Towards Regulatory Criteria for Discharging Iron-rich Mine Water into the Sea

Appropriate criteria are needed for regulating the discharge of mine water into the sea. Simply applying criteria developed for freshwater ecosystems to marine settings would be naïve and highly inappropriate because marine organisms have very different mechanisms for coping with high concentrations of cations than do freshwater species. Furthermore, the hydrodynamics and geochemistry of marine waters mean that certain processes that are very important in determining the magnitude of mine water impacts in freshwater ecosystems are largely irrelevant in the oceans. This is particularly the case relative to benthic smothering by ochre, which does not occur when mine waters are discharged to the sea. Visual impacts of mine water discharge are still important in marine systems, due to the possibility of developing unsightly “slicks” of suspended ochre on the water surface (albeit these are actually innocuous in ecological terms). Avoidance of ochre slicks is a common concern wherever ferruginous mine waters are discharged to coastal waters. Compilation of data from a range of case studies indicates that no visible plume of ochre would be expected where the rate of iron release is less than about 200 kg/day. Maintenance of iron loadings below this critical threshold can be ensured by calculating a target maximum iron concentration (Fe_MAX, in mg/L) for the final effluent (which must be achieved by treatment if necessary) using the simple formula: Fe_MAX = 2,314.8/Q _MAX, where Q _MAX is the maximum anticipated flow rate in litres per second from the mined system (pumped or flowing by gravity).