INVESTIGATION OF PROCESSES AFFECTING SUBSTANTIAL HEAD DECLINES IN A BASALT AQUIFER SYSTEM, MOSIER, OREGON

The Mosier Creek Basin lies on the northeast flank of Mt. Hood, Oregon, draining into the Columbia River near the eastern end of the Columbia River Gorge. The principal aquifers underlying the basin are part of the Columbia River Basalt Group. Varying modes and environments of basalt emplacement, and subsequent folding and faulting have resulted in a complex hydrogeologic framework. Agricultural uses, predominantly cherry orchards, and the town of Mosier are heavily dependent upon ground water. Aquifers have experienced head declines of up to 150 feet over the past 35 years. Factors that contribute to the declines include pumping, limited recharge pathways, hydraulic compartmentalization, cross-connection of aquifers with wells (i.e., commingling wells), and the low storage capacity of basalts. In order to evaluate the ground-water resources, a three dimensional hydrogeologic framework model (geomodel) and numerical groundwater flow simulation model (flow model) have been developed. The geomodel was developed for an area of approximately 150 km² using stratigraphic data from 291 water wells and compilations of geologic and structural mapping for the area. The geomodel was subsequently imported into MODFLOW-2000 for flow simulation. Parameter values were estimated using inverse methods (PEST), but limitations of available hydrologic data to constrain all conceptually important flow features provided non-unique results. As a result, PEST was used in prediction mode to identify the minimal and maximal effects of pumping versus commingling wells. Results indicate that aquifer cross-connection by wells is the dominant process driving the observed head declines in the Mosier Basin. Subsequently, the flow model was used in an automated fashion to create maps of vulnerability to cross-connection of aquifers by wells.