PARENT SESSION
4B Site-specific ecological risk assessment
9:00 AM to 7:00 PM, Tuesday, 08 May 2001
Session Chair:

(T/MF181) Empirical models predicting zinc toxicity as a function of water quality parameters: importance of species and exposure period.

Heijerick, Dagobert¹, Janssen, Colin¹, ¹

ABSTRACT- Various studies have demonstrated that metal toxicity varies as a function of water quality parameters such as hardness, pH and dissolved organic carbon. As Water Quality Criteria (WQC) derivation and risk assessments are predominantly based on laboratory toxicity data, there is an urgent need for a better (quantitative) understanding of the influence of abiotic water quality characteristics on the bioavailability and ecotoxicity of metals. In this study empirical models are presented, developed using a Central Composite design, predicting acute and chronic zinc toxicity as a function of pH, hardness and DOC of the test medium for four three different organisms: the waterflea Daphnia magna, the green algae Raphidocelis subcapitata, the rotifer Brachionus calyciflorus, and the fairy shrimp Thamnocephalus platyurus. For all species, dissolved organic carbon proved to be the major factor reducing zinc toxicity. For D. magna, acute and chronic EC₅₀s decreased with a factor of ± 3 when DOC-levels increased from 2 to 40 mg/L. Similar reduction factors (2 to 4) were observed for R. subcapitata, B. calyciflorus and T. platyurus. Higher hardness also decreased acute toxicity for the different species. However for D. magna increased chronic toxicity was observed at high hardness (> 300 mg/L as CaCO₃) which is possible due to the contribution of Ca and Mg to chronic toxicity. A relatively small increase in acute zinc toxicity is noted for T. platyurus and B. calyciflorus when pH is increased from 6.25 to 8.25 ( ± factor 1.4), but an opposite effect is observed with D. magna (factor 2). For D. magna, the acute-to-chronic ratio remained ± 10, independently of the pH, DOC-concentration or hardness (< 300 mg/L as CaCO₃) of the test medium. From the models it can be concluded that acute zinc toxicity is only influenced by the first order terms of pH, DOC and hardness. This study demonstrates that DOC and, to a lesser extent hardness ions, affect zinc toxicity in a similar way for different species. The effect of pH on zinc toxicity, however, proved to be species-dependent.

Key words: modifying factors, pH, hardness, dissolved organic carbon