Soil Remediation in Coarse Gravelly Soils: Challenges and Lessons Learned

The existing laboratory analytical protocols, including the Canada Wide Standard for Petroleum Hydrocarbons, focus only on the coarse sand and finer textured fraction of the soils. Modern lab equipment (designed to maximize efficiency through minimization of solvent use and extraction times) can arbitrarily limit analyses to only the finer portion of the soil samples by physically restricting the particle sizes analysed. This can create a bias in the analysis as hydrocarbons are often more concentrated in the finer fraction of the soil, which may represent a small proportion of the overall soil mass. To address this problem a modified extraction procedure was developed in co-operation with EnviroTest Labs for gravelly soils to achieve a representative analysis of the whole sample. The soils at the three sites comprise a mix of sands, gravels and cobbles with only minor amounts of silt and clay. Within such soils the hydrocarbon contamination is distributed in different manners. In the cobbles, gravel and sand grains the hydrocarbon contamination is distributed primarily as surface coatings with very little if any of the mass actually penetrating the grains. In contrast, for the finer particle sizes the hydrocarbons may be adsorbed onto organic carbon, diffused into the soil matrix and/or present as a surface coating. This bimodal distribution of hydrocarbon contamination was confirmed through bench scale separation of coarse soils (gravel plus) from finer soils. Once sorted, the gravel plus soils were generally found to meet remedial objectives (using the new extraction method), while the fine soils contained the majority of the contamination. Hence, a remedial program was designed using a mobile screener to