Agricultural activities in general and dairy operations in particular, have been identified as a potentially significant source of nitrate contamination in groundwater. The late 1990s was a period of rapid growth for New Mexico’s dairy industry. The New Mexico State University Cooperative Extension Service reports that the industry grew from 105 producers and 80,000 cows statewide in 1990 to 175 producers and 310,000 cows in 2003, to 145 producers and 323,000 cows in 2015. New Mexico now ranks ninth in the nation in milk production by volume, fifth in the nation for cheese production, and has the largest number of cows per herd in the nation. Hydrologically, these dairy systems represent a complex conglomeration of multiple potential point and nonpoint sources for nutrient and salt leaching to groundwater. The primary groundwater contaminant at dairies is nitrate resulting from disposal of solid and liquid wastes from the feedlots and dairy barns, which is present in the form of organic nitrogen in dairy wastewater. Wastewater that moves downward through the vadose (unsaturated) zone usually encounters conditions that allow the conversion of organic nitrogen to nitrate, a common contaminant in groundwater. Nitrate is the contaminant of primary concern at dairies because the groundwater standard of 10 mg/l for nitrate is based on human health impacts. This study provides descriptive history of each dairy’s wastewater storage and disposal practices, irrigated cropland acreages and irrigation practices over time, and herd size over time. Groundwater samples collected from each dairy at least semiannually were analyzed. The groundwater quality data for nitrate was entered into Excel to perform time-series graphical analyses. The results of the data analyses related to past practices and herd size showed that changes in herd size over time have very little impact on groundwater quality. Land application management practices do seem to have an immediate and long-lasting effect on groundwater quality for nitrate. When over application of dairy wastewater to flood irrigated fields occurs, it negatively impacts groundwater quality. The groundwater quality data downgradient of different dairies in this study shows two ways to reduce the amount of nitrate leaching from land application areas are: 1. Use of more land to control the nutrient loading; or 2. Install infrastructure to irrigate with sprinklers or center pivots to allow more precise use of nutrients. Historical groundwater quality data shows that unlined ponds allow nitrate to move directly into shallow groundwater, but HDPE liners added after the fact often have little to no effect on improving the groundwater quality for many years.
dairy, nutrient and salt leaching, groundwater, dairy wastewater
McDuffie, Nancy J.. "How Do Dairy Feedlot Size and Land Use Practices Affect Groundwater Quality Over Time? A Preliminary Study in New Mexico." (2021). https://digitalrepository.unm.edu/wr_sp/192