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Why is this study important?
This report represents several years of fieldwork and analysis by the U.S. Geological Survey and the Arizona Department of Water Resources. It provides well-documented, fundamental geologic and hydrologic information essential to the more recent development of the northern Arizona regional groundwater-flow model.
Blasch, K.W., Hoffmann, J.P., Graser, L.F., Bryson, J.R., and Flint, A.L., 2006, Hydrogeology of the upper and middle Verde River watersheds, central Arizona: U.S. Geological Survey Scientific Investigations Report 2005-5198, 101 p., 3 plates. The Big Chino sub-basin, in the upper Verde River watershed is 1,850 square miles in area. Within the sub-basin, Big Chino Valley and Williamson Valley encompass about 570 square miles excluding the surrounding mountains and the western part of the Coconino Plateau. The valleys are filled with alluvial deposits eroded from adjacent uplands and interbedded basalt flows. Median thickness of the combined alluvial deposits and basalt flows is about 435feet. Theestimated volume of saturated basin-fill deposits within the valleys is about 155 million acre-feet. Beneath the basin-fill aquifer is a sequence of water-bearing Paleozoic formations that receive recharge where they crop out along the western boundary of the sub-basin. Together, the basin-fill sediments and the Paleozoic formations constitute the regional aquifer in the Big Chino sub-basin. Water-balance calculations indicate that about 1–2 percent of annual precipitation recharges the regional aquifer. Recharge occurs primarily along the Juniper and Santa Maria Mountains, Big Black Mesa, Granite Mountain, and Bill Williams Mountain. Average winter base flow at the Williamson Valley streamflow-gaging station (09502800) was 3.9 cubic feet per second during 1965–84 and 1.7 cubic feet per second during 2002–03. Thedecline is attributed primarily to climate fluctuations. Base flow at the Verde River near Paulden streamflow-gaging station (09503700) averages about 17,700acre-feet per year. It currently (2003) is about equal to the long-term average but has declined at an annual rate of about 380 acre-feet per year since about the mid-1990s. Groundwater outflow from the Big Chino Valley occurs only as base flow in the Verde River. Declines in groundwater altitudes during the past 50 years are attributed primarily to ground-water withdrawals. The Little Chino sub-basin, in the upper Verde River watershed, is the smallest of the three sub-basins in the study area and has had the greatest groundwater development. The regional aquifer underlying the sub-basin is composed of sedimentary, volcanic, and basin-fill deposits of Quaternary and Tertiary age. Interfingering of less permeable units, such as volcanic rocks and cemented alluvium, create confining conditions and artesian flow in some areas. The regional aquifer encompasses an area of about 310 square miles, including Little Chino Valley and Lonesome Valley. Thickness of the basin-fill deposits generally ranges from about 100 to 800 feet; the estimated volume of the saturated basin-fill deposits in the Little Chino sub-basin is 33 million acre-feet. Winter precipitation is the primary source of recharge for the Little Chino sub-basin as well as for other sub-basins in the upper and middle Verde River watersheds. Water-balance calculations indicate that about 1–2 percent of annual precipitation recharges the regional aquifer in the Little Chino Sub-basin. This amount may have been reduced since the predevelopment period (before 1940) by the construction of channel retention facilities along Granite Creek and its tributaries. During predevelopment times a larger volume of ground water flowed north across the southern boundary of the sub-basin than flowed south. Recent numerical groundwater simulations indicate a greater flow of ground-water southward across the boundary than northward. Discharge from DelRio Springs has declined from about 2,800 acre-feet per year between 1940 and 1945 to about 1,000 acre-feet per year in 2003. The 2,500-square-mile Verde Valley sub-basin of the Verde River groundwater basin coincides with the middle Verde River watershed. The regional aquifer in the sub-basin is composed predominantly of Paleozoic units present in the Coconino Plateau and of the basin-fill sediments, including theVerde Formation. The volume of saturated sediments, which are distributed primarily along the course of the Verde River, is about 112 million acre-feet. Recharge to the aquifer occurs predominantly along the Mogollon Escarpment and on the Coconino Plateau. About 4 percent of the average annual precipitation results in recharge to the groundwater system; most of the recharge occurs from winter precipitation. Ground water discharges to the major tributaries and directly to the Verde River. Base flows in tributaries have declined in part because of climate fluctuations. Average winter base flow at the Verde River near Camp Verde streamflow-gaging station (09506000) was 148,600 acre-feet per year for 1936–44and 1989–2003, but base flow declined at an annual rate of about 2,000 acre-feet per year during 1994–2003. Ground-water storage declines are almost entirely caused by groundwater pumping and reductions in natural channel recharge resulting from streamflow diversions. Storage declines are most evident in areas of municipal development where groundwater withdrawals are largest. A geochemical mixing model was used to quantify fractions of groundwater sources to the Verde River fromvarious parts of the study area. Most of the water in theuppermost 0.2 mile of the Verde River is from the LittleChino sub-basin, and the remainder is from the BigChino sub-basin. Discharge from a system of springs increases base flow to about 17 cubic feet per second within the next 2miles of theriver. Groundwater that discharges atthese springs is derived from the western part of the Coconino Plateau, from the Big Chino sub-basin, and from theLittle Chino sub-basin. The relative component of base flow in the Verde River derived from the western part of the Coconino Plateau decreases in the downstream direction, as base flow increases, relative to the component from the Chino Valley sub-basins. By river mile 22, the primary source area is the BigChino sub-basin, and the contribution from the western part of the Coconino Plateau is negligible. Groundwater discharge from the Verde Valley begins to contribute to baseflow between river miles 22 and 30. The increases in base flow in this reach are primarily due to contributions fromgroundwater that has recharge source areas at high altitudes along the Coconino Plateau and Black Hills. Groundwater that has recharge source areas at low altitudes in the Verde Valley, and groundwater from the Coconino Plateau and the Black Hills, also contribute to base flow between rivermiles 30 and 89. Water quality in the study area generally is good for intended uses and shows little effects from human activities. Constituent concentrations in surface water and groundwatergenerally were well below Federal and State regulations. Constituents exceeding U.S. Environmental Protection Agency Maximum Contaminant Levels or Action Levels include antimony, arsenic, fluoride, lead, nitrate, and selenium. Ofthese constituents, arsenic exceeded the MCL inthe greatest number of samples, primarily owing to mineralogy ofthe Supai Group and the Verde Formation. Fluoride and sulfate concentrations exceeded the U.S. Environmental Protection Agency Secondary Maximum Contaminant Levelsin a fewsamples. Average water use in the Big Chino, Little Chino, and Verde Valley sub-basins was about 12,000, 13,000, and 47,000acre-feet per year, respectively, for 1990–2003. Agricultural and residential water use exceeds other water uses; however, agricultural use within the Chino sub-basins hasdecreased since the 1960s and 1970s. CLICK HERE TO READ THE FULL REPORT
