Grande Wash is a tributary of the Verde River and drains an area of 13 square miles within the McDowell Mountains and the Town of Fountain Hills in Central Arizona. The wash enters the Fort McDowell Indian Reservation at the eastern boundary of Fountain Hills and is incised in coarse-grained alluvium that is contiguous with the alluvial aquifer along the Verde River. The aquifer is used by the Fort McDowell Indian Community and the City of Phoenix for municipal water supplies. Episodic flows in Grande Wash, in response to storms, carry potentially hazardous runoff from Fountain Hills onto the reservation. Additional potential hazards to ground water include contamination from a wastewater-treatment plant located less than 1 mile upstream from the reservation boundary, and from a landfill and a cement-processing plant immediately adjacent to the main channel of the wash.

Coarse-grained deposits in Grande Wash also include recent stream-channel deposits, soil backfill, landfill material, and the upper coarse-grained layer of basin-fill sediments. Surface-geophysical surveys and drilling indicated that the coarse-grained deposits are less than 60-feet thick along the wash and in adjacent areas within the reservation, and are underlain by a thick clay and silt unit, the base of which is below the bottom of the deepest monitor well (317 feet below land surface). The coarse-grained deposits form the alluvial aquifer beneath Grande Wash.

Ground water in the alluvial aquifer beneath the wash is shallow and mounded above a less permeable clay and silt unit. Depth to water in the aquifer ranges from 1 to 22 feet below land surface. Saturation of the coarse-grained deposits does not extend laterally for more than about 1,000 feet from the main channel of Grande Wash; the extent varies in response to recharge amounts.

Flux of ground water through the alluvial aquifer beneath the wash is toward the Verde River and is estimated to be about 8,000 cubic feet per day (about 0.2 acre-feet per day). The flow rate is four orders of magnitude less than the flow rate in the Verde River. Vertical flux of ground water through the underlying clay and silt unit is estimated to be 7,000 cubic feet per day (0.17 acre-feet per day). The volume of ground water in storage in the alluvial aquifer beneath Grande Wash is estimated to be about 5.6 million cubic feet (129 acre-feet).

Concentrations of dissolved inorganic constituents in ground water and surface water are high relative to concentrations found in the regional aquifer in surrounding areas and are indicative of salts that can be expected to be mobilized by runoff in the drainage area. Concentrations of nitrate, chloride, and sulfate are near U.S. Environmental Protection Agency Primary or Secondary Drinking-Water Regulations. Concentrations of arsenic, antimony, and strontium are below drinking-water standards but can be attributed to geologic deposits in and near the study area.

Low concentrations of anthropogenic compounds, including chloroform and dichlorobromomethane, were detected. These compounds are disinfection by-products of chlorinated water.

Eight pesticide compounds were detected in the surface water, and two pesticide compounds were detected in the ground water. Pesticide concentrations were below U.S. Environmental Protection Agency Maximum Contaminant Levels. Several other organic anthropogenic compounds that probably originated from commercial activities in the area were detected but at concentrations below laboratory calibration standards.

Concentrations of trace metals in the stormwater sediment collected from the sediment retention basin in the lowest part of the wash were low and several were below the laboratory’s detection limits. Concentrations of most organic compounds in the stormwater sediment were below detection limits. Organic compounds present at concentrations above detection limits were p-cresol and two phthalate esters—bis (2-ethylhexyl) phthalate and dibutyl phthalate. P-cresol is used in pesticides or in disinfectants and deodorizers, and phthalate esters are commonly used in plastics, hydraulic fluid, and electric capacitors.