Routine water quality monitoring within the Grassland Subarea has been performed by the CVRWQCB since 1985 at Crows Landing on the San Joaquin River, in Mud and Salt Sloughs, at sites within the GWD, and at outflow points from the major agricultural water districts. The current monitoring program involves periodic sampling to measure temperature, pH, electrical conductivity (EC), selenium, and boron. Most sites are sampled weekly, others are sampled monthly and in the case of the Crows Landing, daily samples are collected using an autosampler (Table 2.1). Five CVRWQCB sampling stations (MER542, MER531, MER538, STC512, and STC504) correspond to the locations of the primary stations (D,F,G, H, and N), and water quality data from these sites is included in the compliance monitoring program plan. A data summary report and an interpretative report are published annually by the CVRWQCB for these monitoring stations.

Monitoring for the Grassland Water Task Force (GWTF) is collated in an annual report by Summers Engineering. The GWTF is comprised of agricultural water districts, State and Federal refuges and the GWD, all of which discharge to the San Joaquin River. The numerous monitoring stations included in this report are located on the San Joaquin River, along Mud and Salt Sloughs, along conveyances within the GWD, and at outflow points from the major agricultural water districts.  These stations are usually sampled monthly for EC, selenium, and boron; some are also monitored for flow. The chemical analysis performed on these samples measures total concentrations of selenium and boron in the sample and does not discriminate between the dissolved and suspended phases of these constituents. Four GWTF stations correspond to the primary stations in the current monitoring program; two are active (GL-22a and GL-20), one has been eliminated (GL-21 E1 [in 1989]), and one is a station on Mud Slough that was relocated from Highway 140 (GL-19) upstream of the vicinity of proposed Station D. In addition, the GWTF previously monitored Mud Slough at Gun Club Road (GL-15E, eliminated in 1987), which is upstream from proposed Station C. Except for the Gun Club Road Station (GL-15E), all the GWTF stations correspond to CVRWQCB stations. One of the discontinued stations (GL-21E) has been reopened in the current monitoring program.

The San Joaquin River at Fremont Ford (Station G) is a site that has been monitored in the past by USBR for EC, selenium, and boron. USBR has also previously collected data at nine secondary stations (Table 2.1) that may be used to provide background information.