Biogeochemical influence on water quality by efficient phosphorus retention in pH-neutral open-cast mining lakes

Empirical trophic models (Vollenweider-type) are one of the basic "tools of the trade" in management of natural and artificial lakes. They are not applicable to open-cast mining lakes (PML) because they underestimate their resilience to phosphorus (P) inputs. The high iron availability causes an efficient binding for phosphorus (P) in water and sediment of PML. The main objective was use hydrogeochemical modelling to assess the risks of eutrophication in the following way:

i) to develop conceptual (empirical) models (Vollenweider type) that can be used to more accurately estimate tolerable P loading to PMLs

ii) to represent the transition range from conditions with Fe excess to natural conditions in a model structure

iii) to identify specific indicators and tipping points for different P retention.

We analysed hydrological data, loads, and sediment properties to derive a closed P balance for 29 neutral mining lakes. The latter cover a range of water retention times and external P inputs. We distinguished three phases of PML development or maturation and the relevance of P-binding processes or binding forms. The Fe:P ratio in sediment was found to be the most important predictor of P retention. By integrating this ratio into conceptual models it is now possible to predict in-lake P-concentration from P inputs for PMLs and even for lakes with decreasing Fe import at the transition to “natural lake” conditions. We conclude that processes such as Fe~P adsorption and vivianite formation under anoxic conditions most likely ensure high P retention in the long term.