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For many years, the San Joaquin River (SJR) has had low dissolved oxygen conditions intermittently during the late summer and early fall. The low dissolved oxygen conditions are impacting critical fish habitat and the SJR is being regulated under a state of California remediation plan that includes the development of a total maximum daily load (TMDL) allocation for oxygen demanding substances. In support of the development of a scientific TMDL allocation, studies are being conducted to characterize water quality in the many tributaries of the SJR. This study identified the sources of biochemical oxygen demand (BOD) in two western tributaries of the SJR, Mud Slough and Salt Slough, and measured the loads of BOD, algae, and ammonia entering the SJR from wetland and agricultural sources.
Mud and Salt Sloughs drain the Grassland Watershed. The watershed contains seasonal wetlands, irrigated farmland, and other agricultural lands. This drainage is under close regulatory scrutiny, because it produces a majority of the selenium and boron entering the SJR. In this study, wetland and irrigated agricultural drainage were sampled separately and a comparison was made to determine differences in water quality. In addition, water entering the study area was compared to water exiting the study area to determine the effect of water use in the region on water quality.
This study demonstrated that BOD loads from the Grassland Watershed to the SJR were proportional to flow during June�October, the most critical time for dissolved oxygen deficits in the lower SJR. This indicates that Mud and Salt Sloughs are not producing more BOD than other tributaries in the region that are not under close regulatory scrutiny. The BOD concentration of wetland drainage is higher than that of agricultural drainage, but the higher agricultural drainage flows result in a higher mass loading of BOD. Wetland flooding and irrigation of crops both had a negative impact on water quality. Algal growth was identified as the major source of BOD in agricultural drainage and locations where BOD control could potentially be implemented were identified.
