and distribution of arsenic and uranium in the Pomperaug rIVER BASIN aquifer,
Craig J. Brown1,*, Robert A. Zielinski2
in Geology of Long Island and Metropolitan
1* U.S. Geological Survey,
The transport of anthropogenic and natural contaminants
to public-supply wells (PSWs) is being studied in a glacial aquifer system in
Stratified glacial deposits underlie about 13 mi2
Dissolved (filtered) arsenic concentrations in the wells ranged from below the minimum reporting level (MRL) of 0.2 mg/L, to 5.8 mg/L; only five samples were above the MRL. The five samples with the highest dissolved arsenic concentrations had dissolved oxygen levels below 0.8 mg/L, and the two samples with the highest arsenic concentrations had very high dissolved iron levels (above 5 mg/L), indicating arsenic mobility under relatively anoxic conditions. The dissolved arsenic also may be higher in sediments that are derived from the underlying shales of the Newark Supergroup; both the Shuttle Meadow Formation and the East Berlin Formation can be sulfidic, and sulfides can sequester arsenic. The concentration of arsenic in the PSW was below the MRL.
Dissolved (filtered) uranium concentrations in the study area were low, ranging from below the MRL (0.2 mg/L) to 1.3 mg/L; only three samples were above the MRL. Uranium was highest in wells with low dissolved oxygen (<0.8 mg/L) despite the fact that uranium is more soluble under oxic conditions; this indicates that other factors are likely to control uranium concentration than solely dissolved oxygen. Uranium was highest in bedrock wells and generally highest in the deepest well of each cluster, indicating that the proximity to source material (bedrock) affects the dissolved uranium concentration. The concentration of uranium in the PSW was <0.2 mg/L. Activities of Ra-226 and Ra-228 were low (from <0.16 to 0.42 pCi/L and from <0.45 to 0.95 pCi/L, respectively) and are consistent with the uranium concentration data.
Ferric oxyhydroxide coatings on
grains of glacial sediments, which can act as sites for sorption of arsenic or
uranium, were extracted using acid solutions and analyzed for major and trace
elements. Ferric-oxyhydroxide coatings extracted using 10 percent nitric acid
yielded iron concentrations ranging from 2,100 to 2,300 mg/kg (mg of Fe per kg
of bulk sediment). The extracted material also contained 0.09 to 0.38 mg/kg of
uranium and 0.07 to 0.12 mg/kg of arsenic. Fission-track radiography of the
stratified glacial deposits shows evidence of uranium associated with ferric
oxyhydroxide grain coatings. Secondary iron oxides occur as oxidized parts of
primary iron oxides, as hematite coatings and stains on other minerals, as a
hematite-rich matrix of rock fragment grains, and as fracture-fill material.
The distribution of uranium in the aquifer material infers the co-occurrence of
uranium progeny such as radium, assuming that the sorbed uranium is not so
recent that there is a limited build up of daughters by radioactive decay.
Radon-222, which is derived from the decay of Ra-226, had activities in the
glacial aquifer of the
Despite the low concentrations of dissolved arsenic and uranium in the Pomperaug study area, it is important to understand the mechanisms related to their mobilization and transport. A similar terrane with slightly different conditions (pH, redox chemistry, source material) could result in very different behavior of natural contaminants and concentrations in PSWs. Sorption batch experiments will be performed on core samples from the study area to determine the effects of changes in pH on the sorption or desorption of arsenic and uranium.