Biostimulatory Solutions for PHC Contaminated Sites: Effects of C: N: P Ratios on Degrader Prevalence and Potential Activity

Abstract

Optimal aerobic nitrogen (N) and phosphorus (P) concentrations for in-situ biostimulation of petroleum hydrocarbon contaminated sites have been extensively investigated. However, it has not been assessed if C: N: P ratios may differ for anaerobic soils and if lower C: N: P ratios effect the microbial community structure. In addition, there are limited studies that explore if degradation can recover after eutrophic conditions are introduced. Before we were able to make these assessments, we created a bench scale microcosm design that mimics field conditions. Most laboratory studies on biostimulation have proven results in the lab that do not translate to successes in the field. We believe this disconnect is due to the alteration of soil that occurs in microcosm experiments. In most laboratory studies, soils are dried, sieved, and then spiked. This process alters the soil surface area coming into contact with biostimulatory solution, soil structure, fractured flow, microbial population and habitat, and hydrocarbon adsorption and desorption. The objectives were to: 1) design a microcosm experiment to stimulate field conditions; 2) determine how C: N: P ratios effect the microbial community and how soil properties influence C: N: P ratios; and 3) determine if degradation rates recover after high nutrient conditions have been introduced. To mimic field conditions, soil cores were sub-sampled using a 2 x 1.5 (OD) inch slotted polyvinyl chloride (PVC) pipe and each subsample was placed into a sealed 125 mL amber jar with a biostimulatory solution. We demonstrated that C: N: P ratios were not selective for hydrocarbon ratios, but higher P in solution at low contaminant concentrations enhanced benzene degradation more than the other chemicals in the F1 fraction. We also demonstrated that original site conditions and the amount of S and P in solution was more influential on degradation rate than the C: N: P ratio. Lastly, we demonstrated that the microbial community and degradation success was influenced the most by pre-excising site and within site conditions. These results suggest that sites may behave very different even when the same nutrient amendment is applied based on pre-existing site factors.