Ambient Groundwater Quality of tlie Virgin River Basin: An ADEQ 1997 Baseline Study

I. Introduction

The Virgin River groundwater basin (VRGB), located in the remote northwest comer of Arizona (Figure 1) is a region of stuiming natural scenery and a small, but rapidly growing population. The perermial Virgin River, which diagonally bisects the arid VRGB, enters from Utah where it then flows across 35 miles of the northwest comer of Arizona before exiting the state into Nevada (Figure 2).

In the northeast portion of the VRGB, the watercourse flows through the Virgin River Gorge, an area of spectacular geology admired by motorists traveling through Arizona on Interstate 15. The gorge separates the Paiute and Beaver Dam Mountains wilderness areas that are found in the basin. Southwest of the gorge is the broad Virgin Valley which contains small communities historically based Figure 2. In this infra-red satellite image of the Virgin River basin (outlined in white), riparian upon irrigated agriculture. The valley vegetation, irrigated farmland, and, near mountain summits, juniper forest appear in crimson. has experienced recent residential growth that often consists of retirees drawn to the area by a mild climate and II. Background The Virgin River is a major tributary of amenities offered by the nearby Nevada the Colorado River. From its casino resorts. The VRGB encompasses more than 430 headwaters in the Markagunt Plateau square miles in the Arizona Strip above Cedar City, Utah, the Virgin The Arizona Department of section of Mohave County (2). For this River flows through Zion National Park Environmental Quality (ADEQ) report, the VRGB consists of both before eventually discharging into Lake conducted a regional groundwater hydrological boundaries (the Virgin Mead in Nevada. The river's largest quality study of the VRGB in 1997. Mountains and Beaver Dam Mountains tributary in Arizona is Beaver Dam This ADEQ factsheet is a summary of to the east and south) and political Wash, which is perermial for the more extensive, previously boundaries (the Utah border to the north approximately one mile above its published ADEQ hydrology report and the Nevada border to the west) juncture with the Virgin River (2). The available from the agency (1). (Figure 2). Virgin River is called the Pah Roose meaning very muddy stream by the Two major physiographic areas region's original inhabitants, the Paiute intersect within the VRGB. The Virgin Indians. A free-flowing river until it River Gorge (Figure 3) is the reaches Lake Mead, it is characterized demarcation between the Plateau by high turbidity and salinity levels (2). Uplands Province in the northeast and the Basin and Range Lowlands Most land within the VRGB is Province in the southwest. Surface topographically mgged, remote country topography in the latter consists of managed by the U.S. Bureau of Land sloping alluvial fans which extend from Management. Small holdings of private the surrounding mgged mountains to land exist, especially in Virgin Valley. the valley floor. Precipitation averages Some of these parcels contain densely seven inches aimually, increasing settled residences utilizing septic markedly with elevation. Natural systems for wastewater treatment. vegetation varies with topography and Communities found widiin the basin water availability. Salt cedar, include Beaver Dam and Littlefield. Cottonwood, and willow trees are found Mesquite, Nevada and St. George, Utah in river riparian areas; creosote bush, are located nearby. Groundwater is the yucca, and Joshua trees grow in the primary source for municipal, domestic, Figure 1. Location of the VRGB valleys; and juniper forests are found at and livestock uses; however surface within Arizona. the highest mountain elevations. water is also used for irrigation. III. Hydrogeologic Setting

Four VRGB aquifers were examined in this study (Figure 4). These include:

Beaver Dam Wash (BDW) aquifer • Littlefield (LTL) aquifer • Virgin River alluvial (VRA)aquifer Virgin River basin (VRB) aquifer

The BDW aquifer consists of unconsolidated silt, sand, and gravel deposited between steep terraces created by the incision of Beaver Dam Wash (2). The LTL aquifer, located northeast of the town of Littlefield, is comprised of alluvial-fan deposits that rest on a limestone formation (2). The VRA aquifer consists of the floodplain and terrace alluvium southwest of Littlefield (2). The VRB aquifer is composed of the alluvial fan deposits of the Virgin Mountains south of the Virgin River.

IV. Methods of Investigation

The ADEQ Groundwater Monitoring Program, which is authorized by the legislative requirement in Arizona Figure 4. Total dissolved solids (TDS) concentrations are shown at 38 sampling sites. High TDS Revised Statute §49-225 to monitor the is the main limitation in using groundwater for domestic use in the VRGB. quality of the state's groundwater, conducted this study. To characterize The VRGB consists largely of rugged, Secondary MCLs are unenforceable, regional groundwater quality, 38 sites undeveloped wilderness lands. As a aesthetics-based, water quality were sampled for inorganic constituents. result, groundwater sampling was standards that are guidelines for public At selected sites, samples were also concentrated in the Virgin Valley since water systems (4). Water with collected for radiochemistry (10 sites) most other basin areas have few, if any, Secondary MCL exceedances may be and pesticide (3 sites) analyses. wells (Figure 4). Sample sites were unpleasant to drink, but it is not chosen according to a random selection considered to be a health concern. Of process and stratified by aquifer. the 38 sites sampled, 25 had parameters exceeding a Secondary MCL. Sampling protocol followed the ADEQ Secondary MCLs were exceeded for the Quality Assurance Project Plan (3). following parameters: total dissolved Interpretation of the quality control data solids or TDS (25 sites), sulfate (17 indicated that the effects of sampling sites), chloride (15 sites), iron (7 sites), equipment and laboratory procedures on manganese (5 sites), andpH (1 site). the analytical results were not considered Most Secondary MCL exceedances significant. The exception was potential occurred at sites located in the LTL antimony contamination acquired aquifer and the VRA aquifer; these through impurities in filters during exceedances are indicated in Figure 4 sample processing. by TDS levels in excess of 500 milligrams per liter (mg/1). V. Water Quality Standards The 3 samples analyzed for pesticides The collected groundwater quality data had no detections for any of the 152 was compared with federal Safe pesticides or degradation products on Drinking Water (SDW) quality standards the ADEQ Groundwater Protection (4). Primary Maximum Contaminant List. Levels (MCLs) are enforceable, health- based, water quality standards that public Interpretation of these results suggest water systems must meet when suppling that groundwater in the VRGB this resource to their customers. Primary supports drinking water uses. MCLs are based on a lifetime daily However residents, particularly those consumption of two liters of water. Only utilizing the LTL aquifer or the VRA 1 of the 38 sites sampled had parameter aquifer, may prefer to install water levels exceeding a Primary MCL. This treatment units for domestic use or to Figure 3. The Virgin River and Interstate 15 exceedance involved gross alpha levels obtain domestic water from alternative intertwine through the Virgin River Gorge. in a well tapping the LTL aquifer. sources for aesthetic reasons. VI. Groundwater Composition the Kruskal-Wallis test. Many parameter levels followed a typical aquifer pattern: Groundwater in the VRGB may generally be described as slightly LTL > VRA > BDW >, =, < VRB alkaline, fresh or slightly saline, and hard or very hard based on pH, TDS, These differences in aquifer water and hardness levels, respectively. Trace quality are illustrated by graphically elements such as aluminum, barium, comparing hardness levels (Figure 5). beryllium, boron, cadmium, chromium, The highest hardness levels are found in copper, lead, mercury, selenium, silver, the LTL aquifer, its very hard water in and thallium were rarely detected. Only evidence by the calcium carbonate arsenic, fluoride, iron, manganese, and precipitation on plumbing fixtures zinc were detected at more than 10 (Figure 6). Water in the VRA aquifer is percent of the sites at concentrations also very hard but is significantly lower above Arizona Department of Health than water in the LTL aquifer. Although Services Minimum Reporting Levels. both have hard water, the BDW aquifer Nutrients such as nitrate were, with a had significantly higher hardness levels few exceptions, found at levels than the VRB aquifer. Twelve (12) indicating minimal impact from human parameters generally followed this activities. pattern: bicarbonate, boron, calcium, chloride, hardness, magnesium, Groundwater chemistry is useful for potassium, sodium, SC, sulfate, total illustrating differences in aquifers as alkalinity, and TDS. well as tracing recharge sources within the basin. Each VRGB aquifer exhibits Other significant (p< 0.05) patterns a characteristic water chemistry: involved temperature, which was lower calcium-bicarbonate in the BDW in aquifers (BDW and VRA) having aquifer, calcium-sulfate in both the LTL direct contact with perennial surface aquifer and the VRA aquifer, and a flow than in those without direct surface Figure 6. The red arrow points out the mixed chemistry in the VRB aquifer. flow ( LTL and VRA) (Figure 7). precipitation of calcium carbonate on a The groundwater chemistry of the BDW spigot as a result of very hard water pumped aquifer and the VRA aquifer seem by a well tapping the LTL aquifer. strongly influenced by recharge from 3000 the surface water of Beaver Dam data suggest that vertical variation is Wash and the Virgin River, less important than spatial variation for respectively. Beaver Dam Wash parameters in the VRGB. Groundwater exhibits a calcium-bicarbonate depth is significantly (p< 0.05) greater chemistry while the Virgin River has a in the VRB aquifer than in the other calcium-sulfate chemistry. aquifers sampled. Thus, groundwater depth patterns are likely influenced by The strength of association among spatial patterns. Other sources support levels of different parameters was this assertion and indicate that in assessed using Pearson's Correlation Arizona, groundwater parameter levels Coefficient test. Many significant (p^ tend to be a fiinction of flow path 0.05) correlations among parameter evolution more dian vertical mixing (5). levels were detected. Positive correlations occur between TDS, specific conductivity (SC), major ions, BDW LTL VRA VRB VRGB Aquifers total Kjeldahl nitrogen (TKN), boron, and to a lesser extent, iron and manganese. In contrast, these Figure 5. Boxplot comparing hardness parameters had negative correlations levels among four VRGB aquifers. with pH and nitrate. Fluoride had a unique pattern, positively correlated Regression analysis reveals many with only temperature, bicarbonate, parameters such as bicarbonate, calcium, calcium, and potassium. These fmdings chloride, hardness, magnesium, are important because the levels of potassium, sodium, SC, sulfate, total many salts and minerals at a sample site alkalinity, and TDS significantly (p< may be roughly gauged by obtaining an 0.05) decreased with increasing inexpensive parameter reading such as groundwater depth below land surface. SC. In contrast, nitrate, pH, temperature, turbidity, and zinc increased with VII. Groundwater Quality Patterns increasing groundwater depth below BDW LTL VRA VRB VRGB Aquifers land surface. Significant (p< 0.05) statistical differences were detected between Despite these significant parameter Figure 7. Boxplot comparing temperature groundwater quality and aquifers using level-groundwater depth relationships. levels among four VRGB aquifers. A related analysis based on sampling results from three deep wells suggests that a deeper aquifer exists beneath two of the sampled aquifers. Two wells greater than 900 feet deep in the VRA aquifer and one well greater than 650 feet deep in the BDW aquifer had a dissimilar chemistry compared to nearby shallow wells. Calcium-sulfate groundwater with higher TDS levels is found below the shallow BDW aquifer while sodium-bicarbonate/chloride groundwater with lower TDS levels is found below the shallow VRA aquifer.

The deep BDW groundwater sample had many parameters exceeding the 95 percent confidence intervals established for the BDW aquifer. In contrast, two deep VRA groundwater samples had many parameters below the 95 percent confidence intervals established for the VRA aquifer. These findings tentatively suggest that for domestic or Figure 8. Situated in the Virgin River floodplain, a center-pivot irrigation unit tapping the VRA municipal use, relatively shallow wells aquifer frames donnant riparian vegetation and the snow-capped Virgin Mountains. should be used in the Beaver Dam area while deeper wells should be used percent) met aesthetics-based, water —Douglas C. Tovme near the Virgin River. quality standards. Secondary MCL Maps by Larry W. Stephenson exceedances generally occurred at sites ADEQ Fact Sheet 01-02 VIII. Groundwater Impacts in the LTL aquifer and the VRA aquifer March 2001 (Figure 8) while sites in the BDW To evaluate potential impacts to aquifer and the VRB aquifer typically XI. References Cited groundwater quality by human met Secondary MCL standards. activities, upgradient control sample 1. Towne, D. 1999. Ambient Groundwater sites were compared to the 95 percent Each aquifer sampled in the VRGB has a Quality of the Virgin River Basin: A 1997 Baseline Study. ADEQ Open confidence intervals established for unique groundwater composition which each VRGB aquifer. The results File Report 99-04, Phoenix, AZ, 98 p. appears to be related to hydrological and 2. Black, K.R. and S.J. Rascona, 1991. indicate that many parameter levels, geologic conditions within the basin. Maps Showing Groundwater including nitrate, in the control sites for Surface water seems to be a major factor Conditions in the Virgin River Basin, the BDW aquifer and the LTL aquifer affecting groundwater quality in two Mohave County, Arizona, Lincoln and were often below the 95 percent aquifers. The relatively low parameter Clark Counties, Nevada-1991. confidence intervals. This indicates that levels characteristic of the BDW aquifer ADWR Hydrologic Map Series Report the groundwater quality of these two are likely intercoimected with the high- #22, Phoenix, AZ, 1 p. aquifers might be affected by residential quality surface water in Beaver Dam 3. Arizona Department of Environmental development impacts such as nitrates Wash (2). Similarly, the relatively high Quality, 1991. Quality Assurance Project Plan. ADEQ: Phoenix, AZ, from septic systems used for wastewater parameter levels characteristic of the 209 p. treatment by many residents. VRA aquifer are likely influenced by the 4. U.S. Environmental Protection Agency, saline surface flow of the Virgin River. 1993. The Safe Drinking Water Act - A Although nitrate (as nitrogen) levels in Factors influencing the Virgin River Pocket Guide to the Requirements for the VRGB were generally below natural salinity include an initial high salt the Operators of Small Water Systems. background levels of 3 mg/1, this concentration, saline spring discharges USEPA Region 9, San Francisco, CA, parameter exhibits other unique patterns near the community of Littlefield, and 47 p. 5. Robertson, F. N., 1991. Geochemistry that warrant future monitoring. Based irrigation return flows (2). of Ground Water in Alluvial Basins of on statistical correlations, nitrate Arizona and Adjacent Parts of Nevada, appears to originate from a different In contrast, the relatively low parameter New Mexico, and California. U.S. source than other groundwater quality levels characteristic of the VRB aquifer Geological Survey Professional Paper parameters. A unique pattern also are likely the result of high-quality, 1406-C, 90 p. emerged in which nitrate levels in the mountain-front recharge from the Virgin LTL aquifer, which has little associated Mountains. The relatively high For More Information Contact: residential development, were parameter levels characteristic of the significantly lower (p< 0.05) compared LTL aquifer appear to be influenced Douglas C. Towne - ADEQ to the other three sampled aquifers. from contact with limestone known as 3033 N. Central Ave. #360 the Littlefield formation (2). This IX. Groundwater Conclusions horizontal limestone unit is overlain by Phoenix, AZ 85012 (602)207-4412 alluvial fan deposits and is the likely to wne. doug@ev. state, az. us Of the 38 sites sampled in the VRGB, cause of the saline, very hard www.adeq.state.az.us/environ/water/ 37 (97 percent) met health-based, water groundwater found in die LTL aquifer. assess/ambient.html quality standards but only 13 (34