The influence of cold climate seasonal temperature regimes on bioremediation of petroleum hydrocarbon-contaminated soils

Cold-adapted hydrocarbon-degrading bacteria are able to survive and grow in petroleum-contaminated soils in Arctic and sub-Arctic sites, and have been the basis for consideration of bioremediation technologies for clean-up of these sites. Several laboratory studies have also shown that petroleum hydrocarbon biodegradation rates by cold-adapted hydrocarbon-degrading bacteria are influenced by incubation temperatures, and those studies have employed constant incubation temperatures. However, temperatures in cold climates are dynamic due to diurnal, periodic or seasonal temperature variations and variability in incoming solar radiation, and these temperature changes may influence on-site soil microbial activity. To date there has been very little research focused how variations in site temperatures and various seasonal temperature regimes influence biodegradation performance.;Pilot-scale landfarming experiments were performed in a laboratory in soil tanks under site temperature profiles representative of the 3-year site air temperatures in July and August where temperature varied uniformly between 1°C to 10°C over 10 days. The rates and extent of biodegradation of the non-volatile, higher molecular weight hydrocarbons (F3) was significant and comparable to the rates and extent of biodegradation observed for semi-volatile, lower molecular weight fraction (F2). The first-order biodegradation rate constants for the F2 and F3 hydrocarbon fractions were similar to each other in both low initial TPH and high initial TPH landfarms and estimated to be 0.011 to 0.024 day-1 and 0.016 to 0.019 day-11 , respectively. Changes in ratios of residual concentrations of C14, C16 and C18 alkanes with progressive biodegradation was tracked, and showed that as TPH levels declined, the relative abundance of the higher-molecular weight alkanes declined. Soil aggregates with diameters ranging from 0.6 mm to 2 mm contained residual TPH that was not bioavailable and thus controlled the effective endpoint of biodegradation.;Variable temperatures representative of site temperature changes between 1 and 10°C over two months dramatically influenced the rates and extent of biodegradation of petroleum hydrocarbons compared to the rates and extent obtained under a constant average incubation temperature of 6°C. Under the variable site temperature condition, more rapid biodegradation of both semi- and non-volatile hydrocarbons occurred by over a factor of two due to accelerated bioactivity and growth of indigenous hydrocarbon-degrading microbial populations. Preferential biodegradation of semi-volatile hydrocarbons over non-volatile hydrocarbons was significant in the constant average temperature mode, but not under the variable temperature regime. The biodegradation rates determined by the variable site temperature approximation were in better agreement with those determined by an on-site experiment at the same site.;A study was undertaken to quantitatively assess biodegradation of petroleum hydrocarbons and microbial respiration and response during the seasonal transition periods preceding and following summer where freezing and thawing of the surface soil layers occurs at the sub-Arctic site. During the freezing phase, a statistically significant extent of biodegradation of 13% of semi-volatile hydrocarbons occurred, which was correlated with the emergence of Corynebacterineae -related hydrocarbon-degrading bacteria and growth of heterotrophic microbial populations. Petroleum hydrocarbon biodegradation, microbial respiration and changes in the size and composition of microbial community occurred under sub-zero temperatures but only when there was substantial liquid, unfrozen pore waters coexisting with pore ice. A rapid rate of temperature increase caused a burst of microbial respiration activity as previously observed in several studies, and it was found that this microbial activity also resulted in significant reductions of semi- and non-volatile hydrocarbon concentrations of up to 25% and 11%, respectively.;The overall objective of this research was to investigate the rate and extent of total petroleum hydrocarbon (TPH) biodegradation in contaminated soils from a sub-Arctic site under site-relevant temperature regimes in cold climates and to assess the effect of different temperature regimes on TPH biodegradation activity. A series of pilot-scale biodegradation experiments was conducted using field-aged petroleum-contaminated soils shipped from a former military Distant Early Warning (DEW) line site in Resolution Island, Nunavut, Canada (61°30'N 65°00'W).