Formatted Title

Differentiating Sources of High-Salinity Groundwater Contamination Using Geochemical Graphical Tools

Background/Objectives

Groundwater salinity, as measured by total dissolved solids (TDS), is a concern in the Central Valley of California (CV) due to its effects on useability for irrigation or domestic consumption. Groundwater TDS concentrations in the CV are highly variable, and recent studies indicate that TDS concentrations have increased due to declining water levels and agricultural practices. As groundwater sustainability becomes an increasing focus in California’s drought-stressed and agricultural regions, groundwater users may benefit from a more in-depth approach to evaluating groundwater salinity that is able to differentiate TDS sources.

Piper plots and Stiff diagrams, two geochemical graphical tools, were used to complete a groundwater site assessment at a former processing facility in the San Joaquin Valley of Central California. The objective of the site assessment was to delineate the horizontal and vertical extent of groundwater impacted by historic site operations. High-salinity wastewater generated during the facility’s production process was discharged to a series of unlined wastewater ponds, affecting groundwater quality immediately beneath the facility in two complex geological, water-bearing zones. Over time, wastewater impacts on groundwater became less concentrated due to artificial and natural mixing of groundwater. This case study highlights the value in using graphical tools to visualize the relative abundance of different ions in groundwater samples that constitute TDS, thereby differentiating between various influences on groundwater quality.

Approach/Activities

Distinguishing the relative contribution of wastewater compared to other regional factors required consideration of not only the TDS of the groundwater but also the composition and distribution of the ionic components included within the TDS. Data collection and field-testing methods were developed to estimate the relative abundance of different ions in groundwater samples. The field-testing program included collecting depth-discrete vertical profile groundwater samples using Hydropunch™ sampling methods. Field testing was completed using chemistry equipment designed to estimate concentrations of major anions and cations. Groundwater analytical results were then evaluated using Piper plots and Stiff diagrams to characterize the effects of wastewater on groundwater and to make inferences on the sources.

Results/Lessons Learned

Results of the site assessment determined that wastewater impacts on groundwater were co-mingled with naturally saline groundwater; however, the chemical signature of wastewater impacts was distinct. The unique chemical signature of the wastewater, as displayed using Piper plots and Stiff diagrams, was used to define the lateral and vertical extent of wastewater impacts on groundwater. Facilities can use these methods to better calibrate conceptual site models, monitor fate and transport of groundwater contaminants over time, and more precisely target remediation efforts to a well-defined extent.