Long-Term Assessment of Petroleum Hydrocarbon Attenuation at Petroleum Release Sites in California

Currently, there are over 62,000 petroleum-contaminated sites undergoing remediation in the United States. Understanding attenuation rates for dissolved contaminant concentrations and factors that influence these rates is important to effectively manage these sites and move them towards closure. The GeoTracker and California Department of Public Health databases provide more than 15 years of groundwater monitoring results for tens of thousands of monitoring wells and public water supply wells in California. First-order maximum concentration attenuation rate constants (kc-max) were determined for sites with 5 or more years of monitoring data over the time period of 2002 to 2017. kc-max values vary by constituent; across more than 5000 sites, the median kc-max value ranged from 0.30 year−1 for methyl tert-butyl ether (MTBE) (half-life of 2.3 years) to 0.076 year−1 for naphthalene (half-life of 9.1 years). A focused evaluation of remediation technologies at 50 petroleum release sites indicates that dissolved contaminant attenuation typically increased during the time period of remediation. Further, the analysis indicates that biodegradation is an important attenuation process even during periods without active remediation (i.e., periods of natural attenuation). There was little evidence of petroleum and fuel oxygenate impacts to public water supply wells. Overall, the analyses indicate that both remediation and natural biodegradation contribute to site remediation and that these attenuation processes have been sufficient to prevent widespread impacts to public water supply wells.