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 nonaq ueous phase liquid (NAPL) contaminating a shallow sand aquifer. Multip le depth monitoring of oxygen concentrations and air-filled porosity w ere carried out in nutrient amended and nonamended locations to assess the variability of degradation rate estimates calculated from respira tion tests. The field trial consisted of periodic addition of nutrient s (nitrogen and phosphorus) and aeration of a 100 m(2) trial plot. Dur ing the bioremediation trial, aeration was stopped periodically, and d ecreases in gaseous oxygen concentrations were logged semi-continuousl y using data-loggers attached to recently developed in situ oxygen pro bes placed at multiple depths above and within a thin NAPL-contaminate d 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 biodegra dation rates. Air-filled porosity estimates were found to vary by up t o a factor of two between sites and at different times. NAPL degradati on 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 t he high concentrations and weathered state of the NAPL at this test si te. For nutrient-amended sites within the trial plot, estimates of NAP L degradation rates were two to three times higher than estimates from nonamended sites. Rates also increased with depth.