Evaporation basins have been widely used for the disposal of agricultural drainage in areas requiring subsurface drainage in the San Joaquin Valley of California, a highly agricultural production area in USA. The irrigation drainage water contains elevated concentrations of trace elements, including Se, As, B, V, and Mo. Little information is available on As (an environmental risk element) in drainage disposal systems. The objective of this study was to investigate As transformations and accumulation in an agricultural drainage disposal facility for better understanding of As behavior and its environmental fate. The study site was a 726 ha evaporation basin facility (containing serially linked 10 cells) in the hydrologically closed Tulare Basin. Water chemistry, As concentration and speciation in water along the flow path as well as within the cells were examined. Total As [mainly arsenate, As(V)] concentrations in the water increased linearly with increases in Cl, a conservative ion, from evapoconcentration. The reduced As species arsenite [As(III)] and organic arsenic (org-As) also increased with increases in Cl and salinity. Evaluation of water redox conditions indicate reducing conditions developed as salinity increased along the water flow path. Elevated As in sediment profiles indicated a solid-phase sink mechanism, but not significant enough to remove and reduce As concentrations in the water column. These findings mmay apply to environments with similar climatic and hydrogeochemical conditions.