Stratigraphy, hydrogeology, and water chemistry of the Cretaceous aquifers of the Waldorf/La Plata area, Charles County, Maryland
1986, John M. Wilson
Open File Report 86-02-2
Test well Ch-Bf 146 was drilled to 1,650 ft below land surface to explore and evaluate the Patapsco Formation aquifers deeper than 1,000 ft in the Waldorf/La Plata area of Charles County, Maryland . The test hole penetrated two major aquifer systems above a depth of 1,000 ft and another major aquifer system and aquifer below 1,000 ft. For the purposes of this report, the aquifers encountered at the test site have been informally named as noted below.
The deepest aquifer system found in the test drilling, the Lower Patapsco aquifer system, is comprised of a group of sands which lie between 1,140 ft and the base of the Patapsco Formation at 1,417 ft. A major sand, the Middle Patapsco aquifer, occurs between 1,055 and 1,085 ft. Between 610 and 795 ft occurs a sequence of sands (the upper Patapsco aquifer system) that is comprised of a thick sand (620 to 660 ft) and several deeper and thinner sands from 660 ft to 795 ft. Clayey confining layers from 1,085 to 1,140 ft (the Lower Patapsco confining layer) and from 795 to 1,055 ft (the Middle Patapsco confining layer) separate these Patapsco sands.
The Waldorf aquifer system, as used in this report, overlies the Upper Patapsco aquifer system and includes sands in the Waldorf/La Plata area that are referred to as the “Magothy aquifer” by other authors. At the test well site, the Waldorf aquifer system consists solely of sands in the Magothy and Monmouth Formations. The Waldorf aquifer system is separated from the Upper Patapsco aquifer system by a 90-foot thick Patapsco clay confining layer at the test site. At other well sites in the Waldorf/La Plata area, the Waldorf aquifer system contains sands of the Patapsco Formation that are in direct hydraulic contact with sands of the Magothy and Monmouth Formations. The Patapsco sands included in the Waldorf aquifer system are apparently hydraulically separated from the underlying sands of the Upper Patapsco aquifer system.
The three major aquifer systems and the Middle Patapsco aquifer are correlative along the regional strike for at least 10 miles in the Waldorf/La Plata area. Correlations of geophysical logs indicate that individual sands 15 to 25 ft thick can be traced with reasonable confidence between wells. Geophysical log correlations also indicate that, at some well sites, the Waldorf aquifer system and the Upper Patapsco aquifer system are in sand-on-sand contact and function as one major hydrologic system at these sites.
A production well (Ch-Bf 147) was drilled at the site of the test well (Ch-Bf 146). Both of these wells were screened in the same sands of the Lower Patapsco aquifer system and the Middle Patapsco aquifer. The initial static water level of these aquifers was 9.8 ft below sea level. Historic water-level records indicate that the static water level of the Lower Patapsco aquifer system has declined over the past 40 years.
Evaluation of the 24-hour pumping test of Ch-Bf 147 resulted in a transmissivity of 14,000 gpd/ft (1,900 ft2/d) for the first 2 hours of the test and 7,300 gpd/ft (1,000 ft2/d) for the remainder of the test. The decrease of 6,700 gpd/ft (900 ft2/d) in the transmissivity indicates that the well’s expanding cone of depression intersected a transmissivity boundary after 2 hours of pumping. The Patapsco sands tested in Ch-Bf 147 have a storage coefficient of 0.0016. The hydraulic conductivity of the producing sands in Ch-Bf 147 is 127 gpd/ft2 (17 ft/d).
The quality of ground water from the Lower Patapsco aquifer system and the Middle Patapsco aquifer is suitable for most purposes. The water is nearly neutral (pH = 7.4), low in dissolved solids (about 200 mg/l), low in specific conductance (270 to 300 µmhos), slightly warm [22°C (72°f)], very soft (hardness, 1 to 2 mg/l as CaCO3), naturally fluoridated (0.9 mg/l), and moderately high in dissolved silica (33 to 86 mg/l). An analysis of a suite of minor trace metals shows that none of the metals determined occur in concentrations hazardous to human health.
The Lower Patapsco aquifer system and the Middle Patapsco aquifer contain sodium bicarbonate type water while the Waldorf aquifer system waters contain calcium magnesium bicarbonate type water. Water in the Waldorf aquifer system contains an unusually high concentration of potassium in relation to that of sodium. The probable source for the calcium and potassium ions in water from the Waldorf aquifer system is, respectively, dissolution of shell and glauconite in the overlying Brightseat, Aquia, and Nanjemoy Formations. Downward leakage provides a path for the entry of calcium and potassium ions into the Waldorf aquifer system.