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This work presents a simplified approach for estimating ionic concentrations from specific electrical conductance (EC) data in the San Francisco Estuary. Monitoring the EC of water through electrodes is simple and inexpensive. As a result, a wealth of high-resolution time-series data is available to indirectly estimate salinity concentrations and, by extension, seawater intrusion throughout the study domain. However, scientists and managers are also interested in quantifying ionic (e.g., bromide, chloride) and total dissolved solids (TDS) concentrations to meet water-quality regulations, protect beneficial uses, support environmental analyses, and track source-water dominance. These constituent concentrations, reported with lower spatial and temporal resolution than EC, are typically measured in the laboratory from discrete (grab) water samples. We divided the study domain into four unique regions to estimate concentrations of major ions and TDS as mathematical functions of measured or model-simulated EC. Salinity relationships in three of the four regions—regions that represent Sacramento-San Joaquin Delta (Delta) inflow and seawater-dominated boundaries—reflect ionic make-ups that are either independent of or weakly dependent on season and hydrologic condition, and are highly correlated with EC. The fourth region—represented by the interior Delta—exhibits salinity characteristics associated with complex-boundary source-water mixing that varies by season and hydrologic condition. We introduce a novel method to estimate ionic and dissolved solids concentrations within this fourth region, given month, water year type, and (optionally) X2 isohaline position, which allows for more accurate EC-based estimates than previously available. The resulting approach, while not a substitute for hydrodynamic modeling, can provide useful information under constrained schedules and budgets.

The deviation of specific electrical conductance (EC) from conservative mixing behavior is well-established in the scientific literature. This principle is based on the observation that, as salt concentration in a water sample increases, the mobility of individual ions in the sample decreases, and thus their ability to conduct electricity decreases. Despite this fact, some commonly used models for salinity transport in the San Francisco Estuary (estuary) utilize EC as a primary simulation constituent, treating it as a conservative quantity. Such a modeling approach has likely been followed to exploit the wide availability of EC data for model calibration and validation, and to obviate the need to translate between EC and salinity in a domain characterized by multiple source waters with varying ionic make-ups. Arguably, this approach provides a reasonable trade-off between data translation error and model simulation error. In this paper, we critically evaluate this approach, employing an extensive salinity data set that includes measurements of EC and major ion concentrations in the estuary. We demonstrate and quantify EC deviation from steady-state, conservative mixing behavior; review the conservative mixing behavior of three bulk salinity measures (practical salinity, ionic strength, and limiting equivalent conductance); and evaluate their source-dependent correlations with EC in the estuary. We find limiting equivalent conductance—a value that assumes uninhibited mobility among individual ions in a water sample—to be an attractive alternative for salinity transport in the estuary. In addition to being a conservative quantity, it is consistently correlated with EC in the estuary’s dominant source waters, and thus addresses concerns related to data-translation error. We conclude this paper discussing pros and cons of adopting various salinity-transport model constituents.

Accurate estimates of freshwater flow to the San Francisco Estuary are important in successfully regulating this water body, in protecting its beneficial uses, and in accurately modeling its hydrodynamic and water-quality transport regime. For regulatory purposes, freshwater flow to the estuary is not directly measured; rather, it is estimated from a daily balance of upstream Delta inflows, exports, and in-Delta water use termed the net Delta outflow index (NDOI). Field research in the 1960s indicated that NDOI estimates are biased low in summer–fall and biased high in winter–spring as a result of conflating Delta island evapotranspiration estimates with the sum of ungauged hydrologic interactions between channels and islands referred to as net channel depletions. In this work, we employed a 50-year observed salinity record along with gauged tidal flows and an ensemble of five empirical flow-salinity (X2) models to test whether a seasonal bias in Delta outflow estimates could be inferred. We accomplished this objective by conducting statistical analyses and evaluating whether model skill could be improved through seasonal NDOI flow adjustments. Assuming that model residuals are associated with channel depletion uncertainty, our findings corroborate the 1960s research and suggest that channel depletions are biased low in winter months (i.e., NDOI is biased high) and biased high in late summer and early fall months (i.e., NDOI is biased low). The magnitude of seasonal bias, which can reach 1,000 cfs, is a small percentage of typical winter outflow but represents a significant percentage of typical summer outflow. Our findings were derived from five independently developed models, and are consistent with the physical understanding of water exchanges on the islands. This work provides motivation for improved characterization of these exchanges to improve Delta outflow estimates, particularly during drought periods when water supplies are scarce and must be carefully managed.

This work surveys the performance of several empirical models, all recalibrated to a common data set, that were developed over the past 25 years to relate freshwater flow and salinity in the San Francisco Estuary (estuary). The estuary’s salinity regime—broadly regulated to meet urban, agricultural, and ecosystem beneficial uses—is managed in spring and certain fall months to meet ecosystem objectives by controlling the 2 parts per thousand bottom salinity isohaline position (referred to as X2). We tested five empirical models for accuracy, mean, and transient behavior. We included a sixth model, employing a machine learning framework and variables other than outflow, in this survey to compare fitting skill, but did not subject it to the full suite of tests applied to the other five empirical models. Model performance was observed to vary with hydrology, year, and season, and in some cases exhibited unique limitations as a result of mathematical formulation. However, no single model formulation was found to be consistently superior across a wide range of tests and applications. One test revealed that the models performed equally well when recalibrated to a uniformly perturbed input time-series. Thus, while the models may be used to identify anomalies or seasonal biases (the latter being the subject of a companion paper), their use as inverse models to infer freshwater outflow to the estuary from salinity observations is not expected to improve upon the absolute accuracy of existing outflow estimates. This survey suggests that, for analyses that span a long hydrologic record, an ensemble approach—rather than the use of any individual model on its own—may be preferable to exploit the strengths of individual models.

We evaluated two historically important data sets to characterize the San Francisco Estuary’s salinity regime before the State of California began systematic data collection in the early 1920s. One set documents barge travel along the Sacramento and San Joaquin rivers to obtain water of adequate quality for local industry; a second set documents Delta inflow used to compute antecedent outflow. The barge travel distance reported over 2 decades (1908–1929) was well explained by flow–salinity modeling, indicating internal consistency in these measurements. However, absolute salinity intrusion estimated through the barge travel data is systematically lower than suggested by contemporaneous water-quality measurements available since 1921. Through integration of these data sets, our work showed substantial similarities between 1908–1921 and the subsequent period before construction of Shasta Dam (1922–1944). Our analysis reveals an apparent shift in the estuary’s salinity regime, with lesser salinity intrusion occurring in pre-1919 summer and fall months as a result of higher summer Delta outflow; this shift may be related to lower storage and irrigation diversions as well as a preponderance of wet years with higher summer runoff in the pre–1919 period. We found seasonal patterns of wet year salinity intrusion to be comparable over the full study period (1908–1944), indicating that the relative effect of upstream water management is minimal when flows are high, consistent with findings reported in later periods. The barge and flow data provide qualitative insights on early 20th century conditions, when limited data are available. Post–1920 hydrology and salinity data are preferable for quantitative analyses because of better documentation associated with collection and analysis, and sustained reporting over several decades. This work provides a foundation for future efforts to characterize the hydrologic and hydrodynamic changes that occurred in the system between the 1850s (i.e., natural or pre-development conditions) and the 1920s.

Chronic groundwater overdraft threatens agricultural sustainability in California's Central Valley. Diverting flood flows onto farmland for groundwater recharge offers an opportunity to help address this challenge. We studied the infiltration rate of floodwater diverted from the Kings River at a turnout upstream of the James Weir onto adjoining cropland; and calculated how much land would be necessary to capture the available floodwater, how much recharge of groundwater might be achieved, and the costs. The 1,000-acre pilot study included fields growing tomatoes, wine grapes, alfalfa and pistachios. Flood flows diverted onto vineyards infiltrated at an average rate of 2.5 inches per day under sustained flooding. At that relatively high infiltration rate, 10 acres are needed to capture one CFS of diverted flood flow. We considered these findings in the context of regional expansion. Based upon a 30-year record of Kings Basin surplus flood flows, we estimate 30,000 acres operated for on-farm flood recharge would have had the capacity to capture 80% of available flood flows and potentially offset overdraft rates in the Kings Basin. Costs of on-farm flood capture for this study were estimated at $36 per acre-foot, less than the cost for surface water storage and dedicated recharge basins.

A study of the effects of colloids on the transport of two strongly sorbing solutes, nickel and phenanthrene, through sand-packed columns revealed that only some demonstrated enhancement of contaminant transport. Two types of experiments were conducted to highlight the effects of colloids on contaminant transport, namely, with four porous media and two strongly sorbing contaminants. It was found that only when the aqueous chemistry conditions favor a lower deposition efficiency can sorbing colloids substantially enhance transport in a strongly sorbing porous medium.

Several former manufactured gas plant sites (MGP) are plagued with soil and groundwater contamination problems due to past process operations and residuals management practices. Remediation costs for individual MGP sites amount to tens of millions of dollars. Gas Research Institute estimates show that almost 70% of the costs incurred were due to the management of soils and sediments that have been contaminated with tar.

The hydrology of the San Francisco Bay‐Delta estuary (the Delta) has been significantly modified over the past 150 years to serve a variety of human needs for water supply and food production, albeit with adverse ecological impacts. These adverse impacts, in concert with evolving societal values, have motivated change in the estuary's water management to promote ecosystem restoration goals while continuing to support human uses. Understanding historical flow patterns, as well as the environmental functions provided by these flow patterns, is critical to restoration planning. Building upon previous work on outflow trends from the Delta to San Francisco Bay, this paper evaluates historical trends in key interior Delta flows spanning nine decades (1922–2016) and presents an attribution of these trends to various anthropogenic drivers. We reconstructed historical time series records at four key locations in the interior of the Delta; these time series represent tidally‐averaged flows that are intensively managed and are of great importance to the beneficial uses of water in the region. We derived several scenario‐based flow time series at these same four locations to assist in attributing change to a variety of drivers, including reservoir and export pumping operations, in‐Delta barrier and gate operations, and upstream water diversions. Flow changes were measured relative to 1920‐level land use and water management conditions. We find the four interior Delta locations to have distinct seasonal flow trends and, in general, unique responses to various drivers of change. Our work highlights the complex nature of historical flow changes in a highly‐managed estuarine ecosystem, and the types of modifications that would be necessary to reverse these changes. [ABSTRACT FROM AUTHOR]