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• <br />TABLE 2. Classification of waters based on salinity (from Carpenter, 1978). <br /> TDS <br />Fresh water 1000 mg/L <br />Brackish water 1000-10,000 mg/L <br />Saline water 10,000-100,000 mg/L <br />Brine > 100,000 mg/L <br />By this classification, drinking water with an aesthetically-pleasing taste might be <br />considered to have <1,000 mg/L TDS. This says nothing about what comprises the <br />dissolved solids, but rather generalizes water by TDS. <br />Groundwaters with TDS <10,000 mg/L are considered "U.S. drinking water" (USDW) <br />sources under the federal Safe Drinking Water Act regulations. Those waters are protected <br />under state and federal standards through salinity discharge limits. For instance, under <br />state standards, groundwater with 0-500 mg/L background TDS can receive no more than <br />400mg/L TDS or 1.25 times the background level, whichever is least restrictive. Water <br />having 401-10,000 mg/L background TDS can receive no more than 1.25 times the <br />background value. (See, CDPHE-WQCC Basic Standards and Methodologies for Ground <br />Water.) <br />Groundwater from these two pits has relatively low TDS concentrations. <br />Groundwater Composition. <br />Groundwater composition is determined principally by the composition of rock through <br />which it flows. The composition of groundwater is controlled by (a) chemical processes <br />that dissolve and precipitate minerals and (b) mixtures of groundwaters of various <br />compositions. In shallow groundwater particularly, interaction with weak organic soil <br />acids or atmospheric acids can further alter the groundwater composition, leading to other <br />mineral dissolution and/or new mineral precipitation. Dissolution of minerals adds <br />dissolved constituents to the water while mineral precipitation removes dissolved <br />constituents from the water. <br />Relevant to the present issue, waters flowing through the Dolores River alluvium should <br />have a chemical composition reflecting interactions between rainwater or snowmelt and <br />the most reactive rocks in the alluvial gravel combined with bedrock waters draining into <br />the alluvium. The most reactive rocks would include mostly the andesite and latite gravel, <br />s <br />