Geological and mineralogical associations have been used to determine baseline water quality variations in aquifers for well over a century. Geological factors can be misidentified and misunderstood in establishing water quality relationships in aquifers however, especially in groundwater basins undergoing rapid development. The problem is particularly acute when time series and spatial changes in aquifer water quality occur. Time series and spatial changes in groundwater quality may be related to physical changes in aquifer dynamics that are either natural or anthropogenic.

The effectiveness of a generic, but widely accepted model in explaining water quality in one aquifer may be a weak analog for explaining water quality variations in another, apparently similar aquifer. Too often interpreters grasp at the most simple or apparent explanation for a process based on anecdotal evidence; e.g., "agriculture creates the elevated salinity," "industrial activity is the source of the elevated trace elements in the aquifer." A possible misinterpretation might arise merely on account of the fact that there is substantial agricultural or industrial activity in the groundwater basin. Reasons for misidentifying processes related to water quality is often due to a poor understanding of the geologic history of the groundwater basin, mischaracterization and misidentification of flow processes/hydraulics, and incomplete knowledge of the primary and secondary mineralogical assemblages in the aquifer. Insufficient training in evaluating the geological, hydraulic, and water/rock/soil interactions is another factor that comes into play.

The purpose of this presentation is to cover two of our studies where earlier anecdotal models of water quality did not account for the actual water quality processes in the aquifers. It is hoped that these studies will serve as examples of flagrant risks associated with accepting anecdotal models to explain water quality variations in aquifers.

In the first case study, the quality of groundwater in the Rio Grande aquifer is evaluated due to its progressive deterioration due to salinization. Salinization occurs spatially as groundwater and surface-water quality deteriorates progressively downstream in the irrigated regions of the Rio Grande Basin. Historically it had been thought that problems of groundwater salinization are a result of irrigation practices along the Rio Grande. In this model, the irrigation water becomes progressively more saline through evaporative concentration. Concentrated recharge moves down gradient, or discharges into the Rio Grande, and is then re-applied to crops further downstream. In our studies we found that a much more important source of salinity is due to leaching from a previously unidentified buried phreatic playa that formed about 0.75 million years ago. The testing of nested observation wells for halides and chlorine isotopes leaves little doubt about the primary geologic source of salinity.

In the second case study, high selenium and high arsenic concentrations were detected in San Diego Creek Watershed in the Tustin/Irvine/Newport Beach areas. Prior to 1900, the central part of the watershed was marshland, and the edges of the watershed were used for grazing sheep and cattle. After 1900, cattle and sheep grazing were displaced by irrigated agriculture. Drainage ditches and channels were constructed to drain the marshes to "reclaim" the land for use for agriculture. Today, the drainage ditches and channels still exist in the watershed, which is undergoing massive urban growth. Shallow groundwater discharges into these channels and ditches, and the surface water eventually flows into Upper Newport Bay, a thriving ecological habitat.

We found that the highest concentrations of selenium and arsenic in shallow groundwater in the watershed coincide with the marshland areas that were displaced by agriculture. Concentrations of Se in groundwater in the former marsh areas often exceed 25 µg/L, and are as high as 478 µg/L. In areas where marshes were absent, concentrations of Se in groundwater are usually less than 25 µg/L. We found that the elevated concentrations of Se in groundwater where the marshes once existed are natural but a direct result of the destruction of the marsh. Today, oxygenated groundwaters flow through the soils where the marshes once existed, remobilizing Se as selenate, a water soluble and oxidized form of Se that is highly mobile in oxidizing aquifer systems. Arsenic is elevated (up to 175 ug/L) in areas where evapoconcentration occurred in flat lying areas prior to draining of the marsh. Arsenic is leached into shallow groundwater today. Neither of these trace elements is sourced directly from an industrial or agricultural process, but both trace elements are released in shallow groundwater due to land use changes that resulted in changing redox conditions and changing surface-water/groundwater dynamics.

Barry J. Hibbs, PhD, Professor, Department of Geosciences and Environment, California State University, Los Angeles. Barry Hibbs has taught at California State University, Los Angeles, since 1997, where he instructs courses in groundwater hydrology, water quality, watershed analysis, field methods, and groundwater management. Dr. Hibbs received a B.S. in Geology from Arizona State University, an M.S. in Hydrogeology from the University of Nebraska Lincoln, and a Ph.D. in Hydrogeology from the University of Texas at Austin. He is a Fellow of the Geological Society of America and a recipient of Cal-State LA's Outstanding Professor Award (2014). Dr. Hibbs' research focuses on the hydrogeology of arid basins in the Southwestern United States and Northern Mexico; stream/aquifer interactions; isotope hydrology; and trace element hydrochemistry.