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E USGS science for a changing world . - Extent of the High Water Table and Water-Table Fluctuations, 51. Char\es Mesa. Colorado, April 1997 to October 1998 r1"10P- Introduction 0 0 1 q 8 2 St. Charles Mesa (Mesa) is an upland terrace sOLltheast of Pueblo with an area of about 10 square miles (lIg. 1). The Mesa has been irrigated for agricultural purposes since the late I ~H)O's (Dulllcycr. 1(75). The uncolll1ned sand and gravel aquifer. origi- nally deposited by the Arkansas River. overlies an eroded sl1ale- hedrock surface (Scott. 19(9). During the last several decades, the Mesa has become increasingly urbanized as cultivated fields have been converted 10 residential areas. Because the water table is near the land surface in parts of the Mesa, basement flooding has occurred. In addition. the high water table has caused concern that flooding of domcstic septic systems could cause degradation of ground-waler quality. To bcller define the hydrology of the area, the U.S. Geological Survey (USGS), in cooperation with Pueblo County, began a study in 1997. This report describes the extent of .1e high water table and \vater-tabh.: l1uctuations from April 1997 LOCA liON MAP ~~7- - - ,-_//1 , --'J ' 1/ /" DENVII< \ ~ '--\----... \(;"""".,,,, ' I~J ~~'''~~Evl' ~\ ,____ :c____ 104'45' 104'30' High Wqy 50 - Fountain C,-eek 38"20' 38-'15' . o 1 2 3 4 5MILES I I._t I I I rTTl1 I o 1 2 3 4 5 KILOMETERS Figure 1. Location of study area. U.S. Department of the Interior U.S. Geological Survey to October 1998. The measurement of the water table llsed in this study is the depth to the water table below land surface, cal1ed depth to water. The depth to water was measured in 57 wells weekly, biweekly, or monthly. The frequency of measurements depended on whether the well was in an area where (I) basement f~ooding has occuned, or (2) seasonal changes in the depth to water were expected to occur rapidly. Water-table fluctuations The depth to water on the Mesa fluctuatcs because of (1) recharge from precipitation, (2) recharge of inigation water conveyed seasonally through the Bessemer Ditch (fig. I) and its laterals, (3) infiltration of water from lawn and crop irrigation and scptic-system efnuent, (4) discharge of watcr by evaporation or transpiration, (5) discharge of water hy well pumpage, and (6) discharge of ground water through springs on the northcl11 and eastern edges of the Mesa. The depth to water is controlled by the balance between the amount of water that recharges the aquifer and the amount of water that discharges from the aquifer. If more water recharges the aquifer than discharges, the depth to water will decrease, and if more water is discharged than is heing recharged, the depth to water will increase. Long-term fluctuations in the depth to water in wells can occur over several years or decades. For example, the depth to water in well SC021 06403DAC was substantially shallower in wet years (fig. 2A) than it was in dry years (fig. 2B). Depth-to- water measurements in this well were discontinued after 1983. The depth to water fluctuated annually an average of about 2 feet from 1934 to 1982; the maximum change within a year was 6.3 feet between Novembel 1964 and April 1965, and the ditfer- ence between the deepest (in 1977) and shallowest (in 1942) depth to water was 13.3 feet. Seasonally, the deepest depth to water usually occurs in late winter or early spring, and the shallowest depth to watcr usually occurs in summer or autumn (fig. 3). Annual l1uctuations betwecn the deepest and shallowest depths to water for wells measured for this study ranged from more than 15 feet to less than I foot (table I). Depending on the location of a well on lhe Mesa, the seasonal fluctuation in the depth to water in a well can be large (13.76 feet in well 44) or small (2.50 feet in well 24) (fig. 3; table I). AIt welts in which the depth to water fluctuated 6 feet or more between April 1997 and October 1998 were near the southern edge of the Mesa (fig. 4). USGS Fact Sheet 102-99 May 1999