Submitted to: Proceedings of the International Salinity Forum
Publication Type: Proceedings
Publication Acceptance Date: 2/3/2005
Publication Date: 4/25/2005
Citation: Gao, S., Tanji, K., Ryu, J. 2005. Chemical behavior of selenium and arsenic in a saline agricultural drainage disposal pond facility. Proceedings of the International Salinity Forum. pp 185-188. Interpretive Summary: The San Joaquin Valley (SJV) of California is one of the most highly productive agricultural regions in the United States. In addition to irrigation in this semi-arid climatic region, subsurface drainage is also necessary to prevent salts from building up in the rooting zone to sustain agriculture production. Most of the agricultural drainage, especially in the southern part of SJV, 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 is a concern. We are studying the chemical behavior of Se and As in an evaporation pond facility by examining their speciation and distribution in water and sediments. Initial data show that Se and As behaved in a totally different manner in pond waters. There is a positive correlation between increases in As concentrations and increases in salinity due to evapoconcentration while there is no correlation at all between Se concentration and salinity. Trace elements in these ponds are subject to biogeochemical transformations that affect their solubility in water and subsequently toxicity risks. Previously, Se had been implicated as toxic to waterbirds but not As. We are studying the processes and mechanisms that control the fate of Se and As in these highly saline waters because these constituents of concern may affect future drainage disposal options in evaporation basins.
Technical Abstract: The San Joaquin Valley (SJV) is located in the southern part of the Central Valley of California. Both irrigation and subsurface drainage are required for sustaining the agricultural productivity in this semi-arid region. Most drainage water has been disposed into evaporation basins (or ponds). Drainage water, particularly in the west side of SJV, contains naturally high Se that caused waterbird toxicity in disposal sites. Upon evapoconcentration and biogeochemical transformations, trace elements such as Se and As are subject to either accumulation or dissipation in evaporation ponds. Their fate in evaporation ponds is not well understood but may affect the future drainage disposal options. We examined the chemical behavior of Se and As in an evaporation pond facility operated "in-series" by determining their speciation and distribution in pond waters and sediments. An evapoconcentration factor (ECF) is obtained from the ratio of Cl concentration in the evapoconcentrated pond water to that of Cl concentration in the inflow drainage water. The ECF of pond waters increased dramatically following the water flow sequence in the pond cells ranging from 2 to 50. Evapoconcentration of drainage water resulted in increases of EC, alkalinity and soluble ions such as Cl, SO4, Na, and Mg. Selenium and arsenic behaved differently in pond waters as ECF increases. Arsenic concentration increased linearly with salinity but not Se. Selenium concentrations in evaporation pond waters are actually reduced through sink mechanisms (reduction to elemental Se, organic Se and selenite that is sorbed by sediments) and these mechanisms may reduce its hazardous risk to waterbirds to some extent. The accumulation of arsenic may become a future concern if it continues to rise although there is no current evidence of arsenic toxicity to waterbirds. The proportions of reduced species for both Se and As also increased with increase in salinity. Further detailed investigations are required for better understanding of reduction transformations and mechanisms while waters are being evapoconcentrated.