METODOLOGÍA DE EVALUACIÓN DE LAS OFERTAS

West of Everest

4.2.1 Local Hydrostratigraphic Relationships

The Phase II investigation (Argonne 2003) demonstrated that groundwater flow, and hence contaminant migration, across the investigation area is driven predominantly by groundwater recharge to the southeast of the former CCC/USDA facility and by probable groundwater discharge to the intermittent stream west of the Nigh property. The detailed pathways of groundwater flow and contaminant migration are influenced, however, by the hydrogeologic heterogeneity of the aquifer unit, particularly by a region of reduced sand content and hence decreased net permeability near the Nigh property. Groundwater levels measured by hand in 2001–2002, east of the intermittent creek, support this interpretation; see, for example, the results for November 9, 2002 (Figure 4.3).

To achieve a more detailed understanding of the water level variations, automatic water level recorders were used to make continuous water level measurements (every 4 hr) in selected piezometers from July 10, 2000, to June 11, 2001, and again from November 23, 2002, to November 10, 2003. Data for the earlier period were documented previously (Argonne 2003); results for the recent monitoring (Supplement 1, Table S1.1 [on CD]) are summarized in Figure 4.4.

Figure 4.4 illustrates that groundwater levels, and hence the configuration of the potentiometric surface, have been quite stable since fall 2002, showing only a slight net decline over the 12-month observation period. The data indicate that water level responses in the spring and early summer of 2003 were subdued in comparison to transient increases of 5–8 ft at the more eastern observation points (SB01, SB09, SB16, SB18, SB19) during the corresponding seasons in 2001 (Figure 4.13 in Argonne 2003). The recent monitoring data indicate that groundwater levels at the end of 2003 were comparable to those observed during the Phase I investigation in early 2000. The groundwater level data therefore demonstrate that groundwater flow patterns and the expected pathways of contaminant migration within the aquifer unit have remained largely unchanged since 2000.

Five borings installed during the Phase III targeted investigation west of the intermittent stream (Figure 4.5) yielded geologic and hydrostratigraphic data (Appendixes A and E)

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confirming that the Everest aquifer unit can be identified as a groundwater-bearing interval at these locations. The interpreted distribution of the aquifer unit along updated line C–C′, extending from the area north of the former CCC/USDA facility and westward to boring SB71, west of the intermittent stream (location shown in Figure 4.6), is depicted in Figure 4.7.

Water levels measured in the Phase III targeted investigation permit more complete mapping of the groundwater potentiometric surface along the pathway of contaminant migration identified in the aquifer unit, from the former CCC/USDA facility toward the intermittent stream and beyond. The mechanically contoured potentiometric surface shown in Figure 4.8 is based on hand measurements made on November 15–16, 2003 (Appendix E). Piezometer SB77 and monitoring wells MW1–MW3 had not yet been installed as of this date. Groundwater levels subsequently measured at these locations (Table E.1) were consistent with the interpretations presented here.

Groundwater level relationships in the eastern portion of the study area were consistent with observations during the Phase I and Phase II studies. Groundwater levels measured at all locations west of the intermittent creek were higher than those observed at piezometers immediately east of the creek. The November 2003 data demonstrate that apparent groundwater flow pathways along both the eastern and western sides of the intermittent stream are diverted toward the southwest and converge toward the trend of the stream channel.

4.2.2 Visual Reconnaissance along the Creek

Visual reconnaissance of the full length of the creek channel was performed from the vicinity of boring SB64 to the point where the channel crosses Highway 73 southwest of Everest. No clear evidence was found for groundwater seepage from the aquifer unit into the intermittent stream directly west of the Nigh property or along the section of the channel immediately downstream from SB64 (within approximately 1,500 ft of SB64), although several relatively persistent areas of shallow standing water or wet ground were located in the creek bed. These observations are consistent with the groundwater levels (Figure 4.8) measured in the piezometers near the stream and with surface elevations (Appendix H) surveyed at several points along the stream bed, indicating that the potentiometric surface was slightly below the stream bed (west of the Nigh property) in November 2003.

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Further investigation along the stream southwest of the Argonne study area identified significant stream flow resulting from groundwater discharge in a forested area approximately 3,000 ft downstream of boring SB64 and 1,500 ft north of U.S. Highway 73 (Figure 4.9). The observed onset of groundwater discharge and surface water flow coincides with a change in the form of the stream bed from a fairly simple, incised channel upstream to a more meandering morphology. Available topographic maps indicate an elevation of approximately 1,070 ft AMSL (above mean sea level) at the stream bed in this area; this level roughly corresponds to the minimum elevation identified at the base of the aquifer unit in the investigation area.

4.2.3 Summary of the Factors Controlling Downgradient Groundwater Migration

The local hydrologic relationships identified by Argonne are consistent with the more regional observations reported by the Kansas Geologic Survey (KGS; Bayne and Schoewe 1967; Ross 1991) and documented in the Everest Phase I Work Plan (Argonne 2000). The KGS mapping of the regional water table across Brown County (presented as Figure 3.14 of the Work Plan and reproduced here as Figure 4.10) identified a pattern of semiradial groundwater flow westward and northwestward from the former Everest facility, as well as a southwestward shift in flow direction in the vicinity of the intermittent creek west of the town. Identical trends were documented by the local water level monitoring described in Section 4.2.1. The KGS water level data, obtained from relatively widely spaced measurement points, also indicated that groundwater following the plume migration pathway is expected to discharge to the surface along a drainage divide associated with the lower reaches of the intermittent creek and the adjacent portions of the east-west tributary into which this stream drains. The KGS geologic mapping further indicated that the glacial/till sequence that hosts the Everest aquifer unit has been fully penetrated by erosion and hence is absent along Otter Creek (Work Plan Figure 3.3, reproduced here as Figure 4.11), less than 1 mi west of the intermittent creek.

Argonne’s groundwater mapping in the vicinity of Everest and along the intermittent creek provides finer resolution of the water level patterns in this area than the KGS results, but Argonne’s results are fully consistent with the latter data. The Argonne mapping indicates that both the groundwater divide identified by the KGS along the downstream portion of the intermittent creek and the focusing effect of this divide on groundwater flow toward the southwest locally extend farther upgradient along the creek channel than the county-level data alone suggested.

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These findings support Argonne’s previous interpretation that groundwater flow near the intermittent creek is diverted toward the southwest, as a result of groundwater discharge along and near this creek. The available data suggest that the trend of the intermittent creek approximately reflects both local surface water and groundwater drainage divides that will limit the potential westward migration of contaminated groundwater originating at the former CCC/USDA facility. The results indicate that the carbon tetrachloride plume can be expected to continue to migrate toward the southwest and that discharge of contaminated groundwater to the stream north of Highway 73 is ultimately probable.

4.3 Obtain Quantitative In Situ Estimates of Hydraulic Parameters for the