Accumulation of Toxic Trace Elements in Evaporites in Agricultural Evaporation Ponds
Evaporation ponds are utilized to dispose of saline agricultural drainage waters where there are no surface drainage outlets to the San Joaquin River. Presently, 22 ponds in the San Joaquin Valley's west side are receiving about 32,000 acre-feet per year of tile drainage effluents and about 0.8 million tons per year of salt are being deposited. This project investigates the incorporation of potentially toxic trace elements (selenium, boron, arsenic, molybdenum) into evaporite minerals forming by desiccation of drainage waters.
Over 50 samples of salt deposits (evaporites) obtained from seven ponds show that the predominant minerals are either halite (Ne Cl), thenardite (Na2S04) or mirabilite (Na2S04 • 10H20), reflecting the dominant dissolved mineral salts in the drain water. The highest selenium concentrations in evaporites were found in Peck pond, ranging from 3 to 33 mg/kg (ppm) with a mean value of 12 mg/kg for six samples. Evaporite samples from all other ponds contained a maximum selenium level of 1.8 mg/kg and mean value of 0.22 mg/kg or less. The values are well below the 100 mg/kg threshold level of selenium for wastes to be considered hazardous. The concentrations of boron in evaporites ranged from a low of 16 in Barbizon pond to as high as 975 mg/kg in Lost Hills pond. The mean concentration of boron in evaporites from seven ponds ranged from 73 to 690 mg/kg. There are no hazardous criterion for boron threshold level in waste solids.
Laboratory studies indicate that association of boron with halite or mirabilite are influenced by both the nature of the salt crystal surface and the chemical species of boron. For example, boron association occurs mainly by surface adsorption on halite and by both surface adsorption and internal absorption (by diffusion into the crystal lattice) for mirabilite,