Conceptualization and Analysis of Ground-Water Flow System in the Coastal Plain of Virginia and Adjacent Parts of Maryland and North Carolina

U.S. Geological Survey, Professional Paper 1404-F

by John F. Harsh and Randell J. Laczniak

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This report presents the results of a study of the ground-water flow system in the Coastal Plain of Virginia and adjacent parts of Maryland and North Carolina. The ground-water flow system consists of water-table aquifer and an underlying sequence of confined aquifers and intervening confining units composed of unconsolidated sand and clay .Water levels have declined steadily, and cones of depression have expanded and coalesced around major ground-water withdrawal centers. A digital flow model was developed to enhance knowledge of the behavior of the ground-water flow system in response to its development. Transmissivity and vertical leakance maps were developed for each aquifer and confining unit. The model was calibrated to simulate ground-water flow within the system under both prepumping and pumping conditions. Simulated prepumping potentiometric-surface maps indicate that regional movement of ground water was from the Fall Line toward coastal areas and that local movement of ground water was from interfluves toward major river valleys. Maps of simulated prepumping flow across confining units show that most recharge occurred in narrow bands approximately parallel to the Fall Line and under interfluves and that discharge was toward major river valleys and coastal water. Simulated prepumping rates of recharge into the confined aquifer system from the water-table aquifer varied up to 3 .2 inches per year (in/yr), and rates of discharge out of the confined system varied up to 2.8 in/yr.

Ten pumping periods covering 90 years (yr) of withdrawal simulated the history of ground-water development. Simulated potentiometric-surface maps for 1980 show lowered water levels and the development of coalescing cones of depression around the cities of Franklin, Suffolk, and Williamsburg and the town of West Point, all in Virginia. The largest simulated decline in water level, about 210 feet (ft) ,was near Franklin. Water budgets indicate that over the period of simulation (1891-1980) (1) pumpage from the model area increased by about 105 million gallons per day (Mgal/d), (2) lateral boundary outflow increased by about 5 Mgal/d, (3) ground-water flow to streams and coastal water decreased by about 107.5 Mgal/d, (4) lateral boundary inflow increased by about0.7 Mgal/d, and (5) water released from aquifer storage increased by about 1 .6 Mgal/d. The difference between total inflow and total outflow is the numerical truncation error of the digital simulation. Analysis of water budgets for individual confined aquifers shows that the major source of water supplied to wells was vertical leakage induced through confining units by pumping. Simulated rates of recharge into the confined aquifer system at the end of the final pumping period (1980) varied up to 3.8in/yr,and simulated rates of discharge out of the confined system varied up to 2.2in/yr. Results of simulations show an increase of about 110Mgal/d into the confined system from the unconfined system over the period of simulation. This increase in flow into the confined system affected local discharge of ground water to streams and regional discharge to coastal water. Withdrawal of groundwater from the confined aquifers also induced brackish water from Chesapeake Bay into the confined system.

Results of sensitivity analyses indicate that simulated water levels are more sensitive to decreases in aquifer transmissivity and confining unit vertical hydraulic conductivity than to increases in these properties. Lowering the storage coefficient of an aquifer had minimal effect on simulated water levels, whereas increasing the storage coefficient had a much more significant effect. The effect of confining unit storage is shown to be insignificant if it is assumed that the water released from confining unit storage is attributable to the compressibility of water only.

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