2000-05-22_PERMIT FILE - M2000002 (17)

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Holly Bnllnuuid • • Page ? Mubile Prnmi.~~ Concrete. Inc Mtn' 22, 2000 IRA-A. The plume follows the First Creek paleochannel, which Flows northwest from the RMA toward the South Platte River and is northeast of the Tanabe property. The Tanabe property is in what the TCHD refers to as "the gap," or the at'ea between the northern DIMP plume fringe area and northwest DIMP fringe area shown in Figure I. The fringe area is shown on the US Army and TCHD DIMP distribution maps to be hydt'ologicallycoenected to the South Platte River (Figure 3). According to most recent 1998 analytical data and plume maps provided by TCHD, the ground water beneath the norYheas[ern'/z of [he property does no[ have detectable DIMP concentrations. Only the southwestern''/x of the property lies over the fringe area, i.e., areas where DIMP was detected in ground water but did not exceed CGWS. Based on ground water elevation and plume maps, the northern DIMP plume is located hydrologically cross gradient from the Tanabe property. Ground Water Quality D/MP Concentrations. TCHD monitors ground water quality in the Offpost Study Areas for DIMP in both domestic and irrigation wells. According [o TCHD, DIMP currently found in ground water itt the Offpost Study Area is remnant of pre- containment/treahnen[ concentrations because the source of DIMP for the Offpos[ Study Area is removed by [he containmendtreatment systems. In the absence of a DIMP source, it is unlikely that DIMP concentrations would increase under current ground water Flow conditions. The TCHD has also confirmed that the three existing alluvial wells on the Tanabe property (485A, 485B, and 485 C) have been sampled. Because well 485A was once used for drinking water, it has been sampled the most frequently. The three wells have not had DIMP concentrations that exceed 1.3 µg/L since 1990. DIMP concentrations in well 485B may have exceeded the CGWS in 1985. The detection limit used in 1985 (10 µg/L) was above the CGWS. A fourth well is an irrigation well designated as 485D and has never been sampled. Analytical data obtained from three wells located hydrologically upgradient from the Tanabe property show recent concentrations below the CGWS and consistent decreases in DIMP concentrations since 1985, consistent with the original design intentions of the RMA corrective action measures. A summary of obtained ground water quality data is shown in Table I with ground water contours and well locations shown in Figure 4. The decreasing concentrations are consistent with the decrease in size of both the DIMP plume fringe area as well as the northern DIMP plume depicted on maps obtained from the TCHD and the U.S. Army from 1995 and 1997. The maps also show the northern plume following the First Creek paleochannel, north of the Tanabe property. In 1998. TCHD sampled the surface water discharging from two gravel pits located northwest of the Tanabe property and within the plume fringe areas. The first pit, located southwest of the intersection of Brighton Road and 120'" Street, discharged water to the South Platte River a[ an estimated 30 gpm. DIMP concentration of the discharge was 2.3 µg/L. A second, larger gravel pit dewatering operation located south of [he first pit, closer to the plume, discharged water into [he South Platte River at an estimated 8 to 10,000 gpm. No DIMP concentrations were detected in this discharge. ERO Resources Corporation