Simulation of ground water recharge and movement in alluvial aquifers on the Black Mesa

Abstract

Alluvial aquifers associated with ephemeral washes on the Black Mesa of Arizona are an undeveloped and somewhat overlooked water resource in an arid region. A model is developed to simulate conditions on such a stream-aquifer system in order to evaluate characteristics in the natural state, as well as evaluate the effects of a strip mining operation on the watershed and through parts of the aquifer. The model generates precipitation stochastically on an event basis. Storms are considered convective, and duration and areal distributions are calculated. The watershed model computes hydrographs from rainfall on several subwatersheds and routes and adds these outputs in the correct sequence to determine total hydrographs at selected cross- sections. Runoff is determined through use of SCS curve numbers. Recharge is modeled by the Green and Ampt infiltration formula and a one dimensional form of the differential ground water equation. A ground water model then determines head fluctuations between recharge events. Three runs simulating natural, actual post-mining, and hypothetical post-mining conditions show few adverse effects to the streamaquifer system if large portions of the aquifer are not overturned. Examination of the natural conditions shows evapotranspiration dominating drainage losses from the alluvial aquifer.