seepage rate, not recognize that the zone is saturated. The water-bearing zone
identified on the driller's log may reflect only a higher permeability part of the
aquifer. Because of this, it may appear that the static water level is higher than the
water-bearing zone when in fact the entire section is saturated and in hydraulic
Many of the analytical techniques provide for modeling semi-confined situations and such an approach should be used if indicated by the conceptual model.
Granular Versus Fractured Aquifer. The distinction between
granular and fractured relates primarily to how the physical character of the aquifer
impacts groundwater flow. The flow equations used in the analytical models assume that the
aquifer can be modeled as a porous medium, is isotropic in its characteristics and that
groundwater flow direction is in a direct and predictable relation to the hydraulic head
distribution within the aquifer. This is often the case in a granular aquifer, e.g.,
unconsolidated and consolidated sedimentary materials. Rocks hosting closely spaced and
intersecting fractures may in fact be modeled as granular, but may still be anisotropic in
character and therefore not lend themselves to direct application of analytical
techniques. Special techniques are available that will allow the use of analytical methods
to these water systems (Fetter, 1981; EPA, 1994).
Analytical techniques, however, are not generally applicable to those
aquifers that contain widely spaced or discrete fractures, or to rocks that have well
developed preferred orientations (e.g., some metamorphic rocks), or in geologic terrains
consisting of highly deformed interbedded units of differing conductivities. In these
cases, groundwater may be constrained by such anisotropic features to flow in directions
other than 90° to hydraulic
head contours. As a result, the zone of contributions may be significantly different than
that predicted by uniform flow equations, and travel times may be difficult to predict. As
stated above, water systems will be required to supply supporting documentation with
regard to their assignment of the aquifer of concern.
Aquifer Thickness. All models used to delineate WHPAs require input of the thickness of the aquifer. In the analytical models, this should be the average value, however a sensitivity analysis (see below) should be conducted to determine how much difference in the resulting WHPA would be observed by using the range of observed thicknesses.
The analytical methods are based on the assumption that the well is
fully penetrating with respect to the aquifer, i.e., is open to the full thickness of the
aquifer. For wells that are only partially screened in the aquifer it is recommended that
the combined screened intervals or thickness of water bearing zones (whichever is less) be
utilized as the aquifer thickness in the model development. The rationale for this is that
groundwater flow to the well tends to be horizontal, owing to the fact that horizontal
conductivity generally exceeds vertical conductivity by approximately an order of
magnitude or more. A typical ratio of vertical to horizontal conductivity is 0.01.
In areas where the water-bearing zones are in volcanic rock, e.g.,
basalt, it is not uncommon for the well to be cased to the volcanic rock and then
constructed as an open hole for the remainder of the well. Often, there is no information
on the well report to indicate the nature of the water-bearing zones. Clearly it is
inappropriate to use the entire length of open hole as the aquifer when experience tells
us that the water is normally derived from interflow zones, consisting of the more porous
and brecciated (i.e., broken up) flow tops.
Observation indicates that the interflow zones within basalt range from
<5 percent to >25 percent of the total flow thickness. If there is no information on
the well report to indicate the thickness of the water-bearing zones in an open-hole
basalt well, a thickness equivalent to 10 percent of the open hole should be used for
aquifer thickness or open interval.
Hydrogeologic Boundaries. An important step in adequately
determining the ZOC of a pumping well is the recognition of the impacts to groundwater
flow of hydrogeologic boundaries. These boundaries can generally be considered to be
combinations of two limiting cases: constant head and no-flow boundaries:
1. Constant Head Boundary. The most common example of a
constant head boundary is a stream (Figure 3-5). In the case of a
perennial stream, largely supported by surface flow through a drainage basin, the water
level in the stream may show only minor variations over short time intervals. If the ZOC
of a proximal pumping well intersects the stream, a hydraulic connection may be
established whereby a portion of the water being derived from the well originates within
the stream. The net effect is to reduce the size of the ZOC (i.e., less water is being
derived directly from the aquifer). The actual impact will depend on the extent the stream
penetrates the aquifer and how well developed the clogging layer is.
Although activities within the watershed upstream from where the
hydraulic connection is established could impact the water quality of the groundwater
derived from the well, it is unlikely that the community will be able to exert control
over the entire watershed as a means of protecting their drinking water supply. As an
alternative, we recommend that the community include as part of their management strategy
and contingency plan development, a procedure that will protect the aquifer should an
event occur upstream that releases a contaminant to the stream. This may be in the form of
assuring that the public water system is immediately notified of such a release and has
sufficient storage so that the use of the well could be discontinued until the threat of
drawing the contaminant into the aquifer is past.
In the WHPA 2.0 package, the programs MWCAP and GPTRAC will simulate a
stream boundary directly. The program RESSQC is capable as well; however, the boundary
must be simulated by the modeler through the use of image wells. Importantly, the MWCAP
and GPTRAC programs assume that the stream is linear and fully penetrates the aquifer, a
situation that is rarely encountered in real settings. Also, the programs do not consider
the reduced flow to the aquifer from the stream as a result of the presence of the
clogging layer, e.g., fines in the steam bed that reduce the permeability of the materials
through which stream water must flow to reach the aquifer. Approximation of a partially
penetrating stream and the presence of a clogging layer may be accomplished through
variable placement of an image well using RESSQC, supplemented by reducing the recharge
rate of that well.
2. No-Flow Boundaries. These boundaries are those
across which groundwater movement is either prevented or reduced to negligible over the
period of observation. No-flow boundaries are generally associated with groundwater
divides or permeability contrasts (Figure 3-5). A groundwater divide
is analogous to a topographic divide that separates surface water drainage basins. A
groundwater divide (Figure 3-1) is an elevation high on the water
table or potentiometric surface; groundwater moves away from the divide in both
directions. Groundwater cannot flow across the divide, consequently, the divide represents
a no-flow boundary.
Three types of no-flow boundaries formed by permeability contrasts have
been recognized in Oregon. No-flow boundaries resulting from geologic contacts are perhaps
the most common. These occur when the aquifer is limited in its lateral extent, either by
pinching out, i.e., thins to negligible thickness as in a sand lense, or being in contact
with a less permeable rock type:
Modeling Hydrologic Boundaries. In all three of these cases, groundwater flow across the boundary is inhibited because of the difference in the permeability of the materials. Water will not flow across the bedrock-alluvium contact to supply the well. As in the stream boundary, MWCAP and GPTRAC will model the no-flow (barrier) boundaries directly, making the assumption that the boundary is linear and fully penetrating. Because no-flow boundaries do tend to penetrate the aquifer, these assumptions are not as critical in the no-flow case as they are in the constant head boundaries. RESSQC can simulate the barrier boundaries through the use of image wells. Leaking boundaries can also be accommodated through image well modeling (Bair et al., 1991).
Parameter Estimation and Sensitivity Analysis - The key to
a quality delineation clearly is the use of data that is representative of the system
being modeled. Two weaknesses of the analytical approach are: (1) the models generally
assume uniform characteristics within the aquifer (see GPTRAC - Blandford and Huyakorn,
1991), and (2) it is not possible in general to calibrate and validate the model (see
CZAEM - Strack et al., 1994) and CAPZONE (Bair et al., 1991). As a result, it is often not
possible to test whether the assumptions made in the model development accurately reflect
It is not our intent to detail how one estimates all the parameters
utilized within the individual techniques or repeat recommendations made elsewhere (See Appendix A). Rather, we are requiring that a sensitivity
analysis be performed to determine the relative importance of each parameter. This may
provide information on where to expend limited resources to improve the quality of the
data used and make the delineation as representative as possible.
Parameter Uncertainty. In this document, we are using sensitivity analysis to mean the provisional calculations of the WHPA using the range of uncertainty associated with each parameter. Specifically, if estimates or independent measurements of a hydraulic parameter indicate a range of possible values, an assessment of the impact of using one value or the other on the shape, extent or orientation of the WHPA should be evaluated. Unfortunately, as with any estimate, there will be a range of values for most of the parameters of interest. As such, there will be a large number of possible combinations of values that could be substituted into the equations.
As discussed above, the WHPA code MONTEC program is capable of
incorporating these uncertainties into the WHPA delineation for single wells in a
semi-confined or confined aquifer setting and providing a delineation that gives an
estimate of the probability that the true delineation falls within the boundaries of the
calculated WHPA. For other settings, or other programs within the WHPA code package, such
an evaluation is not available.
As an example, you may have specific capacity data from an alluvial
aquifer that indicates a significant range of hydraulic conductivity (K) values or you are
uncertain regarding the value to use for the effective porosity of a basalt aquifer (Table 3-1). You should first try to limit the variability through
careful analysis of the data in the context of the conceptual model to try to determine
which value(s) within the range may be most applicable (EPA, 1994). You should then use
the ranges of values in the appropriate models to determine the sensitivity of the area of
the WHPA. If there is only small variations here, it may be sufficient to simply choose
the most protective version of the WHPA.
If there are large differences in the areas of the calculated WHPAs, it
may be more cost-effective to try to gather additional information (e.g., an aquifer
test), rather than choosing, and then of course, managing the area within the larger WHPA.
Multiple Water-Bearing Zones. An additional example concerns a
situation where a well is screened in two water-bearing zones with significant (i.e., an
order of magnitude) differences in hydraulic conductivity. Using an average transmissivity
value for the two units may yield a WHPA that does not represent the actual time-related
As an alternative, the discharge from the well could be partitioned
between the two water-bearing zones (zones 1 and 2) in the following manner (Golder
Q1 = w1 ´ Q
Q1 represents that part of the total discharge
(Q) that can be attributed to water-bearing zone 1. K1 and K2 and b1
and b2 represent the hydraulic conductivities and thickness of water-bearing
zones 1 and 2, respectively. The discharge derived from water-bearing zone 2 (Q2)
is derived in an analogous manner.
Using the discharge data thus derived and li and
transmissivities (Ti = li ´ Ki) of some water-bearing zone (i), the WHPA associated with each
zone may be determined using Qi and compared with that derived by assuming
average values. In many cases it will be seen that the bulk of the water is derived from
the high-K zone and, as a result, the WHPA associated with that water-bearing zone will be
larger than that derived from average values. Note that in terms of transmissivities:
Modification of WHPA Orientation - The
orientation of the WHPA in the analytical models is based on the input data concerning the
direction of groundwater flow. This information is derived either from regional
considerations or from the direct measurement of static water levels (SWLs) in appropriate
wells at some particular time. It has been the observation of many hydrogeologists in the
state that the gradient in a given area may change direction dramatically (up to 180°?) on a seasonal basis, particularly for
shallow unconfined systems. The change in gradient direction (and magnitude) may be the
result of a change in recharge pattern or a change in the pattern and amount of
withdrawals from the aquifer.
Incorporating Gradient Variations in the Model. Because of the
uncertainty attached to the gradient direction, the state is requiring that the water
system incorporate the potential variability in one of two ways:
1. Evaluation of the gradient quarterly for at least 12 months.
In this step, wells within the aquifer of concern will be monitored for SWLs every 3
months for at least a year. If possible, wells that are screened in the same interval of
the aquifer should be used in the monitoring process. The SWLs will be contoured and the
direction of groundwater flow will be determined. WHPAs will be generated along each of
the gradient directions determined. The area used for wellhead protection will be the
entire area encompassed by the quarterly WHPAs.
2. Arbitrary rotation of the gradient direction. In the absence of quarterly SWL determinations, a community may elect to recalculate WHPAs along gradient directions that are rotated an arbitrary amount from the gradient direction determined in the conceptual model development.
The supplementary delineations in practice do not require a significant
amount of program development. We anticipate that these extra delineations would only
require changing the program input for the gradient and direction, no other modifications
would be necessary (unless demanded by the conceptual model). Time investment at this step
would be minimal.
As an added precaution regarding changing gradient directions, OHD is
requiring that a circular area surrounding the wellhead, with a radius equivalent to the
6-month TOT using the CFR method, be included in the well's WHPA.
Numerical methods provide a high degree of accuracy and can be applied
to almost all types of hydrogeologic situations. Accordingly, these methods are able to
incorporate complex boundary conditions and variations in hydraulic properties within the
aquifer. A large number of numerical models are presently available (van der Heijde and
Developing the Model - The process involves dividing the
area into cells or elements with each element located uniquely by the coordinates of its
corners. Some numerical model codes also allow for vertical division the model into
multiple layers. Each cell is characterized in terms of the aquifer's (or bounding
material's) hydraulic characteristics, e.g., hydraulic conductivity, thickness, storage,
recharge, etc.). Boundaries to groundwater flow can be simulated by specifying cells
(elements) with specific boundary conditions (e.g., constant head) or permeability
characteristics. The modeler is provided with a significant degree of flexibility in the
model development as a result of her/his having the option to design the grid to fit the
conceptual model, e.g., making the grid spacing smaller in areas where significant
variations occur or where detail is desired.
After the model is designed, it is run (probably several times) until it
converges on a solution that is within a specified error criterion. Typically, error
criteria for heads and water balance are evaluated. Discrepancies may indicate that the
error criterion is too small (e.g., heads) or the model does not adequately characterize
the hydrogeologic system being modeled. The numerical model uses the hydraulic data to
calculate head and flux values for each cell within the groundwater flow model.
Calibration - An important characteristic of numerical
models is the ability to calibrate the model to fit observed data. In the calibration
process, the modeler identifies specific calibration targets, usually known values of
hydraulic head or fluxes into and out of the groundwater system. The modeler compares the
head field and fluxes generated by the numerical model to the observed water levels and
fluxes. The comparison is generally done statistically so that the degree of agreement
between the modeled and observed conditions can be evaluated quantitatively. It is also
useful to plot the areal distribution of the differences. When an acceptable comparison is
achieved, the model is said to be calibrated.
Calibrated values for hydraulic conductivity, storage, boundary
conditions, etc., are then systematically changed during a process called a sensitivity
analysis. The sensitivity analysis is done to quantify the uncertainty in the calibrated
model caused by the uncertainties in the estimates of aquifer parameters, boundaries and
stresses (e.g., pumping) on the aquifer. A sensitivity analysis is usually done by
changing one parameter value at a time and observing how much change in the model result
occurs, i.e., how sensitive the model is to that parameter. The results of the sensitivity
analysis should be reported as part of the model development summary.
The calibrated model can then be used to delineate the WHPA, through the
use of particle tracking routine. The particle tracking routine calculates groundwater
flow as a function of the head distribution developed by the numerical method. Capture
zones as a function of time-of-travel can be delineated.
Validation - The numerical model can be further refined
through the validation process. In this step, the impact of some perturbation to the model
is explored, e.g., increased pumping from one or more wells, etc. The resulting new head
field simulation can be compared to the actual situation when it occurs. Modifications to
the input data or to boundary conditions can be made in order to reach agreement between
predicted and observed head distributions.
Once a numerical model has been calibrated and validated, it becomes not
only a tool for WHPA delineation, but also a planning tool for the community. Through the
use of the model, the impact of new wells, infiltration lagoons, potential contaminant
releases, etc., can be evaluated with much greater certainty.
Evaluation of Delineation Techniques
In this section, advantages and disadvantages of the above three
techniques are described.
Calculated Fixed Radius:
Advantages - Simplicity, low cost, does not require significant
amount of data acquisition.
Disadvantages - Generally not representative of the groundwater
system, prone to legal challenges, tends to over protect downgradient and under protect
upgradient, often yields larger area than other techniques.
Analytical Techniques/Hydrogeologic Mapping:
Advantages - Incorporates hydrogeologic characteristics of the
aquifer, groundwater flow and hydrogeologic boundaries into the model, provides for a
defensible delineation of the WHPA, is based on site-specific information. Often produces
a WHPA that is smaller than the one produced using the calculated radius method.
Disadvantages - Assumes a uniform aquifer (note that some
exceptions to this do exist), requires significant expertise and is moderately costly.
Advantages - 3-D modeling of groundwater flow, can account for
evapotranspiration and recharge, groundwater-surface water interaction can be quantified,
provides a more accurate delineation of the WHPA, accounts for variation in hydraulic
parameters and boundary conditions, can be used for predicting the impact of natural and
human-related activities on the flow field. Often produces a smaller area to manage than
Disadvantages - Costly relative to other techniques, requires
significant amount of data collection and high level of expertise to set up the grid.
Factors That May Influence the Delineated Area in the Future:
A WHPA is delineated based on a set of conditions with regard to
hydraulic properties of the hydrogeologic units, fluxes into and out of the system and
stresses applied to the aquifer. Although it may seem unlikely that the hydraulic
properties of the aquifer could change appreciably, it is possible that surface-related
activities could lead to a significant change from conditions assumed to exist in the
initial conceptual model.
Prolonged drought and/or over-drafting of an unconfined aquifer could
result in a decrease in the thickness of the saturated zone or variations in gradient.
Both parameters could cause a change in the size and orientation of the WHPA.
Changes in recharge, either in amount or pattern of application may
alter the dimensions of the WHPA. Recharge could be a function of natural precipitation or
result from changes in irrigation or application of waste water in the area.
Any addition of high-volume wells in the WHPA or in proximity to its
boundary may alter the shape of the WHPA. Significant changes in the pump rate will
obviously have an impact on the size of the well's capture zone. Figure
3-6 portrays the change in shape of the original delineation (a) as a result of the
addition of a single well immediately upgradient (b) and two wells in proximal positions
(c). Details of the models are given in the figure caption.
After the WHPA has been delineated, the community should have as part of
their management strategy a method by which the above influences can be recognized and
The delineations discussed above are based on water movement within the
aquifer only. In fact, a contaminant released at or below the surface must travel or be
transported across the unsaturated zone to the aquifer. Absent a direct communication to
the aquifer, e.g., an improperly constructed well or a through-going fracture system, the
characteristics of the unsaturated zone control the time and probability that the
contaminant will reach the aquifer. Accordingly, it is recommended that a susceptibility
analysis be conducted within the WHPA.
Use of the Analysis - The utility of the susceptibility
analysis is that it indicates areas within the WHPA in which the aquifer is most
susceptible to contamination, i.e., has the highest potential of being impacted by surface
activities. When the susceptibility analysis is combined with the results of the potential
contaminant inventory (Step 4), the highly vulnerable areas can be recognized. A high
vulnerability would be indicated where a high-risk surface practice occurs in an area
where the aquifer has a high susceptibility.
Several methods exist for determining aquifer susceptibility on a
site-specific scale. The EPA has developed a technical document on "Managing Ground
Water Contamination Sources in Wellhead Protection Areas" that incorporates both
susceptibility and risk rankings into its determination. Not all potential sources of
contaminants are considered; however, in some cases the risk triggers are somewhat high.
Oregon State University has developed a decision aid that is designed to
help producers consider groundwater protection when using various agricultural chemicals
in crop production. The Oregon Water Quality Decision Aid utilizes soil characteristics
such as sorption capacity and permeability and chemical characteristics of the contaminant
such as its sorption potential and persistence to arrive at an estimate of the aquifer's
vulnerability to that chemical. Much of the data has been compiled by OSU and the
decision-making process is very user-friendly. (More information can be obtained by
calling OSU at 541-737-5713.) The OHD developed a susceptibility analysis in conjunction
with a monitoring reduction program for public water systems (OHD, 1992). The
determination of susceptibility makes use of information regarding the hydrogeologic
characteristics of the area and the chemical characteristics of the contaminant of concern
(Figure 3-7). Decision points are assisted through the use of
Hydrogeologic Characteristics - The OHD process begins by
using available soil data from Soil Survey Reports. Within the area of a mapped soil type,
the depth to the aquifer and weighted hydraulic conductivity would be estimated from well
reports (or other available information). These two parameters would be compared in a
matrix to yield a traverse potential score. The traverse potential score is then utilized
in conjunction with the hydraulic surplus, the difference between water applied, as
irrigation or precipitation, and the water lost through evapotranspiration and runoff, to
yield an infiltration potential. The infiltration potential is an estimate of the
probability that water will migrate from the surface downward to the aquifer.
Chemical Characteristics - Consideration of the chemical
characteristics begins with using the tendency for the contaminant to sorb (attach itself)
to organic matter along with the amount of organic matter in the soil (as reported in the
soil survey). These parameters combine to yield the mobility potential, the probability
that the chemical will move through the soil zone.
The mobility potential is linked to the infiltration potential (Figure
3-7) to derive the leach potential, an estimate of the probability that water migrates to
the aquifer carrying the contaminant. Combining the leach potential with the persistence
of the contaminant, i.e., how long the contaminant "survives" before processes
in the subsurface cause it to break down. This latter step indicates whether or not the
aquifer is susceptible in that area.
Example - An example of the application of this method is
illustrated in Figure 3-8. The WHPA for a well has been delineated and the soil survey of
the area consulted to determine the principle soil types. In this example, only two types
dominate, soil A and soil B. A cross section has been drawn from A to A¢ in the lower figure. Note that there are
significant differences in soil thickness and depth to the aquifer. The question being
addressed is how does the aquifer susceptibility beneath soil A differ from that below
soil B? The contaminant of concern here is trichloroethylene (TCE).
The comparison is illustrated in the lower table of Figure
3-8. The Traverse Potentials beneath the two soils are quite different based on the
different hydraulic conductivities, the potential of movement of water from the surface to
the water table being lower beneath soil A. Hydraulic surpluses are considered the same
for both soil types, but the lower traverse potential beneath soil A leads to a lower
infiltration potential beneath that soil as well.
Figure 3-7: Aquifer Susceptibility Analysis
Source: OHD, 1992
Figure 3-8: Example of Using The Susceptibility Analysis inWellhead Protection Strategies
Potential Contaminant: TCE with Log Koc = 2.0
Characterics - Type
Soil A - Silt
Soil B - Silty Sand
% Organic Matter
Organic Matter Score
Depth to Aquifer (ft)
Hydraulic Surplus (in.)
Mobility Potential (TCE)
The Mobility Potential for TCE through soil B is higher because of the
lower organic matter content, which in this process is recorded by a higher organic matter
score, in that soil. The combination of the higher
infiltration and mobility potentials beneath soil B lead to a higher leach potential for
TCE in that area. This coupled with the high persistence of TCE indicates that the
susceptibility of the aquifer is greater under soil B than under soil A.
It is apparent from Figure 3-8 that the value of
the infiltration potential provides an estimate of the general susceptibility of the
aquifer while the leach potential, coupled with the persistence of the contaminant, will
provide an estimate of the vulnerability of the aquifer to contamination from a specific
chemical. It is recommended that data from soil surveys and well reports be used to
determine the infiltration potential at individual wells selected to provide good coverage
throughout the WHPA(s). These values can be contoured to indicate areas of high versus low
infiltration potential. General management strategies, particularly those designed to
address future land use, can be developed from this data base. Individual potential
sources can be further evaluated using chemical specific data to determine the potential
of the contaminant of concern migrating to groundwater. Specific management strategies,
designed to minimize risks associated with a specific existing land use, can be developed
from this information.
General Application - Within a wellhead protection area,
communities will want to focus their limited resources on those areas where the risk of
contamination is greatest. Determination of the infiltration potential (see
Figure 3-7) across the WHPA will indicate those regions where rapid infiltration of
water from the surface is most likely. A contaminant release in these areas would pose a
greater risk than in areas where the infiltration potential is low.
OHD's guidance manual suggests selecting representative wells within the
WHPA that penetrate the aquifer of concern and calculating the traverse potential for each
based on well logs and soil survey reports. Ideally the distribution of wells will be
sufficient to provide at least one value of the traverse potential per quarter-quarter
section (i.e., 16 wells per square mile). Using precipitation data and/or irrigation data
(see OHD's guidance), the hydraulic surplus could be calculated and combined with the
traverse potential to yield the infiltration potential (Figure 3-7).
If data is sufficient, the WHPA area could be contoured with respect to the infiltration
potential to more readily identify those regions where the risk of infiltration of a
contaminant release to the aquifer is greatest. Communities could use this information to
prioritize the area for purpose of protective management.
Individual sites could be more specifically evaluated by considering
further the chemical characteristics of the contaminant of concern. This would be done by
combining the infiltration potential at the site with the leach potential of the specific
chemical, i.e., the tendency for the chemical to attach itself to the organic matter
present in the soil, and the chemicals persistence in the environment. OHD's guidance
manual provides pertinent data for those chemicals that are routinely monitored for
drinking water purposes. The procedure will result in an estimate of the susceptibility,
high, moderate or low, of the aquifer for that specific chemical. A facility where the
susceptibility is low will obviously need less oversight than a facility where the
susceptibility is high.
Submitting the Delineation for OHD Certification - The
final delineation should be submitted to the groundwater coordinator of OHD's Drinking
Water Program. The report submitted to OHD should include a map showing the delineated
area and a report that provides documentation of the model parameters used and
justification for the assumptions made in the modeling effort. Well locations and
delineated areas must also be submitted digitally, in DXF or ArcInfo GIS compatible
format. For those delineations involving the development of a conceptual model, the report
should include well logs with locations keyed to a map, cross-sections (fence diagram),
aquifer test results and a discussion of the hydrologic units, boundaries, groundwater
flow direction and gradient, etc. OHD will expect that the issues, concerns and required
tasks and decision points discussed in this chapter will be addressed in the report.
It is recommended that a line of communication between the local
wellhead protection committee, or their consultant, and OHD's groundwater coordinator
(503-731-4010) be established early on in the delineation process. This provides OHD the
opportunity to provide technical assistance and will facilitate the submission and review
OHD will make every effort to review the delineation report in a prompt
and timely manner. In the event that the report cannot be reviewed within 60 days, OHD
will provide the submitter in writing with an estimate of the date in which the review
will be completed.
The final delineation should be submitted to:
Drinking Water Program
Oregon Health Division
800 NE Oregon Street
Portland, OR 97232
Bair, E. S.; Springer, A. E.; and Roadcap, G. S., 1991. Delineation
of Travel Time-Related Capture Areas of Wells Using Analytical Flow Models and
Particle-Tracking Analysis. Ground Water, 29:387-397.
Blandford, T. N.; and Huyakorn; P. S., WHPA: A Modular
Semi-Analytical Model for The Delineation of Wellhead Protection Areas, Version 2.0.
U.S. Environmental Protection Agency, Contract Number 68-08-0003. [Distributed by the
International Ground Water Modeling Center.]
Bradbury, K. R.; Muldoon, M. A.; Zaporozec, A. and Levy, J.,
1991. Delineation of Wellhead Protection Areas in Fractured Rocks. U.S.
Environmental Protection Agency, EPA 570/9-91-009.
Domenico, P. A. and Schwarz, F. W., 1990. Physical and
Chemical Hydrogeology, John Wiley & Sons, New York, 824p.
Golder Associates, 1994. Wellhead Protection Area Delineation
Report, Project No. WHPA-2, Prepared for Springfield Utility Board and Rainbow Water
District, Springfield, Oregon. Golder Associates, Inc., Redmond, Washington.
Haitjema, H. M.; Wittman, J.; Kelson, V. and Bauch, N., 1994. WhAEM:
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Fetter, Jr., C. W., 1981. Determination of the Direction of
Groundwater Flow. Ground Water Monitoring Review, 1:28-31.
Kreitler, C. W. and Senger, R. K., 1991. Wellhead Protection
Strategies for Confined-Aquifer Settings. U.S. Environmental Protection Agency,
Long, J.C.S.; Remer, J.S.; Wilson, C.R. and Witherspoon, P.A.,
1982. Porous Media Equivalents for Networks of Discontinuous Fractures. Water
Resources Reserved Research, V. 18, pp. 645-658.
Oregon Health Division, 1992.
Guidance Document for Phase II/V Use and Susceptibility Waiver
Applications. Drinking Water Program, 92 p.
Strack, O.D.L.; Anderson, E.I.; Bakker, M.; Olsen, W.C.; Panda, J.C.;
Pennings, R.W. and Steward, D.R., 1994. CZAEM User's Guide: Modeling Capture Zones
of Ground-Water Wells Using Analytic Elements. U.S. Environmental Protection Agency,
Todd, D. K., 1980. Ground Water Hydrology. John Wiley and
Sons, Inc., New York.
USEPA, 1987. Guidelines for Delineation of Wellhead Protection
Areas. U.S. Environmental Protection Agency, EPA-440/6-87-010.
USEPA, 1993. Wellhead Protection: A Guide for Small
Communities. U.S. Environmental Protection Agency Seminar Publication,
USEPA, 1994. Ground Water and Wellhead Protection. U.S.
Environmental Protection Agency Handbook, EPA625/R-94/001.
van der Heijde, P. and Beljin, M.S., 1988. Model Assessment
for Delineating Wellhead Protection Areas. U.S. Environmental Protection Agency,
Wuolo, R.W.; Dahlstrom, D.J. and Fairbrother, M.D.,
1995. Wellhead Protection Area Delineation Using The Analytic Element Method of
Ground-Water Modeling. Ground Water, 33:71-83.
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