Agricultural subsurface (tile) drainage water from irrigated lands currently enters the Grassland Water District (GWD) from the south, where it is mixed with variable quantities of surface return flows (tailwater) from the Central California Irrigation District (CCID) and the other riparian diverters.  The commingled water flows northward through the GWD in ditches and canals leading to Mud and Salt Sloughs and eventually to the San Joaquin River.  The proposed GBP would intercept this drainage water at a point between Dos Palos and Russell Avenue, south of the GWD, and convey it through the existing SLD for discharge into Mud Slough (north). This system would allow agricultural drainage flows to bypass the GWD altogether.

The GBP is expected to remove contaminated agricultural drainage from approximately 90 miles of wetland water supply channels, but will introduce all of the drainage waters into 6 miles of Mud Slough (north).

Approval of the GBP was granted with the understanding that certain benefits and risks are associated with the Project. The anticipated benefits are as follows:

Agricultural drainage water will be removed from the GWD water delivery channels, thus allowing refuge managers to receive and apply all of their fresh water allocations according to optimum habitat management schedules.
Removal of agricultural drainage water from the GWD channels will reduce the selenium exposures to fish, wildlife, and humans in the wetland channels and Salt Slough. Concentrations of salinity and other constituents may also be reduced within the wetland channels and Salt Slough.
Combining agricultural drainage flows within a single concrete-lined structure, the SLD, will allow better measurement, potentially leading to a more detailed evaluation and effective control of selenium and agricultural drainage.
The establishment of an accountable drainage entity will provide the framework necessary for responsible watershed management in the Grassland Basin.
These benefits were weighed against the potential risks:
Combining agricultural drainage flows within the SLD will result in an increase in selenium and other constituents which are discharged into Mud Slough. These constituents will be above the levels historically discharged to Mud Slough. Such increases may have an adverse environmental effect on six miles of Mud Slough.
The project includes the development of a monitoring program to provide data with which to evaluate whether the terms and conditions under which the project was allowed to proceed are being met. The plan for the monitoring program has been developed with the coordination and cooperation of several State and Federal agencies including the U.S. Bureau of Reclamation (USBR), the U.S. Fish and Wildlife Service (USFWS), the U.S. Geological Survey (USGS), the U.S. Environmental Protection Agency (USEPA), the California Regional Water Quality Control Board, Central Valley Region (CVRWQCB), the California Department of Fish and Game (CDFG), and the San Luis Delta-Mendota Water Authority (SLD-MWA). Seven irrigation and drainage districts, which will use and operate the SLD, have formed the Grassland Basin Drainage Activity, through an activity agreement, within the SLD-MWA.