SOURCES OF WATER TO WELLS

• Decrease in discharge to surface water - streams and lakes
• Decrease in storage (lowered water levels)

After a long period of pumping around large pumping centers (often on the order of decades), water levels can stop declining or decline very slowly, and all of the water going to the wells is water that would have flowed to surface water.

In general, pumping does not increase or decrease the amount of natural recharge, although total recharge can be significantly increased by human activities such as irrigation.

Sustainable yield is a socio-economic term, not a scientific term. Sustainable yield is usually considered to be that rate of pumping from wells for which the impact is acceptable; however what impact is “acceptable” is subjective. There will always be a hydrologic effect of pumping from wells. In this sense, there is no such thing as “safe yield”.

You can also ask: what is the source of water to wells in a particular area of interest? In that case, besides decrease in storage and decrease in discharge to surface water, there can also be increased ground-water inflow into the area of interest from adjacent areas (or from underlying or overlying formations). But this water also ultimately originates as storage release and reduced streamflow, only outside the area of interest.

Schematic diagram of sources of water to wells in Great Lakes region: A) natural, predevelopment conditions, B) pumping conditions (source: modified from U.S. Geological Survey Circular 1186)

Pumping causes the flow system to change in many respects. It can increase downward flow to a deep well, reduce flow to surface-water bodies, or even cause water to move from a stream or lake to a well.

Thumbnail schematic sections showing diversion of ground water to wells:

The land area contributing recharge to a discharge well, sometimes called the zone of contribution, is the surface area at the water table where water entering the ground-water system eventually flows to the well. This contributing zone must provide an amount of recharge that balances the amount of water being discharged from the well. The lower the recharge rate, the greater is the area of contribution for a fixed rate of pumping.

A) Schematic section showing ground-water circulation from recharge area to well, B) Recharge area in plan view (source: U.S. Geological Survey Circular 1186)

If a well reverses the natural pattern of ground-water discharge and actually induces flow from the stream, then the zone at the land surface contributing recharge to the well is smaller than it would be otherwise because some of the well water is coming from a source other than recharge. In certain areas (typically where coarse alluvial or outwash deposits surround rivers), the supply to shallow wells can consist almost entirely of captured surface water routed underground.

Computerized flow modeling is often used to map the areas from which ground water circulates to surface-water bodies and/or wells. The model calculates the three-dimensional paths that ground water follows from an initial location where recharge enters the water table to a final point where it discharges from the ground-water system. The combined recharge areas that feed a given discharge feature (be it a stream, lake, or wetland) constitutes the zone of ground-water contribution for that feature. Pumping wells compete for ground water with natural discharge locations and change the size and shape of the natural zones of contribution.

Here is an example where a computerized ground-water flow model was used to evaluate the potential effect of proposed pumping wells on shallow surface water discharge in southeastern Wisconsin.

The model area focuses on a part of Waukesha County around Eagle, Wisconsin:

Thumbnail map of study location (source: D.T. Feinstein)

The model results show the predevelopment condition in the absence of pumping (Figure A) and the simulated effect of two shallow wells (Figure B).

	Figure A - Simulated ground-water source areas for surface-water features (source: Tim Eaton, Wisconsin Geological and History Survey, adapted from Figure 8 - Eaton, 2004)
	Figure B - Simulated ground-water source areas for surface-water features - effect of proposed wells (source: Tim Eaton, Wisconsin Geological and History Survey, adapted from Figure 8, Eaton, 2004)

In the absence of pumping, groundwater that recharges at the water table flows either to a stream, a lake, or a wetland. The addition of wells shrinks the size of the zones of contributions for the surface-water bodies. In particular, the simulated pumping decreases the amount of ground water that flows to the wetland by diverting ground water to the wells. It is possible to use these model results to not only simulate how much water is lost to a wetland, but also to estimate the change in the wetland water level.

More information on this example of a hydrogeologic and modeling study aimed at investigating the sources of water to wells is available from the Wisconsin Geological and Natural History Survey. The reference is:

Eaton, T.T. 2004. Construction of a groundwater flow model in the area of the Village of Eagle based on refinement of a regional groundwater flow model for southeastern Wisconsin: administrative report to the Southeastern Wisconsin Regional Planning Commission, Wisconsin Geological and Natural History Survey Open-File Report 04-XX (number to be assigned), 49 p.

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