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

SummaryIn situ denitrification rates were measured in a shallow unconfined glaciofluvial aquifer that had undergone large-scale nitrate contamination. Denitrification rates and isotopic enrichment factors, ε, 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 ε values ranged from −4.86‰ to −9.34‰; a linear relationship between k and ε values showed greater fractionation (more negative ε 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.

► We measure in situ aquifer denitrification rates using two methods.
► Denitrification rates measured using in situ mesocosms (ISMs) are first-order.
► Enrichment for 15N shows greater fractionation with higher kinetic rates.
► Rates from the mass balance approach are similar to those from the ISMs.
► Modeled donors were 4–18% pyrite, 2–43% non-pyritic Fe(II), and 43–92% organic C.