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i STEWART'3 BEACON PH4ASE I REPORT PAGE: 4 contains a clay matrix. From 17 to 14 feet Is a brown,sandy clay,which is firm and wet. Between 14 and about I I or 10 feel is 8 brown,sandy and cleyay silt, that Is water saturated. In 9 southerly direction,this silt grades into a fine grained,ciayey sand Above I i or 10 feet Is a brown,sandy clay that Is capped with a few feet of top soil near the ground surface. The interbedded nature - of these strata Is shown 1n Figure 3. Boring logs for each boring and monitoring "I I are given in Appendix A. The mineralogy of the clay fraction can play an Important rol I In control IIng the movement of contamination in the subsurface. The clay smectite(asweilingclay) acts as both a physical and chemical barrier to migrating contaminants. However, at this point,clay mineralogy&clysis has not been done on these sediments. 3.1.2 Hydrology Ground water was encounte,-ed at a shallow level during the drilling of time soil horIngs,which is typical for the Tracy area. Three monitoring wells were Instal led and the ground water depth and direction of flow determined. Ground water stands at 10 to I I feet depending on the surface elevat Ion. Water level measurements and elevations above mean sea level(MSL)for each monitoring. well taken from the wet I casing top are given in Table 2. Hydraulic gredlent Is in a northerly direction(Figure 2)at a rate of.0043 feet per foot(52/1000 Inch r ` per foot). The monitoringwells �-� upper unit is com penetrate two porous,water bearing strata. The posed+,a clayey silt and clayey sand located between 14 and I I or 10 feet. The lower unit Is composed of a clayey sand and Is located between 20 4 and 17 feet. Separating these coarse grained units Is a sandy clay,that is soft and t very wet. Because of Its thinness,softness,and high degree of water content,we bel leve that this clay does not form a barrier or quitard,and that these water barring strata(from 20 to I I feet)act as one aquifer, Therefore,the lower porosity clay layer between the two coarser grained units wl I I only act as a rets,^dant to vertical water movement and will not prevent water flow across it. The cross section given in Figure 3 shows the relationship of the water gearing strata. _ �.2 Contamination Distribution With the hydraulic grafient going to the w!h,contamination has migrated primarily to the north also. The cross section in Figure 3 i i lustrates our interpretation of the s; contamination distribution In a north--south direction. Soil Contamination has migrated �- laterally from the tank to the north&W east. West of tt- tank at boring 4,motor of is the only compound detected. The source of this motor oft Is unknown. There Is surface --- - --- —evidenee of oil being released on the slaps alonglfSe west edge of the property,but according to Of 11 Stewart this Is only a recent occurrence an there has not been a history of it. Vortical movement of contamination has taken these compounds(gasoline&BTX)down to groundwater. Once it encountered ground water It migrated In the direction of water flow, a