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t t . . . . . . . . . . . . . t . . . t . .' Artificial Recharge of Ground Water in Colorado A Statewide Assessment Lower South Platte River Basin Associatcd with the South Platte River and its tributaries are Pleistocene alluvial and eolian deposits covering an area of over 4,000 square miles (Figure VI-2). These deposits form a vital aquifer in what is referred to as the lower South Platte River basin (LSPRB), which extends from the foothills of the Rocky Mountains east to the state's border with Nebraska. The same geographic region also hosts more than 60 percent of Colorado's population and a thriving agricultural economy that rely on both surface water from the South Platte River and ground water from the underlying alluvial aquifer for crop irrigation, municipal supplies, and industrial uses. A complex system of water distribution canals and ditches has evolved since the late 19th century to distribute that water while several diversion and reservoir projects have been constructed to import and store more than 1.5 million ac-ft of water. The alluvial aquifer is estimated to hold as much as 8.3 million ac-ft of water, and over 3,200 wells tapping the alluvial aquifer extract as much as 0.6 million ac-ft per year (Topper and others, 2003). Furthermore, wells in this alluvium can yield up to 3,000 gpm (CWCB, South Platte River Basin fact sheet). AR is being used extensively throughout the LSPRB as part of a number of augmentation plans and substitute supply plans. Most of the alluvial wells have original water rights that are junior to the majority of the surface-water diversions. The augmentation and substitute supply plans that incorporate AR allow those junior rights to continue to pump ground water, and therefore to continue to irrigate their crops, even when their original water rights are out-of-priority. This application of AR relies on lagged replacement of water to the mainstem of the affected river. Water from ditches, and to a lesser extent from wells, is recharged through infiltration ponds, dry streambeds, leaky reservoirs, and leaky ditches. When possible, distances between point of recharge and the river are selected to time the replacement to the river when natural flows are generally low. Recharge site selection utilizes stream depletion factors (SDFs) that have been calculated and mapped for the entire reach ofthe LSPRB by the USGS (Hurr and others, 1972a, 1972b, 1972c, 1972d, 1972e, 19720. Although the primary objective of AR in the LSPRB is to meet legal obligations, there is also a component of water storage involved. Many of these recharge projects allow greater utilization of the agricultural water in the LSPRB, so that water is available during high demand times (April-October). These projects also promote aquifer restoration by mitigating decreasing water levels. In fact, the original application of AR in the LSPRB at Olds Reservoir (COR-l in Figure VI-2) in ] 939 was done with the objective of rcstoring declining water levels (Skinner, 1963). Watcr levels in the aquifer had decreased by up to 36 feet as of 1970, prior to widespread application of AR (Topper, and others, 2003). Since that time the rate of water-level decline has appeared to decrease, much of which may be attributable to increased use of AR. Artificial recharge in the LSPRB has been an active part of ground-water management since the I 940s, when Olds Reservoir was first utilized, although recharge incidental to agricultural land- use had occurred prior to that. Leaky irrigation ditches and reservoirs have been unofficially recharging the aquifer since they were first constructed. Most of the AR projects in the LSPRB were initiated in the late 1970s to mid 1980s. The actual number of individual AR sites currently active in the LSPRB is difficult to tally. New sites are being added while others are taken out of service (R.Y. Stroud, pers. com., 2003), and the number is always changing. Furthermore, most 41