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<br />shale on the canyon walls, and halite (NaCI) beds located in key parts of the stratigraphic <br />section. <br />Gravel composed of andesite and latite are more reactive than most of the other gravel. <br />Quartz sandstone and quartz siltstone, which comprise much of the canyon wall bedrock, <br />are relatively non-reactive. Shale, which is the other major component of the canyon wall <br />bedrock, has very low permeability and porosity and so has little chance to interact with <br />groundwater except where physically eroded, so in effect may also be considered relatively <br />non-reactive. Thus, it is anticipated that groundwater composition in the vicinity of the <br />proposed gravel operation would be dominated by reaction with andesite-latite, to a lesser <br />extent, shale, and with buried halite. <br />Water composition at any point along a groundwater flow path represents the "steady- <br />state" chemistry of waters at that point along the flow path. Compositions can change <br />seasonally, due principally to influx of other waters or flushing of evaporate salts. <br />Surface Water Quality <br />Limited surface water quality data were examined. This includes: <br />a. Surface water and sediment metals data compiled by the Bureau of Reclamation <br />after 1993 for surface water samples from the Dolores and West Dolores River <br />and select tributaries. These waters and sediments were variously analyzed for <br />field parameters plus arsenic, cadmium, copper, lead, mercury, selenium, and <br />zinc. <br />b. Unattributed surface water quality data collected between 1969 and 1975. This <br />includes all of the regulated metals except aluminum. <br />c. Single sample collected in 1998 from McPhee Lake described as Montezuma <br />Water Co. All regulated primary drinking water parameters were analyzed. <br />d. Major element chemistry for 1995 through 2001 at the first USGS gauging <br />station on the Dolores River several miles north of Dolores. <br />Surface water downstream from Dolores (USGS gauging station at Bedrock) from 1995- <br />2001 has an average overall composition representing mixtures of fluids derived by <br />reaction with several rock types (Table 3). The water at this station is a sodium-calcium- <br />sulfate-bicarbonate water with relatively high concentrations of magnesium and chloride <br />(see Attachment A). There is no single rock having this major element composition, so it <br />can be deduced that these waters are of mixed rock parentage. Though potable and within <br />limits for regulated elements that were measured, this water probably tastes somewhat <br />saline. The chemical composition is compatible with weathering of mixed rock sources <br />(such as the volcanics described above), carbonate rocks, and halite. <br />6 <br />