An evaluation of the toxicity of various forms of chlorine to Ceriodaphnia dubia

Ambient toxicity tests at 15 sites on five small streams that receive discharges from Oak Ridge National Laboratory (TN) showed that chlorine (mostly from once‐through cooling water) was an important toxicant; however, there was not a good correlation between chlorine concentration and toxicity to Ceriodaphnia dubia in the ambient tests. To better understand the toxicity of chlorine, its various forms (hypochlorite ion, hypochlorous acid, monochloramine, and dichloramine) were tested individually. Standard Ceriodaphnia dubia acute toxicity tests (static, 24 h, with added food) showed that the chloramines were very toxic (LC50 < 0.02 mg/L) but that free chlorine (hypochlorous acid and hypochlorite ion) was much less toxic (LC50s of 0.14 and 0.08 mg/L, respectively). Chlorine decay measurements in the toxicity test solutions showed that free chlorine reacts very rapidly (<1 min) with the Ceriodaphnia dubia food (a mixture of digested trout chow, yeast, and Cerophyl^ř) added to the solutions, so the standard toxicity test measured the toxicity of chlorinated food rather than the toxicity of free chlorine. Static tests without food showed LC50s of 0.012 mg/L for monochloramine, 0.016 mg/L for dichloramine, 0.035 mg/L for hypochlorous acid, and 0.048 mg/L for hypochlorite ion; however, the free chlorine still decayed rapidly (< 7 h) in the test solution. Continuous‐flow toxicity tests showed that free chlorine is more toxic than the chloramines, with LC50s of 0.005 mg/L for hypochlorous acid, 0.006 mg/L for hypochlorite ion, 0.016 for monochloramine, and 0.027 mg/L for dichloramine. The LC50s for hypochlorous acid and hypochlorite ion are substantially below the U.S. Environmental Protection Agency's 4‐d average water‐quality criterion for chlorine of 0.011 mg/L