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Watershed Restoration Reduces Selenium Levels in the Grasslands
Watersheds Camp 13 and Agatha Canals
\a/3tprhnr|\' ImnrnvpH T'ie Grasslar|d Marshes, within the 370,000-acre Grasslands
Watershed on the west side of the San Joaquin River Basin, exceed
selenium water quality objectives. With the start of the Grassland Bypass Project in 1996, all tile
drainage from a 97,000-acre area known as the Grassland Drainage Project Area was consolidated
and conveyed to the downstream end of the watershed through San Luis Drain to Mud Slough,
Discharges of drainage water are diverted away from Camp 13 Canal and Agatha Canal, which are
water supply channels that feed the Grassland Marshes, Since 2011, these canals have not exceeded
selenium water quality objectives, building upon previously reported water quality improvements in
Salt Slough downstream of the Grassland Marshes and in reaches of the San Joaquin River.
Problem
The Grassland Marshes are within the 370,000-acre
Grasslands Watershed on the west side of the San
Joaquin River Basin in central California (Figure 1).
In 1988, as part of the water quality assessment
and Clean Water Act (CWA) section 303(d) listing
of impaired waters, 8,224 acres of marshes were
identified as impaired for exceeding selenium water
quality objectives. Selenium is a highly bioaccumula-
tive trace element that, under certain conditions, can
move through the food chain and cause acute and
chronic toxicity to fish and wildlife. Until 1996, the
wetland water supply conveyance channels received
agricultural drainage water in addition to high-quality-
wetland supply water. As a result, the supply channels
periodically conveyed agricultural drainage that
contained elevated concentrations of selenium to
the Grasslands Marshes, Salt Slough and San Joaquin
River. The largest source of selenium was agricultural
subsurface tile drainage.
in 1996, the Central Valley Regional Water Quality
Control Board (CV-RWQCB) re-evaluated the extent of
selenium impairment, determining that the Grassland
Marshes include approximately 75 miles of wetiand
water supply conveyance channels and 61,810 acres
of wetland marshes. In 2000, the CV-RWQCB updated
the Selenium Total Maximum Daily Load (TMDL.)
for Grassland Marshes to reflect the findings of this
evaluation.
Story Highlights
With the start of the Grassland Bypass Project in 1996,
all tile drainage from a 97,000-acre area known as the
Grassland Drainage Project Area is consolidated and
conveyed to the downstream end of the watershed
through San Luis Drain to Mud Slough. Discharges of
drainage water are diverted away from Camp 13 Canal
and Agatha Canal, which are water supply channels
that feed the Grassland Marshes.

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Monthly Mean Selenium Concentrations
-Agatha Canal
rcamp 13 Canal
Monthly Mean MQO
t I

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016
Figure 2. Selenium concentrations in the Camp 13 and Agatha canals meet the measurement quality objective.
Since the initiation of the Grassland Bypass Project, the
selenium load discharged from the Drainage Project
Area into the Grassland Marshes has been reduced
61 percent, leading to improved habitat for wildlife
that use these sensitive resources. The Grassland
Bypass Project is a multi-partner project leveraging
the collective resources of the CV-RWQCB, California
Department of Fish and Wildlife, San Francisco
Estuary Institute, the U.S. Geological Survey, the U.S.
Environmental Protection Agency, U.S. Bureau of
Reclamation, and the San Luis & Delta-Mendota Water
Authority. Water quality monitoring has demonstrated
no selenium water quality standard exceedances since
2011.
The CV-RWQCB issued a Waste Discharge Requirement
that specified the maximum monthly and annual
loads of selenium that the project may discharge into
Mud Slough and the San Joaquin River. In 2016, the
CV-RWQCB issued waste discharge requirements for
growers in the Grassland Drainage Area to reguiate
selenium discharges to groundwater.
Results
Beginning in 1996, the Grassland Bypass Project
separated the Grassland Watershed wetland water
supply conveyance channels from the largest selenium
source: the 97,000-acre Drainage Project Area.
Diverting Drainage Project Area discharges away from
the water supply channels that supply the Grassland
Marshes has reduced the load of selenium discharged
from the Grassland Drainage Project Area by 61
percent (from 9,600 pounds [lbs] to 3,700 lbs), and the
load of salts has been reduced by 39 percent (from
187,300 tons to 113,600 tons).
The concentration-based TMDL (20 micrograms per
liter tftg/L] maximum; 2 |ig/L monthly mean) measured
in wetland water supply conveyance channels has not
been exceeded since February 2011 (Figure 2). Weekly
selenium sampling at the Camp 13 Canal and Agatha
Canal monitoring sites was required through 2015;
currently, only storm event sampling is required.
Partners and Funding
The Grassland Bypass Project is operated by the U.S.
Bureau of Reclamation and the San Luis & Delta-
Mendota Water Authority. The San Francisco Estuary
institute monitors impacts of the use of a portion
of the San Luis Drain for conveyance of agricultural
discharges for the Grassland Bypass Project. Samples
of the agricultural drainage water collected from the
Grassland Bypass Project are sent to the California
Department of Fish and Wildlife Office of Spill
Prevention and Response laboratories to be analyzed
for selenium concentration.
PROl*°
2
o
U.S. Environmental Protection Agency
Office of Water
Washington, DC
EPA 841-F-21-001A
January 2021
For additional information contact:
Holly Grover
Central Valley Regional Water Quality Control Board
916-464-4747 • holly.grover@waterboards.ca.gov

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