ELEVATED LEVELS OF ARSENIC IN THE GROUNDWATER FROM THE PINEY POINT AQUIFER IN THE VIRGINIA COASTAL PLAIN: IN SEARCH OF THE CAUSE AND MECHANISM

The Piney Point aquifer is a hydrogeological system that consists of highly porous, solution-channeled indurated limestone and a valuable primary water source for most rural and suburban areas in certain regions of the Virginia Coastal Plain. The huge amount of withdrawals made from the Piney Point aquifer extend throughout the center of the Northern Neck, Middle, and York-James Peninsulas. These withdrawals make up about five (5) percent of the groundwater supply in all of the Virginia Coastal Plain.

To assess the quality of water in public wells, 26 groundwater samples collected from James City County Service Authority and Newport News Waterworks. Once tested, the results revealed five of the samples collected showed an elevated concentration of arsenic (As) that was higher than (0.01 mg/L) the minimum level of contamination (MCL) set by the Primary Drinking Water Regulation (NDWR).

The present research is focused on understanding the source and the mechanism of release of the elevated levels of arsenic found in the water samples collected from the Piney Point Aquifer. Sediments of the USGS Banbury Cross borehole that represents a section of the Piney Point Aquifer, demarcated by the Old Church formation on the top and the lower boundary by Woodstock Member of Nanjemoy formation.

As the groundwater is not independent of the solid phases of aquifers sediments, thirty-eight (38) sediment samples were selected for further analysis to understand the cause of the elevated arsenic (As) concentration in some groundwater samples of Piney Point aquifer. XRF analysis revealed a high concentration of arsenic in the bulk sediment samples, ranges 42 to 113 mg/Kg. The upper calcareous sequence contains more arsenic (~102 mg/Kg) than lower non-calcareous sequence (~61 mg/Kg) of the Piney Point aquifer. Higher arsenic concentration has a well correlation with TIC, Calcium (Ca) and Sulfur (S), but not with Iron (Fe) in the studied samples. Bulk mineralogical analysis with XRD determined enrichment of Gypsum in some of the samples in the upper carbonaceous sequence. There are instances of the formation of arsenic bearing minerals in carbonate aquifers. Further yielding of groundwater must be carefully monitored as it might increase dissolution of the calcareous phases of the Piney Point aquifer and lead to additional release of arsenic.