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Hydrology of the Lower San Pedro Basin

Groundwater Hydrology

Western Portion

On the western side of the planning area are a group of basins that are tributary to the San Pedro and Gila rivers; Aravaipa Canyon, Donnelly Wash, Lower San Pedro and Upper San Pedro. Groundwater is found in stream alluvium and basin-fill sediments in these basins. 

Lower San Pedro Basin

The Lower San Pedro Basin consists of the northwest-trending San Pedro River Valley bordered by mountains ranging in elevation from 6,000 to over 8,000 feet in elevation. There are two sub-basins; the Mammoth Sub-basin and the smaller Camp Grant Wash Sub-basin (Figure 3.8-7).  The two major water bearing units are stream alluvium and basin fill. Most mining, industrial and domestic/municipal wells are located in the regional basin fill aquifer while most irrigation wells are located in the stream alluvium.  The stream alluvium along the San Pedro River and tributaries can be quite permeable with high well yields but this aquifer is often less than 50 feet thick south of Redington (USGS, 2006a). Groundwater in the alluvium is unconfined. The hydrologic characteristics of the basin fill aquifer vary widely due to the amount of cementation and occurrence of fine-grained layers.  Both confined and unconfined conditions exist. Artesian conditions exist from about five miles north to ten miles south of Mammoth in wells drilled deeper than 500 feet.

Groundwater flow direction is from the mountains toward the valley floor and to the north.  The estimated groundwater recharge ranges from 24,000 to 29,000 AFA (Table 3.8-7) from mountain front recharge, streambed infiltration and underflow from the Aravaipa Canyon and Upper San Pedro basins. Groundwater is discharged by pumpage, evapotranspiration, evaporation from streams, and springs and seeps.  The estimated volume of groundwater in storage ranges from 11 maf to more than 27 maf (Table 3.8-7).  Water level change data between 1990-91 and 2003-04 for 16 wells shows relatively stable water levels in most wells (Figure 3.8-6).  (A water level sweep was conducted in winter 2006-2007 and a hydrologic map series report is expected to be completed by fall 2009).Water quality data from selected sites show that fluoride was the parameter that most frequently exceeded drinking water standards, with elevated levels of cadmium found in the vicinity of Hayden and Dudleyville (Table 3.8-7).

Click to view Table 3.8-6

Click to view Table 3.8-7 Groundwater Conditions in the Lower San Pedro Basin

Click to view Figure 3.8-7

Click to view Figure 3.8-7 Lower San Pedro Basin Groundwater Conditions

Figure 3.0-10

USGS 6-Digit Hydrologic Unit Code (HUC) Boundaries in the Southeastern Arizona Planning Area

Surface Water Hydrology

The U.S. Geological Survey (USGS) divides and subdivides the United States into successively smaller hydrologic units based on hydrologic features.  These units are classified into four levels. From largest to smallest these are: regions, subregions, accounting units and cataloging units.  A hydrologic unit code (HUC) consisting of two digits for each level in the system is used to identify any hydrologic area (Seaber et al., 1987).  A 6-digit code corresponds to accounting units, which are used by the USGS for designing and managing the National Water Data Network.  There are portions of five watersheds in the planning area at the accounting unit level: Middle Gila River; Rio de Bavispe; San Pedro River-Willcox; Santa Cruz River; and the Upper Gila River (Figure 3.0-5). 

San Pedro-Willcox Watershed

The Arizona portion of the San Pedro River Watershed is contained entirely within the planning area.  Approximately 696 square miles of the Watershed extends into Mexico. In Arizona, the Watershed includes all of the Aravaipa Canyon and Upper San Pedro basins, most of the Lower San Pedro and Willcox basins and relatively small portions of the Cienega Creek, Douglas and San Rafael basins.  A few tributaries to the San Pedro River begin on the southwest slopes of the Huachuca Mountains in the San Rafael Basin and drain into Mexico. (ADWR, 2005a) The San Pedro River enters the U.S. from Mexico near Palominas (see Figure 3.13-1) and flows north to its confluence with the Gila River. Major tributaries are the Babocomari River and Aravaipa Creek.

With the exception of Whitewater Draw in the extreme southern end of the basin that drains into the Douglas Basin, most of the surface water drainage in the Willcox Basin is to the Willcox Playa.  The playa occupies about 50 square miles in the center of the basin and is a remnant of Pleistocene-age Lake Cochise. (Oram, 1993)

Some stretches of the San Pedro River are perennial, although recent drought and delay of the summer monsoon has affected some previously perennial stretches for short periods of time, most notably at Charleston in the Upper San Pedro Basin. The Babocomari River, in the Upper San Pedro Basin, is perennial in its upper reach. Aravaipa Creek is perennial within Aravaipa Canyon above its confluence with the San Pedro River as are three of its tributaries in the Aravaipa Canyon Basin (see Figures 3.1-5 and 3.8-6).  Other perennial streams are found in the Lower San Pedro, Upper San Pedro and Willcox basins (Figures 3.8-6, 3.13-5 and 3.14-5).

There are 12 active streamgages in the watershed; two in the Lower San Pedro Basin and 10 in the Upper San Pedro Basin. The gage on the San Pedro River at Charleston has been in operation since 1904. The largest annual flow ever measured in the watershed, (152,798 acre-feet), was recorded at this gage in 1914.  More recently, in 1984, a maximum annual flow of 102,107 acre-feet was measured at the gage on the San Pedro River near Tombstone.  Median annual flow at these gages is 33,203 acre-feet and 29,654 acre-feet, respectively.

The only major springs in the watershed are found in the Lower San Pedro and Upper San Pedro basins. There are 14 major springs in the Lower San Pedro Basin. The largest, Cooks Lake Spring, had a discharge rate of 1,000 gpm when last measured in 1951.  Twelve major springs have been identified in the Upper San Pedro Basin. The largest is Garden Canyon No.1 with a discharge of 134 gpm measured in 1963. Most of the spring measurements in both basins date from before 1980 and may not be indicative of current conditions (see Tables 3.8-5 and 3.13-5).

Fifteen miles of the San Pedro River in the Lower San Pedro Basin from Aravaipa Creek to the Gila River are impaired due to elevated concentrations of E. coli and selenium (Table 3.8-7). In the Upper San Pedro Basin, water quality standards were exceeded in three reaches of the San Pedro River for a total of 53 miles. These reaches are impaired due to elevated levels of E. coli, nitrate and copper (Table 3.13-6).

 

 

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