Addition of bulk organic matter to acidic pit lakes may facilitate closure
Mine Environment Neutral Drainage (MEND)
Place of Publication
Brown, A., Burgess, J., Castendyk, D., Kirk, L., McPhee, J., Seal, R., Williams, D., Wolkersdorfer, C., Bucknam, C., Carballo, M., Figueroa, L., McLemore, V., O'Kane, M., Wiertz, J., & Wilson, W.
Mine Water and Environment Research Centre
Macro nutrients (C, N, P) are essential for sustaining freshwater ecosystems. However, acidic pit lakes are often nutrient-poor a low pH levels and metal complexation (by A1, Fe and Mn) reduces dissolved P and C concentrations. In contrast, N is often not limiting as it is readily available from groundwater and blasting residues.
The Collie basin in Western Australia has a lake district of eleven acidic coal mine pit lakes (pH 3-7, low metals/metalloids and sulfate concentrations). Our previous researches at – microcosm, mesocosm and field scale demonstrated low C and P concentrations limit algal productivity. Addition of organic matter (in the form of terrestrial leaf litter) offered potential bioremediation benefits including habitat creation, sediment formation, the potential for sulfate reduction, and slow release of macronutrients to promote algal growth.
We tested the effect of organic matter additions of straw, lawn clippings and eucalypt leaves on acidic pit lake water and sediment in 1,200 L replicated mescosms. We regularly measured water quality in each mesocosm over a year and concluded by measuring the development of macroinvertebrate communities. The highly labile C found in lawn clippings stimulated algal biomass, macroinvertebrate diversity and abundance, and improved water quality. Straw and eucalypt leaves did not alter water quality substantially but did increase abundance of macroinvertebrates.
Additions of nutrients, particularly as bulk organic matter increased macroinvertebrate abundance but also altered species composition from carnivorous highly mobile opportunists found in the controls to a range of herbivores/omnivores, while not necessarily altering water quality substantially. The presence of labile C (particularly in the grass) was able to increase algal biomass and improve pH. If conservation values are important for closure of pit lakes and eutrophication is not a risk, then addition of bulk organic matter (particularly labile forms) should be considered in closure plans. Organic matter could be actively added or passively through the catchment. As environmental improvements were seen in all treatments, adding many readily available organic matter types to pit lakes, may be helpful to achieving closure objectives associated with biodiversity.