CONCEPTUAL AND NUMERICAL GROUND-WATER FLOW MODELING FOR TRAVERTINE-DEPOSITING FOSSIL SPRINGS, WEST MOGOLLON MESA, ARIZONA

Fossil Springs is the largest spring system discharging in the West Mogollon Mesa area of central Arizona, however its sources have only been preliminarily studied and are poorly understood. This system is characterized by 115 individually mapped spring orifices, an average discharge of 43 cfs, and travertine dams forming along the 14 mile reach from the spring orifices to Fossil Creek's confluence with the Verde River. Spring waters are of the Ca,Mg-bicarbonate type, and gas chemistry of the CO2-rich waters indicates a component of deeply-circulated fluid. The aquifer system of the West Mogollon Mesa is a highly dissected, horizontal, stacked system, composed of two regionally extensive aquifers separated by a confining layer. A recently constructed hydrologic model for the Mogollon Highlands suggests that most regional flow originates in the shallowest regional aquifer by recharge along the Mogollon Rim and that recharge to the lower aquifer occurs as leakage from the shallow aquifer through faults and fractures. Fossil Springs discharges at the contact of two limestone units, at the top of the lower aquifer and intersection of two faults. Two modeling techniques were used to simulate the processes that control groundwater and geochemical transport to Fossil Springs. A digital hydrogeologic framework model of the region was created using EarthVision, a three-dimensional GIS software. Data necessary for the completion of this model were accurate elevations for the tops and bottoms of hydrogeologic units and locations, and strikes and dips of faults. These data were determined using well logs and existing geologic maps which were field checked and then compiled into a database to be assembled into the model. These results were exported to MODFLOW and numerical modeling was used to determine the effect of the complex structure on flowpaths. Results suggest that the juxtaposition of confining units and fault conduits serve to direct a large amount of regional flow to the discharge point at Fossil Springs.