### Hydrogeology, water quality, and stormwater-sediment chemistry of the Grande Wash area, Fort McDowell Indian Reservation, Maricopa County, Arizona

By: John P. Hoffmann

                            https://doi.org/10.3133/wri004116

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#### Abstract

Grande Wash is a tributary of the Verde River and drains an area of 13 square miles within theMcDowell Mountains and the Town of Fountain Hills in Central Arizona. The wash enters theFort McDowell Indian Reservation at the eastern boundary of Fountain Hills and is incised in coarse-grainedalluvium that is contiguous with the alluvial aquifer along the Verde River. The aquifer is used bythe Fort McDowell Indian Community and the City of Phoenix for municipal water supplies. Episodicflows in Grande Wash, in response to storms, carry potentially hazardous runoff from Fountain Hills ontothe reservation. Additional potential hazards to ground water include contamination from a wastewater-treatmentplant located less than 1 mile upstream from the reservation boundary, and from a landfill and acement-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 surveysand drilling indicated that the coarse-grained deposits are less than 60-feet thick along the wash and inadjacent areas within the reservation, and are underlain by a thick clay and silt unit, the base of which isbelow the bottom of the deepest monitor well (317 feet below land surface). The coarse-grained depositsform the alluvial aquifer beneath Grande Wash.

Ground water in the alluvial aquifer beneath the wash is shallow and mounded above a less permeableclay and silt unit. Depth to water in the aquifer ranges from 1 to 22 feet below land surface. Saturation ofthe coarse-grained deposits does not extend laterally for more than about 1,000 feet from the mainchannel 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 isestimated to be about 8,000 cubic feet per day (about 0.2 acre-feet per day). The flow rate is four orders ofmagnitude less than the flow rate in the Verde River. Vertical flux of ground water through the underlyingclay and silt unit is estimated to be 7,000 cubic feet per day (0.17 acre-feet per day). The volume ofground water in storage in the alluvial aquifer beneath Grande Wash is estimated to be about 5.6 millioncubic feet (129 acre-feet).

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

Low concentrations of anthropogeniccompounds, including chloroform anddichlorobromomethane, were detected. Thesecompounds are disinfection by-products ofchlorinated water.

Eight pesticide compounds were detected inthe surface water, and two pesticide compoundswere detected in the ground water. Pesticideconcentrations were below U.S. EnvironmentalProtection Agency Maximum ContaminantLevels. Several other organic anthropogeniccompounds that probably originated fromcommercial activities in the area were detected butat concentrations below laboratory calibrationstandards.

Concentrations of trace metals in thestormwater sediment collected from the sedimentretention basin in the lowest part of the wash werelow and several were below the laboratory’sdetection limits. Concentrations of most organiccompounds in the stormwater sediment werebelow detection limits. Organic compoundspresent at concentrations above detection limitswere p-cresol and two phthalate esters—bis(2-ethylhexyl) phthalate and dibutyl phthalate.P-cresol is used in pesticides or in disinfectantsand deodorizers, and phthalate esters arecommonly used in plastics, hydraulic fluid, andelectric capacitors.

#### Study Area

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