Constructed wetlands are a standard sustainable technology in waste and mine water treatment. Whereas macrophytes actively contribute to decomposition and/or removal of wastewater’s organic pollutants, removal of hydrolysable metals from mine water is not attributable to direct metabolic, but rather various indirect macrophyte-related mechanisms. These mechanisms result in higher treatment efficiency of (vegetated) wetlands relative to (unvegetated) settling ponds. Contribution of macrophytes to treatment predominantly includes: enhanced biogeochemical oxidation and precipitation of hydrolysable metals due to catalytic reactions and bacterial activity, particularly on immersed macrophyte surfaces; physical filtration of suspended hydrous ferric oxides by dense wetland vegetation down to colloids that are unlikely to gravitationally settle efficiently; scavenging and heteroaggregation of dissolved and colloidal iron, respectively, by plant-derived natural organic matter; and improved hydrodynamics and hydraulic efficiency, considerably augmenting retention and exposure time. The review shows that constructed surface-flow wetlands have considerable advantages that are often underestimated. In addition to treatment enhancement, there are socio-environmental benefits such as aesthetic appearance, biotope/habitat value, and landscape diversity that need to be considered. However, there is currently no quantitative, transferrable approach to adequately describe the effect and magnitude of macrophyte-related benefits on mine water amelioration, let alone clearly assign optimal operational deployment of either settling ponds or wetlands. A better (quantitative) understanding of underlying processes and kinetics is needed to optimise assembly and sizing of settling ponds and wetlands in composite passive mine water treatment systems.

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Natural anaerobic biogeochemical processes used for passive treatment of AMD were observed in the extensive shallow water zone of a polymictic pit lake in the former German lignite district of Upper Palatinate. Although continuously fed by acidic metalliferous groundwater, lake-pH increased from 3.5 to circumneutral over a little more than 10 years. The natural attenuation processes were studied and quantified using a regional surface-and groundwater flow model linked with hydrochemical monitoring datasets to establish a simple mass balance. The acidity inflow was estimated at ≈ 5900 kmol over the period 2014-2018, which corresponds to an average inflow of ≈ 1190 kmol/a. This estimate is in very good accordance with an acidity inflow rate for the period 2000-2014 estimated from acidity deposition in the sediment based on sediment core analyses plus the calculated cumulative acidity outflow based on extrapolation of pre-neutralisation acidity levels in the pit lake, together yielding a total acidity of ≈ 15,000 kmol, which corresponds to an inflow rate of ≈ 960 kmol/a. The results strongly indicate that the pit lake self-neutralised due to beneficial environmental and ecological conditions, amplified and potentially initialised by the circumneutral discharge from a chemical mine water treatment plant, and that well-known biogeochemical mechanisms such as natural microbial sulfate reduction were the driving force. The results give rise to perspectives concerning the potential development of pit lakes if ecological considerations are considered.

A three-stage pilot system was implemented for passive treatment of circumneutral, ferruginous seepage water at a former opencast lignite mine in southeast Germany. The pilot system consisted of consecutive, increasingly efficient treatment stages with settling ponds for pre-treatment, surface-flow wetlands for polishing and sediment filters for purification. The overall objective of the multistage approach was to demonstrate applicability and operational reliability for successive removal of iron as the primary contaminant broadly following Pareto’s principle in due consideration of the strict site-specific effluent limit of 1 mg/L. Average inflow total iron concentration was 8.4(± 2.4) mg/L, and effluent concentration averaged 0.21(± 0.07) mg/L. The bulk iron load (≈69%) was retained in settling ponds, thus effectively protecting wetlands and sediment filter from overloading. In turn, wetlands and sediment filters displayed similar discrete treatment efficiency (≈73% each) relative to settling ponds and thus proved indispensable to reliably meet regulatory requirements. Moreover, the wetlands were found to additionally stimulate and enhance biogeochemical processes that facilitated effective removal of secondary contaminants such as Mn and NH4. The sediment filters were found to reliably polish particulate and redox-sensitive compounds (Fe, As, Mn, NH4, TSS) whilst concomitantly mitigating natural spatiotemporal fluctuations that inevitably arise in open systems. Both treatment performance and operational reliability of the multistage pilot system were comparable to the conventional treatment plant currently operated on site. Altogether the study fully confirmed suitability of the multistage passive setup as a long-term alternative for seepage water treatment on site and provided new insights into the performance and interrelation of consecutive treatment stages. Most importantly, it was demonstrated that strategically combining increasingly efficient components may be used for optimisation of treatment performance and operational reliability whilst providing an opportunity to minimise land consumption and overall costs.

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