GROUNDWATER MODEL OF THE REDWALL-MUAV AQUIFER OF THE COCONINO PLATEAU INCORPORATING IMPACTS OF PUMPING AND WATER CONSERVATION ON SMALL SPRINGS OF THE GRAND CANYON

A three-dimensional, steady-state, pre-development (pre-1989), groundwater flow model was calibrated to the few known hydraulic head measurements and to discharge values measured from 20 springs below the South Rim of the Grand Canyon. Root mean squared errors for both hydraulic head and spring discharge calibration for the steady-state model were an improvement over two previous two-dimensional modeling efforts of the Redwall-Muav aquifer. Both of these previous models only included three major springs and not the 17 smaller springs on the South Rim. Subsequently, a transient model was created and calibrated to stresses to the aquifer applied from 1989 to 2002. The model incorporated accurate pumping rates for wells near the South Rim and the impacts of conservation measures and alternative water supplies. The community of Tusayan at the gateway to the South Rim implements rain and snow catchment which decreases volumes of water available to recharge the aquifer. Extensive water conservation and reuse practices further reduce the potential for recharge because of loss of reused water through evapotranspiration by consumptive uses. The transient model was used to gain insight into potential impacts to springs from current pumping rates, conservation practices, and alternative water supplies. The model predicts potentially significantly decreased spring discharges at some small springs from current groundwater use on the Coconino Plateau. These spring discharge decreases are greater with the use of some conservation measures and alternative water supplies. Three-dimensional particle tracking was used to delineate the capture zones of the 20 springs. The particle-tracking shows that capture zones of the low discharge springs are small and close to the South Rim; hence their ability to be easily influenced by pumping near the South Rim.