Differentiating Plume Expansion from Regional Climate Effects for Ground Water Discharge Permits

At facilities with known discharges of contaminants to ground water, a common regulatory requirement is to construct a sentry well network outside the contaminant plume and monitor those wells for plume expansion. Plume expansion is commonly defined by an increase in a contaminant concentration above a threshold based upon background concentrations and/or a risk assessment. Metals and common ions may exist both naturally and as contaminants, thus it is important to differentiate plume expansion from concentration shifts associated with natural phenomena. We present two examples from a site in northern Utah. Ammonia exhibited increases above protection levels in sentry wells, which could be interpreted as indicating plume expansion. Analysis of ammonia concentration and nitrogen isotopic composition in surface water, soil, and shallow ground water demonstrated that the increased concentration reflected regional climate effects rather than plume expansion. A similar analysis for barium in another area of the site yielded a similar result. Demonstration that the increases reflected climate-related hydrologic shifts rather than plume expansion averted compliance issues and additional monitoring requirements dictated by a ground water discharge permit. This example illustrates the need to consider regional climate patterns for similar permitting requirements.