Water quality and temporal variations in chloride concentrations in groundwater in the Maryland Piedmont
2018, VanDerwerker, T.J.
Administrative Report 19-02-01
From the early 1970s to the early 2000s the Maryland Geological Survey and the U.S. Geological Survey sampled many wells in the Maryland Piedmont region, testing the water samples for chloride, major ions, and other constituents. These data provide a valuable baseline against which future water-quality samples can be compared to monitor changes in groundwater chemistry. With expanded urbanization of rural areas of the Maryland Piedmont, including increased road-traffic volumes, there is an elevated risk of chloride contamination from road salt and other anthropogenic sources. The purpose of this study is to: (1) assess overall groundwater quality of the Maryland Piedmont; (2) identify sources, concentrations, and temporal variations in chloride concentrations, and (3) determine if elevated concentrations of other chemical constituents (i.e. trace metals and radionuclides) correlate with elevated chloride concentrations.
The groundwater quality of the Maryland Piedmont was evaluated by collecting samples from 25 wells and analyzing for major ions, trace metals, nutrients, radionuclides, and bromide. Wells sampled in the study are on average 177 feet deep with an average casing depth of 37 feet, and completed in 16 different geological formations, including igneous, metamorphic, and sedimentary rock types. Key results from this study are:
- Groundwater is acidic (median pH = 6.1), oxidized (median dissolved oxygen = 5.9 milligrams per liter), moderately hard (median hardness = 77.1 milligrams per liter), with a median alkalinity of 47 milligrams per liter, median total dissolved solids of 195 mg/L, and median specific conductance of 346 microsiemens per centimeter at 25°C.
- Twelve of the 25 wells sampled had low chloride concentrations (less than ~18 milligrams per liter). Using the chloride to bromide ratio, two of the 25 wells with elevated chloride concentrations (above ~18 mg/L) are likely affected by agricultural activities, 10 of the 25 wells by road-deicing salts, and one by septic effluent.
- The maximum chloride concentration analyzed was 571 milligrams per liter in well CE Bc 56, located approximately 100 feet from Interstate-95. Median chloride concentration for wells sampled was 62 milligrams per liter; average chloride concentration was 114 milligrams per liter. Chloride concentrations in five wells exceeded the U.S. Environmental Protection Agency's Secondary Maximum Contaminant Level of 250 milligrams per liter in drinking water.
- Chloride concentrations have increased by at least 25 percent in 15 of the 25 wells sampled since the initial chloride analyses.
- Groundwater chemistry data collected during this study indicates moderate correlations between chloride and strontium (R2 = 0.51) and nickel (R2 = 0.54). Temporal variations of strontium, lithium, barium, and nickel concentrations in wells MO Dg 34 and MO De 50 generally correspond to changes in chloride concentrations.
- Continuous specific conductance measurements were collected in a monitoring well in Layhill Local Park in Montgomery County since December 2018. Specific conductance ranged from a high of approximately 1,550 microsiemens per centimeter in the winter to less than 1,000 microsiemens per centimeter in the spring. After road-deicing salt applications have ceased for the season, conductivity decreased and stabilized.