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67 Groundwater <br />The data indicate that the chemistry of the valley fill groundwater is similar to the upper <br />bedrock units. Minor variations in alluvial water chemistry were observed as a function <br />principally of the drainage, but also due to location within a given drainage and season, <br />as discussed below. The inorganic chemistry of the valley fill groundwater is illustrated <br />on a trilinear diagram (Figure 5.17). <br />The aqueous chemistry of samples from the Jubb Creek valley fill groundwater varies <br />with the location (distance downdip). Samples from East Jubb Creek (MJ- 95 -02), the <br />most upgradient well, are calcium- bicarbonate, with a very high percentage of <br />bicarbonate ions ( >80 percent). These samples have slightly alkaline pH (7.1 to 7.7) and <br />TDS concentrations of approximately 460 mg /I. Samples from West Jubb Creek <br />(MJ- 95 -01) are calcium magnesium- bicarbonate with circum neutral pH (7 to 7.2) and <br />TDS concentrations between 920 and 950 mg /I. The downgradient alluvial well, <br />MJ- 95 -03, is a magnesium - sulfate, with a comparatively high percentage of magnesium <br />( >60 percent). The pH of the samples are neutral to slightly alkaline (7.2 to 7.4) and the <br />TDS is approximately 1,440 mg /I. The inorganic chemistry of these three wells show an <br />overall chemical evolution from calcium to magnesium and bicarbonate to sulfate. <br />Samples from MJ -95 -01 and MJ -95 -02 exceeded the MCL for iron. Iron concentrations <br />were consistently higher in MJ -95 -01 but decrease to below detection limits downstream <br />at MJ- 95 -03. <br />Samples from the lower Little Collom Gulch sampling location (MLC- 04 -01) are <br />calcium /magnesium - sulfate /bicarbonate with 10 percent or less sodium and chloride. <br />Groundwater from this location has a higher percentage of sulfate compared to samples <br />from MC- 04 -02. Valid samples (the December analysis is suspect) have a narrow range <br />of pHs (7.2 to 7.3) and TDS of approximately 1,100 mg /I. The only exceedance in Little <br />Collom Gulch was for TDS. <br />The groundwater quality in Collom Gulch at both locations (MC -04 -01 and MC- 04 -02) is <br />a calcium /magnesium- bicarbonate with 10 percent or less sodium and chloride. The <br />inorganic chemistry of the water was very similar for all samples, with a pH of 7.2 to 7.4. <br />TDS ranged from of about 600 to 970 mg /l, with higher concentrations at the <br />downstream sampling location. TDS increased at both locations during the spring <br />runoff. At MC- 04 -01, the samples show a gradual increase over time in the ionic <br />percentage of sulfate and decrease in bicarbonate. Upstream sampling location <br />MC -04 -01 generally had low iron concentrations. During the April 2005 sampling event, <br />the iron jumped from the detection limit of 0.05 mg /I to 10.1 mg /I, which exceeds the <br />secondary drinking water MCL. Downstream at MC -04 -2, the trend is reversed, with <br />consistent iron concentrations of approximately 4 mg /I (which exceeds the secondary <br />MCL) during low water periods, and very low concentrations ( <0.05 mg /1) during the <br />runoff. <br />2572 -R2 Colowyo Coal Company <br />Water Management Consultants <br />