Description
The Salton Sea in southeastern California is the recipient of agricultural and wastewater from the extensive farming in the Imperial Valley. It is located in an arid area with no outlets and is totally dependent on these waters to maintain its water level. Consequently it has been experiencing rising salinity, eutrophication, and massive fish and bird deaths. Little has been studied since it was formed in 1906 by accidental diversion of the Colorado River. However, the bottom sediments, if undisturbed by bioturbation, may hold a record of input to this lake. To this end, a laminated sediment core was obtained from the south-central portion of the Salton Sea. Two methods, 210Pb and 137Cs, were used to estimate the sedimentation rate. The low activity of 210Pb and 137Cs precluded the calculation of a sedimentation rate, possibly due to the removal of 210Pb and 137Cs during passage of the inflow water through the surrounding agricultural lands. A sedimentation rate of 0.5 cm/year was estimated by correlating large-scale rainfall events to peak concentrations of elements common in aluminosilicates. A depositional cycle of biologically precipitated calcite, diatom dominated horizons, and gypsum precipitation was observed in laminations in the bottom sediments. This sequence is a consequence of photosynthesis of the large algal mass removing carbon dioxide leading to calcite deposition. As the algal mass dies and falls into the deeper water, oxygen is removed and sulfate is reduced to hydrogen sulfide during periods of stratification. When the lake overturns in the fall, hydrogen sulfide is oxidized by oxygen in the surface waters to sulfate, which then precipitates as gypsum. Diatoms and fish are killed off by the toxic hydrogen sulfide. Elemental concentrations were determined in the bottom sediments of the core. Typical pollution type patterns of metals (decreasing concentrations with depth) were not present indicating no major contamination of the Salton Sea. Elemental concentrations indicate possible precipitation of ferromanganese and phosphate minerals. Phosphorous flux into the Salton Sea appears to be balanced by the removal flux into the sediments. Measured selenium concentrations are high enough as to cause concern if introduced into the biota.
