Why we map, #2: Prescott - Chino valleys

This is the second in a 3-part series explaining the basis for AZGS geologic mapping under the STATEMAP component of the National Cooperative Geologic Mapping Program, administered by the USGS. We were notified last week that partial funding was approved for three areas proposed by AZGS based on recommendations from the Arizona Geologic Mapping Advisory Committee. Here is the description by AZGS geologists Jon Spencer and Phil Pearthree of the mapping project in the Verde River area:

Mapping the region near the city of Prescott was given high priority largely because of concerns about groundwater depletion and renewed concern about arsenic in municipal water supplies. This area has been a GMAC priority in three previous years, and AZGS geologists will have mapped six quadrangles in this area by the end of the current STATEMAP project.

New geologic mapping in the upper Verde River area is consistent with the goals of the Water for America initiative. Proposed mapping is partly directed at better understanding of the surface geology and its relationship to the stratigraphy and structure of groundwater basins, groundwater flow paths, and geologic sources of natural contaminants in groundwater (especially arsenic in the upper Verde River area). Indeed, most of the STATEMAP project areas recommended for by the Arizona GMAC over the past several years were identified because of issues associated with groundwater availability and quality, preservation of riparian habitat, aggregate potential along river beds and flood plains, and flood hazards.

Proposed project #2 map area is within the third province, known as the Transition Zone, which stretches northwest to southeast across central Arizona and lies between the other two provinces. Bedrock consists largely of Proterozoic crystalline rocks that are contiguous with those beneath the Paleozoic sedimentary rocks of the Plateau, with a significant cover of upper Cenozoic volcanic rocks. These rocks are locally broken by Miocene to Quaternary normal faults that have produced generally shallow extensional basins that contain most of the areas groundwater resources. The physiography of the Transition Zone varies from gently sloping basin fill to very rugged bedrock, and the region is drained by several large river systems that supply much of the water for urban and agricultural uses in Arizona.

The proposed mapping areas are in Yavapai County, which has grown from a population of 31,000 in 1960 to approximately 215,500 in 2008. This rapid population growth is due to a number of factors, including the area’s scenic character, and elevations of 4500-5500 feet above sea level which result in cooler weather than in the low deserts where most Arizonans live. The basin aquifer beneath Little Chino Valley, which is utilized by Prescott and the town of Chino Valley, had been depleted at such a high rate by 1999 that the State declared the aquifer to be in an overdraft situation. The City of Prescott and the Town of Chino Valley are now attempting to develop well fields in more distant Big Chino Valley and Williamson Valley. Water-table drawdown in Big Chino Valley might, however, affect water discharge at springs along the perennial Verde River, which would have serious environmental consequences. The quality of water in the basin aquifers is also less than ideal, primarily because of elevated arsenic concentrations. Prescott is currently installing arsenic-removal equipment on six city wells located in Little Chino Valley. Many of the wells in Big Chino Valley contain arsenic above the 10 ppb level allowed by federal guidelines (Blasch et al., 2005).

A primary purpose and justification of new mapping is to identify fault zones and rock units that could serve as conduits for groundwater movement and that should be factored into groundwater flow models. More geologically realistic flow models will serve the interests of property owners in obtaining long-term supplies of clean water, and will provide a better foundation for decisions that affect riparian and aquatic habitats along the Verde River. A Quaternary fault identified in Little Chino Valley by recent STATEMAP mapping (Gootee et al., 2010) projects beneath the Verde and Granite Creek river beds where springs provide most of the recharge in the approximately 15 km reach of the Verde River below Big Chino Valley. The fault may provide a conduit for groundwater movement from the sediment-filled valley to the Verde River. Additional justifications for proposed mapping are as follows: (1) Identification of geologic factors that affect arsenic content of well water, such as association of arsenic with volcanic rocks and derivative sediments. (2) Further characterization of potential earthquake hazards. Quaternary faults have been mapped in the area on a reconnaissance basis, but need careful evaluation of deposits and geomorphic features that may reveal their age of most recent movement. (3) Identification of potentially flood-prone areas in the valleys based on the distribution and nature of young alluvial deposits. (4) Identification of areas with potential for base and precious metal mineralization as revealed primarily by evidence of ancient hydrothermal activity. Existing bedrock geologic maps (Krieger, 1965, 1:48,000 scale; Krieger, 1967a, b, 1:62,500 scale; DeWitt et al., 2008, 1:100,000 scale) are not very detailed.

Groundwater resources. Population growth around Prescott Valley and Chino Valley has placed increasing pressure on groundwater supplies. As a result of concerns that groundwater would be depleted, the Little Chino Valley drainage basin was made part of an Active Management Area (AMA) by the Arizona Department of Water Resources in 1980. The Prescott AMA has a statutory goal of achieving basin-wide safe-yield, balancing annual groundwater withdrawal with natural and artificial recharge, by 2025. According to the legal framework of Arizona water law, recharging water in one part of an AMA can offset groundwater-level declines in another portion of the AMA. In spite of AMA management, however, the water table is declining in 90% of the groundwater wells in the AMA, with an average rate of decline of 2.7 feet per year. Water pumping is now estimated to be 45% greater than recharge.

Knowledge of rates of recharge depends in part on understanding the surficial geology well enough to be able to identify areas of recharge. Proposed detailed geologic mapping will improve recognition of areas where surface water recharges groundwater. This useful aspect of detailed geologic maps was specifically identified by John Rasmussen of the Yavapai County Water Advisory Committee, who indicated his support for detailed geologic mapping in Little Chino Valley and Big Chino Valley. Calculations of changes to the water table resulting from groundwater pumping, and resulting from modifications to the amounts and locations of recharge, require understanding of underground flow paths. Understanding flow paths depends on understanding subsurface basin geometry, the distribution and character of aquifer materials, and potential conduits such as faults.

Proposed geologic mapping is intended to refine understanding of basin-margin geometry as revealed by basin-margin faults and depositional contacts, by recognizing incised basin-filling sedimentary units that represent exposed basin stratigraphy, and by identifying faults in bedrock that are potential conduits for water flow. This will allow better approximation of subsurface basin geology and improved groundwater flow models. Discovery of a Quaternary fault zone on the northeast flank of Little Chino Valley during recent STATEMAP mapping (Gootee et al., 2010) will affect representation of basin geometry in groundwater flow models by creating possible flow paths from basin aquifers to the Verde River.

Verde River base flow. Base flow, which is the amount of flow in a river that results from groundwater inflow and is unrelated to prompt runoff from precipitation, increases along a several-mile stretch of the Verde River in the Chino Valley North 7.5' Quadrangle (Wirt, 2004b). Springs in the streambed, primarily near Upper Verde River Springs and above the Paulden gauging station, emanate from an area of faulted Paleozoic sedimentary units and sustain a base flow of about 25 ft3/s. This base flow maintains critical habitat for the threatened spikedace minnow (Meda fulgida). The Arizona Game and Fish Department recently acquired 796 acres along the upper Verde River in order to protect the area’s native fish (Wirt, 2004b), including at least six native fish species. Proposed mapping is not directly related to the problem of identifying the source of Verde River recharge, but it is directed at better understanding of the basins upstream and up gradient from the springs. Proposed mapping will contribute to a better understanding of regional groundwater behavior and so may influence decisions regarding water use.

Arsenic in groundwater. A recent compilation of well-water chemistry in the Prescott – Chino Valley area outlined the areal extent of wells with elevated arsenic levels (data from Blasch et al., 2005). The highest arsenic levels are within the southeastern end of Big Chino Valley. High levels are also apparent in some spring discharges; all nine samples analyzed from Upper Verde River Springs contained 13 to 29 ppb As. The Quail Ridge Domestic Water Improvement District at the east edge of the Sullivan Buttes 7.5' Quadrangle recently installed arsenic removal equipment in order to reduce arsenic concentrations to below 10 ppb.

Elevated arsenic levels are thought to be derived from lower Paleozoic aquifer units and from fine-grained, basin-interior sediments in Big Chino Valley (Wirt, 2004a). We suspect that Cenozoic volcanic rocks, which are common in the area of elevated arsenic concentrations, could play a currently unappreciated role in contributing contaminants to groundwater. These volcanic rocks are dominated by the Sullivan Buttes latite, a widespread sequence of mafic to intermediate composition volcanics (51-68% SiO2, n=22) with elevated K2O (2.7-6.5%, n=22) and locally abundant lower crustal and upper mantle xenoliths (Tyner, 1984). As part of proposed mapping, we will map the Sullivan Buttes 7.5' Quadrangle and obtain trace-element geochemical analyses, including arsenic, of the latite. (We could not find any geochemical measurements of arsenic from the Sullivan Buttes latite and related volcanic rocks in available geologic literature).

Quaternary faulting. Quaternary normal faults are present in the Jerome Canyon 7.5' Quadrangle and possibly extend northward but are unidentified in the Sullivan Buttes 7.5' Quadrangle. Minor historic seismicity is inferred to reflect approximately east-west crustal extension southward from the more active Hurricane fault and related faults near the edge of the Colorado Plateau in northwestern Arizona (Pearthree and Bausch, 1999). Reconnaissance investigations of the Big Chino fault zone indicate that it has had substantial activity in the middle and late Quaternary (Pearthree et al., 1983). Fault identification is also significant because of implications for groundwater flow.

Flood hazards. Potential flood hazards exist along piedmont drainages throughout the proposed map area. Big Chino, Little Chino, and Prescott Valleys are generally dry, but drainages of all sizes are subject to infrequent to rare floods with deep, high velocity flow. In addition, lateral bank erosion and drastic changes in channel position may occur during floods, especially along banks formed in weakly cohesive Holocene terrace deposits. Mapping of active and abandoned channels and Holocene terraces along these streams defines the corridor that is most likely to be subjected to flooding or bank erosion. In low-relief portions of valleys, sheetflooding on active alluvial fans can cover broad portions of piedmonts. Such floods are infrequent and of short duration, but are potentially devastating to homes because of the extent of inundation and the potential for developing new channels. Mapping surficial deposits on piedmonts will document extensive areas that are covered by young deposits, and thus may be prone to sheetflooding.

Mineral deposits. Mineral deposits and old mining camps are abundant in eastern Yavapai County, and have much to do with the original settlement of the area. The Big Bug and Ticonderoga mining districts at the south edge of the Prescott Valley South 7.5' Quadrangle each have millions of pounds of historic copper production and hundreds of thousands of ounces of historic gold production, as well as substantial lead, zinc, and silver. Better geologic understanding of the bedrock geology in the area could lead to new exploration efforts.

Iron King Mine – Humboldt Smelter superfund site. Arsenic and lead contamination associated with historic mining and smelting activities have resulted in contamination around the Iron King Mine at the extreme southeastern corner of the Prescott Valley South 7.5' Quadrangle and the Humboldt smelter located about a mile down Chaparral Creek and just outside the proposed map area (EA Engineering, Science, and Technology, Inc., 2010). Proposed mapping will clarify the structural geology and rock types under the superfund site area, which will improve understanding of potential pathways for groundwater movement and dispersal of pollutants.


Preliminary Results and Prior Work

The proposed map area was mapped previously at 1:48,000 scale (Krieger, 1965) or 1:62,500 scale (Krieger, 1967), but the Quaternary deposits in the map area were subdivided into only two map units. We will provide much greater differentiation and detail to Quaternary map units. We expect to divide Tertiary volcanic rocks into more units, and to more finely divide other bedrock units if hydrologic properties are likely to be different within previously combined units. Faults also will be mapped with much greater detail. Proterozoic metamorphic rocks in the Prescott Valley South 7.5' Quadrangle are mapped well at 1:48,000 scale (Krieger, 1965), but we expect to add significant detail at 1:24,000 scale.

As noted above, just-completed STATEMAP mapping (Gootee et al., 2010) identified a Quaternary fault zone that projects beneath springs in the Verde River bed that contain elevated arsenic and that provide habitat for native fish. High arsenic levels suggest that spring water is derived from Big Chino Valley, but chemical analysis of water by Wirt et al. (2004a) suggest that spring water is derived from Little Chino rather than Big Chino Valley. Water flow paths may be more complicated than previously appreciated, and perhaps involve subsurface mixing from multiple sources. Further evaluation and study are warranted.