IDENTIFYING THE ORIGIN OF A PERSISTENT ARSENIC PLUME IN A TRANSMISSIVE AQUIFER: APPLICATIONS OF GEOCHEMICAL MODELING

An arsenic plume has persisted in groundwater beneath a commercial phosphate chemical processing facility in the mid-western US, despite the fact that the surficial source (an unlined sludge pond) was removed 30 years ago. The arsenic-phosphate plume exists in a transmissive alluvial aquifer, with arsenic concentrations in the deeper zones of the plume as high as 24 mg/L. Geochemical modeling based on 25 years of groundwater monitoring data helped to identify the origin of the plume, as well as to evaluate the effectiveness of various remedial options such as in situ permanganate injection, pump-and-treat lime addition, and pump-and-treat through a constructed wetland.

Existing data review correlated elevated arsenic (As) levels with low pH conditions and high phosphate (PO₄^-3) concentrations; and identified different redox zones within the As plume. Geochemical modeling based on site-specific data predicted low levels of As in groundwater under oxidizing conditions (arsenate adsorbs on Fe-oxides) and under strongly reducing conditions (As precipitates as sulfides), but high levels (as arsenide species) under moderately reducing conditions.

A conceptual model, consistent with data and modeling results, is as follows:(1) PO₄^-3 and As are transported from the old sludge pond in a dissolved state to the water table by vertical infiltration;(2) the PO₄^-3 added to the aquifer stimulates anaerobic microbial activity, and redox conditions fall below background conditions;(3) sulfate-reducing anaerobic conditions are established within portions of the plume, sulfate is reduced to sulfide, As-sulfide minerals precipitate, and As is immobilized;(4) following removal action 30 years ago, continuing groundwater flow within the aquifer transports oxidizing water of background composition through the plume;(5) the previously precipitated As-sulfides within the plume oxidize, and As becomes mobile once again.

Core samples analyzed by the electron microprobe positively identified As-Fe-sulfide minerals by electron microprobe within the core of the plume, demonstrating the validity of the conceptual model. This investigation shows the applications of geochemical modeling techniques to identify the processes controlling contaminant concentrations, and evaluation of the effectiveness of remedial alternatives in a cost-effective manner.