APPLIED FORENSIC INVESTIGATION OF ELEVATED ARSENIC LEVELS IN A DEEP CONFINED AQUIFER

A recent (2006) regulatory change lowered the EPA maximum contaminate limit (MCL) for arsenic from 50 to 10 ug/l. Doing so pushed many groundwater supply systems over the allowed limit, requiring suppliers to treat or find alternate sources of water to remain in compliance. The lower MCL resulted in the discovery of arsenic “hot spots” in deep confined aquifers. One such “hot spot” occurs in central Texas where suppliers having arsenic issues source water from the Hosston formation of the Trinity aquifer. Because the Trinity aquifer is a deep, confined aquifer at this location, arsenic is thought to occur naturally. This study applied a forensic approach incorporating chemical analysis of raw groundwater, entry point water, well logs, and well cuttings to determine the (1) source, (2) distribution, (3) environments of occurrence, and (4) potential aquifer stresses which might cause elevated arsenic levels.

Chemistry samples taken along the general groundwater flow path from unconfined to confined portions of the aquifer were combined with historical data and determined that arsenic levels above the MCL were constrained to a specific portion of the aquifer. Historical water chemistry data indicated that arsenic concentrations were not changing over time and were not increasing due to deepening cones of depression from pumping. Analysis of cuttings and geophysical logs suggested elevated arsenic concentrations varied among stratigraphic layers, and therefore screen intervals may affect arsenic concentrations in wells. Further work is needed on the depositional environments and stratigraphic distribution of the arsenic in groundwater, but the “hot spot” occurs on a paleo high and down gradient from major faults oriented perpendicular to groundwater flow.