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1.0 BACKGROUND INFORMATION 1.1 Site Description/11istory The site is located at 1145 West Charter Way in Stockton, California (Figure 1). The property is owned by MTS Trucking, who is currently leasing the property to Independent Trucking. There is currently one 1200 gallon septic tank located on the property. This unit is the source of the release of petroleum contamination. The site, consists of office buildings, a series of large truck maintenance/service bays and a parking area for the fleet of trucks. (Figure 2). The site is approximately level and at an elevation of about 10 feet above sea level. It is graded so that surface run-off will drain toward several strategic locations around the site. The septic tank in question and the associated leach field has been in-place for a unknown amount of time. The amount of time or the volume of waste oil that was placed into this tank is not accurately known. The septic tank was used for waste oil storage and disposal until the spring of 1987. At that time all dumping of waste oil ceased and the oil was stored in a above ground waste oil tank. It was in the spring of 1990, the toilets in the maintenance shop began having problems backing up. On a number of occasions, a routine maintenance call on the lines from the toilets to the septic tank cleared the problem of backing up. In September of 1992 the toilets in the maintenance shop began having problems again and this time, pooling of water was noticed in the area of the tank. A maintenance firm was called in again to clear the lines and investigate the reason for the ponding. The investigation exposed a crack in the supply line to the septic tank and the first chamber of the tank was full of septic. A set of portable toilets were brought in to the site and the toilets in the shop were placed out of service. The tank has not been in service since this event. In November of 1992, the tank was pumped of it's contents in the first stage of the sewage and disposed of. After this event was completed the firm of Perish & Sons conducted a excavation at the site to repair and establish a new leach field. During the removal of the piping and leach field drainage rock this contractor encountered oil in and around the area. All work stopped for a reassessment of the situation. The next event occurred after it was discovered that there were more than one quadrant or stage to this septic tank. Sacramento Waste Oil was called to the site and they pumped out 1000 gallons of waste oil from the tank. This product was disposed of off site as a California regulated waste. On November 23, 1992 a limited exploratory excavation was conducted to attempt to define the areal extent of the contamination. 1