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]

Summary

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[3] at the 180-Foot aquifer[4] and is beginning to encroach beyond the city limits of Castroville at the 400-Foot aquifer[5]. The extent of seawater intrusion has moved farther inland due to continuing overdraft conditions for municipal and agricultural uses.

Location

The Salinas Valley[6] basin lies within the boundaries of Monterey County, California[7]. Groundwater is extracted from the four major aquifers: Upper Valley, Forebay, East Side, and Pressure[8]. 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.[8] This high dependence on groundwater for the water needs in the Salinas Valley has led to critical overdraft conditions[9] that have resulted in abandonment of agricultural and urban supply wells due to rising salinity levels[10]. The concern over adverse effects from continuing overdraft of groundwater resources has led to the development of a statewide groundwater management plan[11] and new supplemental water supplies from the Salinas Valley Water Project.

Stakeholders

The agricultural industry in the Salinas Valley is estimated to be worth $8.1 billion[12] and depends heavily on groundwater for almost all of its water needs[10]. Irrigation with high-salinity water damages crop yield, crop quality, and soil health for future use of the land for agriculture.[13]

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. [14]

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[15].

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[16] but has had little authority in regulating groundwater extraction.

On September 16, 2014, Gov. 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.[17]

Hydrology

Along the coast of Monterey County, fresh groundwater flowing 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.[18]

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.[19]

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, growers safely irrigate their crops and reduce pumping of seawater intruded groundwater.[20]
  • The Salinas Valley Reclamation Project 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.[20]

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.[21]

  • 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.[21]
  • 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.[21]

Phase 2 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.[22] As of July 2014, MCWRA had requested resources to conduct an Environmental Impact Report and engaged in initial funding discussions[23].

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.[24][25]

Scientific Tools

  • The State of the Salinas River Groundwater Basin Report [1] and Groundwater Extraction Summary Report [8] 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.[26]
  • The three-dimensional, finite-element code 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 including those made during application of IGSM to the Salinas Valley Groundwater Basin[27].
  • Modeling has been used to help understand Salinas Valley's groundwater hydrologic system. In her Senior Thesis at CSU Monterey Bay, April McMillian used the Salinas Valley IGSM to describe water table changes that have occurred due to water management policies[28].
  • Water Resources & Information Management Engineering, Inc. developed and calibrated a groundwater model and applied the criteria and IGSM to the analysis of alternatives[29].

Future research

How this system works is understood fairly well. What is missing is the knowledge of exact quantity of water being extracted. If resources were not an issue, metering all agricultural wells to measure the exact quantity of water being extracted would be a good start. This information could be linked to the crop that was irrigated to provide information on specific crop water usage. Analysis of this information would assist agencies such as the University of California Co-Operative Extension, National Resource Conservation District, California Resource Conservation District and others to focus on educational outreach to growers on how to maximize their irrigation efficiency. Remote sensing images could be also be used to aid irrigation efficiency.


Hugo A. Loaiciga and Thomas J. Pingel from the Department of Geography at the University of Santa Barbara, conducted a poster presentation addressing the assessment of seawater intrusion potential from sea level rise in coastal aquifers of California. Considering the probable impacts of global climate change which includes the threat of sea level rise, future studies will have to directly address increases in level while examining sea water intrusion into coastal aquifers. The Oxnard Plain aquifer in Ventura County and the Salinas Valley coastal aquifer (Seaside Area) in Monterey County have been studied for the threat of sea-level rise. FEFLOW and ArcGIS were used as modeling and analytical tools. The image on the left from UNEP and WMO shows sea level rise due to global warming.
alt text
The results of this study which began in September 2007 and ended in July of last year will impact future sea water intrusion studies to consider global climate change in their analysis. Click on the link to see the poster presentation File:Spatial poster presentation.pdf.






...

Notes and References

  1. ↑ 1.0 1.1 1.2 State of the Salinas River Groundwater Basin Report (Jan. 2015)
  2. ↑ http://www.mcwra.co.monterey.ca.us/documents/Bulletin_52-B__1946.pdf
  3. ↑ https://en.wikipedia.org/wiki/Salinas,_California
  4. ↑ http://www.mcwra.co.monterey.ca.us/seawater_intrusion_monitoring/documents/01swi180.pdf
  5. ↑ http://www.mcwra.co.monterey.ca.us/seawater_intrusion_monitoring/documents/01swi400.pdf
  6. ↑ http://en.wikipedia.org/wiki/Salinas_Valley
  7. ↑ https://en.wikipedia.org/wiki/Monterey_County,_California
  8. ↑ 8.0 8.1 8.2 http://www.mcwra.co.monterey.ca.us/groundwater_extraction_summary/documents/2014%20Summary%20Report.pdf
  9. ↑ http://www.water.ca.gov/groundwater/sgm/cod.cfm
  10. ↑ 10.0 10.1 http://www.co.monterey.ca.us/planning/gpu/2007_GPU_DEIR_Sept_2008/Text/References/Monterey_Co_WRA_SVWP_Project_2008a.pdf
  11. ↑ http://www.water.ca.gov/cagroundwater
  12. ↑ http://www.miis.edu/about/newsroom/stories/node/41198
  13. ↑ http://www.thecalifornian.com/story/news/2014/03/05/-plan-targets-seawater-intrusion/6046831/
  14. ↑ http://oceanservice.noaa.gov/facts/drinksw.html
  15. ↑ McMillian, April. "Water Table Elevations in the Salinas Valley, California: Animated Visualization using GIS". California State University , Monterey Bay <http://hydro.csumb.edu/Doug/html/salinas_water_table.html> Jan. 2009.
  16. ↑ http://www.mcwra.co.monterey.ca.us/about/documents/MCWRA%20Agency%20Act.pdf
  17. ↑ http://www.water.ca.gov/groundwater/sgm/pdfs/DWR_GSP_DraftStrategicPlanMarch2015.pdf
  18. ↑ http://water.usgs.gov/ogw/gwrp/saltwater/salt.html
  19. ↑ https://pubs.er.usgs.gov/publication/70003372
  20. ↑ 20.0 20.1 http://www.mrwpca.org/about_facilities_water_recycling.php
  21. ↑ 21.0 21.1 21.2 http://www.mcwra.co.monterey.ca.us/salinas_valley_water_project_I/salinas_valley_water_project_I.php
  22. ↑ http://www.mcwra.co.monterey.ca.us/salinas_valley_water_project_II/salinas_valley_water_project_II_overview.php
  23. ↑ http://www.mcwra.co.monterey.ca.us/salinas_valley_water_project_II/salinas_valley_water_project_II_project_status.php
  24. ↑ http://www.sare.org/Learning-Center/From-the-Field/Western-SARE-From-the-Field/Building-Tools-and-Technical-Capacity-to-Improve-Irrigation-and-Nutrient-Management-on-California-s-Central-Coast
  25. ↑ http://cemonterey.ucanr.edu/Custom_Program567
  26. ↑ https://gemcenter.stanford.edu/research/imaging-saltwater-intrusion-along-monterey-coast
  27. ↑ Review of the integrated groundwater and surface-water model (IGSM)
  28. ↑ http://hydro.csumb.edu/Doug/html/salinas_water_table.html
  29. ↑ WRIME Castroville Seawater Intrusion Project


Uncited

[1]

[2] [3].

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