Oil remains the largest contributor to global energy consumption, powering vehicles, heating homes, and fueling the economy. This dependence on oil has led to widespread groundwater contamination, primarily through the release of light non-aqueous phase liquid (LNAPL). These contaminants typically enter the subsurface from leaking underground storage tanks, pipelines, and refineries. Once in the subsurface, LNAPLs migrate through soil and form an immiscible layer that floats on the water table. The formed layer acts as a persistent source of dissolved petroleum hydrocarbons (PHCs) and creates contaminant plumes that last for decades.

PHC compounds are highly toxic and carcinogenic, posing risks to human health and the environment upon exposure. Although groundwater serves as a major drinking water source, drinking water wells are monitored only quarterly or yearly for PHC contamination. Conventional monitoring relies on time-consuming manual sampling and costly laboratory analysis. Sensors exist for directly detecting and measuring PHC levels in the field, but most of these sensors remain experimental and/or expensive.

Microbial communities in aquifers can degrade and thereby attenuate PHCs, making monitored natural attenuation a preferred long-term management strategy for PHC contaminated sites. This is especially the case when active remediation (e.g., pump-and-treat or excavation) is technically challenging or costly. However, there is still a gap for cost-effective tools for real-time, continuous groundwater monitoring for PHC contamination...

Petroleum products have contaminated groundwater with harmful organic compounds, such as benzene, toluene, ethylbenzene, and xylenes (BTEX). Collecting and analyzing polluted groundwater samples is expensive and undertaken infrequently. However, quick remedial action in case of unexpected events requires continuous monitoring. In-situ water quality sensors (pH, EC, DO, ORP) may show correlations with the components of dissolved petroleum hydrocarbon (PHC) such as aromatics and non-volatile mobile fractions. Correlations are prerequisite to ultimately develop real-time prediction models. Since suitable field data sets are limited, we simulated the fate of hydrocarbons in groundwater under various realistic conditions using a reactive transport model as novel approach to explore when, where, and why correlations occur. A stationary oil source zone continuously dissolved at the top of a heterogeneous and shallow sandy aquifer over a two-dimensional cross-section. Our model considered transient conditions (fluctuating water table) and spatially uniform hydrogeochemical composition. We observed a strong correlation between PHCs and water quality sensors (rolling Spearman's correlation > |0.8|) at varying periods. These correlations are strongly affected by the location of observation wells, the aquifer's hydraulic conductivity, and the availability of calcite and oxide minerals, and other electron acceptors. DO and ORP are significant for the early detection of hydrocarbon contamination, whereas pH and EC are important features for the long-term monitoring of hydrocarbons. Our findings lay the foundation for the subsequent development of a data analysis model to detect and estimate in real time PHC levels in groundwater using in-situ water quality sensors.