Groundwater-Surface Water Modeling to Support Instream Flow and Algae TMDL Management in the Ventura River Watershed
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The Ventura River was identified as one of five priority stream systems for flow management efforts in the 2014 California Water Action Plan (WAP).

Per the WAP, the State Water Resources Control Board (SWRCB) and California Department of Fish and Wildlife (CDFW) are identifying potential actions to enhance and establish instream flow for anadromous fish in these priority streams. Furthermore, in 2012, the Los Angeles Regional Water Quality Control Board (LARWQCB) adopted a Total Maximum Daily Load (TMDL) for algae, eutrophic conditions, and nutrients in the watershed, that developed allocations for stormwater, treated wastewater, agriculture, ranching, and onsite wastewater treatment systems (OWTS). Many of these nutrient sources enter the groundwater before ultimately re-emerging as surface water in the Ventura River. At the time of the TMDL development a detailed source assessment for discharges of nutrients to surface water via groundwater was not possible.

These instream flow and water quality management efforts have provided a unique opportunity to cooperatively develop credible, transparent, and scientifically sound models that include groundwater-surface water (GW-SW) interactions and transport of nutrients from the land surface to groundwater and back to surface water. Geosyntec Consultants is working with the SWRCB and LARWQCB in leading and managing the model development together with major partner Daniel B. Stephens and Associates and leading academic advisors.

The presentation will summarize the progress in developing the models, including; • the selection of the USGS Groundwater Surface-Water Flow (GSFLOW) modeling platform, • linkage to an MT3D-USGS nutrient transport model, • the data and information required to develop the models and the efforts to obtain such data, • an overview of hydrologic calibration, • use of results of a forensic study to identify and prioritize regions within the watershed with high-risk of nutrient sources from OWTS to inform the modeling and the use of the model to potentially refine and improve the interpretation of forensic results, • challenges posed by the 2017/2018 Thomas Fire, • benefits of combining modeling efforts for multiple purposes (i.e., instream flow and TMDL efforts), • use of the models to assess effects of potential future changes (e.g., climate change, population change), • stakeholder engagement, and • future efforts to train and transfer the model to SWRCB, CDFW, and LARWQCB staff.

These tools and concepts are applicable to agencies and regulators who face challenges in implementing TMDLs in watersheds where diffuse nutrient (or other pollutant) sources are transported through groundwater and/or where maintenance of instream flows requires careful management of groundwater. In particular, the benefit of developing such sophisticated models will be relevant to agencies operating within groundwater basins subject to the Sustainable Groundwater Management Act (SGMA).

Consistent with the conference theme, the presentation will illustrate application of the latest science and modeling techniques to refine TMDL requirements and optimize groundwater basin management to more efficiently and effectively meet the human and ecological needs of watersheds. The authors will engage the audience through a question and answer period. The authors will discuss lessons learned and draw out bigger picture themes to consider for potential future applications to watersheds statewide.

Publication Summary

  • Geosyntec Authors: Al Preston, Brandon Steets
  • All Authors: Al Preston, Brandon Steets
  • Title: 2019 CASQA Annual Conference
  • Event or Publication: Event
  • Practice Areas: Groundwater Assessment and Remediation
  • Citation: Geosyntec will make substantial technical contributions at the California Stormwater Quality Association (CASQA) Annual Conference at the Monterey Conference Center in Monterey, California on October 7-9, 2019.
  • Date: October 7-9, 2019
  • Location: Monterey, California
  • Publication Type: Platform Presentation