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Section 4 PROPOSED SITE INVESTIGATION OVERVIEW The study proposed in this workplan was developed in response to a request by the Central Valley Regional Water Quality Control Board, dated December 5, 1994, and was designed based on prior site investigation results. The focus of this investigation is primarily on the subsurface soils and groundwater beneath the property immediately adjacent to the gas dehydrator station. The chemicals of concern identified for this study include total petroleum hydrocarbons (primarily in the gasoline and kerosene ranges) and BTEX compounds. Although diethylene, triethylene and tetraethylene glycols were reported in a condensate sample collected from the reboiler unit in 1988, these compounds are not considered chemicals of concern. These glycol compounds are not regulated per Chapter 11,Title 22 of the California Code of Regulations, j nor are they contained in the water quality goals listed Marshack(October 1986; revised September 1991). I These glycol compounds are separate and distinct from ethylene glycol which is listed in the Regional Board document listed above. Ethylene glycol is not used at the gas dehydrator stations and available information indicate that diethylene, triethylene and tetraethylene glycols do not biodegrade to ethylene 1 glycol, but rather degrade to glycolic acids and water. The proposed site investigation consists of borehole soil sampling and groundwater assessment within the property boundaries of the Lathrop Gas Dehydrator Station and in the property located immediately east and north of the site. The work will consist of drilling and sampling 5 exploratory soil borings, installing groundwater monitoring wells in the boreholes, and collecting groundwater samples from the monitoring wells for assessment of groundwater quality (see Figure 5 for the proposed locations of the exploratory borings and monitoring wells). All groundwater monitoring wells will be surveyed and gauged for determining groundwater flow direction and gradient. In addition,groundwater elevations will be logged every 15-min. in one well installed at the site along with water level elevations on the Middle River over a 24-hr period to determine if tidal fluctuations affect groundwater levels beneath the site. If tidal fluctuations are observed in the groundwater levels at the site, the groundwater elevations will be measured in several wells for a 72-1-s period to determine the mean groundwater gradient direction using the methods described in Serfes (1991). J J 1 cca/03/10/95 09:47 AM(0688brpt.doc/ntw12) 4-1 J