Evolution of Ground Water Quality and Source Tracking of Nitrate Contamination in the Seymour Aquifer of Texas
Authors: Payblas Caitlin , 2018 Class of 
Published: June 2019

Introduction: Nitrate contamination of groundwater in the Seymour Aquifer is a well-known issue that has been documented since the 1960's. Concentrations as high as 35 ppm NO3-N have been reported, which is a startling 3.5 times the EPA allowable standard for drinking water. While most water from the Seymour Aquifer is used for agricultural irrigation, a portion is still used for domestic purposes and therefore poses a risk to human health. While this problem has been recognized, the specific source of this contamination remains unknown.

Research Approach: My research combined various analytical and geospatial technologies in order to 1) assess the evolution of groundwater in the Seymour Aquifer since before and after the 1960's, and 2) to determine the source of the high concentrations of nitrate in domestic wells situated on the aquifer. Readily available groundwater quality data from the Texas Water Development Board was used in conjunction with geospatial analysis and chemical analysis to identify changes in the aquifer's water quality over time. In addition to this data, fourteen well sites were sampled in the Spring and Fall of 2017 from selected domestic and irrigation wells situated atop the aquifer to provide an idea of present nitrate concentrations. Three potential sources of nitrate within the aquifer were considered in this study—the geological makeup of the aquifer, the contribution of nitrate from sewage and agricultural fertilizers, and the historical land use change of the area above the aquifer.

Stiff diagrams are a graphical representation of major cation and anion concentrations within water samples. Well concentrations over the course of the past six decades of six major cation and anions typically present in groundwater were averaged and graphed to determine which ion closely matched the metamorphosis of the high levels of nitrate in this portion of the Seymour Formation. This allows one to safely assume that nitrate will behave similarly to that of chloride, and chloride can be used to predict the behavior of nitrate where nitrate data is scant.

Spatial Analysis: An Empirical Bayesian kriging (EBK) analysis was performed via ArcMap in order to interpolate unknown values of Cl-and NO3- throughout the aquifer per known concentrations of well sites in order to map and trace the development of these two ions before 1960 (pre-heavy fertilizer use) and after.

*Note: It is supposed, but not confirmed, that chloride concentrations may have dropped significantly due to the implementation of more efficient irrigation systems that decreased the flushing of salts from soils into groundwater.

Geospatial analyst tools were utilized in order to predict the concentrations at the locations of the 14 sampled well sites for each of the mapped decades. The graph on the right plots the average forecasted outcomes of chloride and nitrate and indicates the correlation between these two ions in the aquifer. 

Isotope analysis: Elements found within chemical compounds exhibit certain stable isotopes, or an isotopic signature, that vary according to the source of the compound itself. The percentage of these unique isotopes within a given analyzed sample can be plotted against a known range of source values in order to determine the origin of the compound. Nitrogen and Oxygen in the compound nitrate (NO3-) have several known possible sources and nitrate contaminants in groundwater in particular could originate from decaying organic matter, soils, fertilizers, or sewage (or sometimes, a combination). Nine samples were taken at various wells across the Seymour Aquifer and were shipped to UC Davis for isotopic analysis.

Conclusion: After mapping well site data provided by the TWDB, chloride and nitrate activity appear to be driven by the northern and southeastern portions of the aquifer. Thanks to isotopic and geostatistical analysis, it is valid to assume that the high concentrations of nitrate could possibly be driven both by the heavy agriculture of the area as well as by the mesquite tree roots left in the ground after the trees themselves were cleared for agriculture before the 1960’s. Mesquite contain deep reaching root systems that support nitrogen-fixing bacteria in its root nodules (and as a result, maintain pockets of nitrogen that extend deep into the earth). As indicated by map data, it is apparent that nitrate levels were still high even before the use of fertilizer in the area, lending support for this rationale. Of the nine samples collected for stable isotopic analysis, it was determined that all of the samples were sourced from either animal waste, soil, and/or septic waste.

Additional study needs to be conducted in order to elaborate upon these findings. For example, isotopic data also indicated that sewage effluent is a contributing factor of nitrate, so further analysis into this discovery must be considered as well. Further research will also include increasing the sample size to provide a clearer view of present aquifer contamination, as well as conducting more detailed isotopic analysis methods to differentiate between origins of NO3-N as soil N and sewage N.