SPECIATION AND DISTRIBUTION OF ARSENIC IN THE AQUIA AQUIFER, MARYLAND

Groundwater samples were collected along flow paths in Aquia aquifer (Paleocene), Maryland as part of a study designed to characterize biogeochemical processes in the deep subsurface that affect “evolution” of arsenic (As) concentrations and speciation. The Aquia crops out near Washington, DC, where it is unconfined, and extends approximately 90 km down dip to the south and east towards and beneath the Chesapeake Bay. Our study focuses on the portions of Aquia located within southern and eastern Maryland. The groundwater flow paths were chosen owing to (i) the number of accessible wells, (ii) differences in total dissolved As concentrations in groundwaters from some the sampled wells, which reach values close to 50 µg/kg (667 nmol/kg; MDE, 2004), and (iii) the distinct difference in total dissolved As distribution in the Aquia between southern Maryland and the Eastern Shore. Dissolved inorganic As [As(III) and As(V)] species in ground water samples were separated in the field using a previously established anion-exchange chromatography. Subsequently, As concentrations were determined by inductively coupled plasma mass spectrometry. In situ measurements of Fe concentrations and speciation, dissolved S2- concentrations, pH, alkalinity, oxidation-and reduction potential (Eh) were determined to establish the oxidation-reduction conditions and solution chemistry along the studied flow paths. Possible mechanisms responsible for the elevated As concentrations in the down-gradient portions of the aquifer will be presented.