Private Domestic-Well Characteristics and the Distribution of Domestic Withdrawals among Aquifers in the Virginia Coastal Plain

Abstract

A comprehensive analysis of private domestic wells and self-supplied domestic ground-water withdrawals in the Coastal Plain Physiographic Province of Virginia indicates that the magnitudes of these withdrawals and their effects on local and regional ground-water flow are larger and more important than previous reports have stated. Self-supplied ground-water withdrawals for domestic use in the Virginia Coastal Plain are estimated to be approximately 40 million gallons per day, or about 28 percent of all ground-water withdrawals in the area. Contrary to widely held assumptions, only 22 percent of domestic wells in the Virginia Coastal Plain are completed in the shallow, unconfined surficial aquifer to which the water is returned directly by home septic systems. Fifty-three percent of the wells are completed in six deeper confined aquifers, and the remaining 25 percent are completed in the Potomac aquifer and confining zone, the deepest units in the confined system. Assuming an equal rate of withdrawal per well, 78 percent of domestic ground-water withdrawal, or about 30 million gallons per day, is removed from the regional confined ground-water system. Domestic ground-water withdrawal from an estimated 200,000 private wells supplies more than 15 percent of the population of the area and provides almost the entire source of water in some rural counties. The geographic distribution of these withdrawals is dependent on the self-supplied population and is highly variable. Domestic-well characteristics vary spatially as well, primarily because of geographic differences in depths to particular aquifers, but also because of well-drilling practices that are influenced by geographic, regulatory, and socioeconomic factors. Domestic ground-water withdrawals in the Virginia Coastal Plain were characterized as part of a larger study to analyze the regional ground-water flow system. Characterizing the withdrawals required differentiation of the withdrawals among the aquifers in the area in addition to determination of the geographic distribution of the withdrawals. Because of a lack of comprehensive data on private-well construction and distribution, a sample of private domestic-well records was used to estimate well characteristics and approximate the proportion of wells and withdrawals associated with each aquifer. Construction data on 2,846 private domestic wells were collected from 29 counties and independent cities (localities) having appreciable self-supplied populations and representing private domestic withdrawals of about 31 million gallons per day. Within each locality, geographically stratified random sampling of well records by tax plat characterized details of well construction for the population of domestic wells. Because neither specific location data nor aquifer elevations were available for individual wells, the primary aquifer in which each well is completed was estimated by cross-referencing the screen elevation estimated from the well record with a generalized configuration of hydrogeologic units underlying the locality in which the well is located. For each locality, summarizing the results of this process allowed the determination of the proportion of wells and withdrawals associated with each aquifer. Additional evaluation of spatial data was used to apply the domestic withdrawal rates developed for each aquifer in each locality to a detailed ground-water study of the portion of the Virginia Coastal Plain east of the Chesapeake Bay, which is known as the Eastern Shore Peninsula. Because domestic withdrawal estimates are based on the self-supplied population, the geographic distribution of withdrawals within each of the Eastern Shore counties was estimated by using population data from the 2000 U.S. Census at the resolution of census block groups and further refining the distribution based on road density. The allocation of withdrawals among aquifers was then determined by cross-referencing the spatial distribut