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

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.