Evaluation of possible alternatives to lower the high water table of St. Charles Mesa, Pueblo County, Colorado

St. Charles Mesa, an upland terrace southeast of Pueblo, Colorado, has become increasingly urbanized as cultivated fields have been subdivided and converted to residential areas. In some areas, the water table in the terrace alluvial aquifer underlying St. Charles Mesa is very shallow. Bessemer Ditch, which delivers irrigation water to farms on the mesa and other areas of the lower Arkansas River Valley, traverses St. Charles Mesa along its southern side and is the principal source of recharge to the terrace alluvial aquifer. The ground-water flow system was assumed to be in a state of dynamic equilibrium (steady-state condition) for this study. A steady-state ground-water flow model of the terrace alluvial aquifer was constructed and calibrated. The model was run in transient state to evaluate possible alternatives of lowering the water table. The possible alternatives evaluated were (1) reducing areal recharge by reducing recharge to irrigated areas by 25 percent, (2) lining Bessemer Ditch from (a) Aspen Street to 21st Lane; (b) Aspen Street to 23rd Lane; (c) Aspen Street to 25th Lane; and (d) Aspen Street to Nicholson Road, (3) installing two drains at a depth of 10 feet below land surface upgradient from the high water table areas, and (4) installing 22 dewatering wells within the high water table areas, each pumping at 80 gallons per minute. All alternatives evaluated were at least partly effective in lowering the water table. As the simulated extent of Bessemer Ditch lining was increased, the extent and magnitude of simulated water-table declines also increased. The maximum simulated declines in the water table were 3 feet when simulated areal recharge to irrigated areas was reduced by 25 percent, 29 feet when lining of Bessemer Ditch was simulated from Aspen Street to Nicholson Road, 6.8 feet when two drains were simulated at 10-foot depth, and 14.4 feet when 22 dewatering wells, each pumping at 80 gallons per minute, were simulated. Lining Bessemer Ditch from Aspen Street to 25th Lane and from Aspen Street to Nicholson Road both resulted in water-table declines of at least 5 feet throughout most of the area. Except for reducing recharge to irrigated areas and installation of the two drains, all the alternatives evaluated probably would lower the water table enough to diminish the ground-water supply available for at least some existing wells.