Grasslands Bypass Project Contaminants Monitoring

Historically, farmers in the Grasslands area of the western San Joaquin Valley have discharged subsurface agricultural drainwater through wetland channels in the San Luis National Wildlife Refuge Complex to the San Joaquin River. This drainage contains elevated concentrations of selenium, salt, boron and other trace elements.

Seining for fish, July 1, 1999

To convey this drainwater more directly to the San Joaquin River, bypassing wetland channels, a portion of the San Luis Drain was reopened in September 1996 as the Grasslands Bypass Project. The San Luis Drain has been modified to allow discharge through Mud Slough, a natural waterway that traverses through the San Luis National Wildlife Refuge Complex and a section of the North Grasslands Wildlife Area.

Analyses completed to date indicate that selenium concentrations have declined in the fish and invertebrates of Salt Slough, the principal wetland channel from which drainwater has been removed by the Project.

After the San Luis Drain was reopened and began discharging into Mud Slough, selenium concentrations in the most common fish (mosquitofish and inland silversides) both upstream and downstream of the discharge point rose substantially during the first six months of operation of the Grassland Bypass Project. These concentrations have more recently declined to levels that are approximately the same as pre-project conditions.

Continued biological monitoring is needed to insure that the project will not have a net negative effect on the ecosystems of the San Joaquin Valley.

Map of biological monitoring sites for the Grassland Bypass Project.

Data

(In the following charts each bar represents an average of composite samples, the line of long dashes marks the Toxicity Threshold, and the line of short dashes marks the Level of Concern threshold (Table 1) in this and subsequent graphs.)

Selenium in small fish in Mud Slough above the San Luis Drain discharge(Site C).

Selenium in medium-size fish in Mud Slough above the San Luis Drain discharge (Site C)

Selenium in small fish in Mud Slough below the San Luis Drain discharge(Site D)

Selenium in medium-size fish in Mud Slough below the San Luis Drain discharge (Site D)

Selenium in small fish in the Mud Slough backwater below the Drain discharge (Site I)

Selenium in medium-size fish in Mud Slough below the Drain discharge(Site I)

Selenium in splittails in GBP sloughs, June 1998

Selenium in invertebrates in Mud Slough above the San Luis Drain discharge (Site C)

Selenium in invertebrates in Mud Slough below the San Luis Drain discharge (Site D)

Selenium in invertebrates in the Mud Slough backwater below the Drain discharge (Site I)

Selenium in small fish in Salt Slough (Site F)

Selenium in medium-size fish in Salt Slough (Site F)

Selenium in invertebrates in Salt Slough (Site F)