Professor, Earth and Environmental Sciences Department
||Current Research in Earth Science Seminar, "Biogeochemistry of Arsenic in Groundwater Flow Systems"
Thursday, September 12, 2013
to 5:00 PM
100 Keith-Wiess Geological Laboratories
6100 Main St
Houston, Texas, USA
Groundwater and sediment samples were collected along a !ow path in the Aquia aquifer(Paleocene), Maryland in order to examine and study the factors in!uencing “evolution” of
arsenic (As) in these groundwaters. The Aquia crops out near Washington, DC, where it isuncon"ned, and extends approximately 90 km down dip to the south and east towards and beneath the Chesapeake Bay. The studied !ow path was chosen owing to (i) the number of accessible wells, (ii) differences in total dissolved As concentrations in groundwaters from some of the sampled wells, which reach values !667 nmol kg-1 or !50 ppb, and (iii) the distinct
difference in total dissolved As concentrations in Aquia groundwaters between the northern and southern portions of the study area. In groundwater samples, in situ separation of inorganic As species [As(III) and As(V)] were performed by using anion exchange chromatography. Subsequently, As concentrations were determined by inductively coupled plasma mass spectrometry. In situ measurements of Fe concentrations and speciation, dissolved S(-II) concentrations, pH, alkalinity, and oxidation-reduction potential (Eh) were determined to establish the oxidation-reduction conditions and solution chemistry along the !ow path.
Concentrations of As in 12 analyzed groundwater samples range from "0.75 to 1 072 nmol kg-1,
and As(III) concentrations ranging from 0.24 to 980 nmol kg-1 appears to be the dominant form
of As in solution. 50% of the studied wells yielded groundwaters with concentrations that
exceed the US EPA's Maximum Contaminant Level for As in drinking water of 133 nmol kg-1 or
10 ppb. In order to examine the solid phase speciation of As within the aquifer sediments, we
collected a number of Aquia sediment samples from a drill core that was archived at the
Maryland Geological Survey. These sediment samples were evaluated using a previously
established sequential extractions procedure. Solid phase As concentrations range between 973
and 2,012 nmol kg-1. Additionally, petrographic, X-Ray diffraction and diffuse re!ectance
spectroscopy analyses of the Aquia sediments reveal presence of glauconite, and smectite along
with goethite and hematite within the samples. Here, we present the possible mechanisms
responsible for the elevated As concentrations in the studied groundwaters of the Aquia aquifer.