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16.10.2012 - 1 -
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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: [email protected]
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
16.10.2012 - 4 -
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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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