Evaluating the Effects of Tailwater Irrigation on Soil Salinity and Discharge Water Quality.
Heather V. Graham1, Nigel W.T. Quinn2, and Kate Hucklebridge2. (1) Occidental College, 1600 Campus Road, Box 833, Los Angeles, CA 90041, (2) Lawrence Berkeley National Laboratory, One Cyclotron Road, MS-70A-3317H, Berkeley, CA 94720
Inorganic salts are a natural component of soil that arise during the process of geologic erosion. Typically, rainwater naturally leaches salts into lower portions of the soil profile. However, in arid regions such as the San Joaquin River Basin of California, lack of rainfall and climatic conditions increase surface evaporation and upward capillary flow resulting in reduced leaching. The geologic composition of this area causes both the soil and surface water to be saline. For this reason, agricultural runoff is highly saline and detrimental to receiving water bodies. The excess salt in water can harm aquatic organisms and cause problems for downstream users. California state regulations limit the amount of salts that agricultural users are permitted to discharge into receiving waters. In order to comply with these regulations irrigation districts are exploring techniques and technologies that can comply with regulations and improve drain water quality without increasing soil salinity. This study examines the efficacy of using agricultural runoff or ‘tailwater' as an irrigation water source. This study also quantifies the change in the quality of discharge water after implementation of the tailwater recycling system. Four sites were chosen within the Patterson Irrigation District in the San Joaquin River Basin that have similar irrigation, surface drainage and crop histories. Each site is supplied by different water sources of varying quality, one of which is from a tailwater collection pond. An electromagnetic salinity survey of each field using a Geonics EM-38 revealed that while irrigation water is generally proportional to soil salinity other factors were affecting the salt level. A field receiving irrigation water 27% more saline than another had the same soil salinity but observations suggested it likely had better subsurface drainage. Comparing flow and EC data collected at the drain site via a Starflow/YSI conductivity meter system indicated a 47% decrease in salt discharge into the river after implementation of the tailwater recovery system. Additionally, there was a very significant decrease in the amount of sediment and boron entering the river. This study suggests that with proper drainage and soil management tailwater can be used as a source of irrigation water. Tailwater recycling decreases quantity of discharge water as well as improving the overall quality.