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Hydrogeological Evaluation of Van 4 Mine <br />Geologic and Hydrogeologic Conditions <br />June, February, December, and January are the driest months of year, with median <br />monthly precipitation totals of 0.34 in., 0.61 in., 0.64 in., and 0.70 in., respectively. The <br />wettest month on record is October 1972 when 5.89 in. of precipitation were recorded <br />at the Uravan station. <br />Figure 2-4 shows the yearly total precipitation data for the period of record. This data <br />is shown in terms of water year totals, from October through September. The average <br />annual precipitation for water years 1962 through 2007 is 12.6 in. A yearly total for <br />water year 1961 could not be computed because precipitation data from October <br />through December of 1960 are not available. Maximum annual precipitation was <br />recorded in 1997 at 19.87 in, while the minimum precipitation was recorded in 1974 at <br />7.66 in. Figure 2-4 also compares the annual precipitation to the average annual <br />precipitation over the period of record, which shows extended periods of particularly <br />high or low precipitation. <br />Table 2-3 shows the pan evaporation data from the monitoring station at Dead Horse <br />Creek near Naturita, Colorado. Figure 2-5 illustrates the relationship between average <br />monthly precipitation measured in Uravan and average monthly pan evaporation <br />from the Dead Horse Creek monitoring station data. This figure shows that monthly <br />pan evaporation is greater than monthly precipitation from April through October. <br />During the late fall to early spring months (November through March), pan <br />evaporation is not measured due to regular freezing temperatures at night and daily <br />thaw. Actual evaporation is likely to be less than pan evaporation due to affects such <br />as available soil moisture, infiltration into the subsurface, variations in surface <br />topography, and differences related to the effect of the evaporation pan on measured <br />evaporation rates. Pan evaporation exceeds precipitation by approximately 4 times <br />based on the available data. <br />Data regarding wind speed and direction are presented in Table 2-4 and Figure 2-6. <br />2.2.2 Surface Water <br />The Dolores River is the major surface water feature in the area of the Van 4 Mine. <br />The river extends for 183 miles from an area northeast of Rico, Colorado, to the <br />confluence with the Colorado River at the town of Cisco, Colorado. The Dolores River <br />Basin is approximately 96 miles long and 34 miles wide, encompassing an area of <br />6,222 square miles. Most of the flow in the river originates on the western slopes of <br />the La Plata and Rico mountains, with additional contributions from snowmelt and <br />springs of higher mesas and mountains. The Dolores River also receives inflows from <br />groundwater, which are estimated to be approximately 0.17 cubic ft per second per <br />mile in the portion of the river extending from the town of Dolores to Cisco (Weir et <br />al. 1983). <br />Surface water bodies are not present in the direct vicinity of the Van 4 Mine. The top <br />of Monogram Mesa slopes gently from the northeast to the southwest and surface <br />drainage would be expected to flow from the ridge overlooking the Paradox Valley <br /> <br />2-5 <br />draft final van 4 hydmgeology report.doc