The Glenn and Colusa Irrigation District and the Rice Research Board provided support funds to the Department of Water Science and Engineering, UCD, in 1973-74 for a major rice area water quality study. Fourteen commercial rice fields were selected in 1974 to represent the rice producing areas in Sacramento and lower San Joaquin Valleys.

þ The non-pesticide quality of the surface outflow from commercial rice  fields in general has been shown to be little different from that of the inflow. There are numerous exceptions when comparing individual values, but also there were many cases in which the outflow value was more favorable. There appears to be little increase in total salts from evapotranspiration. In two fields where the subsoil was saline, surface flows picked up appreciable quantities of salt. Little nitrogen escape was detected in grab samples of surface water outflow. The few exceptions occurred shortly after flooding or aerial broadcasting into the water in midseason. Turbidity of surface outflows was generally less than that of the supply water; of hundreds of samples taken from 16 fields, turbidity exceeded 100 Jackson Turbidity Units in only three samples.

It is not possible to evaluate conclusively the quality of percolating water from composition of subsoil water samples. For example, the electrical conductivity of subsoil water samples from two fields was in the range of 3000 to 6000 μ mhos, or 10 to 15 times that of the irrigation water. The subsoil salts of these particular soils have not been leached out despite several years of flooding for rice, indicating that there is almost no water percolating through. Only where percolation or seepage is relatively rapid can the subsoil water composition be considered representative of the underground outflow.