Unique chlorine chemistry in cooling water affects Daphnia magna toxicity targets

Duc to residual chlorine, the cooling water discharge from two bleached kra ft mills occasionally exceeded the regulatory toxicity limit for Daphnia ma gna But the measured total residual chlorine (TRC) concentration did not co rrelate with the toxicity. Experiments showed that different doses of chlor ine, added to river water, could produce water samples with the same level of TRC. However, the type of compounds measured as TRC differed as the chlo rine dose was increased. This explains why for example, the same concentrat ion of TRC could cause zero or 100% mortality Accordingly, there is no reas on to expect a strong correlation between toxicity and TRC levels in coolin g water.