Analysis into the viability of pea gravel as a diffusing material for biostimulation systems in petroleum hydrocarbon‐contaminated soils

Injecting biostimulatory solutions around underground storage tanks (UGSTs) promotes petroleum hydrocarbon (PHC) bioremediation without the use of intrusive infrastructure. Yet, it is unclear how the interaction between porous bedding material (i.e., pea gravel) and biostimulatory solutions impacts PHC biodegradation rates. Consequently, we assessed whether pea gravel can act as an inert diffusion material that facilitates PHC bioremediation within UGST beds. First, we evaluated how biostimulatory solution elution through pea gravel (i.e., weathered) altered nutrient composition and benzene degradation rates. Suspected microbial activity decreased the proportion (i.e., nutrient concentration after pea gravel elution divided by initial biostimulatory solution concentration) of ammonium (0.45-0.84) and citrate (0.0-0.25) in effluents. In comparison, the proportion of anaerobic electron acceptors sulfate (0.91-1.08) and nitrate (0.78-1.0) were largely unaffected by pea gravel sorption. After 28 days, the weathered biostimulatory solution degraded 25 ± 0.50% of added benzene at rates similar (0.010 ± 0.002 days^-1) to fresh biostimulatory solution (0.008 ± 0.001 days^-1). Next, using positron emission tomography (PET) with [¹⁸F]-fluorodeoxyglucose and [¹⁸F]-fluoride, we evaluated the risks of biofilm fouling to gravel beds. In agreement with the reduced nutrient concentration, PET results show that the biostimulatory solution promoted microbial growth on the surface of the pea gravel. Yet, as the pore space occupied by bacteria remained low (between 8.8% and 13%) following biostimulatory solution elution, biofilm formation is not a concern. Therefore, pea gravel environments in tank nest areas can support PHC biostimulatory solution delivery with minimal consequences.