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• <br />Approximately 75% of all counties in Colorado fall in Zone 1. Counties floored mostly on <br />the San Juan volcanic field (south-central) and on uranium-poor sandstones (north-central) <br />fall into zone 2. <br />On a county-wide scale, Montezuma County ranks in zone 1, where the average indoor <br />radon screening level exceeds 4 pCi/L. This high average concentration is most controlled <br />by rocks in the Uravan uranium-vanadium district. The Dolores River alluvium, which is <br />comprised mostly of rocks derived from volcanics of the San Juans, should have a lower <br />average uranium concentration more like that in counties immediately to the east. Water <br />flowing through sandstone bedrock, which dominates the Dolores canyon walls, likely <br />would be low in uranium. Overall, it is anticipated that uranium and, consequently, radon <br />concentrations in the Dolores River alluvium would fall into Zone 2 according to EPA's <br />rankings, even though the average level for Montezuma County yields Zone 1 levels. <br />Groundwater quality. <br />Limited groundwater quality data are available from discharge permit records of nearby <br />alluvial gravel mines. No other data were investigated other than as appeared in these <br />discharge records and in various objection letters. Two nearby mines, the Tam-Koenig <br />and the Twin Spruce, are monitored for pH, TDS, TSS, oil and grease, and flow under the <br />CDPS/NPDES program. (This program is managed by the Colorado Department of Public <br />Health and Environment, Water Quality Control Division - WQCD). <br />TDS (total dissolved solids) is a chemical parameter regulated in groundwater; pH is a <br />chemical parameter regulated in both groundwater and surface water. TDS can indicate <br />many things: regional degree of weathering; evaporation:precipitation ratios; presence of <br />highly erosive materials such as are found in the Mancos Shale; presence of readily soluble <br />minerals such as halite or gypsum; or a combination of these. (In cases involving warm or <br />hot springs, TDS may indicate the aggressiveness of water and rock interactions. In the <br />present case, these processes are not known to be at issue, but cannot be ruled out.) <br />Local groundwater, exemplified by the values in Table 1, has relatively low TDS, with <br />values generally less than 300 mg/L (expressed as calcium carbonate equivalents). Local <br />groundwater pH is slightly alkaline with most waters ranging between about 7.2 and 8.5. <br />Exceptions appear in streams immediately downstream of acidic mine drainage, where <br />reported pH values reach the 3 range. <br />TSS (total suspended solids) is a physical parameter regulated in surface water in some <br />high salinity areas, though not in the Dolores River. TSS indicates the relative clay <br />content of waters and can measure the effectiveness of site controls to contain sediment. <br />3 <br />