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Delineation of A Drinking Water Protection Area: Groundwater Sources
The 1986 Amendments to the Federal Safe Drinking Water Act required states to develop Wellhead Protection Programs.  Oregon's EPA-approved program, referred to as the Drinking Water Protection Program (DWPP), is voluntary in nature and allows the local community to develop their own plan.  The state provides technical assistance to the community during plan development.  A community-developed Drinking Water Protection plan delineates the drinking water protection area (i.e., the groundwater resource supply drinking water to a public water system), inventories that area for potential contaminant sources, and develops management strategies aimed at reducing the risk of contamination from those sources.  The Department of Environmental Quality and Oregon Health Services have developed a Guidance Manual that describes the DWPP in detail.  This Fact Sheet focuses on the delineation step, describing the process, what data are necessary to do the delineation, and who should do it.

In order to adequately delineate the drinking water protection area (DWPA), there are several questions that we must be able to answer:

How does groundwater occur in the subsurface?  Groundwater occurs in the open spaces and fractures in the geologic materials (e.g., sediment and bedrock) that occur beneath the surface.  Aquifers are any geologic material where the open spaces are filled with water (i.e., are saturated) and through which groundwater can move at a rate and quantity sufficient to supply a well.  In Oregon, typical aquifers include alluvial sediments (river sands and gravels), basalt aquifers, and fractured bedrock.

Where does groundwater come from?  Groundwater is part of the global hydrologic cycle that controls the distribution of water on Earth.  It originates through the infiltration of precipitation that falls on the land surface and sinks through the soil.  For shallow wells, less than 100 feet in depth, the recharge area is often the immediate vicinity around the well.  For deeper aquifers, the recharge area may be several miles or more away.  If the infiltrating water encounters any contaminant at or below the surface, some of the contaminant may be dissolved and carried downward.  Note that it is not necessary for a spill to actually reach groundwater itself; downward percolating water is capable of transporting it to the aquifer.

What controls the movement of groundwater?  The water table is the upper surface of the saturated zone.  Groundwater tends to move from areas where the water table is high to where it is low.  How fast groundwater moves depends on the permeability of the aquifer and the slope of the water table.  Pumping wells can have a significant influence on the direction and rate of groundwater flow because the drawdown that they produce changes the shape and slope of the water table.

What do we mean by delineation?  When a well is turned on, it lowers the water table (drawdown) in its vicinity and begins to intercept water that is flowing downgradient (from where the water table is higher) towards the well.  This creates a capture zone in the aquifer, i.e., that part of the aquifer that supplies water to the well.  Any water molecule or contaminant that is within the capture zone will move naturally downgradient towards the drawdown area.  As it approaches the drawdown area, the water begins to "feel" the effect of the pumping and, as a result, it moves towards the well.  Water that is outside the capture zone may have its path changed as a result of the pumping well, but will move on past.  The area on the ground surface directly above the capture zone is the DWPA.  It is in this area that if a contaminant is released, it can be carried down to the capture zone by infiltrating water and be transported to the well by groundwater movement in the aquifer.

How is the delineation accomplished?  To delineate a DWPA for a water system serving a population of more than 500 people, the following steps must be followed:

  • Identify and characterize the aquifer(s) that supply the well - what is their permeability, their thickness, and extent?  Much of this information comes from well reports (i.e., the reports filed by well constructors when the well is drilled, available from the local Watermaster) or other reports that have been completed.  Wells must be located on a map and the various geologic units identified from the well log and pieced together to get a three-dimensional picture of the subsurface.  To characterize the aquifer, we must rely on pump tests or, in some cases, estimates based on well report descriptions.
  • What is the slope of the water table and what direction does groundwater move naturally?  For some systems, available reports will provide this information.  For others, area wells must be selected for water-level measurements.  The "at rest" water levels (i.e., the water table) are measured area-wide so that the shape of the water table can be identified and mapped.  From this information, the direction of groundwater flow (i.e., down the slope of the water table) can be identified and the velocity of movement calculated.
  • Use a computer model to determine the capture zone.  There are many modeling tools available for accomplishing this task.  The inputs to the model must be carefully selected so that the model closely represents the actual groundwater system being considered.

Who can do the delineations?  As indicated above, a significant amount of hydrogeologic data collection and analysis is required for this process.  Accurate interpretation of the data is essential if the delineation is to be meaningful.  For this reason, only a professional with hydrogeologic experience should complete the task (OAR 333-61-057 (1)(B)).