Impacts of Metal Salt Addition on Water Chemistry of Lake Elsinore, California: 2. Calcium Salts

Abstract

ABSTRACT Laboratory studies using water samples collected from Lake Elsinore in June 2002 demonstrated that addition of CA2+ substantively changed the water chemistry. The addition of agricultural gypsum, rock gypsum and CaCl2 all lowered equilibrium pH and alkalinity levels, while increasing the dissolved Ca2+ concentration. Solution pH decreased linearly with increasing Ca2+ dose, from 9.0 with no added Ca2+, to about 8.4 at a dose of 200 mg·L−1. Alkalinity decreased from 10.5 to about 3.5 meq·L−1, while dissolved Ca2+ levels increased from 20.6 to approximately 100 mg·L−1 over this same dose range. Addition of CaO and Ca(OH)2 had a comparatively small effect on equilibrium chemistry and maintained high pH, high alkalinity and low Ca2+ levels in the water. The kinetics of these changes were quite slow; equilibrium was generally reached between 4–7 days depending upon Ca-salt used and the rate of mixing. The Ca-salt additions resulted in modest (30–40%) reductions in total phosphorus (TP) and chlorophyll levels. Sorption and core-flux experiments demonstrated limited capacity of precipitated CaCO3 to sorb soluble-reactive phosphorus (SRP). More substantial removal was achieved, however, via coprecipitation of SRP with CaCO3 formed in situ from Ca2+-amended recycled water added to Lake Elsinore water, although removal efficiencies remained well below those typically achieved with alum. Based upon these findings, some general recommendations can be made about Ca2+ addition to lakes for the control of phosphorus: (i) Ca2+ addition to lakes will be more effective during periods of high SRP concentrations; (ii) addition of Ca-salts that promote coprecipitation of SRP with CaCO3 formed in situ is more effective at removing SRP from the water column than adsorption to preformed CaCO3; and (iii) the form and dose of the Ca-salt dramatically influences the final chemical condition of the water.