Aquifer denitrification and in situ mesocosms: Modeling electron donor contributions and measuring rates

In situ denitrification rates were measured in a shallow unconfined glaciofluvial aquifer that had undergone large-scale nitrate contamination. Denitrification rates and isotopic enrichment factors, epsilon, were measured using three tracer tests in two aquifer in situ mesocosms (ISMs). Denitrification rates were also measured using a mass balance method using water samples from multiport samplers. First-order kinetic rates (k) best described the denitrification rates measured. ISM kinetic rates ranged from 0.00049/d to 0.0031/d and epsilon values ranged from -4.86%0 to -9.34%o; a linear relationship between k and epsilon values showed greater fractionation (more negative E values) associated with higher rates. For the mass balance method, k values ranged from 0.0028/d to 0.0041/d. Combined mineralogical analysis, water quality data from the ISMs, and geochemical models using PHREEQC indicated that contributions of major electron donors to denitrification were 43-92% by organic carbon, 4-18% by pyrite, and 2-43% by non-pyritic ferrous iron, depending on the sample date and the type of amphibole used as the electron donor for ferrous iron. ISMs show promise as a tool for hydrogeochemical investigations. They are large enough to allow long-term sampling of aquifer denitrification tracer tests (>2 years), they may be used, with the modeling methodology shown herein, to estimate relative e(-) donor contributions, and they limit the influence of advection and mechanical dispersion on the amended water within the chamber. (C) 2012 Elsevier B.V. All rights reserved.