Consideration of Hydrogeologic Factors in Designing Wellhead Protection Areas

Abstract

Legislation was enacted in 1986 to establish the Wellhead Protection Program in the United States. This is a Federal program directing the U.S. Environmental Protection Agency to assist the states in designating and protecting areas surrounding public water supply wells from contaminants that may adversely affect human health. Under this program, the states will delineate the Wellhead Protection Area (WHPA) for each public well or wellfield. The WHPA is defined as the surface and subsurface area surrounding a well or wellfield through which contaminants can reasonably be expected to move toward and reach the well. The monitoring of WHPAs can be an integral component of the wellhead protection program. The components of monitoring network design in wellhead protection areas include geohydrologic and geochemical factors of the aquifer flow system; the hydraulics of the wellfield; external hydraulic stresses on the flow system, that is, stresses other than those of the wellfield; anthropogenic and natural sources of potential contaminants; and the chemical and organic nature of potential contaminants. The monitoring of wellhead protection areas should be designed to determine sources of potential contaminants in the WHPA and to detect migration of contaminants toward the wellfields in time to provide remedial or mitigating action to protect the water supply, or in time to provide alternative public water supplies before the source is contaminated. Another role of monitoring is to refine the delineation of the WHPA. The contributing area to the wellfield is not static, but subject to change as it is influenced by increase or decrease in withdrawal from the public supply and as the flow system is influenced by other stresses, including climatic changes that affect recharge from infiltration of precipitation and streamflow, and withdrawal for agricultural or industrial supplies. These factors and others may influence the size and location of the zone of contribution to the well or wellfield. It is therefore important to initially delineate a WHPA that is large enough to adequately protect the water supply under different discharge-recharge conditions.