Electronic Supplementary Material Mine Water and the Environment

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1 Electronic Supplementary Material Mine Water and the Environment In-Lake Neutralization: Quantification and Prognoses of the Acid Load into a Conditioned Pit Lake (Lake Bockwitz, Central Germany) Kai-Uwe Ulrich 1*, Christian Bethge 1, Ina Guderitz 1, Ben Heinrich 1, Volker Neumann 1, Claus Nitsche 1, Friedrich-Carl Benthaus 2 1 BGD Soil and Groundwater Laboratory GmbH, a company of GICON Group, Tiergartenstraße 48, Dresden, Germany; 2 LMBV mbh, Lausitzer und Mitteldeutsche Bergbauverwaltungsgesellschaft, Knappenstraße 1, Senftenberg, Germany * Corresponding author, Phone , Fax , kulrich@bgd-gmbh.de Content 1 Precipitation, Surface Runoff and Soil Erosion 2 Interflow and Groundwater Recharge - Parameterization of the HYDRUS Hydraulic Transport Model 3 Acid Release Potential from Overburden Substrates 4 Approach to Calculate Diffusive Transport near the Sediment Water Interface 5 References 4 Tables 12 Figures 12 Pages S1

2 1 Precipitation, Surface Runoff and Soil Erosion The photos in Figure A1 show details of the bank area of Lake Bockwitz. Seepage water entering the lake above the water table demonstrates evidence of interflow through the bank substrates (Fig. A1a). Soil erosion from uncovered bank area is visible in Figure A1b. The monthly totals of precipitation, soil erosion, and runoff recorded at the monitoring plot on the western bank area of Lake Bockwitz are depicted in Figure A2 (Schapp et al. 2010). Figure A1. Bank area of Lake Bockwitz visualizing (a) seepage of interflow from bank area into the lake, (b) erosion relief area (Photography: BGD company, Dresden 2010). Figure A2. Monthly totals of rainfall, soil erosion, and runoff recorded at the monitoring plot on the western bank area of Lake Bockwitz. Input information for the E3D code included relief data from laser scanning with a spatial discretization of 5 m x 5 m (Fig. A3a), soil properties, and land use / vegetation data (Fig. A3b). By spatial intersection of this information, soil area-specific erosion or deposition was calculated (Fig. A4). The model output is summarized in Table A1. S2

3 Figure A3. Input data for modeling soil erosion: (a) Laser scanning data, spatial discretisation 5 m x 5 m, (b) spatial distribution of land use and vegetation type. Figure A4. Mapping of soil erosion and deposition at the drainage area of Lake Bockwitz. According to the E3D model, soil erosion occurred on 11 % of the total drainage area, in particular on sparsely and non-vegetated areas. S3

4 Table A1. Annual totals of calculated runoff and soil erosion for whole drainage area based on observed events 2007 and Calculated runoff (m³) Calculated soil erosion (t) 3 94 Erosion rate for total catchment (t/ha) Total rainfall (mm) Erosion relevant rainfall (mm) Number of erosion relevant events (-) 8 5 Max. intensity for single event (mm/10 min) Effective acidity input by runoff (kmol) Effective acidity input by soil erosion (kmol) Interflow and Groundwater Recharge - Parameterization of the HYDRUS Hydraulic Transport Model A common approach was used to describe the highly nonlinear soil water retention and hydraulic conductivity functions. Basically, the Richards equation was solved based on van-genuchten parameters (Šimůnek et al. 2006). The parameters were obtained by fitting the Mualem-van Genuchten model (van Genuchten 1980) based on datasets of multi-step outflow experiments carried out in the laboratory (Table A2). These datasets represent the typical soil substrates and the zonation of the bank slope profiles. On these profiles small-scale calculations of interflow and percolation processes were carried out with the HYDRUS software. Table A2. θ s θ r Layer α (1/m) n K s (m/s) WC (m) Van-Genuchten parameters for soil substrates from different soil depth below surface obtained from ultrasonic drilling and used in the HYDRUS hydraulic transport model. Depth van Genuchten Parameters (m below surface) θ s θ r α (1/m) n K s (m/s) saturated water content residual water content shape parameter of retention function shape parameter of retention function saturated hydraulic conductivity water column S4

5 Figure A5. Soil water retention functions for soil substrates from different soil depth below surface. Small-scale calculations of the water transport processes at the bank slopes were done with HYDRUS software for several cross-sections representing the varying geological stratification of the bank slope around the lake. The boundary conditions of these profiles are consistent with the local situation of the study site (lake right side, groundwater level left side, atmosphere on top, including evapotranspiration, interception by vegetation, and precipitation from a twenty-year climate data record on a daily basis). The software output generated the time-dependent distribution of pressure head (Fig. A7a) and flow velocities (Fig. A7b) relative to atmospheric pressure along the cross section. Scale intervals and color grading were chosen for visualization of the saturated zones (= groundwater) and vadose zones (= percolating water, interflow). Red to orange colors indicate the saturated soil with positive pressure heads and flow velocities >0.125 m/s, while yellow to blue colors indicate soil of the vadose zone with negative pressure heads and flow velocities <0.125 m/s. Due to the zonation of the substrates (compare illustration in Figure A6) and the effects of processes at the upper boundary (soil surface of the profile), a heterogeneous distribution of water tension follows. The flow velocities within the saturated zone increase towards the lake. Figure A6. Geologic cross-section of the eastern bank slope area of Lake Bockwitz. S5

6 Figure A7. Cross-section of the eastern bank slope of Lake Bockwitz from lake (left) to hilltop (right) demonstrating the spreading of calculated (a) hydrostatic pressure head (in m of water column) and (b) flow velocities (in m/s) from small scale calculations. Depicted geometry not to scale; color grading adapted to a range of values. 3 Acid Release Potential from Overburden Substrates The high potential of acid release through oxidation was demonstrated for bank substrate (soil profile in Fig. A8) from the eastern slope area by soil eluates and quick-weathering tests using H 2 O 2 as a strong oxidizing agent (Table A3). Based on these results, soil liners were selected to run intermittent-flow column experiments. S6

7 Figure A8. Soil profile of ultrasonic drilling carried out on the eastern bank slope area (Fig. 1) in December 2009 (image taken from expert study of BGD company). S7

8 Table A3. Results of the quick oxidation tests on soil samples obtained from the eastern bank slope area (Fig. 1) by ultrasonic drilling. Corresponding soil profile displayed in Figure A8. Soil sample Depth below surface ph ph ph ph ph Notes Lab code (m) 0 h 1 h 3 h 24 h 48 h BW-1 Blank Blank sample BW-2 Blank / Vadose zone 0689/ / / / / / / / Saturated zone 0696/ / / / / / Approach to Calculate Diffusive Transport near the Sediment Water Interface Because convective inflow of groundwater near the lake bottom was found to be insignificant, a simplified approach was chosen to describe the solute exchange processes at the lake bottom. A 1D soil column (Fig. A9) was parameterized in the physical transport model HYDRUS based on FICK s diffusion equations (Table A4). The model was site-specifically calibrated for a cluster of 600 sediment layers to obtain the diffusive proton flux from the sediment into the lake. S8

9 Figure A9. (a) Model concept of instationary diffusive solute transport across the sediment-water interface and within the sediment porespace as a function of depth (z i ) and time (t i ). REV i means representative space unit. D means diffussion coefficient, ρ means substrate density. (b) Sediment liner from Lake Bockwitz showing the sediment-water interface overlain by an illustration of the model concept of instationary diffusive solute transport. Turbid brownish water layer caused by sediment-water exchange processes. S9

10 Table A4. Input parameters of the hydraulic transport model of lake sediment (Ulrich et al. 2010). Input parameter Value Bulk density of dried sediment 1.0 Fraction of cation exchange sites in contact to mobile water (nondimensional) Frac = 0.45 Fraction of immobile water ThImb = 0.05 Effective diffusion coefficient of protons in pure water at given water temperature, validated Water temperature T = 10 C D H+,10 C,val = m 2 day -1 Temporal discretization (weighting: Crank-Nicholson) Spatial discretization (weighting: Galerkin): Initial acid inventory of water body Initial acid inventory of lake sediment Validation period: t = 3650 days (based on field data from 2003 to 2007) Prognosis period: t = days, t 0 = z = 600 x m ACY net = 0.18 mol m -3 (mean water quality at the lake bottom if ph >6 is maintained in the water body) ACY net 50 mol m -3 (data collected at deepest sampling point RBS4). Short-term calculations after model calibration show a relatively good trend fit when comparing calculated data and measured ACY net data from the deepest point of the lake (Fig. A10). Fluxes of net acidity (ACY net ) from the sediment into the lake water and daily loads of ACY net into Lake Bockwitz were predicted over 100 years since the onset of flooding in 2004 (Fig. A11). Annual loads of iron and aluminum into Lake Bockwitz from the major sources are exemplified in Figure A12 for the years 2010, 2020, and In this figure, loads from erosion, runoff, and sediment-water exchange were neglected. S10

11 Figure A10. Comparison of the ACY net inventory in the lake sediment (0 20 cm layer) determined at the deepest point of the lake from 2003 to 2011, and the effective acidity in different layers (5 cm, blue line; 20 cm depth, dashed red line) calculated from FICK s diffusion equations. Figure A11. Calculated flux of net acidity (ACY net ) from the sediment into the lake water (blue line) and daily load of ACY net into Lake Bockwitz (red line) as a function of time since the onset of flooding in 2004 (set to zero). S11

12 Figure A12. Annual loads of iron and aluminum from major sources into Lake Bockwitz calculated for the years 2010, 2020, and Loads from erosion, runoff, and sediment-water exchange are neglected. 5 References Schapp A, Heinrich B, Pokrandt KH, Nitsche C, Biemelt D, Grünewald U (2010) Modelling soil erosion for the catchment of a lignite mining dump using monitoring data. Presentation held at European Geoscience Union conference, Vienna Šimůnek J, van Genuchten MT, Šejna M (2006) The Hydrus Software Package for Simulating the Two- and Three-Dimensional Movement of Water, Heat and Multiple Solutes in Variably-Saturated Media, Technical manual, V 1.0, Prague. Ulrich KU, Guderitz I, Menzel U, Heinrich B, Weber L, Pokrandt KH, Häfner K, Nitsche C (2011a) Quantification and prognosis of acid release from the sediment of a mining pit lake (Zwenkauer See) by means of field and laboratory experiments (in German). Deutsche Gesellschaft für Limnologie (DGL) Erweiterte Zusammenfassungen der Jahrestagung 2010 (Bayreuth), Hardegsen 2011, Van Genuchten MT (1980) A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J., 44: S12