Estimation of Biodegradation Rates Using Respiration Tests During In Situ Bioremediation of Weathered Diesel NAPL

Respiration tests were carried out during a seven month bioremediation field trial to monitor biodegradation rates of weathered diesel non‐aqueous phase liquid (NAPL) contaminating a shallow sand aquifer. Multiple depth monitoring of oxygen concentrations and air‐filled porosity were carried out in nutrient amended and nonamended locations to assess the variability of degradation rate estimates calculated from respiration tests. The field trial consisted of periodic addition of nutrients (nitrogen and phosphorus) and aeration of a 100 m2 trial plot. During the bioremediation trial, aeration was stopped periodically, and decreases in gaseous oxygen concentrations were logged semi‐continuously using data loggers attached to recently developed in situ oxygen probes placed at multiple depths above and within a thin NAPL‐contaminated zone. Oxygen usage rate coefficients were determined by fitting zero‐and first‐order rate equations to the oxygen concentration reduction curves, although only zero‐order rates were used to calculate biodegradation rates. Air‐filled porosity estimates were found to vary by up to a factor of two between sites and at different times. NAPL degradation rates calculated from measured air‐filled porosity and oxygen usage rate coefficients ranged up to 69 mg kg‐1 day‐1. These rates are comparable to and higher than rates quoted in other studies, despite the high concentrations and weathered state of the NAPL at this test site. For nutrient‐amended sites within the trial plot, estimates of NAPL degradation rates were two to three times higher than estimates from nonamended sites. Rates also increased with depth.