Increased reproductive toxicity of landfill leachate after degradation was caused by nitrite

Leachate from the landfill Lindbodarna was suspected to cause reproductive effects on fish in a Swedish lake, called Molnbyggen. The acute toxicity of this landfill leachate is caused by ammonia. In the present study the acute and chronic toxicity of the leachate from the landfill was tested with Ceriodaphnia dubia before and after treatment, either with (inoculated) or without addition of microorganisms from activated sludge, in both 2000 and 2001. On both occasions, the acute toxicity decreased after treatment, more rapidly with inoculum than without, and the cause of the decrease was mainly explained by decreasing concentrations of ammonia. However, the chronic toxicity decreased after treatment with inoculum but increased after treatment without inoculum. Therefore, we performed a series of acute and reproductive tests with ammonia, nitrite and nitrate on C. dubia, and the 24-h EC50s were 1.0, 2.7 and 59 mM, respectively, which are consistent with literature data. However, the chronic toxicity of these compounds gave quite a different picture with 8-day EC50s for reproduction of 3.0 mM for ammonia, 0.016 mM for nitrite and 1.5 mM for nitrate. Thus, the acute-chronic ratios for these compounds were 0.33 for ammonia, 170 for nitrite and 39 for nitrate. These findings show that reproduction is more sensitive than survival for both nitrite and nitrate, and that nitrite is the more hazardous of the two. This implies that the chronic and reproductive toxicity of nitrite and nitrate on zooplankton may in fact increase effects of eutrophication. In this study the toxicity of the fresh leachate was dominated by ammonia, but after treatment the contribution of nitrite increased, and especially the chronic toxicity of the treated landfill leachate was dominated by nitrite toxicity.