Effects of calcium magnesium acetate (CMA) on dissolved oxygen in natural waters

Abstract

Comprehensive laboratory and field experiments were performed to determine the potential effects of the experimental deicing agent calcium magnesium acetate (CMA) on aquatic and terrestrial ecosystems. A companion paper covers the overall results of these experiments. This paper concentrates on the leading concern to emerge from the testing, the potential for CMA to deplete dissolved oxygen (DO) in water bodies. In laboratory biochemical oxygen demand (BOD) experiments, 100 mg CMA/1, at the high end of the concentration routinely expected in highway runoff, completely depleted oxygen within 2 days at 20°C. A concentration of 10 mg/1 caused a net depletion of about 4.5 and 7.0 mg O 2/l for reagent-grade and corn-based CMA, respectively. This extra oxygen demand was created by the corn-based CMA even though the two products had equal acetate concentrations. Most of the extra demand can probably be attributed to the presence of butyrate as a contaminant in the corn-based CMA. The rate of DO depletion was strongly dependent on temperature. While the rate appeared to follow an Arrhenius relationship with temperature, the classic, first-order BOD equation did not represent initial DO depletion well at low temperatures. An alternative model was proposed for this region, where depletion better followed a logistic-type curve, probably in direct proportion to the growth of bacteria. Oxygen depletion after CMA additions was also observed in microcosm ecosystems and field ponds, although reaeration reduced net depletion compared to the BOD experiments. When ice covered one of the ponds, a large DO drop occurred following a relatively small CMA inflow. It is recommended that CMA applications be avoided in situations where receiving waters are close to the road, have a low dilution potential, or support populations of fish sensitive to low oxygen levels. Applications during late spring storms and where there is potential for CMA runoff to get into ice-covered water bodies are also discouraged.