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1995 Technical Memorandum on Soil Boring Investigation
Section 10 - Summary of Results
10.1 - Geology and HydrogeologyThe following summarizes the geology and hydrogeology at the seven SWMU sites from ground level to 30 ft below ground level (bgl).
The thickness of soil overlying Cretaceous-age Glen Rose Formation limestone ranges from 0 ft at SWMUs B-19 and O-1 to 5.5 ft at SWMU B-28. The soil was described as dark grayish brown clay (CL) with little limestone gravel and some silt.
The color of the underlying Glen Rose limestone varies from white to tan and light to dark gray. Where the limestone varies from white to tan, the amount of marl varies from very little to almost total marl. Limestone cores were often fossiliferous, containing forams, pelecypods, gastropods, echinoderms, and crinoids. The Corbula bed or Salenia zone (Atnipp, 1986) marking the boundary between the upper and lower members of the Glen Rose Formation was not positively identified in any of the soil borings drilled during this investigation. Based on the presence of the Salenia zone in Salado Creek south of SWMU B-28, it appears that the upper Glen Rose Formation is very thin or nonexistent in this area.
Common structural and textural features in the core samples included high-angle to vertical fractures from 1 inch to 1 foot in length, voids partially filled with calcite crystals, solution channels, and dissolution of fossiliferous zones. Evidence of water movement is indicated by the calcite-filled voids.
Sufficient volumes of perched groundwater to obtain an analytical sample were encountered at six of the seven SWMU sites and seven of the thirty-four soil borings (less than 20%). Two groundwater samples were collected at SWMU B-2. Water levels in the open boreholes ranged from 12.42 to 29.1 ft bgl.
Two sites, SWMUs B-3 and O-1, appear to have the highest potential for being the source area for the contamination in wells 2, 3, 4, 16, and D. Perched groundwater at B-3 contained all the chlorinated hydrocarbons (PCE, TCE, and cis- and trans-1,2-DCE) detected in well 16, plus the biodegradation product vinyl chloride. Concentrations of PCE, TCE, cis-1,2-DCE and vinyl chloride exceeded MCLs in perched groundwater at B-3. PCE, TCE, and cis- and trans-1,2-DCE were also detected in soil samples collected at B-3. Soil TCE concentrations exceeded comparison criteria in three of the four soil borings drilled at B-3/ The oxidation pond had the highest levels of PCE detected, but these values did not exceed the MSCs for soils less than 2 ft bgl. This initial characterization did no determine the lateral and vertical extent of contamination from these sites in reference to well 16. Potential contaminant pathways from SWMUs B-3 and O-1 to well 16 include a network of fracture systems and solution cavities.
Although the contaminant detected in well 16 were not detected at SWMUs B-1, B-2, B-4, B-19, and B-28, SWMUs B-2 and B-4 should not be completely ruled out at this time as potential sources and may warrant further investigation for SWMU closure purposes. Toluene was detected at B-2 below TNRCC comparison criteria in three soil borings. This compound has been associated with past waste disposal activities, such as burning and even though chlorinated hydrocarbons were not detected, it may still present a potential source of contamination. Since drilling the five soil borings at SWMU B-4, three additional subsurface geophysical anomalies have been detected at SWMU B-4A (Parsons ES, 1995a).
Seven out of thirty-four borings yielded sufficient volumes of perched groundwater (3-4 liters) to obtain an analytical sample at six of the seven SWMU sites. One soil boring at B-19 only produced enough water to sample VOCs. This implies that perched groundwater in the vicinity of well 16 is discontinuous. VOCs were only detected in one groundwater sample from B-3. With the exception of B-1 and B-2, all sites are downgradient of wells 2, 3, 4, 16, and D. However, a change in the gradient, caused by pumping groundwater, could pull the contaminants toward the wells. Since the occurrence of perched groundwater was discontinuous, the lateral extent of contamination was not determined. Lithologic information from the soil borings indicated the sites contained numerous small vertical fractures and solution cavities for groundwater to migrate vertically, as well as horizontally. Water in perched zones does not necessarily flow in the same direction as the regional groundwater gradient, but follows the path of least resistance. During the wet season, the extent of the perched groundwater zones may increase and potentially carry contaminants to well 16.