Arsenic in ground water in the Coastal Plain aquifers of Maryland
2010, Drummond, D.D. and Bolton, D.W.
Report of Investigations 78
A study was conducted to determine the extent and range of arsenic in the major aquifers of the Maryland Coastal Plain, and to identify possible hydrochemical controls on arsenic distribution in these aquifers. This study was undertaken in response to a lowering of the U.S. Environmental Protection Agency’s Maximum Contaminant Level for arsenic from 50 to 10 micrograms per liter.
Arsenic data from more than 4,200 wells were evaluated to determine the geographic distribution of arsenic in the Coastal Plain aquifers of Maryland. These data indicate that the Aquia and Piney Point aquifers are the only Coastal Plain aquifers with water that exceeds the Maximum Contaminant Level of 10 micrograms per liter on a widespread basis. There were no exceedances of the Maximum Contaminant Level in the Potomac, Magothy, or Columbia aquifers, and only one well in the Miocene aquifers exceeded the Maximum Contaminant Level. Arsenic concentrations in the Aquia aquifer range from below detection limits (generally 2 micrograms per liter) to 131 micrograms per liter (in Anne Arundel County). Arsenic concentrations in the Piney Point aquifer range from below detection limits to 33 micrograms per liter (in Dorchester County). The lack of elevated arsenic concentrations in the shallow aquifers, the age of water in the Aquia and Piney Point aquifers, and the distribution of arsenic in the Aquia and Piney Point aquifers indicate that the overall arsenic occurrence is a natural phenomenon, and is not caused by anthropogenic contamination.
Elevated arsenic concentrations (those that exceed 10 micrograms per liter) in the Aquia aquifer form a band that approximately parallels strike (northeastern/southwestern trend), and extends from the Eastern Shore, beneath the Chesapeake Bay, and into Southern Maryland. An additional area of high arsenic concentrations was identified on the Mayo Peninsula in Anne Arundel County, about 10 miles northwest of the main area of elevated arsenic concentrations. No evidence indicates vertical variation of arsenic concentrations in the main band in the Aquia aquifer, although insufficient data on vertical zonation are available. However, in Anne Arundel County, elevated arsenic concentrations appear to be restricted to the depth interval of 70 to 100 feet below land surface. In this area, the aquifer is unconfined, and surface contamination cannot be ruled out at this location.
Elevated arsenic concentrations in the Piney Point aquifer form a band similar to the one in the Aquia aquifer, only narrower and farther to the southeast. It also extends from the Eastern Shore, beneath the Chesapeake Bay, and into Southern Maryland. Additional smaller areas of elevated arsenic concentrations occur north of the main band in Talbot and Queen Anne’s Counties. As in the Aquia aquifer, there is no evidence of widespread vertical zonation of arsenic concentrations in the Piney Point aquifer, although more data are needed to confirm this.
Speciation analyses indicate that arsenite is generally the dominant dissolved arsenic species, with about 80 percent of total arsenic as averaged for samples from the Aquia aquifer and about 70 percent for samples from the Piney Point aquifer. The organic methylated arsenic compounds monomethylarsonate and dimethylarsinate ranged from below detection limits (0.1 micrograms per liter for both) to 0.2 and 0.6 micrograms per liter (as arsenic), respectively, in the Aquia aquifer, and from below detection limits to 0.2 micrograms per liter (as arsenic) for both species in the Piney Point aquifer.
The lack of correlation of arsenic concentrations with most other solutes and a lack of data on the geochemistry of aquifer material preclude the development of a unique hydrochemical model that fully explains arsenic distribution in the Aquia and Piney Point aquifers. Arsenic distribution in ground water may be controlled by distribution of lithologic components in the aquifer material, by mobilization mechanisms, or a combination of both. The area of elevated arsenic roughly coincides with the highest percentages of medium to coarse sand in the Aquia aquifer. Possible aquifer components that could provide a source for arsenic in the Aquia and Piney Point aquifers include calcareous shell material and cement, glauconite grains, phosphate pellets, goethite pellets, and iron oxyhydroxide coatings on mineral grains. However, insufficient data are available on the distribution of these components within the aquifers and arsenic composition of these materials to identify lithologic controls on arsenic distribution.
Mobilization controls that could partially determine arsenic distribution in ground water include oxidation-reduction reactions, pH variations, adsorption/desorption reactions, reductive dissolution, sulfate reduction, and ionic competition (and enhancement). Reductive dissolution is a likely mechanism for the mobilization of arsenic. In reductive dissolution, an arsenic-bearing substrate (either incorporated in the mineral structure or adsorbed on the substrate surface) such as iron oxyhydroxide, is solubilized due to increasingly reducing conditions, and arsenic is mobilized into solution. Sulfate reduction may also play a role in arsenic mobilization. In the Aquia and Piney Point aquifers, the highest arsenic concentrations occur in wells where sulfate concentrations are below 10 milligrams per liter, indicating that sulfate-reducing bacteria may produce sulfide, which precipitates arsenic-bearing sulfide minerals, and limits arsenic mobility. Competition for adsorption sites with other solutes, such as phosphate, may mobilize arsenic in the Aquia and Piney Point aquifers, and enhancement of adsorption by solutes such as calcium and magnesium may demobilize arsenic. Chemical evolution of ground water and leakage of water from adjacent confining units may also influence arsenic distribution.