Maddox & Associates, Inc. - Hydrogeology Company   Request a Free Consultation

Phase II: Site Investigation Case Study

Site Investigation

Car Wash

Southern California

This Phase II Site Investigation was conducted at a car wash in Southern California.  When two USTs, associated piping, and gasoline dispensers were removed from the site, it was determined that leakage had occurred from beneath one of the dispensers.

The Riverside County Department of Environmental Health ordered the property owner to conduct a subsurface investigation to evaluate the lateral and vertical distribution of  contaminants in the soils and to determine whether gasoline had impacted the underlying groundwater.

M&A installed soil borings and groundwater monitoring wells to characterize the distribution of gasoline contaminants beneath the site.  The primary contaminant detected in the soil column was MTBE (a fuel oxygenate).  Additionally, another oxygenate, TBA, was detected at the very bottom of the soil column (the capillary fringe) and also was present (along with MTBE) in the groundwater.

M&A reviewed the records of the ARCO AM-PM mini-mart directly across the street and in the APPARENT down-gradient direction of groundwater flow.  Calculations of the  groundwater flow direction by M&A indicated that the actual direction of groundwater flow was, unexpectedly, from the ARCO station toward the car wash site.  M&A further determined that the gasoline contaminants present at the car wash site were primarily the result of vapor transport, with the vapors originating at the ARCO station.  Both site and historical data indicated that TBA had never been a constituent in the gasoline dispensed at the car wash, but was present in high concentrations within the soil/groundwater contaminant plume beneath the ARCO site.

M&A prepared a Site Conceptual Model/Closure Request Report and submitted it to the Regional Water Quality Control Board.  The site is currently under review for closure, with closure anticipated in the near future.