Reports
Evaluation of the geohydrology and water-supply potential of the Lower Patapsco and Patuxent aquifers in the Indian Head-Bryans Road area, Charles County, Maryland: initial findings
1998, Andreasen, D.C. and Mack, F.K.
Open File Report 98-02-9
Key Results
The study area is located in northwestern Charles County and includes Bryans Road, Chapman's Landing, and Indian Head. Two major aquifers occur in the area, the lower Patapsco aquifer and the underlying upper Patuxent aquifer. The lower Patapsco aquifer is currently the primary source of potable water; approximately 2 million gallons per day (Mgal/d) were withdrawn in 1997 from the Indian Head-Bryans Road area. Increasing population may require an additional 5 Mgal/d by 2020. The upper Patuxent aquifer is a potential source for increased ground-water production in the area. The upper Patuxent aquifer generally produces water of good quality. The water type is sodium bicarbonate with pH (field), dissolved solids, and dissolved iron ranging from 7.7 to 7.9, 214 to 278 milligrams per liter (mg/L), and 17 to 100 micrograms per liter (μg/L), respectively.
Analyses of aquifer tests, geophysical logs, and drill cuttings and core samples from wells completed in the upper Patuxent aquifer in the study area suggest relatively low to moderate productivity. The lithology consists of fine to coarse sands separated by dense, chiefly green and brown clays. Transmissivity, based on aquifer tests, ranges from 113 to 2,600 feet squared per day (ft2/d). Aquifer thickness ranges from 70 to 90 feet (ft) in the vicinity of the Chapman's Landfog test site and the Bryans Road area and is generally less than 60 ft at other locations drilled in Charles County. Flow boundaries that diminish recharge to the aquifer may occur in places. Few data are available that characterize the hydraulic properties of the upper Patuxent aquifer in areas of Charles County outside of the study area, but indications are that the aquifer has a relatively low productivity.
The lower Patapsco aquifer consists of alternating beds of fine to coarse sand interbedded with layers of dense clay. Generally, it has a higher transmissivity than the upper Patuxent aquifer. The two aquifers are separated by the Arundel Clay, a confining bed.
Pumping from the upper Patuxent aquifer in the Indian Head-Bryans Road area has no short-term effect on water levels in the lower Patapsco aquifer, as demonstrated by a five-day aquifer test at the Chapman's Landing test site. Long-term water-level records outside the study area at Douglas Point and Waldorf, however, indicate that the two aquifers are hydraulically interconnected, although the water-level decline in the upper Patuxent aquifer caused by pumping from the lower Patapsco aquifer is relatively small to date (generally less than 10 ft).
To evaluate the supply potential of the upper Patuxent aquifer, geohydrologic data, including data collected from three test wells drilled during the study, were compiled to develop a three-dimensional, finite-difference ground-water-flow model (MODFLOW). An enlarged model area was specified in order to take into account the effects of pumpage from outside the study area. The model was used to assess the availability of ground water to meet projected water-supply demands in northwestern Charles County by simulating water-level drawdowns caused by future increases in pumpage.
In 1997 the remaining available drawdown (difference between current water level in a well and the 8O-percent management level) in the upper Patuxent aquifer in the Indian Head-Bryans Road area ranged from approximately 350 to 650 ft. The upper Patuxent aquifer is capable of supplying 3.4 Mgal/d out to 2020 in the Indian Head-Bryans Road area before the 80-percent management level (80 percent of the difference between pre-pumping water level and top of aquifer) is reached. An additional 1.3 Mgal/d (total of 4.7 Mgal/d) could be pumped before water levels drop below the top of the aquifer. Pumping at a rate of 3.4 Mgal/d results in significant drawdowns. For example, the water level immediately outside a pumping well in the area of the model w1th the deepest water level (535 ft below sea level) is estimated to be 650 ft below sea level. The drawdown is only somewhat reduced when the pumpage is divided between additional wells. D1viding the same pumpage (3.4 Mgal/d) evenly between 16 wells spaced approximately 0.5 mile (mi) apart, as opposed to 10 wells, results in a pumping water level as deep as 560 ft below sea level immediately outside of the well.
Three additional model simulations were made to estimate the impact on water levels in 2020 of three projected pumping rates for the upper Patuxent aquifer based on residential and commercial growth scenarios. Pumping the upper Patuxent aquifer at 2.1 Mgal/d in the Indian Head-Bryans Road area resulted in 30 to 50 percent of available drawdown remaining. This simulation represents a population increase between 1995 and 2020 of 23,000 in the Indian Head-Bryans Road area. However, increasing the upper Patuxent aquifer pumpage to 4.0 and 4.7 Mgal/d (representative of population increases of 38,000 and 47,000, respectively) caused declines in water levels that exceeded the available drawdown within much of the study area. Pumping at a rate of 4.7 Mgal/d results in model cell heads below the top of the aquifer within a narrow band along the Potomac River shoreline near Chapman's Landing.
Model simulations indicate that pumping the upper Patuxent aquifer within the Indian Head-Bryans Road area will cause minimal head decline in the lower Patapsco aquifer. Drawdown in the lower Patapsco aquifer caused by 3.4 Mgal/d withdrawn from the upper Patuxent aquifer was less than 18 ft in the Indian Head-Bryans Road area. Drawdown elsewhere in the model area was less than 10 ft.
In 1997 remaining available drawdown in the lower Patapsco aquifer ranged from approximately 0 to 200 ft in the Indian Head-Bryans Road area and approximately 30 to 400 ft elsewhere in the study area. The 8O-percent management water level was exceeded along the Potomac River shoreline in the central part of the Indian Head peninsula. Less available drawdown occurs in the updip section of the aquifer along the Potomac River because the top of the aquifer is relatively shallow. Further development of the lower Patapsco aquifer, therefore, should be located downdip (southeast) of the Indian Head-Bryans Road area and include a reduction in pumpage in the updip areas. A model simulation indicates that pumpage from the lower Patapsco aquifer can be increased by 0.6 Mgal/d over the current amount of approximately 2 Mgal/d if 0.5 Mgal/d is shifted from areas near the Potomac River to areas along the southeastern border of the study area.
Model simulations indicate that water levels in the lower Patapsco aquifer may decline by as much as 40 to 50 ft in the Bryans Road area and less than 25 ft in the Indian Head area between 1997 and 2020 as a result of lower Patapsco pumpage elsewhere in the model area. This drawdown is based on a total withdrawal in the model area of 9.5 Mgal/d, 5.8 Mgal/d of which is withdrawn from the Waldorf area. The available drawdown was exceeded in two areas along the Potomac River shoreline, in the central part of the Indian Head peninsula and northwest of Bryans Road. Maintaining pumpage in the Waldorf area at 1997 levels (2.6 Mgal/d) results in approximately 20 to 30 ft of drawdown in the Bryans Road area between 1997 and 2020. The percentage of available drawdown remaining ranges between 10 and 40 percent in the Bryans Road area and 0 and 40 percent in the Indian Head area.
A summary of pumpage rates and results of predictive model simulations are given in the following table, entitled "Summary of results of model simulations I, II, III, IV, and V."
