Formatted Title
Optimizing Biostimulatory Solutions Using Site-Specific Conditions to Enhance Petroleum Hydrocarbon Degradation Rates
Background/Objectives
Former retail fuel facilities are often subject to petroleum hydrocarbon (PHC) impacted soils due to faulty or leaking underground storage tanks (USTs). Historically, contaminated soils have been excavated and disposed of at an off-site treatment facility or landfill. As costs associated with equipment operation and off-site disposal continue to rise, environmental managers are increasingly turning to in situ remediation technologies and techniques to deal with PHC impacts. A particularly effective in situ technique for petroleum hydrocarbon remediation is bioremediation, specifically, biostimulation which involves the delivery of nutrients and electron acceptors to the impacted area below ground surface. Standardized biostimulatory recipes have been used across North America for many years with varying levels of success. This variability can be attributed to the heterogenous nature of soils and site-specific geochemistry. Soils in the Canadian prairies typically have high clay and carbonate content resulting in high sorption of the nutrients delivered in the amendment solution, specifically phosphate. A key to unlocking the increased effectiveness of biostimulation in Canada’s calcareous soils is understanding soil buffering capacity and how it contributes to the weathering of the amendment solution.
Approach/Activities
A flow through microcosm was developed to determine the influence of site-specific soil properties on the effectiveness of a series of four biostimulatory solutions. Each microcosm consists of two sections: a bottom section where a 20 cm segment of intact core is inserted and a top section that contains a sensor pack that detects soil gases including total PHC, carbon dioxide (CO2), methane (CH4), and oxygen (O2). The biostimulatory solutions are pumped using a peristaltic pump set to a flow water of approximately 0.3 mL/min. Biostimulatory solutions were continuously pumped through the cores for 30 days. Effluent samples from every optimizer were collected after 1, 3, 5, 7, 14, 21 and 30 days. Eluent samples were taken to the University of Saskatchewan for ion chromatography (IC) analysis. Anions (NO2-, NO3-, SO4-2, PO4-3) and cations (Ca2+, Mg2+, K+, Na+ and NH4+) will be quantified in samples. The ion data were then correlated with the soil gas information collected from the sensor pack. Statistical analyses are currently underway to determine which biostimulatory solutions are most effective for each core tested using the soil gas and IC data. These results were then compared to site-specific soil and groundwater data to determine which parameters may be driving increased PHC degradation rate.
Results/Lessons Learned
Calcium dissolution was used as a surrogate for soil buffering capacity due to the calcareous nature of the soils. Dissolution kinetics were modeled using a power function with calcium dissolution capacity, as a response variable to evaluate site differences. Results indicated that there were site differences in calcium dissolution capacity. Site-specific groundwater routine groundwater parameters, including cations, anions and nutrients, were then compared against the calcium dissolution capacities for each site to determine how the parameters may influence calcium dissolution. Initial statistical analysis indicated that calcium, magnesium, potassium and total organic carbon concentrations in groundwater may be driving calcium dissolution. Laboratory are still being conducted to determine how these parameters condition the biostimulatory solution and how the conditioning ultimately impacts hydrocarbon degradation rates. The ultimate goal of this project is to develop a predictive model using soil and groundwater parameters to determine which optimal biostimulatory solutions for PHC degradation. Once the drivers are identified they can be used to adjust biostimulatory solutions to allow for increased hydrocarbon degradation rates and thus faster remedial timeframes utilizing a more sustainable, in situ strategy.