Using constructed wetlands (CWs) for treatment of acidic and neutral waters is well documented, but their application for treatment of alkaline drainage has received less focus. In addition, there is limited available data for long-term employment of CWs treating alkaline drainage, with most studies in small scale systems or pilots <5 years old.
Bauxite residue (BR) formed as a waste of alumina production, is inherently alkaline (pH 9-13) and contains elevated concentrations of Al , As and V, with Na the dominant cation. Leachates from Bauxite Residue Disposal Areas (BRDAs) have similar characteristics and will be generated for decades, thus requiring appropriate treatment prior to discharge.

A pilot scale single cell leachate treatment system has been operational for 10 years, with influent and effluent monitoring to assess its ability to reduce pH. Sediment and water analyses across the operating period give insight to system performance (pH, conductivity, soil elemental fractionation, microbial community) with this system possibly having one of the longest running datasets for an alkaline leachate CW.

Results indicate the 10-year-old system consistently reduces pH in the BRDA leachate to an average of 7.14 (±0.18) at outflow, far below the target of pH<9, with average reductions in concentrations of Al (96%), As (85%) and V (71%). Sodium also decreased (22%), with Na mostly found in soluble forms in the substrate (up to 60% of total).

New, larger multicell CWs have been constructed on a separate site on the same BRDA. The two new systems (3 cells each) began operating in spring 2024, using differing substrate organic matter content to investigate the effects of substrate content on pH and metal reduction, and whether lab-based trace element removal studies are replicated in the field.

The new system with the higher organic content has to date shown a greater pH reduction in the effluent (pH = 7.1) when compared with the system with the lower organic content (pH=7.41). Analysis is ongoing to establish whether effluent elemental concentrations differ, with longer monitoring needed over the first year and further to determine whether pH reduction across time is maintained as the systems age.

Current results of the pilots demonstrate CWs are an effective treatment method for at least a decade for alkaline BRDA leachates. Future monitoring of the new CWs will ascertain whether performances are affected using different substrate mixes, but initial findings indicate increased organic content improves the extent of pH reduction.

Acid mine drainage (AMD) is widely recognized for its environmental impact, but in carbonate-rich systems, neutral mine drainage (NMD) can occur, posing distinct risks to water quality despite its neutral pH. Effectively managing post-mining sites in the long term requires a deep understanding of the hydrogeochemical processes governing the water chemistry. While numerous studies have explored abandoned iron and coal mines, the geochemical processes are often highly site-specific. Therefore, extensive localized studies are essential to determine the main water-rock interactions, sources of dissolved elements, and the role of bacterial populations in the geochemical cycles of each abandoned mine.

This work focuses on the Gardanne lignite mine, located near Marseille (Southern France), which operates in an NMD context. The flooding of galleries in 2003 caused a noticeable deterioration in water quality, notably with substantial increases in iron and sulfate concentrations. Currently, water is pumped through the Gérard shaft, the main outlet for the mine water reservoir, to control water levels and avoid overflow into the former discharge gallery, which directly leads to the port of Marseille. To identify the main spatial sources of mineralized water in the mine, physico-chemical measurements are conducted throughout the water column of the Gérard shaft, from the water table surface down to a depth of 700 m. Based on these results, samples are collected at specific depths for detailed chemical, isotopic, dissolved gas (CFCs, SF6) and microbiological analysis.

A previous similar physico-chemical log, conducted in 2011, identified four gallery levels likely contributing water to the Gérard shaft. The current campaign aims to confirm those results or detect any changes in the groundwater flow regime within the mine reservoir. Furthermore, chemical and isotopic analyses will be interpreted in light of similar studies from nearby aquifers, which could potentially sustain the mine system through deep underground sources.

To complement this study, column and batch leaching experiments will simulate water-rock interactions using rocks from the Gardanne mine, refining our understanding of these processes. The ultimate goal is to develop a geochemical model of the mine, either semi- or fully spatialized, by integrating fieldwork and laboratory results. This work is expected to provide essential insights for the long-term management of flooded lignite mines, particularly in neutral mine drainage contexts.

In areas affected by mining, such as the Ruhr region (Germany), mine gas releases are regularly observed at the surface. These gases are composed of mixtures of methane, carbon dioxide, carbon monoxide, oxygen, nitrogen and other higher hydrocarbons emitted from a fossil deposit. Methane in particular is a very climate-harmful greenhouse gas.

Mine gas release does not end with the closure of the coal mine and can continue for decades. After the shutdown of a mine, the mine water pumps are stopped or at least continued to be operated at a reduced pumping rate. The result is a rise of the mine water level in the underground mine. As the mine water level rises, the cavity volume decreases. Mine gas will therefore be pushed to the surface by a piston effect. The migrating gas will either be caught in traps or migrate further upwards through mine workings/shafts, permeable strata or faults. The emission on the surface and release into the atmosphere can either be wide spread and diffuse or focused.

In August 2024, the EU regulation 2024/1787 on the reduction of methane emissions in the energy sector went into force. Companies are now obliged to continuously measure and quantify methane emissions and develop measures to reduce these methane emissions into the atmosphere. Against the background of several thousand locations in the former coalfield in the Ruhr area alone where methane is degassing, this requirement represents a major challenge for companies in the European Union. Only a few techniques for dealing with mine gas, even in low concentrations, have been tested and utilised to date.

An integrated monitoring, which includes hydraulic and geochemical monitoring, can help to explain the mine gas release at the surface and can identify the processes in the mine workings that influence the gas during the mine water rebound. A comparison with maps of the mine workings and continuous measurements of the mine water level provides conclusions about potential migration pathways for the mine gas. Measurements of the gas concentration and composition as well as the analysis of isotopic markers reveal processes that consume or generate methane. The evaluation of the integral monitoring can make an important contribution to optimise future mine water rebounds with regard to the regulation of mine gas release behaviour and thus directly contribute to the requirements of the EU methane regulation.

Underground mine flooding is an important consideration of mine closure due to discharges that can occur when the water table reaches specific outlets as this has potential to modify the water quality of local water sources. Understanding the rate of reflooding is important to inform water management strategies. Presented in this study is a comparison between an analytical and a numerical method for estimating the natural flooding of an underground mine.

Using an Excel-based analytical method, the flooding rate was estimated by dividing the total underground void volume by the current total groundwater inflow. This method, however, assumes a constant inflow rate which is not akin to natural conditions in the study area due to a reducing hydraulic gradient developing as water level rises in the mine relative to the water level in the surrounding rock. Therefore, numerical modelling was incorporated to scrutinise this initial estimate considering inflow rate decreases as the mine floods.

The flooding process was simulated using the existing calibrated FEFLOW model and a Python script (FEFLOW API) that relies on Volume-Elevation curves estimated from available mine wireframes. The FEFLOW-based workflow estimates a total flooding time of around 4.5 years. This result contrasts with the 1-year flooding period estimated using the analytical method, due to the assumption of a constant inflow of 5,900 m3/d which is considered too conservative. According to the simulation, the mean inflow during the first 4.5 years of flooding will be approximately 1,300 m3/d. Additionally, FEFLOW also accounts for the required time to re-saturate the surrounding rock.

Although there were benefits to the analytical method such as lower computational costs, data requirements and assumptions, the conservative approach resulted in an overestimation of the daily inflow rate. Representing more realistic conditions via transient simulations has been effectively demonstrated using numerical modelling, increasing confidence in the estimation.

The formation of pit lakes during coal mine closure poses complex environmental, hydrological, geological, and socio-economic challenges that require integrated and proactive planning. This study develops a strategic framework for pit lake formation in Indonesia by combining hydrological modelling, geochemical assessment, and regulatory analysis. A key focus is the use of nearby river water for controlled pit filling to mitigate acid mine drainage (AMD) and enhance long-term landscape sustainability. The framework is applied to Pit D2 in the Binungan River watershed, evaluating two scenarios: precipitation-only and river-supplemented filling. Hydrological modelling using HEC-RAS identifies an optimal diversion channel elevation of +4 masl to manage flood risks. Water quality modelling with PHREEQC demonstrates that river supplementation significantly improves water chemistry, reducing Fe concentrations from 211.57 to 0.94 mg/L, sulphate from 459.46 to 28.83 mg/L, and increasing pH from 5.64 to 6.07—approaching regulatory standards. Beyond improving water quality, the study highlights the potential of pit lakes as flood retention systems in high-rainfall areas such as Kalimantan. Given limited regulatory guidance on river use for pit lake filling, this research underscores the importance of considering such approaches within mine closure planning. Future efforts should further integrate pit lake development into early-stage mine design to support sustainable, multi-functional post-mining land use.