In situ stimulation of groundwater denitrification with formate to remediate nitrate contamination

In: situ stimulation of denitrification has been proposed as a mechanism to remediate groundwater nitrate contamination. In this study, sodium formate was added to a sand and gravel aquifer on Cape God, MA, to test whether fo rmate could serve as a potential electron donor for subsurface denitrificat ion. During 16- and 10-day trials, groundwater from an anoxic nitrate-conta ining zone (0.5-1.5 mM) was continuously withdrawn, amended with formate an d bromide, and pumped hack into the aquifer. Concentrations of groundwater constituents were monitored in multilevel samplers after up to 15 m of tran sport by natural gradient flow. Nitrate and formate concentrations were dec reased 80-100% and 60-70%, respectively, with time and subsequent travel di stance, while nitrite concentrations inversely increased. The field experim ent breakthrough curves were simulated with a two-dimensional site-specific model that included transport, denitrification, and microbial growth. Init ial values for model parameters were obtained from laboratory incubations w ith aquifer core material and then refined to fit field breakthrough curves . The model and the lab results indicated that formate-enhanced nitrite red uction was nearly 4-fold slower than nitrate reduction, hut in the lab, nit rite was completely consumed with sufficient exposure time. Results of this study suggest that a long-term injection of formate is necessary to test t he remediation potential of this approach for nitrate contamination and tha t adaptation to nitrite accumulation will be a key determinative factor.