CHEMICAL EFFECTS OF SALINE DRAINAGE WATERS ON IRRIGATED SAN JOAQUIN VALLEY SOILS

Samples of the Twisselman soil which underlies a field experiment on irrigation water re-use conducted near Lost Hills were taken from plots receiving irrigation waters of differing quality and from a site leached extensively with California Aqueduct water.Soluble and exchangeable cations and carbonate/gypsum analyses were done on all samples.The bulk sample from the leached site was put into columns for a replicated glasshouse leaching experiment that simulated the field experiment over a one-year period.Analysis of the field soil samples showed that exchangeable and soluble sodium and magnesium were increased by the application of saline irrigation water, particularly when California Aqueduct water was used for pre-irrigation.Exchangeable calcium decreased with increasing salinity of the water applied; some precipitation of calcium carbonate may have been involved, also.The soil columns contacting saline "groundwater" and leached with saline irrigation water showed an accumulation of soluble and exchangeable sodium at the soil surface, whereas soluble calcium and magnesium accumulated at about 20 cm below the soil surface.Soil columns leached with saline irrigation water and allowed to drain freely did not show sodium accumulation at the soil surface nor the calcium/magnesium "bulges" at depth, but instead showed accumulation of all three cations at or near the bottom of the column.After one year of leaching the column soils appeared to be in equilibrium with the applied waters.In the field soil samples, the exchangeable sodium percentage tended to exceed the sodium adsorption ratio in the soil solution, varying from 16% at a site receiving California Aqueduct water to 24% at a site receiving saline irrigation water.A similar increase was seen in the soil columns leached with the two waters, but the effect was less pronounced when saline "groundwater" was present.The charge fraction of exchangeable magnesium was generally equal to the charge fraction of soluble magnesium in both the field and column soils, in agreement with previous laboratory studies by the authors.1. Field soil samples 58 2. Column soil samples C. Conclusions