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(70-86) Evaluating Copper Toxicity to Ceriodaphnia dubia in Waters of Hardness Greater Than 400 mg/L. Gensemer, Robert1, Naddy, Rami1, Stubblefield, William*,1, Hockett, Russ1, Brix, Kevin2, DeForest, David3, Paquin, Paul4, Santore, Robert4, 1 ENSR, Fort Collins, Colorado2 EcoTox, North Bend, Washington3 Parametrix, Kirkland, Washington4 HydroQual, Mahwah, New Jersey ABSTRACT- The mitigating effect of increasing hardness on metal toxicity has been incorporated into United States regulatory standards and criteria over a range of >25 to <400 mg/L (as CaCO3). However, in waters draining regions of the arid west of the U. S., hardness frequently exceeds the upper limit of 400 mg/L that can be used to modify water quality criteria for metals. The applicability of these hardness-toxicity relationships are largely unknown. Therefore, studies were designed that quantify the relationships between copper toxicity to Ceriodaphnia dubia and water hardness at concentrations >400 mg/L. This study was conducted as part of the Arid West Water Quality Research Project (WQRP) in Pima County, Arizona. The WQRP conducts research to improve the scientific basis for developing appropriate water quality criteria that will better protect the species and habitats characteristic of ephemeral and effluent-dependent stream ecosystems in the arid and semi-arid West. Acute toxicity tests were conducted at hardness levels ranging from ca. 300 to 1000 mg/L using reconstituted waters that mimic three kinds of natural waters with elevated hardness: 1) waters with a molar Ca:Mg ratio = 2, but with elevated TDS ions (desert Southwest streams such as Las Vegas Wash), 2) waters with strongly elevated Ca:Mg ratios (e.g., treated mining discharge), and 3) EPA synthetic hard water. Although copper toxicity still decreased with increasing hardness at levels > 400 mg/L, the hardness-toxicity relationships differed as a function of ion composition. In particular, increasing alkalinity and, to a lesser extent, magnesium concentrations could explain most of the variation in copper toxicity regardless of the ion composition of the reconstituted hard waters. Key words: copper toxicity, hardness, metals, alkalinity |
