University of California Water Resources Center

Agricultural drainage from Delta islands is known to be a significant contributor of DOC (Dissolved Organic Carbon) that forms TI-IMs (Trihalomethanes) when drinking water is chlorinated. The current agricultural practices create seasonal wet-dry cycles in the fields so that salinity, sodicity, temperature and moisture content of soils are varied. This study was carried out to understand the influences of the current agricultural practices on the production of DOC and THM from surface (oxidized) and subsurface (reduced) peat soil of the Sacramento-San Joaquin Delta. Both abiotic and biotic factors are examined independently in order to identify the major DOC and THM precursors production processes. Abiotic factors examined with successive batch soil solution extraction were salinity (EC from °to 4 dS/m) and sodicity (SAR from °to co) that affect the coagulation and dispersion of soil organic matter (SOM). Biotic factors examined with S-week batch incubation were temperature (lO, 20, 30 oC) and moisture (30, 70, 200 % moisture) that affect the rate of microbial decay of SOM, producing CO2 and DOC.

In the abiotic experiment, the results showed that the increase of the soil-water salinity decreases the amount of Doe and decreases its aromaticity. A decrease of SAR also decreases the amount of DOC and decreases its aromaticity. The results of the abiotic experiment showed that the salt accumulation of the summer irrigation is not the major production of DOC; instead, the salt accumulation may reduce the DOC leaching from the peat soil, if we consider the salt effects alone.

In the biotic experiment" the 8-week incubation experiments showed that temperature, moisture content and wet-dry cycles affect the microbial activities in soils, but only the flooded and the wet-dry cycle incubations increase the DOC concentration in the oxidized peat soil. However, the extracted DOC from the incubated peat soils showed lower STHMFP although there was increases in the concentration and SUV254' In summary, the current agricultural practices alter the soil salinity and create the wet-dry and flooded conditions in the fields. The summer irrigation increases the soil salinity. However, the increase of salinity decreases the DOC productions but increases the STHMFP. On the other hand, the wet-dry cycle in the summer and flooded conditiOlf in the winter did produce the DOC, but the STHMFP of DOC produced in these conditions decreased.