Impacts of Metal Salt Addition on the Chemistry of Lake Elsinore, California: 1. Alum

Abstract

ABSTRACT A series of laboratory studies were conducted to evaluate the impacts of alum addition on the water chemistry of Lake Elsinore. Lake Elsinore is a warm, shallow, polymictic and eutrophic lake, located in southwest Riverside County in Southern California, that is plagued with chronic poor water quality conditions due in part to large internal nutrient loadings. The lake also possesses a high pH and high alkalinity. Control of internal phosphorus loading by alum was considered as a possible control method, although most alum applications have been made in relatively soft water lakes. The changes in water chemistry resulting from alum and high-acid alum treatments were quantified for 3 different Al doses (9, 13 and 18 mg L−1). The high alkalinity in the lake limited the capacity for alum at the 9 mg L−1 Al dose to reduce the equilibrium pH below 8.7. High acid alum formulations and higher Al doses were somewhat more successful at lowering the pH, however. For example, Al doses of 9 mg L−1+20% H2SO4 and 18 mg L−1 (no free acid) were able to lower the pH to approximately 8.5. The highest dose and free-acid content (18 mg L−1+20% H2SO4) was most effective, lowering the pH to about 8. Dissolved Al concentrations varied strongly as a function of Al dose, percent free acid in the high-acid alum formulations, and time. Dissolved Al concentrations increased with increasing pH, and approached (or exceeded) 2 mg L−1 in some samples, but generally remained below levels predicted by the solubility of amorphous Al(OH)3. pH and dissolved Al concentrations both varied markedly over time, due to kinetic limitations to CO2 outgassing. Dissolved Al concentrations monitored in longer-term core experiments decreased over time, suggesting transformation of amorphous, high-solubility Al phase(s) to more crystalline, lower-solubility phases. The high Al solubility and potential toxicity makes alum treatment inappropriate for Lake Elsinore and other strongly alkaline lakes.