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Source identification for groundwater arsenic in the Verde Valley, Central Arizona, USA
aDepartments of Chemistry and Environmental Science and the Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ 86011-5698, USA
Available online 1 November 2007.


The chemical composition of water samples collected from Montezuma Well and Verde Hot Springs in the Verde Valley of central Arizona, USA, is compared to the chemical composition of groundwater from the Verde Formation to identify the original As source for Verde Valley groundwater. Verde Hot Springs water (As = 719 μg/l, Li = 1040 mg/l, B = 9100 μg/l) is very different chemically from Montezuma Well water (As = 100 μg/l, Li = 180 mg/l, B = 760 μg/l) and Verde Formation groundwater (As = 30 μg/l, Li = 50 mg/l, B = 60 μg/l), being similar in composition to geothermally influenced groundwater. As expected, Verde Formation groundwater shows the chemical characteristics of low-temperature groundwater. Strontium isotope ratios (87/86Sr) were measured by ICP–MS spectrometry. 87/86Sr isotope ratios of 0.700 for Montezuma Well water indicate contact with Precambrian rocks, while Verde Formation groundwater has 87/86Sr ratios greater than 0.720, indicating that this water is from recent sediments. Lithium and boron concentrations were used to estimate the percentage of Montezuma Well water that can be attributed to geothermally influenced groundwater at 10.5%. This is the first evidence that the As in Montezuma Well water and Verde Formation groundwater may have originated in Precambrian rocks, 800–1000 m below the surface.

3.1. Introduction

Arsenic problems in groundwaters are an international issue affecting the health of millions of individuals. Accounts of epidemic levels of As poisoning from Bangladesh ( [Mukherjee and Bhattacharya, 2001] and [Kinniburgh et al., 2003] ) and the West Bengal region of India (Das et al., 1995) have been followed with reports of similar problems in Vietnam (Berg et al., 2001) and the Inner Mongolia region of China ( [Luo et al., 1997] and [Ma et al., 1999] ). The As in each of these studies is attributed to shallow wells that draw on groundwater from Holocene epoch deposits (Smedley and Kinniburgh, 2002). The Verde Valley of central Arizona, USA, also suffers from As-contaminated groundwater [Owen-Joyce and Bell, 1983] , [Compton-O’Brien et al., 2003] and [Foust et al., 2004] , and the groundwater As has been attributed to the Verde Formation (Owen-Joyce and Bell, 1983). Lindberg (2001) studied sinkhole development and groundwater channelization in the Verde Valley, and suggested that some groundwater may contact deep Precambrian deposits (800–1000 m) before surfacing in travertine springs. The purpose of this study was to test Lindberg’s hypothesis by chemically analyzing water known to be in contact with regional Precambrian rocks, and compare the analyses with Montezuma Well water.

3.2. Geology of Verde Valley

The Verde Valley of central Arizona is located approximately 160 km north of Phoenix and 85 km south of Flagstaff. Approximately triangular in shape, 50 km on two sides and 33 km on the third, the Verde Valley is bounded by the Colorado Plateau to the north and east, and the Black Hills on the southwest. The valley floor is approximately 1000 m above the sea level and 1200 m below the bordering highlands, and covers an area of 900 km2 (Twenter and Metzger, 1963). An area map of the Verde Valley is shown in Fig. 3-1, and a geologic map of central Arizona is available (Blakey, 2002b). Figure 3-2 is a geologic cross-section of the Verde Valley, referenced to the area map in Fig. 3-1, showing the major geologic features (Kamilli and Richard, 1998). A stratigraphic column of the Verde Valley is shown in Fig. 3-3 ( [Blakey, 2002a] and [008] .

Fig. 3-1.
Area map of the Verde Valley in central Arizona

Fig. 3-2.
Geologic cross-section of the Verde Valley in central Arizona (modified from [Kamilli and Richard, 1998] and [008] )

Fig. 3-3.
Stratigraphic column of the Verde Valley in central Arizona (modified from [Kamilli and Richard, 1998] and [Blakey, 2002a] )
Metamorphic and igneous rocks of Precambrian age are exposed along the southwestern border of the Verde Valley, owing their elevated position to the Verde fault, which separates the Black Hills from the Verde Valley and the Colorado Plateau. These Precambrian rocks are assigned to the Ash Creek group by Anderson and Creasey ( [Anderson and Creasey, 1958] and [Lehner, 1958] ) and consist of basaltic, andesitic, rhyolitic and dacitic flows. The ore body at Jerome, Arizona, associated with these rocks yielded 3 736 000 t of copper between 1883 and 1952, in addition to economic amounts of gold, silver, lead and zinc.
Sedimentary rocks of Paleozoic age overlay the Precambrian rocks on the valley floor and on the plateau to the north and east. The Paleozoic sections consist of Tapeats Sandstone, Martin Limestone, Redwall Limestone and the lower part of the Supai Formation (Twenter and Metzger, 1963). The upper member of the Supai Formation and Coconino Sandstone occur next, covered with the Verde Formation, the principal regional aquifer of the Verde Valley (Owen-Joyce and Bell, 1983). Depth to water in the Verde Formation varies from 150 m at Cottonwood to flowing on the surface at Rimrock and Lake Montezuma.

3.2.1. Verde Formation

The Verde Formation consists of fluvial deposits intermixed with evaporates from an ancient lake that was created when uplifting and volcanic activity in the Hackberry Mountain area that dammed the Verde River. Intermittent streams from the surrounding highlands carried a load of very fine to very coarse fragments to the valley where they deposited on the floor and along the perimeter. Limestone deposited in the deeper parts of the lake and evaporates (salt, dolomite and gypsum) formed in shallow isolated ponds during dry periods. The southern end of the Verde Valley consists of tuffaceous sediments derived from volcanic activity, deposited directly from the air or carried in flowing streams. The tuffaceous rocks contain a mixture of ash, lava and fragments of Precambrian and Paleozoic rocks and decrease in thickness moving north along the valley floor. Clastic sediments and evaporites interbed with the tuffaceous rocks, increasing thickness as the tuffaceous rocks thin, resulting in a flat valley floor (Twenter and Metzger, 1963).

3.2.2. Precambrian rocks

Two major groups of Precambrian rocks occur in Arizona, which for convenience are called Older Precambrian and Younger Precambrian. Older Precambrian rocks are found at the bottom of the Grand Canyon and in large portions of the central part of the state, including the Verde Valley (Nations and Stump, 1981). Older Precambrian rocks are associated with the ore-bearing deposits in Jerome and other parts of Arizona, and consist of metamorphosed basalts and rhyolites ( [Anderson and Creasey, 1958] and [Lehner, 1958] ). Although Anderson and Creasey report the bulk chemical composition of the Older Precambrian rocks, they did not report any trace metal data. Arsenopyrite, however, was identified as present with the economically recoverable metals. The distance between the surface and Older Precambrian rocks at Montezuma Well is estimated at 800–1000 m (Kamilli and Richard, 1998).

3.2.3. Montezuma Well

Groundwater in the Montezuma Well vicinity originates from both the Verde Formation and the Supai Formation. The Supai Formation varies from 0 to 580 m in thickness and consists of sandstone and siltstone, mudstone, and a conglomerate of sandstone and limestone. The Verde Formation overlays the Supai, and drillers’ logs indicate an average thickness of 16 m for the Verde Formation. Near Montezuma Well, the Verde Formation is eroded away, exposing the Supai Formation in several locations (Konieczki and Leake, 1997). Montezuma Well is a collapsed travertine spring with distinctly different water chemistry from groundwater in the Verde or Supai Formations. Field observations by Lindberg (personal communication) suggest that the current spring surface area, ∼8100 m2, is much smaller than the original spring, because the opening narrowed as travertine deposited. A caveat to this observation is that the volume of water passing through Montezuma Well on a daily basis was greater in the past and may be responsible for depositing minerals into the surroundings.

3.2.4. Verde Hot Springs

The Verde Hot Springs (34°21′25″N, 111°42′38″W) are located approximately 32 km south of Montezuma Well (34°38′56″N, 111°45′03″W) and 25 km southeast of Camp Verde (34°33′25″N, 111°50′50″W). Whereas Montezuma Well and Camp Verde are situated in the Verde Formation, the Verde Hot Springs sit 30 m from the Verde River where alluvium and Verde Formation rocks contact Older Precambrian rocks (Flora, 2004). Classified as a rheocrene spring, the water emerges from the bedrock and flows along the slopes until it is collected in two man-made enclosures, where it has been used for recreational purposes since the 1920s. Verde Hot Springs water has a constant temperature of ∼39°C, dissolved solids levels of approximately 3500 mg/l and an As content that ranges between 700 and 1100 μg/l. The composition of Verde Hot Springs water is very close to that expected for geothermal water (Smedley and Kinniburgh, 2002), and the 510-mg/l Cl value agrees with high As geothermal waters from Yellowstone National Park (Webster and Nordstrom, 2003). Verde Hot Springs is located approximately 50 km southeast of Jerome and Clarkdale, where the United Verde mine operated. The site has functioned as a recreation area as long as the Verde Valley has been settled; there is no indication that recreational use or the mining operation has had an effect on the spring’s chemical composition. It was for these reasons that Verde Hot Springs was taken as an example of Verde Valley water in contact with Older Precambrian rocks.

3.2.5. Chemical composition of Verde Valley groundwater

Owen-Joyce and Bell (1983) sampled the water from 259 Verde Valley wells, identifying the rock unit each well tapped. Complete chemical analyses were done for each well, providing an excellent database describing the typical chemical composition for groundwater for each rock unit. They also provided chemical data from drill cuttings, and spent considerable effort evaluating the extent of As contamination in sediments and groundwaters. The median concentration of As in selected drill cuttings from the Verde Formation is 43 mg/kg, and the median As concentration in Verde Formation well water (188 samples) is 30 μg/l. Water from the alluvium along the Verde River (7 samples) has a median As concentration of 27 μg/l, and the other four water-bearing formations (158 samples) have As values below 10 μg/l. The highest As values in drill cuttings (88 mg/kg) were found in red clay taken from a well drilled in the Lake Montezuma area, 7 km from Montezuma Well.

3.2.6. Local channelization of groundwater

Six active sinkholes in the Sedona Arizona area were studied by Lindberg (2001) and shown to result from active groundwater movement. The groundwater flows along active fault systems in Mississippian Redwall Limestone and Devonian Martin Dolomite, beneath the insoluble sandstone surface layers, forming solution caves that collapse when the cave grows too large to support the mass above. These sinkholes provide a conduit for meteoric water to funnel into the water-bearing rocks below; the water emerges from springs in the Redwall and Martin Formations. The Redwall Limestone and Martin Formation are separated from Older Precambrian rocks by a relatively thin layer of Tapeats Sandstone, and parts of the Verde Valley actually sit on Precambrian rocks (Nations and Stump, 1981). Under artesian conditions, then, it is quite reasonable to expect spring water that may have been in contact with older Precambrian rocks at some point.

3.3. Experimental

3.3.1. Cation and anion analyses

Two water samples were collected at each sampling site [a field-filtered (0.45 μm) acidified sample and a non-filtered non-acidified sample]. Cations were determined from the acidified samples and the non-acidified samples were used for anion analysis. Dissolved oxygen, temperature and conductivity readings were taken at the site. Alkalinity was determined titrimetrically in the laboratory. Anions (View the MathML source, Cl, View the MathML source and View the MathML source) were determined by ion chromatography using Dionex 200 ion chromatograph. Calcium, magnesium, sodium, potassium, iron (Fe), copper, zinc, manganese and nickel concentrations were measured by atomic absorption spectroscopy, using either a Perkin–Elmer model 560 flame AAS or a Perkin–Elmer Analyst 600 graphite furnace atomizer equipped with Zeeman background correction and an autosampler for reproducible sample delivery. Operating conditions for each element were those specified by the U.S. Environmental Protection Agency (1998). Fluoride was determined with an Orion ion selective electrode. Each analysis was checked for accuracy by comparing charge ratios between cations and anions (Hem, 1985). Agreement between cations and anions averaged 5% for all analyses. Accuracy of trace constituents was verified with National Institutes for Standards and Testing reference materials. ICP–MS analysis
A reagent blank was prepared by adding 27 ml of concentrated nitric acid (J. T. Baker, trace metal grade) to an acid-washed 1-l flask, and diluted to volume with de-ionized, re-distilled water. A germanium stock solution, 100 μg/l, was prepared daily by serial dilution of a 1000-μg/l germanium standard (SPEX Industries, 2% HNO3).
Germanium was used as an internal standard for all ICP–MS analyses. Samples were prepared by adding 500 μl of 100 μg/l germanium stock to an empty acid-washed volumetric cylinder, adding 100 μl digestate, and diluting to 25 ml with 2.7% nitric acid blank. Four arsenic standards (0, 1, 5 and 10 μg/l) with germanium as an internal reference, were prepared by serial dilution of 1000 mg/l arsenic (SPEX Industries, 2% HNO3).
Concentration data were collected with VG Elemental Axiom ICP–MS operating in electrostatic scan mode. The instrument was set to collect 3 points per peak width and 5 points in total, with an average of 30 scans (10 scans per run, 3 runs per sample).

3.4. Results and discussion

The chemical composition for water samples from (1) the Verde Formation, (2) the Lake Montezuma and Rim Rock region, (3) the Middle Verde region, (4) Montezuma Well and (5) Verde Hot Springs are shown in Table 3-1. The Verde Formation composition data are median values from 188 wells scattered throughout the Verde Valley, all drawing water from the Verde Formation (Owen-Joyce and Bell, 1983). Chemical composition data for Lake Montezuma and Rim Rock and Middle Verde are median values from all wells in these two regions. The Lake Montezuma and Rim Rock region is the area directly adjacent to Montezuma Well. The Middle Verde region is 13 km west of Montezuma Well. Differences in water chemistry are clearly evident, with the Verde Formation, Lake Montezuma Well and Rim Rock, and Middle Verde waters being in good agreement with each other. Verde Hot Springs water varies greatly from the other four water types with higher magnesium, bicarbonate, sulfate, chloride, fluoride, dissolved solids and arsenic. Montezuma Well water represents a middle ground between these two extremes. The calcium, magnesium, bicarbonate, sulfate and chloride values for the Verde Formation, Lake Montezuma and Rim Rock, and Middle Verde waters represent expected values for water in contact with rocks that make up the Verde Formation (Hem, 1985).
Table 3-1. Comparison of the chemical composition of well water from the Verde Formation, the Lake Montezuma and Rim Rock region, the Middle Verde region, Montezuma Well and Verde Hot Springs
SubstanceVerde FormationaLake Montezuma and Rim RockaMiddle VerdeaMontezuma WellbVerde Hot Springsc
Ca2+ (mg/l)688059110110
Mg2+ (mg/l)3432483645
View the MathML source (mg/l)344027501520
View the MathML source (mg/l)350441330460580
Cl (mg/l)27282222510
F (mg/l)
Dissolved solids (mg/l)4244524165733130
As (μg/l)303643100719
a Data sources: Owen-Joyce and Bell (1983).
b Data sources: Foust et al. (2004).
c Data sources: This study.
Smedley and Kinniburgh (2002) describe the general characteristics for four types of high-As water. Verde Hot Springs water most closely matches the characteristics of geothermally influenced groundwater, as shown in Table 3-2. The 39.5°C temperature, sodium (950 mg/l), chloride (510 mg/l), boron (9100 μg/l), lithium (1040 mg/l), fluoride (1.5 mg/l) and sulfate (580 mg/l) are strong indicators that Verde Hot Springs water is correctly classified as a geothermally influenced groundwater. Smedley and Kinniburgh classify Verde Formation and Montezuma Well waters as low-temperature groundwater.
Table 3-2. Comparison of the trace chemical constituents from Verde Formation Wells, Montezuma Well and Verde Hot Springs with geothermally influenced groundwater, low-temperature groundwater and sulfide mining as potential As sources
ParametersGroundwater influenced by geothermal sourcesaLow-temperature groundwateraSulfide miningaVerde FormationbMontezuma WellcVerde Hot Springsb and d
Temperature (°C)Increased temperature

Na+ (mg/l)Increased salinityIncreased salinity
Cl (mg/l)Increased salinityIncreased salinity
B (μg/l)HighPossibly high
Li+ (mg/l)High

F (mg/l)HighPossibly high
pH>7>8Possibly low7.47.26.5
Fe (μg/l)High
View the MathML source (mg/l)High

b Data sources: Owen-Joyce and Bell (1983).
c Data sources: Foust et al. (2004).
d Data sources: This study.
Konieczki and Leake (1997) concluded, as a result of modeling studies and diving observations, that the water source for Montezuma Well was the Verde Formation and the underlying Supai Formation. Two fissures at the bottom of Montezuma Well contribute water that differs from bulk water in the well by having a warmer temperature, lower specific conductance (400 μS vs 1000 μS), lower pH and lower dissolved solids (200 mg/l vs 500 mg/l). Two water sources were invoked to explain the water chemistry. Infiltration of water from the Verde Formation is considered a primary source for Montezuma Well water, and the water entering through the two fissures may come from the Supai Formation and underlying rock units (ibid.). If we assume that the water from underlying rock units contacted Older Precambrian rocks, foundation rocks at this point of the Verde Valley (Kamilli and Richard, 1998), then Montezuma Well water should have some of the chemical characteristics of Verde Hot Springs water.
Montezuma Well water contains lithium (180 mg/l) and boron (760 mg/l) at levels significantly greater than Verde Formation water, but less than Verde Hot Springs water. Assuming that all the Montezuma Well water lithium and boron were contributed from Precambrian water, it is then possible to calculate the percentage contribution Precambrian water makes to Montezuma Well. Lithium gives a composition of 13% Verde Hot Springs water and 87% Verde Formation water for Montezuma Well, and boron gives a composition of 8% Verde Hot Springs water and 92% Verde Formation water for Montezuma Well. Approximately 10.5% of the water in Montezuma Well can then reasonably be attributed to deep water, 800–1000 m (ibid.), which has contacted Precambrian rocks.
Using the same logic, it is possible to then calculate the amount of As in Montezuma Well water that is contributed from water in contact with Precambrian rocks. The resulting value of 114 μg/l compares very favorably with the experimental value of ∼100 μg/l. The implication of this result is that Montezuma Well, as well as other springs in the Verde Valley, may have contributed substantial quantities of As to the environment through this mechanism.
Strontium isotope ratios (87Sr/86Sr) are often used to study groundwater movements and weathering processes. Isotopic ratios 87Sr/86Sr of 0.7000 to 0.7040 are indicative of mantle-derived, older rocks, 87Sr/86Sr values of 0.7100 are generally interpreted as representing crustal material and 87Sr/86Sr ratios greater than 0.7200 usually mean young rocks (Faure, 1986). The ICP–MS results of five Montezuma Well samples and five Verde Formation wells are shown in Table 3-3. All Verde Formation samples have 87Sr/86Sr ratios greater than 0.7200, indicating that the strontium in these samples was derived from recently formed sediments. Montezuma Well samples, on the other hand, contain three values near 0.7000, indicating contact with older rocks. These ICP–MS results independently suggest that a portion of Montezuma Well water contacted Precambrian rocks before surfacing at Montezuma Well, supporting the hypothesis that the primary source for As in Montezuma Well is deep Precambrian rocks.
Table 3-3. 87Sr/86Sr isotope ratios for Montezuma Well water and Verde Formation water
Montezuma Well samplesVerde Formation samples
The water in Montezuma Well undoubtedly comes from several sources, as pointed out by Konieczki and Leake. Infiltration of meteoric water through the Verde Formation supplies chemical constituents typical for low-temperature groundwater in contact with limestone, dolomite and gypsum. Additional water from the Supai Formation, entering through two fissures in the well floor, blends with Verde Formation water to give the chemical composition of Montezuma Well. A small contribution (∼10%) of deep, Precambrian water is apparently mixed with the Supai water, adding lithium, boron and arsenic, and possibly other trace metals. Over millions of years, it is likely that the high As levels observed in Verde Formation deposits originated from Precambrian rocks, and was transferred to the surface by artesian movement of deep groundwater through channelization, sinkhole development and spring formation.

3.5. Future directions

Additional stable isotope work to examine this hypothesis is planned. Helium isotopic studies should confirm that Montezuma Well contains some Precambrian water. Lead isotopes should provide a fingerprint for geothermal water that can be used to trace Precambrian water in the Verde Valley.


We acknowledge the financial assistance of the National Science Foundation for grants DBI-0244221 and CHE-0116804, and the US Department of Energy, through cooperative agreement No. DE-FC02-02-EW15254, administered by the HBCU/MI Environmental Technology Consortium. This work could not have been accomplished without the technical assistance of Mr Thomas Huntsberger and Dr Michael Ketterer, Northern Arizona University chemistry department. We acknowledge many helpful discussions with professors Larry Middleton, Abe Springer and Ron Blakey, NAU geology department. Finally, we thank Dr Paul Lindberg for his insight into this problem and sharing his ideas with us.


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