Hydrocarbon Contamination Contributes to Increases in Soil Organic Carbon: Impacts on Risk Assessments of Contaminated Land in the UK

In the UK, contaminated land risk assessments using the Contaminated Land Exposure Assessment (CLEA) model rely on soil organic matter (SOM) values to determine acceptable thresholds for contamination caused by anthropogenic pollution for human health. Soil organic carbon (SOC), total organic carbon (TOC) or loss on ignition (LOI) are routinely used as a proxy for SOM in the industry, both by contaminated land consultants and laboratories who rely on conversions such as the Van Bemmelen factor. Many standard laboratory methods for measuring SOC or TOC do not differentiate between natural organic carbon and petroleum hydrocarbons. This study investigates the interference of total petroleum hydrocarbons (TPHs) with SOC measurements by analysing 2377 brownfield soil samples. A positive correlation was observed between the two variables; when TPH concentrations increase by 1000 mg/kg, the reported SOC increases by 0.46 percentage points when outlier samples (>10,000 mg/kg TPH or >30% SOC) are excluded. When converted to SOM for risk assessment purposes using the Van Bemmelen factor, the calculated SOM rises by 0.79 percentage points per 1000 mg/kg TPH. This can push soils into higher assessment bands, generating less stringent generic assessment criteria (GAC). While the entire 0.46-percentage point increase cannot be accounted for entirely by a TPH rise of 1000 mg/kg (or 0.1%), it is hypothesised that other contaminants containing carbons are likely to rise alongside TPH and contribute to the increase in SOC. This study finds that relying on SOC as a proxy for SOM in hydrocarbon-impacted soils masks the absence of the natural organic matter required to sorb contaminants, leading to an underestimation of human health risks in contaminated land risk assessments.