STEP 7

Planning for Future Public Water System Needs


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Water systems may find it necessary, as a result of either existing or projected increased demand, to explore the development of additional groundwater sources for drinking water. Wellhead protection provides a mechanism that can be used to help select the best site and to identify areas that should be protected now in order that they will provide quality drinking water in the future when they are needed. Additionally, it should be realized that the development of a new groundwater source in the vicinity of existing sources may modify the movement of groundwater in the subsurface, perhaps changing the shape and orientation of existing wellhead protection areas (WHPAs). Evaluation of the significance of those changes is necessary in order to ensure that the management strategy that is in place will continue to protect the community's drinking water supply.

In this section the procedures are outlined for systems adding new groundwater sources or modifying existing sources. Water systems are reminded that the development of any new source, or a major modification to an existing source, requires prior approval by the Oregon Health Division, as part of the existing plan review requirements (OAR 333-61-060). The work must also comply with appropriate construction standards as described in OAR 333-61-050.

New Groundwater Source:
A groundwater source may be a well, wellfield or spring. A new groundwater source is defined as either an additional groundwater source, or an existing groundwater source that has been modified in a manner to increase its capacity or discharge to the system. The guidance in this section applies only to those sources that are owned by the water system.

A.   New Groundwater Sources Outside an Existing WHPA Boundary:  If the water system is planning on developing a previously unexploited groundwater resource that occurs outside the boundary(ies) of already delineated sources, there are several steps that should be followed. If more than one potential site is available, the system should conduct a provisional delineation and preliminary potential contaminant source inventory for each site being considered. By provisional delineation it is meant the application of the existing model for already delineated wells to the considered sites. Depending on the size of the system, the following procedures are recommended:

1.   Systems using wells and serving a population of 500 or less.

2.   Systems serving greater than 500 population and using wells as a drinking water source.

3.   Systems deriving their groundwater from springs.

B.   The Development of New Sources Within a System's Existing WHPA.

If more than one potential site is available for the new source, the system should proceed in its evaluation of those sites according to the discussions above. If a system develops a new well, or increases the capacity of an existing well that is within an already delineated WHPA, it is likely that the new or modified source will have a significant impact on the existing WHPA (see Section 3-3 in this document). In all cases, the affect of the new well on the already existing WHPA geometry and orientation should be evaluated.

1. Calculated Fixed Radius Method. The impact of proximal wells on the individual WHPAs is accounted for in the delineation process by determining the "capture zone" associated with a hypothetical single well having the combined pumpage of the other wells and located according to the individual pumping rates (see Section 3-3 and Appendix A).

2. Analytical and Numerical Methods. In these methods, the impact of proximal wells can be accounted for within the models themselves, provided the location and pump rate of these wells are known. The models calculate capture zones based on the input provided. If such a model has been used to previously delineate the WHPA of a system's well(s), the model can be rerun and new or adjusted WHPAs identified by modifying the input to the model. The system may wish to contact the consultant who performed the original delineations for this task.

The adjusted WHPA boundaries should be compared to the existing WHPA boundaries. If significant differences are observed, the system should consider modifying the existing wellhead protection plan to encompass the new delineation.

C.   Future Sources:

Systems may recognize that as a result of growth, diminishing sources or both, there will be a need for additional groundwater supplies, beyond their current capacity, in the future. These systems may choose to identify the area(s) where this future supply will be obtained for protection purposes. These areas may be identified through a regional hydrogeologic study designed to assess the availability of groundwater resources. This study will involve the development of a conceptual model similar to that required by the delineation step. Existing wells and perhaps test wells will be utilized to evaluate water quantity issues.

Once an area or site for future groundwater development been identified, it will be in the best interest of the system to develop a protection strategy to apply in the area in order to ensure that the groundwater will be usable for drinking water when the future need arises. Provisional delineations may be utilized in order to recognize the more critical areas needing protection. With this information in hand, future development can be directed in a manner that will allow for growth but will provide a layer of protection for the system's future drinking water needs.



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