Submitted to: Chemosphere
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/9/2006
Publication Date: 1/23/2007
Citation: Gao, S., Ryu, J., Tanji, K.K., Herbel, M.J. 2007. Arsenic Speciations Accumulation in Evapoconcentrating waters of agricultural evaporation basins. Chemosphere 67: 862-871. Available: www.sciencedirect.com Interpretive Summary: Irrigation and drainage are essential for agricultural production in the San Joaquin Valley (SJV), California. Most of the drainage, especially in the southern part including the Westside of SJV and the hydrologically closed Tulare Basin, are disposed into constructed evaporation basins (or ponds). Accumulation of salts and toxic trace elements such as selenium (Se) and arsenic (As) in drainage disposal sites has been an environmental concern. Little information is available on the fate of As in agricultural drainage. In this study, we examined chemical behavior of As in pond waters and sediments. The data show that As behaves in a totally different manner from Se in pond waters. As concentration increased with Se due to evapoconcentration. A reducing environment was developed as water Se increased resulting in reduced forms of As. We evaluated the processes and conditions affecting the fate of As in the saline pond waters. The information is useful for future evaluation of drainage disposal options.
Technical Abstract: To sustain agricultural productivity, evaporation basins (or ponds) have been widely used for the disposal of agricultural drainage in areas requiring subsurface drainage in the San Joaquin Valley of California, USA. The drainage water contains elevated concentration of trace elements including selenium (Se) and arsenic (As). Unlike Se, little information is available about As, a potentially high risk element. The objective of this study was to characterize the chemical behavior of As and acquire data for better understanding of biogeochemical processes and conditions affecting As fate in evaporation ponds. The study site was a 726 ha evaporation basin facility (containing 10 cells with water flowing in series) in the hydrologically closed Tulare Basin of California. We examined water chemistry, As concentration and speciation along the flow path between cells as well as within the cells. As concentrations in the water increased linearly with Chlorine (Cl-), a conservative ion from evapoconcentration. Reduced As species as arsenite (As III) and organic arsenic (org-As) also increased with increases in Cl- and salinity. Water samples with elevated electrical conductivity (EC) (i.e., towards the end of flow path) had high dissolved organic matter, low dissolved oxygen, and elevated sulfide concentrations, indicating the development of reducing conditions. We hypothesize that such changes could facilitate the reduction of arsenate (As V) to As(III) and org-As. Elevated As in sediment profiles indicate a solid phase sink mechanism, but not significant enough to remove and reduce As concentrations in the water columns. These findings help us better define the processes that affect As in drainage facilities and contribute to our understanding of how As behaves in other regions of the world that have similar climatic and hydrogeochemical conditions.