MtBE Tracer Study To Define Groundwater Transport Parameters: Estero Marine Terminal

Abstract

Methyl Tertiary Butyl Ether's (MTBE) chemical properties lead to rapid transport in groundwater systems. A short-duration spill of MTBE-bearing oil to a well-studied and monitored hydrogeological system allows determination of chemical and hydrogeologic conditions in the vicinity of the spill site. The measured rate of expansion of the MTBE plume, coupled with known soil and hydrologic properties, allows determination of field-scale MTBE transport parameters; the trajectory of the MTBE plume provides independent, chemical, confirmation of groundwater flow paths inferred based upon hydrogeologic data. Chevron's Estero Marine Terminal, located on California's Central Coast near Morro Bay, received crude oil from Kern County production and loaded it to ships for transport from the 1930s to 1999. The terminal has been subject to assessment and monitoring activity on at least a quarterly basis for 5 years. A plume of cutter stock (a thinner approximately equivalent to a diesel composition) mixed with heavy crude oil is at the water table, but the low solubility material has produced a very small halo of hydrocarbon-bearing groundwater. No benzene or MTBE had been detected in site monitor wells. During pipeline abandonment activity, ∼200 gal of a gasoline-like light-cycle oil was spilled to the ground and infiltrated 2–3 ft to groundwater over several days. The light-cycle oil contained ∼2 mg/L MTBE. Subsequent analysis of groundwater samples from the monitoring well array sequentially provided detections of MTBE and benzene. These data constrain calculations of field-scale transport parameters at the site and provide a tracer of the groundwater flow path from the spill area. The observed transport rate for MTBE is approximately twice that of benzene. This relative velocity agrees well with predictions based upon the organic carbon content of the soils. The observed MTBE transport rate is approximately equivalent to the predicted groundwater flow velocity. This observation indicates little or no retardation of MTBE. The plume of MTBE appears to have stabilized over an ∼3-month period, suggesting that some attenuative mechanism is at work. The site is located adjacent to a coastal stream and the Pacific Ocean. Past study has indicated a three-component flow field in the area of the spill, with the coastal stream discharging to groundwater, and a net zero gradient towards the ocean caused by symmetric tidal variation. The track of the MTBE plume is along the flowpath inferred based upon hydrologic data alone. The track of the plume also suggests that the groundwater system ultimately discharges to the Pacific Ocean, but the flowpath is deflected strongly to the south based upon discharge from the coastal stream to groundwater.