C150094 Red Mesa Aquifer Groundwater Study 2002

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Executive Summary The objective of the Red Mesa terrace gravel aquifer study is to better understand the hydrogeology of Red Mesa and its relationships with associated surface water flows and irrigation practices. The aquifer could be used temporarily to store water in an effort to yield increased base flows in Long Hollow. This would enable Colorado to meet the La Plata River Compact obligations by delivering required water to New Mexico. Additionally, through improved water management, this temporary storage would provide irrigators with a more dependable water supply and assist in sustaining domestic water well supplies. To better understand the hydrology of Red Mesa it is imperative to determine timing and direction of groundwater movement. One primary purpose of this study is to collect information that can be used to develop knowledge pertaining to the Red Mesa aquifer. Understanding where and when to irrigate or artificially recharge the Red Mesa aquifer when water is available is paramount to the success of managing limited water supplies. • A comparison of the timing and amount of annual flows of various watersheds for dry, average and wet years indicates that Long Hollow is supplied primarily by groundwater discharge while the La Plata River and Cherry Creek are supplied by snowmelt and precipitation. A total of 42 wells were selected for study within Red Mesa on the basis of location and availability (see Figure ES 1). Additionally, some ditches and their diversion practices were incorporated into this study. The wells were monitored on 14 dates from April 13, 2001 to October 10, 2002. During each site visit, static water levels were measured and compiled into a database. From this database, groundwater contours were generated for the Red Mesa terrace aquifer. Using these groundwater table maps groundwater flow paths were derived. Groundwater travel times from each well to its discharge point at a surface drainage were estimated. Some key assumptions for these calculations include a porosity value of 30 percent and a hydraulic conductivity value of 500 gallons per day per square foot(gpd/ft2). The source of recharge for the Red Mesa aquifer is the deep percolation of both direct precipitation and irrigation water. Hydrographs for 42 wells monitored in the study area indicate that water table fluctuations in the Red Mesa aquifer can be correlated to irrigation practices. Irrigation recharge comes from losses in conveyance of water through ditches and deep percolation. Ditch diversion data were compared with the change in groundwater levels in nearby wells. The groundwater elevations rise shortly after irrigation begins and continue to rise for a period after irrigation ceases, showing a one-to-three month lag in irrigation recharge. Low water table conditions typically occur during the winter and early spring. During the study period, the amount of groundwater in storage in the Red Mesa aquifer was estimated to be 165,000 acre-feet (af) with approximately one-half, or 82,500 af, available for withdrawal and use. Artificial recharge of the aquifer could reasonably be expected to increase the stored water available for withdrawal and use to 125,000 af, with a maximum of 200,000 af. An Document/2002/991-077/031/Executive Summary