Towards understanding how geographic, hydrologic, and chemical processes interact to produce trends in groundwater quality

The purpose of this study is to develop methods and guidelines to help understand how geographic (land use and resource development), hydrologic (directions and rates of groundwater flow), and chemical processes (reaction rates) interact to explain historical changes in the distribution of natural and anthropogenic constituents in and across major aquifer systems and how these factors might affect groundwater quality in the coming decades. This study will include contribute to the understanding of how geologic heterogeneity and data/model uncertainty affect the quality of predictions made using large-scale groundwater models. An ancillary purpose is to make recommendations for sampling USGS National Water-Quality Assessment Program water-quality networks to enhance the detection and understanding of incipient groundwater quality trends. This study is in the early stages of development. Although the study encompasses work at multiple sites, this presentation will focus on an effort in the Salt Lake Valley, Utah. Groundwater quality is spatially variable in this basin-fill aquifer, primarily as a result of rock-water interaction and variations in recharge water quality. Recharge water quality is influenced by human activities (such as the use of de-icing chemicals) that tend to contribute water with relatively high dissolved solids and by natural processes (such as the infiltration of meteoric water from adjacent mountains) that tend to contribute water with relatively low dissolved solids. Human activities and natural processes are not stationary, and changes in water-quality distribution over time are expected; documented changes in groundwater quality include local increases in nitrate, sulfate, chloride, and total dissolved solids. These changes affect the public-water supply that is pumped from the deeper part of the basin-fill aquifer and should be considered in the future management of that supply. An existing groundwater flow model was recalibrated using more than 100 tritium samples. Flow-weighted travel times were calculated by backward-tracking particles from the simulated vertical position of the sampled well screens. Advective travel times calculated in this way were subtracted from the date of sampling, giving the simulated date of recharge. The atmospheric tritium value on this date was decayed, using the advective travel time, to calculate simulated equivalents for comparison to measured tritium concentrations. Future work includes testing a variety of approaches to account for some of the mixing, dilution, and transformation that occurs; assessing the predictive ability of the model by comparison of simulated historical trends with historical data for selected constituents; assessing the possible effect on future groundwater quality from processes, rates, and feedback loops arising from climate change and increased development; and assessing prediction uncertainty using Monte Carlo simulations.