Ambient Groundwater Quality of the San Rafael Basin: An ADEQ 2002 Baseline Study I. Introduction The San Rafael Groundwater Basin (SRF) is a rustic landscape composed of large cattle ranches and public lands situated along the Arizona/Mexican border between the cities of Nogales and Sierra Vista (Figure 1). The basin is renown for the beauty of its oak- dotted, rolling hills that contain some of the most pristine remnants of shortgrass prairie in Arizona. From a hydrology perspective, it's most noteworthy as the headwaters of the bi-national Santa Cruz River (Figure 2). This factsheet reports upon the results of groundwater quality investigations in the SRF and is a summary of the more extensive report Mexico produced by the Arizona Department of Figure 3. Infrared satellite image (June 1993) in which forested mountains appear in crimson, Environmental Quality (ADEQ).' grasslands are in blue, and irrigated or riparian areae along the lower stretches of the Santa Cruz River are depicted In bright red. The San Rafael Basin has a very email, dlepereed population. For II. Background most services, residents must travel outside the basin to Patagonia, 5onolta, or Nogales as Washington Camp, Puquesne, and Sunnyside are old mining camps with little commercial activity. The SRF is a small basin encompassing 172 square miles.' It includes the broad Elevations in the SRF range from 7,900 Limited groundwater occurs in hardrock San Rafael Valley, the eastern slopes of feet at Peterson Peak in the Huachuca areas that are most productive where the the Patagonia Mountains, the southern Mountains to 4,500 feet where the bedrock is fractured and faulted.' The slopes of the Canelo Hills, and portions Santa Cruz River enters Mexico near SRF is predominantly sedimentary rock of the western slopes of the Huachuca the town of Lochiel. Vegetatively, the with basaltic rock formations in the Mountains (Figure 3). To the south, SRF supports 90,000 acres of shortgrass Canelo Hills, and granitic rock outcrops the Mexican border creates an arbitrary prairie as well as valuable cottonwood in the Patagonia Mountains (Figure 4). physical boundary. riparian habitat along the Santa Cruz River and its major tributaries. The Uplands in the SRF are managed by the surrounding uplands support an U.S. Forest Service while the San evergreen woodland ecosystem.^ Rafael Valley is mostly private land. Although the majority of the basin is in III. Hydrology Santa Cruz County, the extreme eastern portion is in Cochise County. Groundwater in the SRF is found in an alluvial aquifer consisting of three basic units: streambed alluvium, pediment gravels, and basin-fill alluvium." The streambed alluvium consists of well- sorted silt, sand, and gravel that forms the narrow floodplain of the Santa Cruz River and its major tributaries. The pediment gravels form terraces along the valley's eastern side. The remainder of the valley consists of basin-fill alluvium composed of clay, silt, sand, and gravel deposits." ^*Groundwater in the San Rafael Basin, particularly in areas not in proximity to historic mines in the Patagonia Mountains, Figure 2. A bi-national watercourse, the San Rafael River begins in the SRF, flows into appears to be largely suitable Figure 1. The 5an Rafael Basin is located in Mexico and turns back into the U.S. 32 miles a relatively isolated part of southeastern for domestic use." later near the city of Nogales. From there it Arizona enabling it to largely escape the continues north to Tucson. The author is pictured along a perennial stretch of the river transformations that have altered the near the hamlet of Lochiel, Arizona. landscape of other rural areas of the state. Groundwater movement in the SRF is toward the Santa Cruz River and then south into Mexico. Groundwater depth is shallowest near the major waterways, Granitic averaging 10 to 25 feet below land Rock ^ surface (bis), increasing to over 100 feet bis in other parts of the basin." The Santa Cruz River is one of the few grassland streams in the Southwest with stretches of perennial flow. Although ephemeral in its headwaters in the Canelo Hills, it becomes perennial for a three-mile reach, five miles north of the international boundary with Mexico. Numerous springs and creeks contribute to this perennial flow. The Santa Cruz Primary & Secondary MCL O Secondary MCL River has an average annual discharge No MCL Exceedance A USGS Gauge into Mexico of 2,900 acre-feet." IV. Methods of Investigation Figure 4. Locations of 20 sample sites and the U.S. Geological Survey gaging station on the Santa Cruz River are shown. Primary, health-based water equality standard exceedancee (shown This study was conducted by the ADEQ in red) occurred only In the Fataqonla Mountains In or near areae of granitic rock. Ambient Groundwater Monitoring Program, as authorized by the V. Water Quality Sampling Results lead, gross alpha, and uranium legislative mandate in Arizona Revised concentrations each exceeded Primary Statutes §49-225. To characterize The collected groundwater quality data MCLs at one site apiece. regional groundwater quality, 20 sites were compared with Environmental (15 wells and 5 springs) were sampled. Protection Agency (EPA) Safe Drinking EPA SDW Secondary MCLs are Samples were collected for inorganic Water (SDW) water quality standards. unenforceable, aesthetics-based water constituents and isotopes of hydrogen quality guidelines for public water and oxygen at all sites. At selected Primary Maximum Contaminant Levels systems. Water with Secondary MCL sites, samples were also collected for (MCLs) are enforceable, health-based exceedances may be unpleasant to drink radiochemistry (5 sites), radon gas (5 water quality standards that public and/or create unwanted cosmetic or sites), and volatile organic compounds systems must meet when supplying laundry effects but is not considered a (2 sites) analyses. water to their customers. Primary health concern. MCLs are based on a lifetime daily Sampling protocol followed the ADEQ consumption of two liters of water. Of the 20 sites sampled, 3 had Quality Assurance Project Plan. Based constituent concentrations exceeding a on quality control data, the effects of Of the 20 sites sampled, 2 had Secondary MCL (Figure 4). sampling equipment and procedures constituent concentrations exceeding a Constituents above Secondary MCLs were not considered significant. Primary MCL (Figure 4). Antimony, included total dissolved solids (TDS), sulfate, manganese (two sites apiece), and iron (one site). VI. Groundwater Composition Groundwater in the SRF is generally slightly alkaline (pH > 7 standard units), fresh (TDS < 1000 milligrams per liter or mg/1), and hard (> 150 mg/1) (Figure 6). At 85 percent of sites, nitrate (as nitrogen) was found at levels under 3 mg/1, which is often interpreted as representing no impact from human activities. "SRF groundwater chemistry is generally calcium-bicarbonate which is common in Arizona and typical of recharge areas. Two sites in the Patagonia Mountains with calcium-sulfate chemistry Figure 5. John Russell Partlett toured the 5an Rafael Valley in 18>5] as part of the U.S. appear to be impacted by historic boundary Commission and wrote that "this valley was covered with the most luxuriant herbage and thickly studded with live oaks; not like a forest, but rather resembling a cultivated park.'"^ mining activity." This deecrlptlon is still appropriate as shown in this photo taken near the 5an Rafael Ranch; the Huachuca Mountains are in the back/ground. alluvial deposits found in the SRF. '^The surface flow of the Santa These factors suggest that recharge occurs not only along mountain fronts Cruz River near Lochiel tends to in the basin but also along the Santa have lower mean constituent Cruz River and its major tributaries. concentrations than SRF groundwater sites. This may be VIL Groundwater Quality Patterns explained by the dilution of the stream's base flow by direct runoff Groundwater quality constituent from precipitation." concentrations were statistically compared among SRF aquifers, watersheds, and geologic types. No significant differences were found Most sample sites exhibited a calcium- except that sodium concentrations at bicarbonate chemistry, though the two sites in the Santa Cruz River watershed sites with Primary MCL exceedances in in the central portion of the SRF were the Patagonia Mountains had a calcium- significantly higher than at sites in the sulfate chemistry. Barium, fluoride, and Parker Canyon watershed located in the zinc were the only trace elements eastern portion of the basin (Kruskal- detected at more than ten percent of Wallis in conjunction with the Tukey sites. Others such as antimony, arsenic, test, p < 0.05). beryllium, boron, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, selenium, silver, and thallium were rarely, if ever, detected. Figure 8>. The San Rafael Valley contains some Many significant correlations were of the most pristine desert grasslands in the found among concentrations of water Southwest. Livestock q^raisnc^ is the main land quality constituents. Generally, levels use. Pictured is the windmill, barn, and rolling grasslands of the Arizona State Park's San of major ions (calcium, magnesium, Rafael Ranch. sodium, chloride, and sulfate), TDS, and hardness (Figure 7) were positively correlated with one another (Pearson VIII. Study Conclusions Correlation Coefficient test, p < 0.05). Groundwater in the SRF generally The SRF appears to be an open meets drinking water standards