Estimation of Self-Neutralisation Rates in a Lignite Pit Lake

Joscha Opitz,Wolfgang Schäfer,Matthias Alte,Martin Bauer,Thomas Söll

doi:10.1007/s10230-020-00692-9

Abstract

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.

Highlights

Mining activities, especially open-pit coal and lignite mining, are often associated with the extensive formation of acid mine drainage (AMD)
In 1995, a mine water treatment plant was installed in close vicinity to Knappensee for chemical treatment of seepage water from an adjacent former open pit that was subsequently used as a landfill for waste rock and lignite ashes
Acidity was calculated for all samples based on the analytical results (pH, Fe(II), Fe(III), Mn, Al, Zn, Ca, Mg, SO4, Cl, N H4, Na, K, and N O3 considered for ionic strength) and corrected for CO2 using PHREEQC broadly following Kirby and Cravotta (2005) and Peine (1998)

Summary

Introduction

Especially open-pit coal and lignite mining, are often associated with the extensive formation of acid mine drainage (AMD). Neutralisation of pit lakes is usually the first step in pollution control, since solubility and mobility of hydrolysable metals (Fe, Al, Mn) as well as other contaminants closely associated with the former in post-mining waters are predominantly pH-dependent (Lee et al 2002; Zänker et al 2003). Most of the formerly 50 m deep pit was backfilled, resulting in an unusually shallow pit lake with a surface area of ≈ 550,000 m2 and a water volume of 1.2 × 106 m3. In 1995, a mine water treatment plant was installed in close vicinity to Knappensee for chemical treatment of seepage water from an adjacent former open pit that was subsequently used as a landfill for waste rock and lignite ashes. The treated water is discharged into the pit lake at the shallow northern end (Fig. 1)

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Conclusion