Understanding Zone Of Influence

WHY 7000 FEET?

The conditions set forth in the permit granted by NH DES to USA Springs, Inc., mention a 7000-foot circle centered on the extraction wells. Why 7000 feet?

The zone of influence from pumping the wells at 310,000 gallons per day was shown by Gradient Corporation in their 2003 pump test report as a small comma-shaped area around the USA Springs wells, extending west along Rt.4 (shown in gray on the map below). This area is more correctly described as the observed zone of influence., that is, the area within which water levels dropped during the pump test.

The potential zone of influence is actually larger, extending beyond the last well that experienced drawdown to a point where drawdown will be zero, estimated at 7000 feet in the direction along Rt.4 (see graph). The reasoning is that a well within that distance in any direction from the production wells might experience some drawdown, so a circle has been drawn on the map. This is the circle within which anyone noticing a loss of water can expect some help according to the conditional Large Groundwater Withdrawal Permit. The Nottingham Planning Board insisted that this larger ZOI also be used in dealing with water quality complaints (see Domestic Well Water Supply Contingency Plan, Step 2).

The recharge area extends farther beyond the potential zone of influence, because water will be moving toward the production wells even where the rate of recharge keeps up with the rate of withdrawal. The circle approach works only for aquifers that allow water to flow at the same rate in all directions, which certainly is not the case for crystalline bedrock aquifers. The water will be moving along fractures in the rock, following paths that are very hard to predict. I have proposed that the recharge area might follow the zone of interconnecting lineaments (see map) as described in a report by the USGS, though we do not know which of those lineaments actually correspond to bedrock fractures.

Summary by Peter J. Thompson, Ph.D., P.G.