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Acidification of Agricultural Evaporation Ponds: Effects on Trace Element Chemistry in Sediment-Water Core Microcosms

Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616.

^* Corresponding author (kktanji{at}ucdavis.edu).

ABSTRACT

Accumulation of potentially toxic trace elements in agricultural evaporation ponds in California's San Joaquin Valley poses a hazard to wildlife. Some evaporation pond cells have evapoconcentrated Se resulting in reduced reproduction and deformity of waterbirds. One option to reduce toxicity hazards to waterbirds is to break the aquatic food chain by acidification of pond waters. Effects of pond water acidification on trace element chemistry and chlorophyll-a concentrations in sediment-pond water cores from three representative pond facilities were studied using pH stat experiments. Three acid treatments were investigated during the 4-mo experiment: HS4, HS5, and FS4, the latter oxidizing ferrous iron (Fe^II) to ferric iron (Fe^III) which released protons. Changes in trace element speciation due to acidification were examined using a revised version of SOLMINEQ.88. FS4 treatments were the most effective in terms of requiring the smallest number of additions to maintain the pH and reducing aqueous concentrations of many trace elements. Arsenic, Mo, and Se concentrations significantly decreased in the water column for FS4 treatments. Iron and Mn concentrations increased for all treatments, and Al, B, Cu, and Zn concentrations either were relatively static or were too low in concentration to discern any effect. Chlorophyll-a concentrations (an index of pond water productivity) showed decreases for all treatments in Pond 1, but were variable in Ponds 2 and 3. An economic analysis indicated a high cost for the initial acidification, which may inhibit implementation of acidification as a method to reduce contaminant hazards to wildlife.