FIELD DOCUMENTS AND WORK PLANS 1989

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Gary A. Reents 4- 24 February 1989 C. Isoconcentration contours for the shallow and intermediate zones were drawn. Chromium migration is more rapid in the intermediate tone than in the shallow zone. D. The areal extent of the chromium plume (to 0,05 mo/1 ) , in the shallow zone, has been established . This plume extends downgradient only 2001 - 300' from the southwest corner of the South Yard . E. The areal extent of the chromium plume (to 0.05 mg/1 ) , in the intermediate zone, has not been completely established . The downgradient extent of the plume has been verified as extending about 4000 feet to the south (to East Main Street) , the known lateral dispersion of the plume is minimal (600 to 800 feet) , The narrow, elongated distribution of chromium within the intermediate tone is attributed to the dominance of advective transport within the highly transmissive sands and gravels . F, The vertical extent of contamination into the deep zone has not been completely established . Phase V Ground Mater Investigation Proposal : H&A recommends the installation of three (3) monitoring wells within the 100-foot sand, six (6) monitoring wells within the intermediate zone, and five monitoring wells within the deep zone. in addition, H&A recommends that on exploratory borehole be drilled and geophysicaliy logged at each monitoring Yell location (to determine ) ithology and appropriate screen intervals) , and that one (1) borehole be continuously cored to 260 feet BGS. H&A proposes that these fourteen (14) "Stage 1" monitoring wells be installed at the known extent of the chromium Plume; and that subsequent monitoring well installations proceed downgradient and laterally, in an iterative fashion, until the 0.05 mg/1 concentration of chromium is reached. H&A proposes that if a Stage 1 monitoring well contains chromium in concentrations greater than 0,05 mg/1 , then three (3) Stage 2 monitoring wells will be installed one ( 1 ) downgradient ; one ( 1 ) laterally outward ; and one adjacent but deeper than the contaminated Stage 1 well , A monitoring well location map is presented that depicts the location of the Stage 1 wells. H&A' s rationale for the proposed Stage 1 monitoring weiis is presented in Table 6 of their report, Comments and Recommendations: 1. Beneath the North Yard, H&A measured and mapped a ground water mound within the shallow zone, and a around water high within the intermediate zone, These features were first noted on the July 1988 water level contour map. However, the possible causes and implications of the mound on plume migration from the site are not described . The presence of a around water ridge and/or mound , at this location, implies that around water, within these zones, can move northerly, diametrically opposite to the known plume movement.