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Hydrology of the Coconino Plateau Basin

The Redwall-Muav (R or limestone) aquifer is the primary water-bearing unit of the Coconino Plateau Basin.  The Kaibab, Coconino and Supai formations comprise the regional Coconino Aquifer (C-aquifer) that overlies the R-aquifer.  The Moenkopi Formation volcanic rocks and unconsolidated sediments overlie the C- and R-aquifers and provide locally important sources of water.  A generalized stratigraphic section of the Coconino Plateau that illustrates the relationship between these various units is shown on Figure 6.0-5.  Perched aquifer zones in association with volcanic rocks occur primarily in the central and southern part of the basin and in consolidated sedimentary rocks west and northwest of the volcanic fields. These perched aquifers are dependent on recharge from precipitation and runoff and may be undependable water supplies.  An exception is the “Inner Basin Aquifer” of the San Francisco Peaks where a water-bearing zone is contained in glacial outwash and volcanic rocks and is used by the City of Flagstaff as a water supply (USBOR, 2006).

The R-aquifer underlies the entire Coconino Plateau Basin with depths of greater than 3,000 feet below land surface (bls) in most areas (Bills and others, 2007).  Relatively few wells have been completed in the R-aquifer in the basin due to its extreme depth.  In the northeast part of the basin, the R-aquifer is in partial hydraulic connection with the C-aquifer through faults and other fractures. Shale units within the R-aquifer impede downward flow. 

Figure 6.0-5 Generalized stratigraphic section of the Coconino Plateau

Figure 6.0-5

The C-aquifer, consisting of hydraulically connected sandstones, limestones and shales occurs primarily in the eastern portion of the basin. Although perched zones occur, it is largely drained of water in the rest of the basin, coincident with the northeast-southwest trending Mesa Butte Fault (Bills and others, 2007).  Infiltration of precipitation through volcanic rocks and the Kaibab Formation is the primary source of recharge to the C-aquifer.

Lateral movement of groundwater in the R- and C- aquifers occurs through fracture zones and solution cavities.  In the northeastern portion of the Coconino Plateau Basin, groundwater moves relatively rapidly from the C-aquifer to the R-aquifer through solution channels and fractures (USBOR, 2006). Regional flow is generally northward toward the Grand Canyon where springs discharge along the Little Colorado and Colorado rivers and Havasu Creek (see Figure 6.1-7).  Widely-spaced faults and folds also affect groundwater movement in the region. The Mesa Butte Fault and the Cataract Syncline direct flow to major discharge areas on the lower Little Colorado River at Blue Springs and in Cataract Canyon (Montgomery and others, 2000).  The Blue Springs area is considered the primary groundwater drain from the Little Colorado River Basin, although the primary source of the water is not well known (Hart, and others, 2002). Local flow characteristics are poorly understood because of the complex geologic structure and because aquifer depths limit exploratory drilling and testing. The varying chemistry of springs and residence time for groundwater discharge suggests that water discharging from the R-aquifer is from many different recharge areas and follows different flow paths. (USBOR, 2006)

Click to view Figure 6.1-7

Click for Figure 6.1-7 Coconino Plateau Basin

Groundwater Conditions

An annual natural recharge rate is not available for the basin. ADWR estimated that as much as 3.0 million acre-feet (maf) of water may be stored in basin aquifers based on assumptions by Montgomery and others (2000) of the plateau’s area of about 10,000 sq. mi., average saturated thickness of 800 feet and an average specific yield of 0.1%. Their study area was larger than the basin but included most of it. Well yields in the basin are relatively low and depend on the occurrence of fractures, faults and solution channels. The median of well yields reported from 16 large diameter (>10 inches) wells was 45.5 gallons per minute (gpm).

Water levels in basin wells are typically quite deep. Tusayan’s water supply plan reports water level depths of 2,347 and 2,425 feet in two system wells with well yields of 65-80 gpm (HydroResources, 2007).  While water has been found in perched aquifers near Williams at depths less than 950 feet bls, yields from these more shallow wells are generally less than five gpm.  At Williams, three of the four water system wells are drilled to depths exceeding 3,500 feet bls.  Water level depths in these wells are between 2,740 and 2,875 feet.  Water in the deepest of the Williams wells is of poor quality with elevated metal concentrations, including arsenic, and high corrosivity (City of Williams, 2007).

Water quality is generally good in the basin but poor locally where there is leakage from overlying units or other factors.  Water quality in the upper and middle parts of the C-aquifer is good, but generally degrades due to salts at increasing depths. Most of the water quality data shown in Table 6.1-7 is from springs where elevated levels of arsenic and total dissolved solids (TDS) were most commonly detected.

For information on surface water hydrology see the Little Colorado River and Lower Colorado Lees Ferry to Lake Mead watersheds.



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