Respiratory quotients as a useful indicator of the enhancement of petroleum hydrocarbon biodegradation in field-aged contaminated soils in cold climates

Abstract

The Respiration Quotient (RQ) is a substrate-dependent respiration indicator. The determination of RQ-values from soil gas data obtained during the bioremediation of contaminated soils provides useful information about whether or not microbial respiration activity is specific to the microbial degradation of contaminants. Less attention has been paid to the uses of RQ-values for petroleum hydrocarbon-contaminated cold-climate soils under a wide range of low temperature regimes including sub-zero temperatures. In this study, RQ-values were determined for biotreated petroleum hydrocarbon-contaminated soils shipped from two cold-region sites and were validated using pilot-scale aerobic bioremediation experiments with various soil treatments, low temperature regimes and soil types. The RQ-values (0.57 to 0.78), empirically determined from CO2 and O2 in soil gas, were consistently close to the theoretical RQ-values for target hydrocarbon fractions (0.65–0.8) only when the decrease in hydrocarbon concentration was statistically significant in the biotreated soils. In addition to the RQ-values, a strong linearity was observed in the RQ plots for CO2 production and O2 consumption, but only for the soils in which hydrocarbon biodegradation activity was significantly enhanced. Using confirmed hydrocarbon and microbial data for the occurrence of hydrocarbon biodegradation, the RQ plot characteristics (RQ-values and linearity) proved to be considerably relevant to hydrocarbon biodegradation. Due to the substrate-dependency of the RQ-values, conventional soil gas monitoring activities involved in the bioremediation and management of contaminated soils at remote cold sites can be more effective and meaningful by indicating enhanced microbial activity specific to petroleum hydrocarbon biodegradation.