Salinas Valley Seawater Intrusion

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A watershed-related issue examined by the ENVS 560/L Watershed Systems class at CSUMB.

Seawater intrusion into groundwater aquifers from 1944 through 2013. The 180-Foot (left) and 400-Foot (right) aquifers are the two main water supply aquifers within the Salinas River Groundwater Basin. Images from State of the Salinas River Groundwater Basin Report (Jan. 2015) [1]


Seawater intrusion into the Salinas Valley groundwater aquifers has advanced since it was first measured in 1944.[2] Currently, elevated salinity levels have been recorded less than 1/2 mile from the city of Salinas at the 180-Foot aquifer[3] and seawater is beginning to encroach beyond the city limits of Castroville at the 400-Foot aquifer[4]. The extent of seawater intrusion has moved farther inland due to continuing overdraft conditions for municipal and agricultural uses.


The Salinas Valley basin lies within the boundaries of Monterey County, California. Groundwater is extracted from the four major aquifers: Upper Valley, Forebay, East Side, and Pressure[5]. These aquifers create an interconnected groundwater system that supplies the bulk of the irrigation and municipal water usage in the Salinas Valley.

Resources at stake

Over 524 thousand acre-feet of groundwater was extracted during the 2014 water year. 91.55% went to agricultural use and 8.45% for urban use.[5] This high dependence on groundwater for the water needs in the Salinas Valley has led to critical overdraft conditions[6] that have resulted in abandonment of agricultural and urban supply wells due to rising salinity levels[7]. The concern over adverse effects from continuing overdraft of groundwater resources has led to the development of a statewide groundwater management plan[8] and new supplemental water supplies from the Salinas Valley Water Project.


The agricultural industry in the Salinas Valley is estimated to be worth $8.1 billion[9] and depends heavily on groundwater for almost all of its water needs[7]. Irrigation with high-salinity water damages crop yield, crop quality, and soil health for future use of the land for agriculture.[10] The municipalities that lie within the Salinas Valley watershed, such as Salinas, Castroville, Marina, and unincorporated area residents rely on the aquifers for urban use. When water salinity rises too high, it becomes unpalatable for drinking water and poses significant health threats.[11] In addition, the businesses and communities surrounding Lake Nacimiento rely on the lake for recreational use and energy production. Despite the lake's location within neighboring San Luis Obispo County, the Monterey County Water Resources Agency (MCWRA) manages it for the purpose of aquifer recharge and mitigation of saltwater intrusion.

Laws, policies, & regulations

In 1961, Nacimiento Dam was completed to supply irrigation water to the Salinas Valley agricultural community and to recharge the aquifers. Attempts to change structural and operational practices at the dam have meet severe local opposition resulting in several lawsuits[12]. A special act district, Monterey County Flood Control and Water Conservation District, was formed in 1947 for the Salinas Valley due to early concerns about seawater intrusion and flooding problems throughout the region. It later became the MCWRA in 1991, which monitors groundwater quality and levels[13] but has had little authority in regulating groundwater extraction. On September 16, 2014, Governor Jerry Brown signed into law a three-bill legislative package collectively known as the Sustainable Groundwater Management Act (SGMA). The SGMA mandates the formation of Groundwater Sustainability Agencies (GSA) and will significantly increase the role and responsibilities of local and state agencies to support sustainable groundwater management. As of March 2016, a draft plan has been developed for the GSA's key actions over the next several years that includes an outline for mitigating seawater intrusion.[14]


Along the coast of Monterey County, fresh groundwater flows from inland aquifers to meet with seawater from the ocean. The fresh groundwater flows from the Salinas Valley towards the coast where elevation and groundwater levels are lower. Due to the higher salinity of seawater, it is denser than fresh groundwater and has a higher hydraulic head. When the fresh groundwater aquifers within the Salinas Valley have a lower hydraulic head, seawater moves inland in a wedge shape under freshwater until head levels return to equilibrium. Seawater and fresh groundwater then mix along the transition zone through dispersion and diffusion, raising salinity levels for wells that tap into these areas.[15] Once seawater intrusion has taken place, the effect on drinking and irrigation water quality within the aquifer is long-lasting. Wells are typically abandoned when salinity exceeds appropriate water quality standards.[16]

Mitigation Strategies

  • The Monterey County Water Recycling Projects were designed to retard seawater intrusion and protect drinking water supplies via wastewater recycling at a combined cost of $75 million.
    • The Castroville Seawater Intrusion Project began construction in 1995 and started delivering recycled water to 12,000 acres of farmland near Castroville in 1998. By using recycled water pumped from the Monterey Regional Water Pollution Control Agency plant, growers safely irrigate their crops and reduce pumping of seawater intruded groundwater.[17]
    • The Salinas Valley Reclamation Plant treats wastewater to advanced tertiary level. The resultant recycled water meets all California State Standards for recreational and irrigation uses. The facility can produce a maximum of 91 acre-feet per day. It is the largest sewage treatment installation in the world to recycle wastewater for freshly edible food crops.[17]
  • The Salinas Valley Water Project was a $33 million project designed to provide long-term management and protection of groundwater resources by stopping seawater intrusion and providing adequate water supplies and flexibility to meet needs in 2030.[18]
    • The Nacimiento Dam Spillway Modification Component enlarged the original spillway to increase the amount of flood flow that could be controlled by raising and strengthening chute walls, anchoring channel walls. Other modifications included the installation of a Obermeyer (rubber) Gate System and strengthening the bridge pier with steel reinforced concrete.[18]
    • The Salinas River Diversion Facility was was constructed to provide treated (filtered and chlorinated) water from the Salinas River, significantly reducing the need to pump groundwater except in periods of extremely high demand, through use of pneumatically controlled diversion dam.[18]
    • Phase II of the Salinas Valley Water Project has been proposed to construct two additional water capture and diversion facilities along the Salinas River. The two water diversion points, will be located near the Soledad and south of Salinas.[19] As of July 2014, MCWRA had requested resources to conduct an Environmental Impact Report and engaged in initial funding discussions[20].
  • Several other irrigation efficiency studies have been, and are continuing, to help reduce agricultural impact by reducing the quantity of water being applied to crops.[21][22]

Scientific Tools

  • The State of the Salinas River Groundwater Basin Report [1] and Groundwater Extraction Summary Report [5] developed by the MCWRA are useful tools for understanding how groundwater is utilized in the Salinas Valley.
  • Monitoring wells at selected sites within the Salinas Valley are tested to assess early indicators of seawater intrusion. Elevated sodium to chloride (Na/Cl) ratios indicate that numerous wells on the landward side of the seawater intrusion front have likely been affected, even though the chloride concentration has not increased to the 500 mg/L level used by MCWRA to delineate seawater intrusion.[1]
  • Reseachers at Stanford University use electrical resistivity tomography to noninvasively image depths of ~490 feet. Monitoring the electrical properties of water salinity along the the coast between Seaside and Marina provides a large area of coverage to map the location of seawater and fresh groundwater.[23]
  • The three-dimensional, finite-element-based Integrated Groundwater and Surface-Water Model (IGSM) was originally developed by Dr. Young S. Yoon in 1976 at the University of California, Los Angeles. Designed to simulate confined ground water flow, IGSM later underwent major revisions and modifications[24] including those made during application of IGSM to the Salinas Valley Groundwater Basin (SVIGSM)[25].
  • Modeling has been used to help understand Salinas Valley's groundwater hydrologic system. In her Senior Thesis at CSUMB, April McMillian used the SVIGSM to describe water table changes that have occurred due to water management policies[12].
  • RMC Water and Environment developed and calibrated an SVIGSM model and applied the criteria to the analysis of alternatives[26].

Ongoing Research

Compilation of paleo sea level data, tide gauge data, altimeter data, and estimates for likely projection range of global mean sea level rise scenarios, all relative to pre-industrial levels. Image sourced from IPCC Climate Change 2013[27]

Sea level rise must be included in future coastal aquifer studies due to probable impacts from global climate change. In 2013, the Intergovernmental Panel on Climate Change (IPCC) predicts about 1 meter of global mean sea level rise that will affect hydraulic head levels and seawater intrusion in Salinas Valley aquifers[27]:

  • Research conducted in the Department of Geography at UC Santa Barbara addressed the assessment of seawater intrusion potential from sea level rise in coastal aquifers of California. The Seaside Area sub-basin, adjacent to the Salinas Valley aquifers, in Monterey County was simulated with sea-level rise of up to 0.9 meters using FEFLOW and ArcGIS as modeling and analytical tools.[28]

Managed aquifer recharge projects provide a method of increasing groundwater supply:

  • UC Santa Cruz researchers are involved in a series of studies on managed aquifer recharge using infiltration ponds in central coastal California. They aim to quantify variability in infiltration, recharge, groundwater movement, and water quality. Results suggest that managed recharge systems might be operated for simultaneous improvements to both water supply and quality.[29]

Irrigation management aims to reduce the amount of groundwater extracted for agricultural use and slow the rate of seawater intrusion:

  • The Terrestrial Observation and Prediction System (TOPS), a NASA modeling framework developed to monitor and forecast environmental conditions, is being developed to support the use of satellite data to provide rapid assessments of current crop conditions. The TOPS Satellite Irrigation Management Support web interface project will translate the satellite data into formats that are useful to agricultural producers in maximizing irrigation efficiency[30].
  • CropManage, a UC Cooperative Extension web application, is currently being developed using evapotranspiration data from the California Irrigation Management and Information System to accurately estimate the appropriate applied irrigation to meet crop needs and minimize potential leaching losses of nitrates. The online interface is directed toward growers to track irrigation and nitrogen fertilizer schedules based on field trials of high-yield production crops.[31]

Groundwater level monitoring provides insight into the quantity of groundwater being extracted within the Salinas Valley:

  • MCWRA actively monitors key wells for monthly fluctuations, and annual measures an established network of wells to determine relative changes in groundwater storage. A survey is also conducted each August to monitor changes in coastal groundwater zones that will affect the inland movement of seawater. In addition to these three surveys, the MCWRA maintains a network of dedicated monitoring wells instrumented with electronic data loggers that record data at regular intervals.[32]


  1. 1.0 1.1 1.2 State of the Salinas River Groundwater Basin Report (Jan. 2015)
  2. Bulletin No. 52-B Salinas Basin Investigation Summary Report
  3. Historic Seawater Intrusion Map for Pressure 180-Foot Aquifer
  4. Historic Seawater Intrusion Map for Pressure 400-Foot Aquifer
  5. 5.0 5.1 5.2 Groundwater Extraction Summary Report 2014
  6. California Department of Water Resources: Critically Overdrafted Basins
  7. 7.0 7.1 Salinas Valley Water Project Description
  8. California Department of Water Resources: Sustainable Groundwater Management
  9. MIIS: Agriculture Contributes $8.1 Billion to Local Economy
  10. Pajaro Valley plan targets seawater intrusion
  11. NOAA: Can humans drink seawater?
  12. 12.0 12.1 Water Table Elevations in the Salinas Valley, California: Animated Visualization using GIS by April McMillian
  13. Monterey County Water Resources Agency Act
  14. California Department of Water Resources: Groundwater Sustainability Program Draft
  15. USGS Groundwater Resources Program: Saltwater Intrusion
  16. USGS: Saltwater intrusion in coastal regions of North America
  17. 17.0 17.1 Monterey Regional Water Pollution Control Agency: Recycled Water
  18. 18.0 18.1 18.2 MCWRA: Salinas Valley Water Project (SVWP) Overview
  19. MCWRA: Salinas Valley Water Project Phase II Overview
  20. MCWRA: Salinas Valley Water Project Phase II Project Status
  21. Sustainable Agriculture Research and Education: Building Tools and Technical Capacity to Improve Irrigation and Nutrient Management on California's Central Coast
  22. UC Cooperative Extension: Irrigation, Water Quality & Water Policy
  23. Stanford University: Imaging Saltwater Intrusion Along The Monterey Coast
  24. Review of the integrated groundwater and surface-water model (IGSM)
  25. MCWRA: Salinas Valley Integrated Ground Water and Surface Model Update
  26. Salinas Valley Integrated Regional Water Management Functionally Equivalent Plan Summary Document UPDATE
  27. 27.0 27.1 IPCC Climate Change 2013: The Physical Science Basis chapter 13
  28. Sea Water Intrusion by Sea-Level Rise: Scenarios for the 21st Century by Hugo A. Loaiciga, Thomas J. Pingel, and Elizabeth S. Garcia
  29. California Institute for Water Resources: Improving groundwater recharge
  30. TOPS Satellite Irrigation Management Support
  31. CropManage Overview: A web application for managing water and nitrogen fertilizer in lettuce
  32. MCWRA: Groundwater Level Monitoring Overview



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