2009-05-19_REVISION - M1977285 (28)

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Hydrogeological Evaluation of Sunday Mines Group Geologic and Hydrologic Conditions Figure 2-7 compares the annual precipitation to the average annual precipitation over the period of record, which shows extended periods of particularly high or low precipitation. This data is shown in terms of water year totals, from October through September. The average annual precipitation for water years 1962 through 2007 is 12.6 in. Note a yearly total for water year 1961 could not be computed because precipitation data from October through December of 1960 are not available. Maximum annual precipitation was recorded in 1997 at 19.87 in, while the minimum precipitation was recorded in 1974 at 7.66 in. Table 2-3 shows the pan evaporation data from the monitoring station at Dead Horse Creek near Naturita, Colorado. Figure 2-8 illustrates the relationship between average monthly precipitation measured in Uravan and average monthly pan evaporation from the Dead Horse Creek monitoring station data. This figure shows that monthly pan evaporation is greater than monthly precipitation from April through October. During the late fall to early spring months (November through March), pan evaporation is not measured due to regular freezing temperatures at night and daily thaw. Actual evaporation is likely to be less than pan evaporation due to affects such as available soil moisture, infiltration into the subsurface, variations in surface topography, and differences related to the effect of the evaporation pan on measured evaporation rates. Pan evaporation exceeds precipitation by approximately 4 times based on the available data. Data regarding wind speed and direction are presented in Table 2-4 and Figure 2-9. 2.3.2 Surface Water Surface water bodies are not present in the direct vicinity of the Sunday Mines Group. The affected areas of the mines are located on the south side of Big Gypsum Valley approximately 1/2 mile from an ephemeral drainage called Big Gypsum Creek, which drains into the Dolores River several miles downstream of the affected areas. Surface water is generally not present at the Sunday Mines Group, except during periods of snow melt or in response to episodic precipitation events. Storm water retention ponds are present at the mines, which are designed to retain all storm water generated by a 100 year precipitation event at the mines. The Dolores River is the major surface water feature in the area of the Sunday Mines Group. The river extends for 183 miles from an area northeast of Rico, Colorado, to the Colorado River at the town of Cisco, Colorado. The Dolores River Basin is approximately 96 miles long and 34 miles wide, encompassing an area of 6,222 square miles. Most of the flow in the river originates on the western slopes of the La Plata and Rico mountains, with additional contributions from snowmelt and springs of higher mesas and mountains. The Dolores River also receives inflows from groundwater, which are estimated to be approximately 3 liters per second per kilometer in the portion of the river extending from the town of Dolores to Cisco (Weir et al. 1983). `m 2-8 T:\64986-Denison Mines\Task Order 3 - DMO Sampling and Analysis Plan\Task 3.14 - Hydrogeology Report\hnal sunday hydro reporNextTINAL Sunday Hydrogeology Report.doc