Guadalupe River Watershed Mercury TMDL

Ten Year Report / Background / Mine Sites / Reservoirs / Monitoring / TMDL Documents

Ten Year Report:

The Water Board has developed a Ten Year Report describing TMDL implementation actions as well as subsequent water quality and fish tissue monitoring resulting from the Guadalupe River Watershed Mercury TMDL. The report has an executive summary with relevant highlights.


The Guadalupe River Watershed supports many beneficial uses, such as drinking water supply, sport fishing, and habitat for wildlife and endangered species. Santa Clara County has issued a fish consumption advisory for mercury contamination. Mercury concentrations in fish tissue that exceed the U.S. EPA human health mercury fish criterion (0.3 mg/kg), have been measured at numerous creeks and reservoirs in the Guadalupe River Watershed. Elevated mercury concentrations in fish tissue may also pose a threat to wildlife, such as birds, amphibians, and mammals.

The Guadalupe River Watershed Mercury TMDL examined this water quality problem and provided a watershed-wide mercury management strategy. The main source of mercury in the Watershed is the New Almaden Mining District, the largest-producing mercury mine in North America. Other sources include atmospheric deposition from global and local sources, soil erosion from areas not known to contain mines, urban stormwater runoff, seepage from landfills, and Central Valley Project water inputs to Calero Reservoir. In addition to being the primary regulatory means of achieving water quality goals in the watershed, the Guadalupe River Watershed Mercury TMDL will simultaneously reduce the amount of mercury in the Bay in accordance with the San Francisco Bay Mercury TMDL's proposed requirements.
Abandoned mercury mine at Mine Hill in the Guadalupe River watershed.

TMDL Implementation Status (updated June 2019)

This update covers implementation actions at mercury mine sites, oxygenation to suppress methylation of mercury in reservoirs and lakes, and monitoring.

Mercury mine sites:

As planned, we started implementation at the top of the watershed by requiring mine site owners to evaluate and report on the potential for mining waste to erode from their properties. These reports are posted below. The following erosion control work is underway:

  • In 2009, the Midpeninsula Regional Open Space District and San Francisco Estuary Partnership jointly received a grant from the Water Board and U.S. EPA Clean Water Act Section 319(h) funds. In fall 2014, the grantees completed remediation of an eroding slope of mercury mining waste at Hicks Flat. Project information can be found on the Partnership’s website.
  • In June 2013, the Board adopted Site Cleanup Requirements Order No. R2-2013-0024 (SCRs) for Guadalupe Mine, owned by the Guadalupe Rubbish Disposal Company (GRDC). The SCRs require GRDC to complete appropriate mercury mining waste cleanup and stabilization measures. Current status of site cleanup is available at: Search for name (Guadalupe Mercury Mine) and select from list.
  • The Santa Clara County Department of Parks and Recreation and San Francisco Estuary Partnership have jointly received several grants for mine cleanup projects from U.S. EPA’s San Francisco Bay Water Quality Improvement Fund. These projects are complete.
    • Senador Mine: This project was completed in 2016. The project restored a headcut and other targeted features in a creek channel, removed five times more calcine piles (piles of roasted mercury ores) than anticipated, and reduced erosion of mercury mining wastes. Nearly 300 pounds of mercury were removed or stabilized.  Project information can be found on the Partnership’s website.
    • Calcine-paved roads: This project was completed in 2017. The project excavated calcines used during the mining era to pave approximately 4 miles of mine roads. The roads were re-surfaced with imported gravel. An estimated 290 pounds of mercury were removed. Project information can be found on the Partnership’s website.
    • Upper Jacques Gulch: This project was completed in 2017. This grant was for the portion of Jacques Gulch that was not previously remediated. (The lower portion of Jacques Gulch was remediated in 2010 by Santa Clara Valley Water District under a Natural Resource Damages Assessment legal settlement unrelated to the TMDL.) An engineering firm prepared a Feasibility Report, which can be found on the Partnership’s website. The engineers determined that it is not feasible to remove the calcines from Upper Jacques Gulch.
  • The Santa Clara County Department of Parks and Recreation is working on another mine cleanup project.
    • Creeks adjacent to Hacienda Furnace Yard: Under a Natural Resource Damages Assessment legal settlement unrelated to the TMDL, County Parks is developing designs to restore creeks adjacent to the Hacienda Furnace Yard, by far the largest mercury ore processing facility at New Almaden. This would remove calcines from about 500 feet of creek bed, and restore these stretches of creeks. More information on the New Almaden Mine CERCLA Site is available at: In 2015, the design was nearly completed and permits initiated. However, construction was delayed. County Parks plans to complete the designs and permits in 2019 and begin construction in 2020.
  • Reports from mine site owners  regarding the potential for mining waste to erode from their properties are available here:

Guadalupe Rubbish Disposal Company

Santa Clara County Department of Parks and Recreation

Midpeninsula Regional Open Space District

See next section (Reservoirs – Dam Seismic Strengthening) regarding additional mercury cleanup and erosion control.


Dam Seismic Strengthening

California dam safety is regulated by the California Department of Water Resources, Division of Safety of Dams (DSOD). In compliance with DSOD, Valley Water, formerly the Santa Clara Valley Water District ("Water District"), had their dams inspected. Engineers determined that several dams—of reservoirs subject to the TMDL—require strengthening.

These dam construction projects may increase mercury methylation unforeseen during TMDL development. The Water Board recommends mitigation for that impact by surveying dewatered areas for mercury hotspots and possibly removing or capping sediment hotspots (details in the Water Board letters below).

Mercury-laden mining wastes upslope or upstream of reservoirs have been and will continue to be addressed to reduce mercury in reservoirs.

Almaden Reservoir: Cleanup actions are already completed in areas of Almaden Quicksilver County Park that drain to Almaden Reservoir. Those cleanups also protect Calero Reservoir, which receives mining wastes from Almaden Reservoir via canal.

Guadalupe Reservoir: There are nearly 50 mining waste features above Guadalupe Reservoir. In 2018, Water Board staff used a portable x-ray fluorescence (XRF) mobile metals detector for mercury concentrations and visual assessment of erosion. Reservoir dewatering for dam seismic strengthening may provide an opportunity for coordinated cleanup of upland mining wastes and capping of areas of reservoir sediment with elevated mercury levels.

  • Waterboard August 2018 XRF Survey of Almaden Quicksilver County Park near Guadalupe Reservoir (XRF Technical Memo - XRF Monitoring Data)
  • In summer 2019, Water Board staff plans to collect additional XRF mercury measurements above Guadalupe Reservoir.

Oxygenation to suppress methylation of mercury in reservoirs and lakes:

Valley Water, formerly the Santa Clara Valley Water District ("Water District"), continued its voluntary methylmercury production and control studies, which it initiated in 2005. Initially, the Water District tried solar powered circulators, which were effective in suppressing methylmercury production in Lake Almaden but not in Almaden or Guadalupe Reservoirs. Subsequently, the Water District installed more powerful hypolimnetic oxygenation systems in Calero, Guadalupe, and Almaden Reservoirs, and in Stevens Creek Reservoir as a treatment control site not affected by mercury mines.

Recent Water District reports are available here and on Valley Water's website.

In 2014 and 2015, low water levels due to the drought interfered with these studies. More information is available in the Water District’s December 2015 biennial report, and previous December 2013 biennial report, and December 2011 biennial report. (Their next report is due December 2019).


The four entities discussed above have established a coordinated monitoring program for methylmercury levels in fish in creeks and Lake Almaden, and mercury loads discharged to San Francisco Bay. The first five-year cycle of monitoring spanned 2011 to 2016 and included severe drought conditions; monitoring and reporting was led by Santa Clara County Parks (see March 2017 Report). Prey fish were monitored at five locations in 2011, 2012, and 2016; consistent trends over time in fish methylmercury concentrations were not found. The mercury loads monitoring occurred in 2015 and provided drought-year loads data; an estimated 6.3 kg of mercury were discharged over the year. Previously, the program submitted two interim reports, the January 2012 and January 2013 annual data reports. 

The second cycle of coordinated monitoring will span 2018–2023, and Valley Water will lead this effort. The monitoring plan was approved by Water Board staff. In the 2018-2019 wet season, two storms were monitored in accordance with the plan.

January 2017 Storm Monitoring: The Regional Monitoring Program (RMP) for San Francisco Bay funded monitoring of mercury loads in Guadalupe River at Highway 101 during a large storm series in January 2017. This one storm series transported an estimated 70 kg of mercury. (There is no mercury data for the larger and more significant storm that occurred in February 2017.) The Coordinated Monitoring Program (see above) will monitor large storms during 2018–2023. 

Report: RMP Technical Report: Guadalupe River Mercury Concentrations and Loads During the Large Rare January 2017 Storm.

TMDL Approved by U.S. EPA

U.S. EPA gives final approval to TMDL and Water Quality Objectives for Mercury in the Guadalupe River Watershed
On June 1, 2010, the U.S. Environmental Protection Agency approved the Basin Plan amendment adopted by the Regional Water Board in October 2008. These actions establish New Water Quality Objectives and Total Maximum Daily Loads (TMDLs) and an Implementation Plan for mercury in waters of the Guadalupe River Watershed. The amendment is now part of the Water Quality Control Plan for the San Francisco Bay Basin (Basin Plan), the master planning document for water quality in the Bay Area.

TMDL Documents:

Previously, the State Office of Administrative Law (OAL) approved the amendment on February 24, 2010, the State Water Board approved the amendment on November 17, 2009, and the Regional Water Board adopted the amendment on October 8, 2008.

Materials from Sept. 11, 2008 meeting with Alamitos Creek property owners

Reference documents
Final Conceptual Model Report, May 22, 2005 (Tetra Tech, Inc.)
Due to the size of the original PDF document (47.98 MB), we have made both the full document and each separate chapter available below for download. All these PDF files were created using Adobe Version 6. If you do not have this version (or higher), you will not be able to view the files.

Full Document (47.98 MB)

Individual Files (smaller-size documents):
Cover Page, Table of Contents, and Executive Summary (1.4 MB)
Chapter 1. Introduction (233 K)
Chapter 2. Watershed Characterization (15 MB)
Chapter 3. Data Summary (2.3 MB)
Chapter 4. Estimated Mercury Loads (4.3 MB)
Chapter 5. Conceptual Model of Mercury (5.9 MB)
Chapter 6. Summary (203 K)
Chapter 7. References (233 K)

Derivation of Numeric Wildlife Targets for Methylmercury in the Development of a Total Maximum Daily Load for the Guadalupe River Watershed (USFWS 2005) (2.3 MB)

Reservoir Sediment Sampling, April 2005 (Tetra Tech, Inc., 203 KB)

Draft Project Plan, June 2004 (407 KB)

Preliminary Project Definition, August 2004 (45 KB)


Santa Clara Valley Water District (home page)
Information on the Water District’s treatment systems in reservoirs and first of two phases of angler survey at mercury impaired reservoirs and lakes are available on this website.

San Francisco Bay Mercury TMDL


For more information contact:
Gerardo Martinez
Water Resource Control Engineer
San Francisco Bay Regional Water Quality Control Board
1515 Clay St., Suite 1400
Phone: (510) 622-1015
Fax: (510) 622-2460