Alternate Source Demonstration (ASD) for Arsenic In Groundwater
Arsenic in groundwater is naturally occurring in the carbonate bedrock aquifer underlying northwest Ohio. In some cases, naturally occurring concentrations of arsenic can exceed the arsenic drinking water standard or MCL. Under an integrated groundwater monitoring program, which integrates the monitoring of RCRA regulated units under the RCRA post-closure monitoring program and solid waste management units under the RCRA Corrective Action program, arsenic was detected above its statistically-derived interwell background limit. Resampling also indicated an exceedance of the background limit. Rather than conclude that a release of arsenic had occurred, Cox-Colvin conducted an alternate source demonstration (ASD) as allowed by the RCRA post-closure monitoring rules.
The ASD involved literature review, development of a site conceptual model, groundwater sampling, and statistical analyses using both historical site-wide groundwater data and data both upgradient and downgradient of the units being monitored. The ASD concluded that natural variability in the fractured dolomite aquifer system and a naturally occurring, complex redox controlled geochemical process has resulted in arsenic concentrations that vary both spatially and temporally. This natural variation was identified through the ASD as the source of the sporadic detections of arsenic above its background limit.