Simulating Water‐Quality Trends in Public‐Supply Wells in Transient Flow Systems

Abstract

Models need not be complex to be useful. An existing groundwater-flow model of Salt Lake Valley, Utah, was adapted for use with convolution-based advective particle tracking to explain broad spatial trends in dissolved solids. This model supports the hypothesis that water produced from wells is increasingly younger with higher proportions of surface sources as pumping changes in the basin over time. At individual wells, however, predicting specific water-quality changes remains challenging. The influence of pumping-induced transient groundwater flow on changes in mean age and source areas is significant. Mean age and source areas were mapped across the model domain to extend the results from observation wells to the entire aquifer to see where changes in concentrations of dissolved solids are expected to occur. The timing of these changes depends on accurate estimates of groundwater velocity. Calibration to tritium concentrations was used to estimate effective porosity and improve correlation between source area changes, age changes, and measured dissolved solids trends. Uncertainty in the model is due in part to spatial and temporal variations in tracer inputs, estimated tracer transport parameters, and in pumping stresses at sampling points. For tracers such as tritium, the presence of two-limbed input curves can be problematic because a single concentration can be associated with multiple disparate travel times. These shortcomings can be ameliorated by adding hydrologic and geologic detail to the model and by adding additional calibration data. However, the Salt Lake Valley model is useful even without such small-scale detail.

The effects of numerical-model complexity and observation type on estimated porosity values

“…In this paper, model results are compared by using simple and complex models calibrated with either tritium concentrations or tritium concentrations plus interpreted tritium/helium apparent ages. Tritium concentrations above background levels have persisted in the Salt Lake Valley aquifer, Utah (USA) for decades, a time period over which changes in total dissolved solids concentrations also have been observed (Starn et al 2014); therefore, one would expect that calibration to tritium data should serve to decrease uncertainty in model predictions of changes in water quality.…”

“…Only one parameter, effective transport porosity, is required to convert a groundwater-flow model for advective transport simulation; however, prior estimates of effective porosity vary widely and often are not available for most sites (Konikow 2011). Effective transport porosity values have been determined through calibration to tritium/ helium apparent ages (Reilly et al 1994;Sheets et al 1998;Murphy et al 2011), and, less often, directly to tritium concentrations (Herweijer et al 1985;Engesgaard et al 1996;Starn et al 2014).…”

“…Large and long-term changes in recharge or discharge relative to groundwater residence times can lead to significant long-term changes in water quality (Reilly and Pollock 1996). Relatively few studies have considered short-term (annual) transient flow in analyzing waterquality trends (Furlong et al 2011;Starn et al 2014). In transient flow, age distributions are time dependent (Manning et al 2012).…”

“…In transient flow, age distributions are time dependent (Manning et al 2012). Time-dependent source areas (Rock and Kupfersberger 2002;Masterson et al 2004;Starn and Brown 2007;Starn et al 2014) can contribute different solutes to groundwater, resulting in complex mixtures of water at pumped wells.…”

Using environmental tracers to determine the relative importance of travel times in the unsaturated and saturated zones for the delay of nitrate reduction measures

“…Baillieux et al, 2014). Thus, in most cases, travel times of groundwater, as well as these additional processes affecting nitrate are determined by means of hydrogeological modeling (2D or 3D) and particle tracking, environmental tracers, or a combination of both (Alikhani et al, 2016;Green et al, 2008;Jeffrey Starn et al, 2014;Kaown et al, 2009;MacDonald et al, 2003;McMahon et al, 2006;Osenbrück et al, 2006;Visser et al, 2009;Wang et al, 2012;Zoellmann et al, 2001). Environmental tracers are a complementary tool to hydrogeological models and time series of nitrate or other tracers for determining travel time distributions (or age distributions) of groundwater and dilution (Turnadge and Smerdon, 2014;Visser et al, 2009 Ar,14 C,or 4 He are commonly interpreted with lumped-parameter models (LPM) to obtain travel time distributions: The concentrations of the tracers in a well are calculated as the convolution integral of the input history of the tracers and a relatively simple shape of the age distribution (Zuber and Maloszewski, 2001).…”