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In-Lake Neutralization: Quantification and Prognoses of the Acid Load into

a Conditioned Pit Lake (Lake Bockwitz, Central Germany)

Kai-Uwe Ulrich1, Christian Bethge

1, Ina Guderitz

1, Ben Heinrich

1, Volker Neumann

1, Claus

Nitsche1, and Friedrich-Carl Benthaus

2

1BGD Soil and Groundwater Laboratory GmbH, Tiergartenstraße 48, 01219 Dresden, Germany;

2LMBV

Lausitzer und Mitteldeutsche Bergbauverwaltungsgesellschaft mbH, Knappenstraße 1, 01968 Senftenberg,

Germany; corresponding author’s e-mail: kulrich@bgd-gmbh.de

Abstract: The formerly highly acidic pit Lake Bockwitz south of Leipzig (Germany) has been

repeatedly treated since 2004 with soda ash to meet water quality criteria for the lake effluent. Intense

monitoring of water quality parameters showed that previous predictions underestimated the acid load

into the lake. Field research and lab experiments were designed to identify and quantify the processes

responsible for re-acidification. Monitoring data and key parameters from intermittent-flow column

experiments were integrated in hydrogeochemical and physical transport models. The combined lake

budget model indicated that re-acidification was dominated by leaching of acid sulfide mineral

weathering products from the Tertiary bank substrates. High inputs of iron, aluminum, and sulfate

were generated by infiltrating rain water, interflow, and groundwater recharge. In contrast, acid loads

from surface runoff and soil erosion were minor at this particular site. Based on this work, a

methodology is proposed to obtain critical parameters from field and lab investigations and integrate

those into hydrogeochemical and physical transport models. These process-based models offer tools to

reliably predict the water quality of mining pit lakes, develop appropriate treatment measures for the

rehabilitation period, and plan the requirements for cost-effective lake water conditioning.

Keywords Acid mine drainage • Lignite mine pit lake • Rehabilitation • Re-acidification • Acid-base

balance • Predictive modeling

1 Introduction

The development of a post-mining pit lake amenity is in the best interest of mine operators, public

authorities, and regional stakeholders. In densely populated areas of Central Europe, lake ecosystems

are welcome landscape components for the purposes of recreation, nature conservation, and river basin

management. Depending on the planned use of a pit lake and its environment, common problems to be

solved include (i) the management of local/ regional water balance (type and duration of lake filling,

adjustment of groundwater level, regulation of lake discharge), (ii) prevention of landslides on the

slopes of overburden dumps during rewetting, and (iii) the degradation of water quality from acid

mine drainage (AMD). As soon as a flooded pit lake produces a discharge into the connected river

basin, its water quality has to be adopted to stringent limits of German water laws based on the scope

of the European Water Framework Directive, which requires prevention of any deterioration in the

water quality. Hence the discharge of acids (measured by pH), salts (concentrations of sulfate and

ammonium), and metals, including Fe and Al, from mining areas is limited.

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However, most of the recently created lignite mining pit lakes in Germany do not meet the water

quality requirements. Acid load from connected aquifers is due to weathering of sulfide minerals,

mainly in the overburden substrate (Blodau 2006; Schultze et al. 2010). Hence, water treatment is

required either ex situ (e.g. treatment plants for discharge) or in situ (e.g. in-lake neutralization) (LUA

Brandenburg 2001). Such treatment measures are extremely expensive, and in situ treatments interfere

with use of the lake.

Prediction of the hydrology and water quality of a lake is essential for developing appropriate and

cost-effective treatment measures. Modeling is an indispensable tool for assessing the potential range

of water quality parameters. The basic principles and challenges of the conceptual design and the

processes to be considered in water quality models of pit lakes are well known (e.g. Castendyk 2009;

Vandenberg et al. 2011). However, as pointed out in the latter reference, more effort is needed to

improve the accuracy of such predictions, and to rank the sensitivity of pit lake water chemistry to

environmental processes to determine which are most significant.

Approaches to predict the water quality of pit lakes are usually site-specific and have been categorized

into four study types by Castendyk and Webster-Brown (2007). According to their concept,

geochemical predictions of Type I studies are based on detailed observations of climate, bedrock

mineralogy, and existing water chemistry and/or bedrock leaching tests, plus predictions of

groundwater hydrology, lake water balance, and physical limnology. Type I studies are uncalibrated

(in particular, when used prior to pit lake genesis). Using the identical input data as Type I studies,

geochemical predictions by Type II, Type III, and Type IV studies are calibrated, making them more

robust. In Type II studies, the Type I geochemical model is adjusted until the predicted geochemical

data match the observed data from laboratory experiments in which representative input waters are

mixed. In Type III studies, the geochemical prediction based on a Type I model is calibrated with

water chemistry data from a pre-existing (temporary) lake within the pit. Type IV studies compare

geochemical predictions to post-closure pit lake observations to calibrate and validate the model.

References for each of these four study types are given in Castendyk and Webster-Brown (2007).

However, no study is referenced that integrates results from process-oriented field investigations and

laboratory experiments to pit lake monitoring in order to calibrate a multiple compartment model.

Apart from one other study in which several processes and compartments were incorporated, mainly

through empirical approaches and theoretical considerations (Werner et al. 2001a, b), the present site

study is likely to add a new category to the proposed classification.

To the best of our knowledge, Lake Bockwitz is the first lake in Germany of its size that has been

treated in situ to accelerate the natural neutralization process and overcome the acidity maintained by

the iron buffer. The amount of alkalinity (281∙106

mol equivalents, i.e. 14,620 metric tons (t) of soda

ash) added between 2004 and 2007 even exceeds that applied in the largest previously reported liming

action in a single lake (Lake Orta, Italy: 214∙106 mol equivalents, i.e. 10,700 t of pure CaCO3 added as

powdered limestone; Bonacina 2001). Lake Bockwitz is a good example of a pit lake that was

expected to naturally establish neutral water quality within a few decades, and where this process

could be accelerated by adding alkaline materials. Soda ash was chosen for the purpose of both

neutralizing the lake water and providing enough buffering capacity against the ongoing, but

dwindling loads of acidity from the drainage area. The demand of soda ash was calculated by means of

geohydrological modeling based on groundwater quality data collected since 1997. In-lake supply of

soda ash was proposed as more economical than construction and operation of a treatment plant for the

runoff to realize compliance with the regional state authority criteria (DGFZ 1998; Guderitz et al.

2003).

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However, the neutralization concept of Lake Bockwitz failed in that the acid loads to the lake were

higher than predicted and consumed all of the excess alkalinity, leading to re-acidification of the lake

water (Neumann et al. 2008). This rapid re-acidification led to the hypothesis that major sources of

acidity within the drainage area had been underestimated. Potential reasons for underestimating the

acid load into Lake Bockwitz include:

higher acid loads from surface runoff, abrasion, and soil erosion than initially expected;

change of groundwater quality during transit from permanent monitoring wells to the lake

(distances of >100 m);

ongoing acidification of uncovered overburden substrate due to sulfide oxidation;

ion diffusion and exchange processes at the sediment-water interface, with Na+ migrating into

the sediment and protons being released.

The goal of this study was to identify and quantify the predominant sources (and sinks) of acidity and

alkalinity of Lake Bockwitz. The methodology that we developed combines process-oriented

monitoring of multiple field compartments with laboratory experiments. These are designed to

determine key parameters of process-based sub-models and integrate those results into a complex

hydrogeochemical mass balance model for the lake. The aim of this approach is to retrace and predict

the lake water quality at a higher level of reliability, and to determine the future demand of

conditioning material. Unlike most other acid pit lakes, a few site-specific features of Lake Bockwitz

had to be considered as well, e.g. the small watershed and groundwater inflow, mainly to the

epilimnion. This article builds on a recently published conference paper (Heinrich et al. 2011).

2 Study Site

2.1 Geology, Lake History, and Current Status

Open-cast lignite mining took place in the periphery of Leipzig (Saxony) for about a century.

Operation of the Borna-East Mine lasted from 1961 to 1992 and stopped after excavation of the lignite

seam II, leaving behind several pits and an untouched deeper seam beneath a natural barrier layer of

Tertiary clays. Founded on a shelf plate of Elster-Pleiße till, the open pit is confined by dumps to the

south and west, and by undisturbed terrain to the north and east. This area consists of glacial and

fluvial sedimentary deposits that are truncated by the bank slopes and laterally connected to the dump

terrain. The pit bottom belongs to the lignite seam II, which was only partially excavated. Fluvial

sands located between seam II and seam IV act as aquifers that are connected to upper aquifers due to

erosional processes in the Tertiary and Quaternary age deposits. The soils of these aquifers are

characterized by sulfide concentrations around 0.2 % of dry weight (d.w.) and elevated Darcy perme-

ability (kf ≈ 1·10-4

– 1·10-5

m/s). Alternate layers of clay and lignite serve as aquitards (kf ≈ 1·10-8

–

1·10-9

m/s) and facilitate lateral groundwater seepage into the lake from the natural terrain. The bank

slopes consist of mixed substrates of the truncated aquifers with highly variable sulfide concentrations,

from <0.005 to 0.74 %

d.w. The dumped soils are characterized by low Darcy permeability (kf ≈ 1·10

-6

to 1·10-7

m/s). Precipitation on the dump site mainly enters the lake as runoff, and infiltrated water can

be stored for relatively long periods of time.

Lake Bockwitz formed as the last lake downstream and the largest lake in a series of smaller lakes that

formed within the former mining area. Flooding lasted from 1993 to 2004 and was solely based on

groundwater originating from the Tertiary fluvial aquifers above seam II. About 85 % of all inflow

entered the lake’s epilimnion laterally from the southeastern and eastern terrain. Surface water from a

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series of three smaller pit lakes located upstream within the southern dump area enters Lake Bockwitz

to the south (Fig. 1, aerial view). To the north, the lake effluent feeds the Saubach creek, which has

been drained off into the Eula and Wyhra creeks since 2007 (Carmienke et al. 2011). The water table

of Lake Bockwitz has to be balanced at roughly +146 m above sea level to support the bank slopes.

Currently, Lake Bockwitz has a maximum depth of 19.5 m, a volume of about 18.4·106 m

3, and a

surface area of ≈1.7 km2 (Neumann et al. 2008).

The above-ground drainage area (≈3.1 km2) mainly comprises the bank slopes on which several

trenches exist. About 30 % of this drainage area is uncovered (blank substrate and flutes), while

≈30 % is covered by scattered pioneer vegetation and grassland, ≈15 % by comprehensive grassland,

and ≈25 % by young stands of trees (birch and pine). Much of this area has been designated for nature

conservation since 2003.

Fig. 1 Aerial image (courtesy of LMBV) of the Lake Bockwitz monitoring site with locations of sensing and

sampling sites. Of 11 permanent groundwater wells (yellow circles) located 100–200 m upstream from the lake

shore, only 10 are visible on this display detail. Six temporary wells can be seen at three locations (red circles)

on the bank slope close to the shoreline.

2.2 Hydrology

The climate data used in the present study was provided by the German weather service (Deutscher

Wetterdienst, DWD), recorded at the weather monitoring station Leipzig-Schkeuditz located about

40 km northwest of Lake Bockwitz. Consistent with the temperate climate zone of Central Europe,

annual precipitation is highly variable. While the annual precipitation in 2009 (620 mm/a) was close to

the 30-year average of 585 mm/a, annual precipitation was substantially lower in 2008 (490 mm/a),

and almost 25 % higher in 2010 (720 mm/a) than the average.

The amounts and percentages of surface runoff, evapotranspiration and infiltration, as well as

interflow and groundwater recharge were determined from data collected at an erosion monitoring plot

located on the western slope of Lake Bockwitz (Fig. 1). Based on the long-term modeling of the local

water balance, inflow values predicted for 2010 amounted to ≈2.3 m3/min for the average groundwater

inflow (including seepage water and interflow) and ≈0.6 m3/min for surface water from the southern

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inlet. Discharge of Lake Bockwitz was predicted to be ≈0.8 m³/min for groundwater effluent to the

NW direction and ≈2.0 m3/min for surface effluent into the Saubach creek on the north side of Lake

Bockwitz (IBGW 2010). The median surface effluent recorded from 2008-2011 was ≈3.8 m3/min,

likely due to the elevated precipitation in 2010.

2.3 Treatment

The initial water of Lake Bockwitz appeared strongly acidic (pH 2.7, Fetot ≈55 mg/L, Altot ≈19 mg/L)

and did not meet the criteria permitted by the water authority (pH >6, Fetot <3 mg/L, Altot <0.5 mg/L).

The Lausitz and Central-German Mining Admin Company (LMBV mbH) was responsible for

establishing an adequate water quality for the effluent and deciding on appropriate treatment

technology. Based on conclusive water balance modeling and groundwater monitoring data collected

since 1997, expert studies (DGFZ 1998; Guderitz et al. 2003) predicted that after neutralization of the

established lake, continued groundwater inflow should outweigh the estimated external loads of

acidity, leading to self-sustaining neutral water. Adding soda ash to the lake was identified as being

more economical than constructing a treatment plant for the lake effluent.

Treatment of Lake Bockwitz began in March 2004. Light soda ash (99.4 % Na2CO3) was poured into

the lake just below the surface through a floating pipeline located in the southern part of the lake

(Neumann et al. 2007; Rönicke et al. 2010). Within two years (2004-05), a total of 12,870 t of soda

ash was used to neutralize the free acid and the iron buffering, increasing the pH to ≈5 in the lake,

corresponding to a unit-area addition of ≈7.6 kg/m2 and achieving an alkalinity of ≈140 mol/m

2 (given

in terms of equivalents unless otherwise indicated). The acidity of the lake water (also stated in terms

of equivalents) was 8.1 mol/m3 prior to the soda treatment. This corresponds to an acid inventory of

≈147,000 kmol in the whole water body, or a unit-area value of 86.5 mol/m2. Based on the ratio of

acid inventory in the lake water to the added alkalinity, the initial soda treatment in 2004-05 had a

maximum efficiency of 62 %. A more detailed balance calculation in 2007/08 indicated an efficiency

of 65 %. Furthermore, it was estimated that ≈10 % of the added alkalinity neutralized the ongoing acid

load from the surrounding overburden slopes and subsurface sources, ≈20 % were consumed by

neutralization processes in the upper part of the lake sediment, and ≈5 % was assumed to be lost by

calcite precipitation (Neumann et al. 2007, 2008; Rönicke et al. 2010). Considering the additional

alkalinity sinks, the soda treatment efficiency was between 90 and 95%.

The in-lake neutralization has been monitored from its onset by an extensive program of sampling and

analysis of groundwater, lake water, and sediments (Table 1). After stopping the initial soda treatment

in late autumn of 2005, the continued monitoring of water quality parameters revealed a rapid decrease

of pH and increase of acidity (base neutralizing capacity) of the lake water within the first few months

of 2006. Therefore, in-lake treatments have been continued at irregular intervals to maintain the pH >6

in the lake water.

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3 Material and Methods

3.1 Field Investigations and Monitoring

Initial constraints for planning the field investigations of Lake Bockwitz and its drainage area were

that the lake had reached its final water table, was repeatedly treated to overcome the Fe and Al

buffers, and generated effluent into the Eula creek. Figure 2 shows a scheme of processes and system

components that were considered as potential sources of acid loads. Table 1 summarizes the number of

sampling locations, sampling periods, and intervals, and the sensing devices used for determining

input fluxes. Most of these sampling locations are roughly visible on the aerial view of Lake Bockwitz

(Figure 1).

Fig. 2 Schematic drawing of major system components investigated to determine fluxes of acidity and alkalinity

into Lake Bockwitz after its neutralization. Positive and negative values indicate the BNC and ANC fluxes in

kmol/day, predicted for 2010 according to Table 6. Acid release in the vadose and saturated zones was

determined by intermittent-flow column experiments.

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Acid and alkaline input from surface runoff and abrasion was quantified on a 30 m2 field plot by an

automated erosion recording and sampling device. Technical components included a tailrace/ trench, a

flow measure, an automated sample rotor for 24 bottles (1 L per sample), a rain gauge with integrated

sampling, and a frequency domain reflectometry sensor for continuous recording of soil temperature

and soil moisture content. Runoff samples (water and soil suspension) were automatically taken during

precipitation events that induced surface runoff, transported biweekly to the laboratory, and analyzed

for the set of parameters described below.

Data on long-term groundwater composition of specific aquifers were obtained from the groundwater

monitoring network operated by the LMBV since 1997. Eleven permanent groundwater monitoring

wells were sampled and analyzed once a year according to the mining-related hydrological monitoring

standard (MHM, LMBV 2007). To investigate the change in groundwater quality between permanent

monitoring wells and the lake shore, six temporary groundwater monitoring wells were installed at

three locations close to the shoreline (Fig. 1, red circles). At each location, the upper filter screen

represented groundwater of the vadose zone likely affected by sulfide mineral weathering, while the

lower filter screen characterized the saturated aquifer unaffected by sulfide oxidation (Fig. 2). This

filter level was almost equal to the level of the upstream permanent monitoring wells. Such level-

oriented sampling enabled us to determine the actual quality of groundwater reaching the lake and to

study the partly recharge-driven change of groundwater composition on its transit through the

overburden substrate. The six wells were sealed with packers to avoid penetration of oxygen into the

groundwater and subsequent change of the groundwater chemistry. To collect seepage water

originating from interflow and groundwater recharge exfiltrating out of the bank slopes above the

water table (Fig. A1a, Electronic supplementary material (ESM)), four water samplers were installed

and sampled four times in 2010 (Table 1).

Ultrasonic drilling was applied to obtain soil cores and explore the stratigraphic structure of the bank

slopes from the surface to the aquitard. At both ends of each liner, subsamples were taken on site to

gain soil eluates in which pH and electrical conductivity (EC) were measured. In the laboratory, soil

slurries were shaken with 1.5 % hydrogen peroxide (H2O2) solution and the pH values were recorded

after 1, 3, 24, and 48 h to determine the acid release by oxidation (“quick-weathering” test). Based on

the batch results representative soil cores were selected for running column experiments. In addition,

homogenized soil samples were analyzed with respect to acidity, alkalinity, total metals, total sulfur,

and sulfide contents.

Lake water and sediment composition were characterized quarterly near the maximum depth

(Table 1). Additional water samples were taken fortnightly or monthly at distinct locations and water

depths (0, 5, 10 m, and above lake bottom) along the longitudinal axis. Water composition and

discharge of the main inlet and outlet of Lake Bockwitz were monitored monthly. Sediment samples

were collected at least once a year by undisturbed cores at the same locations used for water sampling.

All analytical methods followed the MHM code of practice (LMBV 2007), which includes sampling

methods as well as analytical methods for all relevant types of samples based on international (ISO,

EN) or German (DIN) standards of examination. For some well-founded exceptions, AMD specific

methods are listed. This MHM code of practice guarantees high quality data and their comparability

among all the monitoring networks within the LMBV scope of responsibility. Samples were analyzed

with respect to pH, EC, redox potential (EH), dissolved oxygen, KB4.3 (if applicable), KB8.2 (BNC, base

neutralizing capacity; titration without addition of tartrate-citrate buffer), KS4.3 (ANC, acid neutralizing

capacity), metal cations (ICP OES), anions (ion chromatography), ammonium, and ortho-phosphate

(UV-VIS spectrophotometry).

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For calculation of the fluxes and lake budgets of acidity and alkalinity, chemical equilibrium with

dissolved O2 and CO2 in the lake water was achieved in PHREEQC modeling based on the measured

water quality data set (see “Modeling” section). Finally, net acidity (ACYnet) was calculated as the

difference of the calculated values of BNC minus ANC. Net alkalinity (ALKnet) is the absolute value

of the negative net acidity.

3.2 Laboratory Column Experiments

An intermittent-flow column experiment (ICE) was designed to mimic on a truncated timeline the

combined processes of sulfide weathering within bank substrates and leaching of acid weathering

products accumulated in the pore space and porewater. Air-saturated synthetic rain water was

percolated in an upward direction into a soil column of uncovered Tertiary material (11.0-

11.5 m below surface) from the vadose zone (Fig. 2). The length and diameter of the column were

50 cm and 5 cm. The effluent percolate water was collected in evacuated air-tight Tedlar bags. Ten

pore volumes were exchanged by intermittent infiltration flow; i.e. the pump speed was adjusted to the

exchange of one pore volume within the first 24 h, followed by 24 h of stagnancy. Scaled up to the

field site, one column exchange translates into a time period of 20-25 years. Hence, compared to the

natural conditions of the vadose zone, transport of dissolved oxygen through the column could be

assumed faster than gas diffusion under naturally unsaturated conditions.

3.3 Modeling

The framework of all model calculations was set with a regional large-scale geohydrological model

(HGMS) based on the finite-volume groundwater flow and transport model PCGEOFIM (Müller et al.

2003, 2008). This 3-dimensional model uses hydrological and geological data to calculate the regional

water balance including the flows of groundwater, precipitation, surface runoff, discharge from Lake

Bockwitz and its tributaries as well as upstream lakes. The consistency of the water balance has been

validated by the measured levels of the groundwater and lake water tables as a function of time

(Guderitz et al. 2003). The model has also been used to calculate the groundwater-borne fluxes to and

from Lake Bockwitz by multiplying the calculated discharge with measured solute concentrations (i.e.,

Qcalc x Cobs). Mass conservation of all components including influents and effluents has been checked

and validated as a function of time. Additional submodels were implemented for calculating mass

transport from other system components (Fig. 2) into Lake Bockwitz, which was ultimately treated as

a continuously mixed reactor in equilibrium to the atmosphere (Fig. 3). The type of submodels and the

input data used to feed them are described hereafter.

The monitoring data from the erosion field plot were used to feed the code E3D (Schmidt 1996). This

model quantifies rainfall-induced soil erosion in catchments based on physically founded, process-

oriented algorithms. The model was calibrated and validated for the monitoring plot using rates of

surface runoff and loss of matter recorded during the 2008-2009 sampling period. Detailed mapping of

sub-areas with similar hydrological characteristics and relief analysis based on GIS (Geographic

Information System) analysis allowed an upscaling of the processes to the whole drainage area of the

lake. The model calculated the distribution and concentration of the runoff including detachment,

transport, and deposition of solids on the bank slopes along the shoreline (see ESM).

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Fig. 3 Model framework used to predict the lake water quality of Lake Bockwitz. Calculated discharge data

(Qcalc) and observed concentration parameters (Cobs) fed four sub-models by which individual fluxes of water

chemistry parameters were calculated as the input variables of the LAKE budget model. Using the open-source

code PHREEQC and assuming a mixed water body in equilibrium with atmospheric conditions (at given partial

pressure of O2 and CO2), the LAKE model output generated calculated concentrations as a function of time

[C(t)calc]. By validating the LAKE model to concentrations [C(t)obs] observed over a period of 3.5 years, the

model was approved as suitable for generating a water quality prognosis of Lake Bockwitz.

In order to model small-scale effects such as precipitation and groundwater recharge on the bank

slopes of Lake Bockwitz, a transient 2D hydraulic transport model based on the software HYDRUS

(Šimůnek et al. 2006) was adapted for this purpose. The overall bank slope area was divided into five

plot areas, for each of which a characteristic cross section was implemented in HYDRUS. Each cross-

section ranged laterally (x axis) from lake to hilltop and vertically (z axis) from surface to the

saturated zone and reflected the individual geology, soil stratigraphy, physical soil properties, natural

slope geometry, surface discharge coefficients, and groundwater saturation conditions. Another

boundary condition was the surface vegetation, which was divided into four categories: (i) blank

substrate (no vegetation), (ii) scattered pioneer vegetation or grassland, (iii) comprehensive grassland,

and (iv) young stands of trees. The hydraulic transport model was parameterized for the vadose zone

of bank slopes using soil water retention functions determined in the laboratory (see Electronic

Annex). For each plot area, the daily amounts of infiltrating precipitation were calculated as a function

of evapotranspiration potential (Haude 1955) and interception (Hoyningen-Huene 1983) based on a

20-years (1991 to 2011) dataset of daily climate records. The model output generated flow velocities

for groundwater recharge, interflow, and exfiltration at the lower parts of the bank slopes. From this

data and the measured porosity of the bank slope substrates, we calculated the retention time (duration

of one pore volume exchange). By then relating the leaching and weathering functions obtained from

the ICE (dependent on the pore volume exchange) to the exchange of pore volumes calculated for the

bank slopes substrates, we were able to calculate the input fluxes to the lake for different hydrologic

conditions with respect to weathering and elution processes.

The driving force for ion exchange processes across the sediment-water interface is molecular

diffusion against concentration gradients. Given the relatively low Darcy-permeability of the lake

sediment (< 10-7

m/s), hydrolysis of Fe3+

and Al3+

and ion exchange processes on the substrate surface

can be assumed faster than the physical transport of solutes within the porous medium. Under this

assumption, the acid release from the sediment can be approximated using Fick’s laws of diffusion.

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The hydraulic transport model HYDRUS 2D was recently used to calculate the release rate of acidity

as a function of time across the interface of submerged overburden substrate at another pit lake south

of Leipzig (Lake Zwenkau, Ulrich et al. 2011a, b). This transport model was adapted to the average

properties of Lake Bockwitz sediment to calculate the time-dependent loss of bulk acidity as a

function of sediment depth based on a vertical resolution of 1 mm (see ESM).

The fluxes in all of the above-mentioned flow and transport models were calculated from monthly or

annual median or mean values dependent on the available data sets. The fluxes were then merged into

the LAKE model, a hydrogeochemical budget model of the lake water chemistry based on PHREEQC

(Parkhurst and Appelo 1999). This open-source code contains a widely accepted thermodynamic

database of minerals’ and gases’ solubility, redox chemistry, sorption equilibria, and ion complexation

reactions. The added alkalinity was also considered in the LAKE model (Fig. 3). The model ran on a

monthly time step and assumed dynamic mixing of the water body and chemical reactions at

equilibrium in the lake water. The LAKE model was validated with water quality data obtained bi-

weekly at 5 to 20 monitoring sites (Table 1) over a period of 3.5 years (see “Model Validation” in the

“Results” section).

4 Results

4.1 Compartments and Processes Contributing to Acid and Alkaline Load of Lake Bockwitz

4.1.1 Surface Water Inflow

The monthly or bi-monthly monitored water chemistry data and recorded discharge values of the inlet

were used to calculate the loads of acidity including sulfate, iron, and aluminum based on PHREEQC.

Table 2 presents a statistical summary of the water quality for the 2008-10 monitoring period,

indicating strongly acidic water quality and highly variable dissolved iron and sulfate concentrations.

For 2010, an average net acidity load of ≈6 kmol/day was calculated for the direct inflow of surface

water into Lake Bockwitz.

4.1.2 Precipitation and Surface Runoff

Monthly accumulated precipitation and runoff recorded at the erosion monitoring plot are shown in

Figure A2 (ESM). Of the average annual precipitation, 30 % was lost to evapotranspiration, 20 %

formed surface runoff, and 50 % infiltrated into the ground. The infiltration flow calculated for the

given vegetation coverage was 0.48 ± 0.07 m3/day, of which 90 % moved laterally as interflow, while

10 % percolated and ultimately contributed to groundwater recharge. As expected, the rain water was

unbuffered, and direct precipitation on the water table was neither a source of acidity nor of alkalinity

(Table 3). The average surface runoff was slightly acidic (Table 3) and delivered an average net

acidity of ≈0.1 kmol/day. The annual total surface runoff showed considerable variability, ranging

from 29·103 m

3 in 2007 to 380·10

3 m

3 in 2008. These data translate into annual loads of net acidity of

15 kmol (2007) and 200 kmol (2008) (Table A1, ESM).

4.1.3 Soil Erosion

Soil erosion monitored at the field plot is displayed on a monthly basis in Figure A2 of the ESM. By

GIS-based intersection of soil properties and land use data, sparsely and non-vegetated areas were

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identified as prone to soil erosion, comprising altogether about 11% (0.46 km2) of the total catchment

(Figure A4, ESM). Consistent with the high variability of precipitation and runoff, the input of solid

matter and associated net acidity amounted to 3.0 metric tons with 0.6 kmol in 2007 and 94 metric

tons with 20.0 kmol in 2008, respectively (Table A1, ESM). The average load of acidity amounted to

≈0.1 kmol/day.

4.1.4 Interflow and Groundwater Recharge

The critical water quality parameters from temporary wells on the bank slope near the shoreline

(Table 4) indicated a substantial change in groundwater quality compared to the upstream monitoring

wells located 100-200 m away from the lake (Table 5). While the latter groundwater was characterized

by neutral pH und net alkalinity, the groundwater close to the lake shore revealed high acidity. A

general trend showed that close to the shoreline, the excess acidity of groundwater from the upper

filter screen (10-20 mol/m3 on average) was about 3 to 20-times higher than that of groundwater from

the lower filter screen (0.2-6 mol/m3 on average) (Fig. 4). Even higher ACYnet mean values up to

45 mol/m3 were analyzed in seepage water collected from the bank slopes in 2010. The high ACYnet

values correlated with elevated concentrations of Fe, Al and sulfate, suggesting leaching of mineral

weathering products. In contrast, the far-upstream groundwater revealed an average excess alkalinity

of 3-5 mol/m3.

To validate the model assumptions of vertical zonation within the bank substrates, cross-sections of

the hydrostatic pressure head and flow velocity were calculated (see ESM). Whereas positive pressure

heads indicate the saturated aquifer, the vadose zone was characterized by negative pressure heads. In

this zone, which spanned vertically up to 12 m, the pressure head varied due to instationary flow

conditions and the perpetual change of precipitation, infiltration, evapotranspiration, and desiccation.

Consequently, the mean flow velocity of water through the vadose zone was about two to three orders

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of magnitude lower than that of groundwater within the saturated zone. However, not only was there a

steep gradient in the flow velocity between vadose and saturated zone obvious, but also an increasing

flow gradient from the landside to the lake (Fig. A7b, ESM).

Combining the calculated flow velocities within the vadose zone, the water quality data obtained from

temporary monitoring wells in 2007-10, and the bank seepage samples from 2010, an average load of

net acidity of 19.6 kmol/day was attributed to infiltration based on the vegetation coverage given in

2010. The acid load split into 18.0 kmol/day of direct interflow to the lake and 1.6 kmol/day of

groundwater recharge to the saturated zone. Ultrasonic drilling and on-site analyses of soil eluates

combined with quick-weathering tests using H2O2 as a strong oxidant indicated a high potential of acid

release for each stratigraphic unit (see ESM). All soil samples collected from 10 m to 21.5 m below

the surface showed a rapid decrease of pH from ≈7.5 to ≈2.5 on average and therefore can be

considered prone to sulfide mineral weathering and subsequent release of acidity if exposed to oxygen

(see ESM, Table A5).

Based on these results, soil cores were selected to carry out intermittent-flow column experiments. The

column fed with synthetic rain water showed an initially high net acidity in the effluent sharply

diminishing from about 12.6 to 1.5 mmol/L after the second pore volume exchange (Fig. 5). This

sharp decline was accompanied by a substantial decline in EC values (not shown) and the

concentrations of dissolved oxygen, sulfate, and Fe(II), and a rise in pH from pH 2.9 to 5.3. Along

with subsequent pore volume exchanges, the effluent was mostly anoxic and the concentrations of

ACYnet and sulfate steadily declined to a much lower level. After the 10th pore volume exchange, the

effluent revealed almost steady ACYnet of ≈1.3 mmol/L and low concentrations of sulfate

(≈40 mg/L), Fe(II) (1.3 mg/L) and dissolved Al (<0.1 mg/L). Very similar concentration curves were

observed in an ICE using Tertiary substrate from a nearby lignite mining pit lake area (Lake

Störmthal, unpublished results), showing that these experimental trends were reproducible.

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While the initially sharp concentration decline indicates leaching of sulfide weathering products

accumulated within the pore space, the subsequent generation of net acidity and sulfate on a roughly

2:1 molar ratio (corrected for the influent sulfate concentration of 22.4 mg/L), the smooth

concentration decline and the consumption of dissolved oxygen can both be attributed to slow

oxidation of a limited sulfide pool within the bulk. Given the soil volume of the column (1.02 dm3),

the reaction time of a full pore volume exchange (48 h), and the simplified sum reaction of pyrite

(FeS2) oxidation (Eq. 1), a weathering rate of 5.2∙10-7

molFeS2 m

-3 s

-1 was calculated for the saturated

conditions of the ICE.

FeS2 + 15/4 O2 + 7/2 H2O ↔ Fe(OH)3 + 2 SO42-

+ 4 H+ (Eq. 1)

Based on the ICE results, one can expect a decline of net acidity by 80 % of the initial inventory from

the first to the second pore volume exchange. This leaching behavior had to be extrapolated to the

transient processes of infiltration, groundwater recharge, and spatially variable vegetation coverage.

Based on the lateral distances and the flow velocity, one pore volume exchange of the ICE scale

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translates to ≈20 to 25 years on the field scale. Assuming continuity of the present vegetation and

substrate conditions as well as mean climate conditions, the infiltration-driven load of net acidity by

interflow and groundwater recharge was predicted to halve by the year 2025 (< 10 kmol/day on

average).

Fig. 4 Increase of ACYnet mean concentrations and decrease of mean pH values during the groundwater (GW)

transit in a overburden substrate on the western slopes, and b Tertiary aquifer of the eastern slope as indicated by

samples from far upstream GW (permanent wells), bank slope GW collected at lower and upper filter screens of

temporary wells, and seepage water collected near the lake shore in 2010.

Fig. 5 pH values and concentration of dissolved oxygen (DO in mg/L), ACYnet and sulfate (in mmol/L) in the

effluent of the intermittent-flow column experiment performed on a soil sample obtained from the oxic vadose

zone sample as a function of exchanged pore volumes. One pore volume exchange at column scale translates

into 20–25 years at field scale.

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4.1.5 Lake Sediment

Sediment quality data of different sampling locations across Lake Bockwitz (Fig. 1) revealed high

heterogeneity with respect to location, sampling depth, and time (data not shown). The most consistent

results were found near the deepest point of the lake, where 10 to 20 cm of fine-grained sediment had

settled on top of lignite seam II. At the onset of water conditioning, the acid inventory of this sediment

area yielded 300 to 500 mmol/kg d.w. (Fig. A10, ESM). Along with the soda additions, the acid

inventory decreased and the sediment pH increased from about pH 3 to 6.5 (not shown). Between

2004 and 2006, the loss of acidity (i.e. H+) in the sediment at this location was found to be in the same

range as the increase of total sodium and calcium concentrations in the sediment bulk (on the basis of

equivalents). Over 80 % of Natot in the sediment belonged to the exchangeable fraction. From

extrapolation to the whole sediment area it can be assumed that about 10 % of the total Na+ supply

with soda ash has been taken up by the sediment surface (Neumann et al. 2007). The most consistent

explanation of this phenomenon is cation exchange of Na+ and Ca

2+ ions with protons sorbed to the

sediment, thus enhancing the upward flux of acidity into the lake water.

Another possible acid-generating process in the sediment could be mineral transformation from

schwertmannite to goethite (Blodau 2005, 2006). Irrespective of the contribution of each process, the

sediment diffusion model was able to fairly well describe the loss of acidity observed in the sediment.

Based on Fick’s diffusion equations, the cumulative load of ACYnet into Lake Bockwitz yielded, on

average, 2.3 kmol/day for the 1995-2004 decade, 1.0 kmol/day for the 2005-2014 decade,

0.9 kmol/day for the 2015-2024 decade, 0.8 kmol/day for the 2025-2034 decade, and 0.7 kmol/day for

the 2035-2044 decade. Over 50 years, the acid release from the whole lake sediment is predicted to

sum up to approximately 2.1·104 kmol.

4.1.6 Conditioning with Soda Ash

In 2007, lake water conditioning with soda ash was continued to increase the lake water pH from ≈5 to

pH >6 to meet the permitted pH level and the concentrations of Fetot <3 mg/L and Altot <0.5 mg/L for

the lake effluent. Despite some fluctuations in the data monitored from 2007 to 2010, these threshold

levels were met throughout. The annual averages (±1σ standard deviation) in 2010 amounted to

(0.39 ± 0.17) mg/L for Fetot and (0.13 ± 0.05) mg/L for Altot. The average net alkalinity was

(0.16 ± 0.05) mmol/L in 2010. However, the soda additions did not provide lasting buffering capacity.

For instance, in the winter period from December 2009 to March 2010, the ALKnet diminished by

0.14 mol/m3 or 2.3∙10

6 mol (Fig. 6). The loss of alkalinity proceeded during summer 2010 and was

more pronounced in the epilimnion, coinciding with heavy rainfall events in August 2010. The loss of

alkalinity had to be compensated by soda supply in fall 2010. A similar effect was observed in 2011.

4.2 Acidity / Alkalinity Budget of Lake Bockwitz

4.2.1 Model Validation

After integrating the calculated fluxes of acidity and alkalinity for each compartment, the

comprehensive water quality model of Lake Bockwitz was validated for a period of 3.5 years

(01/2008-06/2011), implying complete mixing of the lake water. The consistent agreement between

measured and modeled lake water parameters within the validation period, exemplified by pH and

ANC values in Figure 7, validated the LAKE model. Hence, the model can be used as a tool to assess

and compare the contribution of each compartment to the overall load of acidity and alkalinity into

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Lake Bockwitz under the present conditions (years 2010-11), and predict the water quality of Lake

Bockwitz for the long term (next 50-90 years). Because most compartments did not reach equilibrium

yet, different scenarios with predefined assumptions about process rates and equilibrium conditions

have to be modeled. The scenarios considered in the present study focused on the time period

necessary to achieve pH >6, either naturally (no further soda supply from 06/2011 on) or by continued

conditioning with soda ash, the calculated amount of soda needed to maintain pH >6, the time period

for conditioning, and the water chemistry of the lake expected under stationary conditions (around the

year 2100).

Fig. 6 Monthly soda supply to Lake Bockwitz from 2008 to 2010 (in metric tons, left ordinate) as well as pH of

lake water and inventory of net alkalinity determined during lake circulation (right ordinate)

4.2.2 Current Situation (2010)

The modeling results clearly demonstrate that among all sources of acidity considered, interflow from

the bank slopes was the primary source, delivering two-thirds of the total net acidity into Lake

Bockwitz in 2010 (18 kmol/day, Table 6). While the sheer magnitude of this load appeared somewhat

unexpected with regard to the overall water balance, in which the volume of seepage water exfiltrating

from the bank slopes was almost meaningless, this result is highly consistent with the expected

significance of AMD caused by sulfide mineral weathering. As demonstrated by the ICE, the acid

release was most intense during the first (and second) exchange of the pore volume and diminished to

a much lower level following subsequent pore volume exchanges. This release behavior indicates

leaching of acid weathering products (in particular sulfate, protons, and Fe2+

) that already existed in

the porewater of the investigated bank substrate. Regardless of the individual processes facilitating

sulfide mineral weathering (e.g., infiltrating rain water or temporary exposure to air) the ICE results

indicate an advanced stage of sulfide weathering in the tested substrate. However, limitations on the

experimental results arise in two major respects: i) the patchiness of the sulfide content within the

overburden substrate, making the extrapolation of the column result to the field site somewhat

arbitrary, and ii) the highly variable intensity of sulfide mineral weathering due to its dependency on

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hydrological and mineralogical factors. These include the amount of infiltrating rain water, the

alternation of desiccation and rewetting, the type of sulfide minerals present, and their particle size and

surface reactivity. Contrary to some other field sites (e.g. Grützmacher et al. 2001) the sulfide mineral

inventory of the investigated bank substrate from Lake Bockwitz (0.2 – 0.3 % d.w.) appeared likewise

low.

Fig. 7 Validation of the LAKE budget model on basis of water quality data measured from 01/2008 to 07/2011

as illustrated by pH value (left ordinate) and ANC (right ordinate). Note the different scales. Negative ANC

values reflect KB4.3 values, which ranged around 5 mmol/L prior to in-lake neutralization

A secondary, but still important source of acidity was surface water inflow from two upstream lakes,

delivering about one-fifth of the total net acidity in 2010 (≈6 kmol/day, Table 6). The acid load from

the lake sediment was estimated at ≈1.3 kmol/day, which is about 5 % of the total acid load. This

release rate was mainly dependent on the concentration gradient between the overlying water body

(low BNC) and the porewater near the sediment surface (high BNC). These gradients were more

pronounced prior to in-lake neutralization.

Compared to the above-mentioned sources of net acidity, the contributions of erosion and surface

runoff to the lake’s acid load were minor (0.1 kmol/day each) and even lower than initially expected

given that considerable part of the bank area was uncovered or sparsely vegetated (Figs. A1b and A3b,

ESM). Obviously, erosion and transport of solids into the lake as well as dissolution of acid products

into the runoff water were fairly moderate processes at this particular site due to the low sulfide

mineral inventory of the investigated bank substrate. This finding can hardly be generalized because

the intensity of these processes will always depend on the physical and chemical properties of the

substrate, the declination of the bank slopes, and the occurrence of storm events, the latter being

unpredictable.

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4.2.3 Prognosis of Lake Water Chemistry to 2050 and Beyond

Based on the validated hydrogeochemical LAKE model and the collected monitoring data, a reference

scenario showed that with abandonment of further conditioning treatments, Lake Bockwitz will suffer

a substantial decrease of pH, reaching pH 4 as early as August 2012. This decrease in pH will be

accompanied by rising Fe and Al concentrations in the lake water. The low pH level will be sustained

until the year 2030 and slowly rise thereafter due to the decreasing load of acidity from seepage water

and the neutral surface water inflow expected at this time. Hence, the lake water shall reach pH 6

around 2050. Beyond this time, the alkalinity of groundwater inflow will outweigh all inputs of acidity

and lead to self-sustaining neutral water quality in Lake Bockwitz. However, according to the current

quality criteria permitted for the lake discharge, the reference scenario would not be acceptable.

Therefore, a second scenario included in-lake water conditioning by soda ash to maintain pH ≥ 6 in

Lake Bockwitz (with all other conditions identical to the reference scenario). A total of 5,600 t of soda

ash would be needed with annually decreasing quantities until about 2035. The model calculations

show that about 10 years beyond the validation period, i.e. in 2020, the constant load of alkalinity

from groundwater (≈11 kmol/day) is predicted to just outweigh the combined acid load from

interflow, groundwater recharge, surface runoff, and erosion (Table 6). However, Lake Bockwitz will

still receive net acidity from the upstream pit lakes and sediment exchange processes (together

≈4.1 kmol/day). In the year 2050, the lake budget will be the opposite (Table 6). By then, the

alkalinity load from near-lake groundwater (≈10.7 kmol/day) will be higher than the combined acid

loads from interflow (≈2.4 kmol/day), upstream lakes (≈0.8 kmol/day), sediment-water exchange

(≈0.6 kmol/day), erosion, and surface runoff (≈0.1 kmol/day each). Hence, Lake Bockwitz is

forecasted to receive net alkalinity of ≈6.7 kmol/day, making further soda additions unnecessary.

According to these predictions, the natural loads of acidity and alkalinity into Lake Bockwitz will be

balanced around the year 2035.

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Consistent with the decrease of acid load, the loads of Fetot and Altot into Lake Bockwitz are also

predicted to diminish over time. Compared to the significant loads in 2010 (ca. 1,000 t/a Fe and 25 t/a

Al), the loads of Fe and Al will decrease by 80 % or more until 2050, mainly due to the diminishing

leaching of weathering products by interflow and seepage water (Fig. A12, ESM).

A third scenario assumed pasturing on the bank slopes with scattered vegetation and grassland. To

maintain pH ≥ 6 in Lake Bockwitz under such conditions, a total of 6,800 t of soda ash would be

needed up to 2038, i.e., 21 % more alkaline material on a time scale prolonged by three years

compared to the second scenario. A fourth scenario assumed intensified vegetation growth with

comprehensive grassland and young tree stands. To maintain pH ≥ 6 in Lake Bockwitz under these

conditions, a total of 3,800 t of soda ash would be needed up to 2028, i.e. 32 % less alkaline material

due to a time scale shortened by 10 years compared to the second scenario.

5 Discussion

5.1 Uncertainty and Limitations of Model Predictions

Predictions of the development of complex hydrogeochemical systems, such as in this study, can only

be obtained by system modeling. However, simplification of the model complexity and assumptions

on the relevance of processes, interactions, and boundary conditions are necessary to keep the cost-

benefit relationship balanced. A balance between the financially limited technical efforts and the

request for highly reliable prediction of future actions must be obtained. Several general or site-

specific limitations and simplifications have to be considered:

1. Unknown or ignored system components and interactions, e.g. the effects of biological activity on

water chemistry. According to a simplified reaction (Eq. 2, Redfield 1958) in which E is the solar

energy input,

106 CO2 + 122 H2O + 16 NO3- + 1 HPO4

2- + 18 H

+ + E → <mass of algae> + 138 O2 (Eq. 2)

the production of biomass in lakes consumes protons (H+). The oxidative mineralization follows

the reverse direction. Hence, a continuous increase of alkalinity can only occur through

sedimentation and burial of part of the biomass that is not oxidized. After the initial neutralization

of Lake Bockwitz, the phytoplankton biomass was very low. This is consistent with oligotrophic,

young pit lakes (Rönicke et al. 2010). Assuming a net production rate of organic carbon of

100 g C m-2

a-1

(according to the oligotrophic Lake Stechlin, Klapper et al. 1992) and a burial of

10 % of this organic matter (Werner et al. 2001a), protons on the order of 2.4·105 mol H

+ a

-1 would

be consumed in Lake Bockwitz. Compared to the overall load of net acidity (3.46 mol m-2

a-1

in

2010, Table 6), the in-lake alkalinity production of 0.14 mol m-2

a-1

would be negligible. A similar

conclusion has been drawn for Lake Senftenberg (Werner et al. 2001b).

2. Simplification of the system scales by using a coarse model grid size with respect to space and

time. This will, for instance, apply to the quantification of inflow, the spatial variability of the

sulfide mineral content in the bank substrates (expected highly variable due to hydrologic and

pedogenic factors as well as the mining and dumping history), the seasonal stratification of the

lake, and the seasonal fluctuation of aquatic organisms.

3. Focusing on processes and reactions, e.g. negligence of differentiation between the metastable iron

oxyhydroxides schwertmannite and ferrihydrite with respect to proton release through precipitation

and transformation to goethite. While the molar precipitation of schwertmannite produces fewer

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protons than the precipitation of ferrihydrite, the proton generation balance is approximately

equalized through the transformation of schwertmannite to goethite (Blodau and Peiffer 2002;

Knorr and Blodau 2006). Thus, reaction rates become critical for the time-dependency of the

individual reactions if further differentiated.

4. Estimation of reaction and transport rates that cannot be assumed constant and can hardly be

determined in field scale and only with limited representativeness in short-term lab experiments,

e.g. mineral weathering or transformation rates, sulfate reduction rates, groundwater flow

conditions (velocity and direction), and solute transport rates. As a consequence, groundwater flow

conditions and reaction rates are often estimated and assumed constant over the prediction period.

5. Projection of highly stochastic processes from the past into the future, e.g. meteorological

conditions. As a consequence, either averaged meteorological conditions are assumed constant over

the prediction period, or long-time series records of the past are mirrored into the future. Such

assumptions can have tremendous impact on the water balance and geohydrologic conditions. For

example, the annual regional precipitation recorded in 2010 was 25 % higher than the 30-year

average, leading to increased fluxes by elevated interflow, groundwater recharge, and seepage.

6. Assumptions about direct and indirect human activities including lake and land use (e.g.,

settlement, pasture, or forestation on the bank slopes, fisheries, boat traffic, water sports), water

management, and conditioning practices. For example, Werner et al. (2008) attributed

underestimation of the acid load into the pit lake Bärwalde to underpredicted groundwater inflow

because water management practices had changed in an unforeseen way.

The contributions of such limitations to the overall uncertainty of prediction can hardly be quantified.

Instead, rough estimates are to be obtained based on expert experience. Hence, even if a model has

been successfully calibrated and validated, forward predictions can only generate a range of results

based on various scenarios. In the present work, three likely scenarios were compared with respect to

the type of vegetation on the bank slopes. Depending on the vegetation coverage, these scenarios

predicted a total of 3,800 to 6,800 t of soda ash needed until 2028 or 2038 to maintain pH ≥ 6 in Lake

Bockwitz. While this prediction on the amount of conditioning material differs by ±28 % around the

mean, the prediction on the time scale of conditioning differs by ±22 % around the mean, which would

be [23 ± 5] years from 2010.

5.2 Comparison of Modeling Approach to Previous Studies

The initial prediction of the water quality of Lake Bockwitz failed because Guderitz et al. (2003) could

only use groundwater quality data from permanent wells installed 100 to 200 m away from the lake.

However, as shown in the present study, these wells did not cover the area of aquifers that transported

the vast bulk of acidity into the lake. The predominant sources of acidity were interflow and seepage

water from the bank slopes, identified by means of temporary wells installed near the shore line for the

purpose of this investigation. Thus, water quality modeling of pit lakes with similar geohydrology

should always integrate groundwater quality and flow through the near-lake bank slopes, in particular

the vadose zone. On the other hand, acid contributions from surface runoff and soil erosion appeared

meaningless in the present case study. While the contribution of acid release from the lake sediment to

the overall acid load ranged on the order of 8 – 10 % for Lake Bockwitz, this proportion was

substantially higher for Lake Zwenkau (Ulrich et al. 2011a, b). Studies on other pit lakes reported

predominant acid load from erosion (Müller and Eulitz 2010; Müller et al. 2011). Hence, the

significance of acid-generating system compartments and processes, including sediment-water

interactions, has to be determined site-specifically.

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A similar comprehensive study on hydraulic fluxes of acidity and alkalinity was published by Werner

et al. (2001a, b) for Lake Senftenberg in the Lusatian lignite mining district. Similar to our study, the

hydrologic sub-model HYDRUS 2D based on van-Genuchten parameters was used, fluxes for

different processes and compartments were calculated and put into equilibrium with atmospheric

conditions using an equilibrium speciation code. Contrary to our study, in which process parameters

were measured in the field and obtained from lab experiments, assumptions were made on the depth

and sulfide content of the vadose zone, the infiltration rate and groundwater recharge, the mass flux

into the lake by erosion, and the alkalinity generation by biological activity. Werner et al. (2001a)

calculated a pyrite weathering rate for near-shore substrate from weathering experiments that are not

further described. Interestingly, their rate of 6∙10-7

molFeS2 m

-3 s

-1 was similar to the pyrite weathering

rate of 5.2∙10-7

molFeS2 deduced in the present study from an ICE carried out under saturated

conditions. Another important difference is that the present study demonstrates the validation of the

lake budget model based on field-measured water quality parameters.

Hence, following up on the Castendyk and Webster-Brown (2007) categorization of study types

mentioned in the Introduction, we suggest a fifth category represented by our study, in which

geochemical prediction based on a Type I model is founded on process parameters obtained from field

investigations and lab experiments, and then calibrated with water chemistry data from post-closure pit

lake monitoring.

5.3 Quantification of Acidity and Alkalinity

The fluxes of acidity and alkalinity from each system compartment into Lake Bockwitz (Fig. 2) were

determined with PHREEQC. The monthly or annual means of major cations and anions were entered

into the code, charge-balanced with sulfate, and set into equilibrium with dissolved O2 and CO2

analyzed in Lake Bockwitz. Then, the equilibrium pHeq and the net acidity or alkalinity fluxes were

calculated (see Table 5). In addition, potentiometric titrations with acid or base were carried out to

determine the actual (approximately in situ) KS4.3, KB4.3, and KB8.2 values (KS8.2 being irrelevant).

While these parameters characterize the actual sample properties and can be used to cross-validate the

ion analyses, a reliable measure of effective acidity or alkalinity can only be obtained when oxidation

of ferrous iron and Mn2+

is achieved and hydrolysis reactions to ferric salts and Al hydroxides are

completed, and excess CO2(aq) has degassed. These reactions rarely reach equilibrium during titration.

Another approach to characterize the acid inventory of soil and overburden substrate is a quick-

weathering test in which batch slurries with added H2O2 (1.5% final concentration) are shaken over

48 h at room temperature or gently boiled for 5 min to determine the proton release by complete

sample oxidation. Although this approach does not guarantee that all sample material is oxidized, the

results can be used for method-specific comparison among different substrates to identify and locate

sensitive substrates in the field. Of course, such experimental data have no predictive value because

they do not refer to natural conditions and potentially metastable equilibria (see also discussion by

Morin and Hutt 2009).

5.4 Acid Release from the Vadose Zone

The ICE using bank substrate from the vadose zone of Lake Bockwitz was performed under saturated

conditions. As expected, two major effects were observed:

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rapid leaching of acid weathering products (H+, Fe

2+, sulfate) within the first two pore volume

exchanges;

a continuous, slowly declining generation of sulfate and protons accompanied by consumption

of dissolved oxygen, attributed to sulfide oxidation.

The infiltration of air-saturated rain water every 24-48 h transported more oxygen into the soil than

would be expected by diffusion of air under unsaturated or partially saturated conditions. Thus, the

pyrite weathering rate obtained from the last three pore volume exchanges may still represent an upper

bound of the oxidation rate to be expected under natural, stochastically changing conditions. In a

complementary ICE experiment using Tertiary bank substrate from the saturated zone of Lake

Störmthal percolated with anoxic groundwater from the field site, no signs of sulfide weathering were

found (unpublished results). While the sulfide oxidation rate within the vadose zone is limited by the

penetration of air, the transport of dissolved oxygen with groundwater recharge limits the oxidation

rate within the deeper vadose zone and the saturated zone (Werner et al. 2001b). In the case of Lake

Bockwitz, we have strong evidence to postulate that most of the initial sulfide inventory within the

upper vadose zone is already oxidized, and hence leaching of the acid reaction products by interflow is

the predominant source of acid load of the lake.

5.5 Advective Groundwater Flow

Compared to other field studies (Blodau 2005; Fleckenstein et al. 2009; Knorr and Blodau 2006), it

appears that the process of advective groundwater flow through the lake bottom and sediment is a site-

specific process, primarily dependent on the geohydrological environment of the lake. While the

present study found a hydraulic barrier (aquitard) near and below the lake bottom and identified lateral

groundwater inflow from Tertiary aquifers into the lake epilimnion to be dominant, the above-cited

studies describe advective groundwater inflow into the deep water body of pit lakes. Site-specific

investigations on the geology and geohydrology of the lake basin and its environment as well as in-

lake seepage measurements using, for instance, caissons (Fleckenstein et al. 2009) are important for

the investigation of groundwater flow conditions.

6 Conclusions and Outlook

The management of water quality and the planning of water treatment of mining pit lakes require

detailed knowledge of the processes contributing to the anticipated water quality. As demonstrated in

the present study, this objective can be achieved by focusing standardized field monitoring on the

collection of process-oriented data and combining those with process-based lab experiments to

determine critical parameters needed for appropriate modeling of the relevant processes that affect

water quality. Applying this approach to Lake Bockwitz, we were able to identify and quantify its

major sources of acidity, Fe, Al, and sulfate. The following conclusions summarize the main lessons

learned from this case study.

1. Local groundwater monitoring by permanent wells located 100-200 m away from the shore line

was insufficient for determining the subsurface loads of critical substances into the lake. The

reason was that the groundwater quality substantially changed during transit through previously

drained Tertiary aquifers and/or overburden substrate, both exposed to oxygen during the

mining activities. Temporary monitoring wells near the shore line installed for the purpose of

this study were much better suited to determine the quality of groundwater infiltrating Lake

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Bockwitz. By these means, we were able to quantify the infiltration-driven interflow through the

bank slopes and to then rank this process as the predominant source of acid load for Lake

Bockwitz. This result appears fundamental for improving the reliability of water quality

prognosis.

2. Column experiments with local bank substrate help identify the kinetics of key processes

providing acid to the interflow and seepage water. In the analyzed Bockwitz substrate the

sulfide mineral inventory was relatively low due to advanced weathering. The predominant

process was leaching of acid weathering products accumulated within the pore space and pore

water. The actual rate of sulfide oxidation is expected to depend on the transport of oxidants

within the vadose zone, the type of sulfide minerals, their particle size and surface area, and

other environmental conditions.

3. Despite visual observations from the field site that would indicate otherwise (Fig. A1, ESM),

erosion of solid matter from the bank slopes and surface runoff were insignificant sources of

acidity for Lake Bockwitz. This unexpected finding was made possible by considerable

investments and efforts in installing an erosion field plot equipped with automated sensing and

sampling devices near the shoreline of Lake Bockwitz, as well as continuous inspection,

support, and monitoring.

4. Lake sediments can act as a source of net acidity through ion exchange processes, initially

enhanced by soda treatments. However, comparably low acid loads were calculated at each time

interval because diffusion is a slow process. The affected sediment depth will be limited

(< 0.5 m), and residues of continued soda treatments can provide buffering capacity on the

sediment surface.

5. In summary, the acid load into Lake Bockwitz will not terminate as soon as initially predicted

from spatial budget calculations. It will slowly diminish over time as a result of complex

interacting processes. Process-oriented investigations combining field monitoring and lab

experiments are crucial to obtain the fundamental parameters for modeling. Moreover, process-

oriented models are indispensable tools for reliable predictions. We tried to cover the most

important processes for post-mining pit lakes, but the relevance of each process depends on the

local environmental properties and conditions, history of mining, reclamation, and treatments,

and thus is site-specific.

To further verify the results of this study and enable a better adaptation of the set of models to other

field sites, future investigations should focus on the upscaling of the infiltration-driven acid generating

processes, for instance by lysimeter tests. Using substrate from the respective field sites, this approach

can improve the model-assisted description of sulfide mineral weathering and solute transport in

unsaturated or partially-saturated porous media. With regard to lake rehabilitation, mitigation

measures like barrier layers or optimized vegetation coverage on top of the bank slopes to decrease the

quantity of interflow should be investigated.

7 Acknowledgements

This paper is dedicated to Karl-Heinz Pokrandt on the occasion of his retirement from the

LMBV mbH; we acknowledge his commitment, persistent cooperation, and helpful support on many

projects. The authors are indebted to all associates of the Lausitz and Central-German Mining Admin

Company involved in this research. Special thanks are due to Eckhard Scholz, head of the Geotechnics

department. We thank Bob Kleinmann and four anonymous reviewers for helpful suggestions on an

earlier draft, and Thomas Voltz for linguistic improvements.

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