Geochemical speciation as a predictor of copper toxicity in low alkalinity, soft waters of East Texas

The biotic ligand model (BLM) provides a way to produce site-specific ambient water quality criteria for metals through an equilibrium geochemical modeling framework. Since copper is now the third most used metal in the world, more information is needed to understand its speciation, and its toxicity to organisms in soft water with neutral to low pH and low alkalinity. Waters from eight East Texas rivers and streams which exhibited these characteristics was used in this study. Analysis confirmed the pH of the samples ranging from 3.75--8.65 with alkalinity ranging from 22--56 mg CaCO3·L -1 and total hardness ranging from 40--78 mg CaCO 3·L-1. Concentrations of K+, Na+, Ca2+, Cl- and Cu 2+ were measured in the samples with ion selective electrodes. The water-quality parameters found in each of the natural waters were used in the chemical speciation program, Visual MINTEQ, to evaluate how copper complexes and in what concentrations in these waters. The two copper complexes in the highest concentrations were Cu2+ and CuCO3. Titrations were preformed on the natural waters to see how much CuSO4 was needed to reach three concentrations of ionic copper (1.57x10-8 , 1.57x10-7 and 1.57x10-6 MCu2+) and a statistical difference was found between what was measured and what Visual MINTEQ reported at the two lower concentrations. EC50 values of the toxicity tests in pH 5 ranged from 0.54--2.57 microg·L-1 Cu2+, pH 6 ranged from 3.73--6.6 microg·L-1 Cu2+ and pH 7 ranged from 6.21--17.53 microg·L-1 Cu2+. At pH 5 and 6 an over prediction occurred by the BLM and at pH 7 an under prediction occurred.