Nitrates in Rural Groundwater Not From Farming; An Unexpected Finding Based on Forensic Geochemistry

Sharptown is located in an agriculturally dominated portion of Maryland and is supplied by four wells. Groundwater quality compliance samples reflect elevated nitrate concentrations for years. Non-point agricultural sources were thought to be the source, but detailed study of geochemical data trends has revealed a differing and surprising interpretation.

Two of Sharptown’s four wells are screened in an unconfined aquifer and one is screened in a confined aquifer. A fourth well is of uncertain design and might be open to both aquifers, either through a lack of adequate grout or the presence of an additional screen. As a component of other work being performed for the Town, we analyzed available water quality compliance data via End-Member Mixing Analysis. EMMA is a common surface water investigative technique; its application to groundwater is less common. We used chloride and nitrate as tracers; the two aquifers differ in their concentrations of these two constituents.

The EMMA revealed a previously unknown, probable point source of nitrate contamination. This interpretation was based on sharply disparate nitrate concentrations, and nitrate-to-chloride ratios, between two closely-spaced and similarly constructed wells. This suggested that the variable-concentration in one of the wells is affected by a seasonally-operative and proximal (to that one well) nitrate point source.

The lack of an equally plausible explanation suggests that the nitrate in this one well may originate more from a local sewer system leak than from historic and regional farming. Work is ongoing. and we are seeking to confirm this theory by sampling for Methylene Blue Active Surfactants (MBAS), an indicator of detergents and thus, of domestic sewage. MBAS is synthetic and any detectable presence would indicate a proximal point-source of detergent-laden wastewater. If the sewer system leak is confirmed, its repair may offer a more economical and assured means of nitrate reduction than either engineered treatment or agricultural source reduction through farmer education.