Spatially Explicit Mapping of Hydrologic Residence Time Paired with Water Quality Measurements to Determine the Effects of the Emergency Drought Barrier

Science Center Objects

The purpose of this study is to assess the distribution of water residence times across the central Delta using rapid water isotope measurements (δ2H, δ18O) made with a boat-based flow-through instrument.

nitrate data

Map of study area showing example spatially explicit nitrate data from a ~5-hour mapping campaign from Sac River at Rio Vista, CA, through the study area, returning to Rio Vista Aug, 2015. Variation in nitrate concentration gradients are seen throughout the transect. Lower nitrate concentrations are seen (dark green) in Franks Tract.

(Public domain.)

Installation of the emergency drought barrier (EDB) in False River is designed to limit salinity penetration into the central Delta and to the water intakes in the south Delta. The EDB alters the tidal exchange in Frank's Tract, resulting in a cascade of hydrologic and environmental changes in the region. Changes in dispersive transport are expected to move brackish water landward and move freshwater seaward restricting salt flux from west to east in the Delta, affecting foodweb dynamics. Resultant changes in water velocities and tidal exchange in Old River and Fisherman's Cut have already been documented, and are in line with projected changes in water circulation and residence time in Frank's Tract. These hydrologic changes have potential ecological consequences for native fish species in the central Delta because of effects on water residence time, nutrient gradients, phytoplankton productivity, and water quality.

The purpose of this study is to assess the distribution of water residence times across the central Delta using rapid water isotope measurements (δ2H, δ18O) made with a boat-based flow-through instrument. By pairing this instrument with a suite of high frequency water quality sensors that continuously measure nitrate, chlorophyll, dissolved organic matter fluorescence, and ancillary water quality parameters (temperature, pH, specific conductance, dissolved oxygen, turbidity), linkages can be made between physical and biogeochemical processes. This approach is particularly relevant to the tidally complex area surrounding the EDB, because measurements made over relatively short time scale (hours) over a broad spatial scale (tens of miles) are needed to adequately resolve changes associated with tidal forcing (flood tide, ebb tide, slack tide). Data generated by this approach provides information about areas in the Delta that are not being monitored by other programs, and can be directly linked to existing fixed monitoring stations, discrete sampling programs, and remote sensing efforts combining hydrodynamic modeling with investigations of copepod distributions to estimate the flux of copepods into the low salinity zone (LSZ) with and without the barrier. This effort will combine ongoing model development with previous field work and limited additional field work to be conducted in fall of 2015.

This boat-based measurement approach provides spatially-explicit data representing a temporal "snap shot" of conditions that will augment existing monitoring networks and monitoring programs in the study region. The study will also provide opportunities for coordination and collaboration with other data collection efforts. The specific timing and route the boat takes can be designed to enhance other efforts including fixed stations, discrete sample collection, and remote sensing. In this study, we are collaborating with other research groups (USGS, SFSU RTC, Stanford, SFEI, UCSC) to collect and analyze discrete samples for phytoplankton and zooplankton enumeration, algal pigments, primary productivity, nitrate and ammonium uptake rates, stable isotope compositions, microcystis, and water quality data for ground truthing remote sensing models. This project addresses the ecosystem and climate strategic directions in the USGS Science Plan, for example 'Understanding Ecosystems and Predicting Ecosystem Change' as well as 'Climate Variability and Change' (U.S. Geological Survey, 2007). This is accomplished by improving our understanding of effects and changes in water quality in the Sacramento-San Joaquin Delta due to extreme drought conditions and installation/removal of the flow barrier in the False River. The spatially explicit water quality data and calculated water residence times, will lead to improved hydrodynamic and water quality models that will aid in planning potential future responses to drought related issues in the Delta.

The Objectives of this project are:

  1. Assess feasibility and effectiveness of using surface water mapping of water isotopes and water quality parameters, coupled with discrete samples for validation to characterize water quality for a range of parameters in Franks Tract.
  2. Evaluate spatial heterogeneity, and assess the need for and/or benefit of using this approach as a component of routine monitoring in Franks Tract and other regions of the Delta.
  3. Characterize water quality in Franks Tract via high-speed mapping in Franks Tract under different tidal regimes while the EDB is in place and after the EDB is removed:
    1. One transect covering ebb to flood transition after EDB installed.
    2. One transect covering flood to ebb transition after EDB installed.
    3. One transect covering ebb to flood transition after EDB removed.
    4. One transect covering flood to ebb transition after EDB removed.
  4. Interpretation of spatial data with data from continuous moored sensors at fixed stations to develop a system-scale biogeochemical picture and support ongoing remote sensing (Landsat-8) efforts.