Irrigation with Sulfate-Enriched Mine Effluent Does Not Affect Alfalfa Quality

Water scarcity in the Western United States has been an on-going issue for irrigated agriculture. Exploration of additional and potentially viable sources of water for irrigation (such as reuse of agricultural, municipal and industrial waste waters) is essential for the longevity of the western agricultural industry. One potential source of wastewater is a sulfate-rich water from a mine-shaft near Superior, Arizona. This water is pumped from over 1,500 meters below the surface to remove it from the mine before copper mining operations can commence. This mine effluent water, naturally abundant in sulfate, is treated to remove heavy metals and is further enriched with additional sulfate via inputs from the treatment process raising its concentration by 3.6 meq/L higher than that of the Central Arizona Project (CAP) water. The treated water is conveyed via a gravity fed pipeline to an irrigation district where it is blended with the existing CAP water to achieve an average blended SO4 content of 9.1 meq/L in one portion of the irrigation district (C-System), and 5.7 meq/L in a smaller portion of the irrigation district (B-System). These two areas served as the treatments that were compared against neighboring irrigation systems that did not receive the treated mine water (CAP water = 5.5 meq/L . SO4) Reuse of this high-sulfate water for irrigation of crops would enable both water savings and mitigate environmental impacts. In this study, blended water is used for irrigation of approximately 600 hectares of alfalfa (Medicago sativa ) within the New Magma Irrigation District in an effort to determine the viability of reclaimed mine water as a supplemental source of irrigation water. Soil and plant samples were collected and analyzed for this study over the span of one year (June 2011 to March 2012) for both nutrient status (soil and plant tissue) as well as forage quality (plant tissue). In addition to potentially impacting the protein content and quality in the forage, elevated plant tissue sulfur concentrations can have adverse impacts on animals that consume it if fed exclusively over the long term. The purpose of this study was to assess the impact on forage quality of irrigating alfalfa with treated mine effluent that was higher in sulfate than what had been historically supplied. Soil samples were collected at the beginning and ending of the study, and plant tissue samples were collected during four separate sample events over the course of the year. The data indicate that irrigation with treated mine water high in sulfate had little bearing on the quality of the forage. Data collected from the untreated, control plots indicated that plant tissue sulfur concentrations were high (2930-6240 mg/kg) presumably because of historic and current management practices of lowering pH through the addition of sulfuric acid to neutralize the native calcite. Forage samples collected from treated plots contained 3195 -6002 mg/kg plant tissue total sulfur, but values were not significantly different from the untreated control plots. Other forage quality parameters, specifically, crude protein, neutral detergent fiber and acid detergent fiber, averaged 29.6, 27.8 and 23.9 percent, respectively, for the fields that received blended irrigation water and 29.1, 28.1 and 24.2 for the unblended, control fields. There were no statistical differences among these parameters as well as between irrigation water sources. Additionally, a seasonal effect on forage quality was noted where samples collected during hotter periods of the year (late summer) tended to result in poorer forage quality, i.e., lower protein content and higher fiber content. Sulfate-rich treated mine water appears to be a suitable supplemental source of water for irrigation of alfalfa over a year duration. Its use as a supplemental source will reduce the pressures placed on existing water supplies and will inevitably result in water savings over the long term. However, when native soil and plant tissue concentrations are already elevated as is the case here, close attention should be paid to forage quality changes over the longer term, particularly increases in tissue S.