U:\2021\BM210482_IBo_Waihi_North_Project\Documents\WNP_Freshwater_Report_May_2022\FINAL_Freshwater_Report_FOR DELIVERY_June 2022\FINAL DELIVERED 17 JUNE 2022\WAI-985-000-REP-LC-0007_Rev0_Freshwater_20220621_FINAL.docx 20.6 Effects of sulphate on ecological values Sulphate33 occurs naturally and ubiquitously in the aquatic environment, and is released by many aquatic, terrestrial and geological processes. As a result, natural concentrations of sulphate can vary depending on the prevailing catchment geology and hydrology. Sulphate is considered one of the least toxic of the major ions present in surface waters and is often linked more to osmotic stress rather than sulphate toxicity itself. A recent global review of sulphate in freshwater ecosystems found that very little is known about the concentration thresholds at which sulphate becomes toxic (Zak et al. 2021). Nevertheless, tests of lethal and inhibitory concentrations of sulphate have also shown that sensitivity of aquatic organisms to sulphate is decreased with increasing water hardness (and chloride). It was also evident that the removal rate of sulphate increases with water temperature. The reported range of concentrations for effects of sulphate on a range of aquatic organisms is wide (~137 to 3,231 g/m3) depending on species, life-stage, type of test and water hardness. Little data is available for the effects of sulphate on New Zealand aquatic organisms. However, some of the reported overseas concentrations that cause effects in some organisms are reached (here reported as annual median and annual maximum) within the Ohinemuri River at Waihi (Table 46). Notwithstanding specific water hardness measures at a point in time, we note that annual maximum in-river sulphate levels range from 4.1 to 580 g/m3, and annual median ranges from 3.7 to 320 g/m3 within the Ohinemuri River in the vicinity of Waihi. The range narrows slightly in the lower reaches below the OGNZL discharge at Waihi (67 to 580 g/m3 for annual maximum, and 17 to 320 g/m3). Notable increases in concentrations of sulphate are evident downstream of the treated water discharges. In their review, Zak et al (2021) report sulphate concentrations that cause inhibitory and/or lethal effects on a variety of organisms. Most relevant to the aquatic ecological values of the Ohinemuri River are the concentrations that effect salmonids (rainbow trout (Oncorhynchus mykiss), embryo and fry). Reported concentrations for rainbow trout ranged from 174 g/m3 (IC/LC 1034) at low water hardness (i.e., 6 mg CaCO3/L) to 3,670 g/m3 (IC/LC 5035 at 100 mg CaCO3/L). Sulphate concentrations effecting other aquatic organism survival (e.g., amphipods, molluscs, algae) were typically higher (cf. 1,759 g/m3 for cladoceran Ceriodaphnia dubia (IC/LC 10), 1,502 g/m3 for mollusc Sphaerium simile (IC/LC 10). On the other hand, sulphate concentrations were lower and within the range experienced within the Ohinemuri River for inhibitory effects on reproduction and/or embryos. Ryder (2019b) reported toxicity testing for sulphate that has been conducted using Taieri flathead galaxias (Galaxias depressiceps). The most sensitive stages of the flathead galaxias (eggs and larvae) were exposed to a range of concentrations of diluted mine waste rock seepage. The principal constituent of seepage is sulphate and the testing used sulphate concentrations ranging from 100 to 3,000 g/m3. No impact was identified on ova and there was no evidence of a toxicity effect during any of the egg development stages. Actual mortality effects did not occur until sulphate concentrations of between 1,640 and 1,920 g/m3. Sulphate concentrations recorded downstream of the OGNZL discharge to the Ohinemuri River at Waihi are below these concentrations recorded as impacting on Taieri flathead galaxias survival, but exceed the concentrations set out by Zak et al. (2021) as inhibitory or lethal for rainbow trout embryo and fry. Accordingly, there is the potential for the sulphate levels within the treated 33 Sulphate is the anion of Sulphuric Acid. 34 IC/LC 10 = Inhibitory concentrations affecting 10% of sample/experimental populations. 35 IC/LC 50 = Inhibitory concentrations affecting 50% of sample/experimental populations.
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