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Investigation and Modeling of Nitrate Loading from Municipal Treatment Wastewater Lagoons and Mitigation Alternatives for Groundwater

Client: Municipal Client, California

The Challenge

The municipality and several golf courses within its boundary depend on groundwater pumped from a regional aquifer. The municipality also operates a wastewater treatment plant that was thought to be a source of a regulated contaminant, nitrate, which had been found in wells. A portion of the treated waste water was recycled to irrigate publicly-owned golf courses. However, the majority infiltrated the ground beneath storage lagoons. The objectives were to characterize the extent and degree of groundwater contamination, to identify sources, to quantify contributions from the public waste water treatment plant, and to evaluate by computer modeling the potential benefits of alternative mitigation efforts. The groundwater flow and transport model developed for the project is regional in scope, extending across the breadth of the regional aquifer and incorporating two regional aquifer layers, each of 500 feet thickness.

    Significant challenges to the project included:
  • Catalog, interpret, and map chemical analyses of groundwater samples from the region.
  • Estimate historic treated water infiltration rates and related nitrate loading from 15 years of treatment plant effluent and recycling data.
  • Consult with golf course operators to derive annual and seasonal irrigation and fertilization requirements.
  • Consult with University of California, Riverside, and researchers to evaluate nitrate demand and uptake by Bermuda turf grass under applicable local conditions.
  • Develop a conceptual model of nitrate loading from regional sources and the resulting nitrate plume in groundwater.
  • Develop a three-dimensional groundwater flow and transport model capable of replicating the existing plume from estimated historic source loading.
  • Project future outcomes for nitrate plume dispersion based on various mitigation scenarios.

The Interdisciplinary Approach

The project team worked with the office of the municipal manager and the treatment plant operator to catalog and evaluate treatment plant flow and sampling data. Well information and groundwater sample data were obtained from the two water districts that supplied the region with water. Golf course irrigation and fertilization requirements were evaluated in consultation with the manager of the publicly owned golf courses. These were compared to similar parameters used in research studies nitrate uptake by turf grass being done at the University of California. These data fed the development of a conceptual model of nitrate loading rates and the potential benefits of reusing treated water as irrigation water. Development of the groundwater flow and transport model required an understanding of the hydrogeologic characteristics of the regional aquifer as well as groundwater abstraction by water supply wells.

The Sustainable Result

    The study provided the municipality with:
  • A quantitative understanding of the past and potential future contributions of treated waste water effluent to nitrate contamination of groundwater.
  • A conceptual model of nitrate plume sourcing and the disposition of the plume relative to potable water source wells.
  • Alternative methods of mitigation that involved increased recycling treated waste water for irrigation and abstraction of nitrate contaminated groundwater during periods of peak\ irrigation demand. Computer simulations of three-dimensional nitrate plume dispersion based on future scenarios including varying degrees of treated water recycling.
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    Project Highlights:
  • Current and past distributions of nitrate in groundwater - evaluated from sample data.
  • Nitrate loading rates (past and future) – analyzed from plant flow-through and subsurface monitoring data.
  • 3D Flow and transport model used to evaluate mitigation alternatives, including expanded reuse of treated effluent for golf course irrigation. Irrigation scenarios based on usage data provided by course managers and nitrate uptake studies by U.C. Riverside.