Quality of water and estimates of water inflow, northern boundary area, Fort McDowell Indian Reservation, Maricopa County, Arizona

Increased agricultural and recreational activities and recent growth of population centers within the Verde River basin have led to concerns about the quality and quantity of water flowing onto the Fort McDowell Indian Reservation. The purpose of this study was to determine the quality and quantity of water in the Verde River and in the shallow stream-channel deposits in the vicinity of the northern boundary of the reservation.

The quality of surface water entering the reservation at the northern boundary and of ground water in the shallow stream- channel deposits beneath the flood plain is suitable for most purposes. Concentrations of dissolved solids and major ions did not exceed water-quality standards. Dissolved oxygen and pH generally were in acceptable ranges for all designated uses. Total coliform counts and nutrient concentrations also did not exceed water-quality standards. Six organic compounds were detected; however, concentrations of these compounds were below U.S. Environmental Protection Agency Maximum Contaminant Levels. The presence of these organic compounds indicates that the water has been affected by anthropogenic activities. Concentrations of all trace metals were below the applicable State of Arizona Water Quality Standards for Surface Water and U.S. Environmental Protection Agency Maximum Contaminant Levels. Arsenic concentrations were below the Maximum Contaminant Level of 50 micrograms per liter at the time of collection and analysis; however, in January 2001, the U.S. Environmental Protection Agency set a new Maximum Contaminant Level of 10 micrograms per liter. All arsenic concentrations in surface water were 10 micrograms per liter or greater. Arsenic concentra-tion in ground water ranged from 6 to 9 micrograms per liter. The source of arsenic is probably oxidized arsenic compounds that are typically found in basin-fill sediments in southern Arizona.

Surface-water flow onto the reservation was determined from recorded discharge at the Verde River below Bartlett Dam and Verde River near Scottsdale streamflow-gaging stations from 1962?99. Average annual flow onto the reservation is about 13 cubic feet per second less than average annual discharge at the Verde River below Bartlett Dam and 17.5 cubic feet per second more than the average annual discharge at the Verde River near Scottsdale for years in which rainfall is less than 20 inches. Average daily flow onto the reservation also is about 13 cubic feet per second less than discharge at the Verde River below Bartlett Dam and 18.7 cubic feet per second more than discharge at the Verde River near Scottsdale for May through November. Flow onto the reservation for December through April is less predictable because of a large variation between dis-charges at the Verde River below Bartlett Dam and the Verde River near Scottsdale.

Correlation between the nine instantaneous discharge measurements made between January 1998 and August 1999 at the northern boundary and recorded discharge at the two streamflow-gaging stations is high; correlation coefficients were greater than 0.99. Instantaneous inflows onto the reservation can be related to discharges at the Verde River below Bartlett Dam and the Verde River near Scottsdale using the equations:

instantaneous inflow = 0.861(instantaneous discharge at the Verde River below Bartlett Dam) + 20 cubic feet per second, and,

instantaneous inflow = 0.893(instantaneous discharge at the Verde River near Scottsdale) + 36 cubic feet per second

for discharges that range from 102 to 704 cubic feet per second at the northern boundary.

The direction of ground-water flow is
approximately parallel to the flow of the Verde
River. Ground water has a minor component of
flow into the river when the river stage is lower
than about 5 feet above the local datum. There is a
minor component of flow into the stream-channel
deposits when the river stage is higher than about
5 feet above the local datum. Ground-water inflow
to the reservation in the Verde River streamchannel
deposits is about 1.1 cubic feet per second
and is minor compared to the average annual
streamflow of 675 cubic feet per second at the
Verde River below Bartlett Dam on the basis of
111 years of record.