Leslie F. Grober, Joe Karkoski, Tim Poole1
Wetlands in the Grassland Water District of the San Joaquin Valley, California are flooded in the fall and drained in the spring to provide habitat for waterfowl and shorebirds. The spring release of water from these wetlands has an impact on water quality in the San Joaquin River. Samples of wetland soils and water were collected over a period of one year and analyzed to quantify and understand the movement of salts through the system. This information is used to ascertain the potential effect of management practices employed to improve wetland soil and water quality and minimize adverse impacts to the San Joaquin River.
Electrical Conductivity, boron, and selenium concentrations were measured for wetland supply water, wetland releases, wetland soils, and a San Joaquin River tributary. Boron concentrations of wetland releases were found to be elevated relative to wetland supply water. These elevated boron concentrations occur contemporaneously with elevated boron concentrations in San Joaquin River tributaries. Data obtained from soil sampling of wetland soils during the dry summer season indicates that these soils are the likely source of additional boron. Monitoring of wetland pond water quality during the fall floodup shows that boron is leached from wetland soils during this period. The areal distribution of boron in the soils demonstrates the impact that poor drainage and past land and water management practices have had on soil quality; appropriate wetland management can now be used to improve soil quality.
Management practices, such as improved drainage, use of higher quality delivery water, early partial drawdown, flushing, and extended gradual drawdown, are suggested to mitigate adverse water quality impacts of wetland releases on San Joaquin River water quality. Further, modification of the timing of wetland releases, consistent with wildlife needs, can be used to mitigate other San Joaquin River water quality problems.
