Cost-effective abatement of stochastic metal loading to water recipients

Relevant implementation of the EU Water Framework Directive with regard to mine water pollution of downstream water bodies requires pressure-impact assessment and catchment-scale economic optimisation of abatement measures. However, pressure-impact assessment is bound to be uncertain. This is because mine water pollution is often an unknown combination of pollution loads, from both known mine waste sites (point sources) and diffuse mine water sources, such as old mine wastes, abandoned mine voids and/or desorbing pollutants from immobile water zones and solid aquifer/sediment material. We investigate and quantify the influence of such pressure-impact assessment uncertainty on the allocation of cost-effective measures for targeted mitigation of mine water pollution loading. We use a stochastic description of total pollution loading from different possible polluted mine water sources and quantify the effect of random load variance on minimum abatement costs for a targeted 50 percent Zinc load reduction to the Swedish Dal River. Consideration of stochastic mine water pollution loading implies that cost-efficient abatement measures and associated minimum total costs within the Dal River Basin increase considerably with increasing load variance and desired probability to reach the targeted load reduction. However, this increase is only step-wise continuous because the cost-efficient abatement solutions include both continuous (e.g. constructed wetlands) and discrete (e.g. soil covers) abatement measures.