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<br />Sandstone beds within the Mesaverde Formation in the study area generally <br />have minimal primary permeability, although secondary permeability, such as <br />fractures, can yield enough water for domestic-well production. Transmissivi- <br />ties for the Mesaverde Formation in the North Fork of the Gunnison River <br />valley are between 0.3 and 16.7 ft2/d for typically unfractured rock (Brooks, <br />1983). Wells producing from the Mesaverde Formation, which is exposed in the <br />eastern part of the study area, yield between 0.7 and 24 gal/min. <br /> <br />Specific-conductance values for water samples from 43 springs issuing <br />from consolidated sedimentary-rock formations other than the Mancos Shale <br />ranged from 42 to 18,500 ~S/cm with a median of 932 ~S/cm. Specific- <br />conductance values for ground water in this formation ranged from 325 to <br />5,390 ~S/cm. Two samples from springs issuing from the Mancos Shale had <br />specific-conductance values of liD and 7,540 ~S/cm. Two wells had specific- <br />conductance values of 629 and 1,900 ~S/cm. <br /> <br />Alluvial Aquifers <br /> <br />Sand, silt, and gravel of Quaternary age form aquifers in stream valleys <br />and terraces throughout the basin (see pl. 2); these materials will be <br />referred to collectively as the alluvial aquifers in this report. Springs are <br />common at higher elevations; wells generally are located in the valleys. Most <br />inventoried wells completed in alluvial aquifers are completed in glacio- <br />fluvial materials or alluvium. These deposits are rarely more than 200 ft <br />thick, and most often are less than 100 ft thick. <br /> <br />Saturated alluvial deposits are the most productive aquifers yielding <br />water that is suitable for most uses. Well yields reportedly range from 1 to <br />750 gal/min. Most wells yielded a calcium sulfate bicarbonate type water. <br />Salinity, water chemistry, and relative position of bedrock and alluvial <br />aquifers indicate that some alluvial aquifers receive discharge from bedrock <br />aquifers. Salinity and water chemistry also indicate that flow paths and <br />residence times are longer for most well waters than for spring waters. <br /> <br />Spring waters predominantly originate from precipitation and snowmelt <br />infiltration. Alluvium, glacial drift, and landslide deposits on steep slopes <br />may be saturated only seasonally and are intermittent sources for springs. <br />Some valley springs issue from the base of extensive terrace gravels overlying <br />bedrock and have a large continuous discharge. These springs are less saline <br />than most well waters and probably derive much water from the infiltration of <br />streamflow and irrigation water on the terraces. <br /> <br />Specific-conductance values for samples from 10 wells ranged from 80 to <br />32,200 ~S/cm with a median of 1,750 ~S/cm. Specific-conductance values for 16 <br />of 18 sampled springs were less than 500 ~S/cm. Most spring waters are a <br />calcium sulfate bicarbonate type, but some are of a calcium magnesium <br />bicarbonate sulfate type. <br /> <br />10 <br />