Mine Waste and Modeling

1. ORD Mine Waste Scientist to Scientist Meeting15 June 2000, Las Vegas, NV Mine Waste and Modeling David G. Jewett, Ph.D. USEPA/ORD/NRMRL Subsurface Protection and Remediation Division, Ada, OK Co-Director, Center for Subsurface Modeling Support (CSMoS)

2. Presentation Outline • Modeling Basics • Definition • General Approach • Types of Models • Source • Flow • Reactive Transport • Other • Modeling Resources

3. Definition Mathematical model: simulates ground-water flow and/or solute fate and transport indirectly by means of a set of governing equations thought to representthe physical processes that occur in the system (Anderson and Woessner, 1992).

4. Modeling Approach • Establish purpose • Develop conceptual model • Select appropriate mathematical model • Model design (iterative process) • Calibration (including sensitivity analysis) • Verification • Prediction (including uncertainty analysis)

5. Types of Models • Source models • Unsaturated and saturated flow models • Reactive transport models • Other models

6. Source Models • Estimate mass flux from a source area • Characterize source areas in terms of source mineralogy and subsurface hydrogeological and geochemical conditions • Describe loading as steady-state or as a function of time

7. Source Code Example • PYROX • Univ. of Waterloo (Wunderly & Blowes, 1996) • 1-D finite element code • Sulfide mineral oxidation model • Based on conceptualization and mathematical derivation of Davis and Ritchie • Oxygen diffusion is rate-limiting factor • Simulates release of Fe, SO42-, and H+

8. Source Code Example • MINTEQA2 • NERL-ERD (Allison et al., 1991) • Versatile equilibrium solution chemistry code • Calculates equilibrium mass distribution of dissolved and adsorped species and multiple solid phases • Extensive thermodynamic database

9. Flow Models • Define water movement and variability of soil water tension or hydraulic head across domain of interest • Unsaturated versus saturated flow • Steady-state versus transient simulations

10. Unsaturated Flow Code Example • SOILCOVER • Univ. of Saskatchewan (Wilson, 1994) • 1-D finite element code • Simulates water flux at atmosphere-soil interface and water movement in the near surface unsaturated zone • Designed for developing soil covers for mine tailings and acid generating waste rock

11. Unsaturated Flow Code Example • HELP • USACE for USEPA (Schroeder et al., 1991) • Quasi-2-D layered water budget model • Rapid estimation of surface runoff, subsurface drainage, and leachate production • Extensive climate and soil characteristic databases

12. Unsaturated Flow Code Example • VS2DT • USGS (Healy, 1996) • 1-D or 2-D finite difference code • Simulates water and solute movement in variably saturated porous media • Distributed in VS2DI package as of 2/2000 (combined with VS2DH)

13. Saturated Flow Code Example • MODFLOW • USGS (McDonald and Harbaugh, 1988; Harbaugh and McDonald,1996) • 3-D finite difference ground-water flow code • Modular structure for easy adaptation • Ground-water flow model work horse • Widely used and extensively tested • GMS, Groundwater Vistas, Visual Modflow

14. Saturated Flow Code Example • FEMWATER • ORNL (Yeh and Ward, 1979) and Penn State (Yeh, 1990) • 3-D finite element ground-water flow code • Saturated and unsaturated flow • GMS

15. Reactive Transport Models • Define temporal and spatial distribution of dissolved contaminant mass in the model domain • Incorporate the physical, chemical, and biological processes controlling solute fate and transport (sorption, abiotic transformations, biologically mediated transformations)

16. Reactive Transport Code Example • MINTRAN • Univ. of Waterloo (Walter et al., 1994) • Couples PLUME2D (2-D finite element solute transport code; Frind et al., 1990) and MINTEQA2 • Simulates multicomponent reactive transport in spatially discrete ground-water systems • MINTOX = MINTRAN + PYROX

17. Reactive Transport Code Example • PHREEQC • USGS (Parkhurst and Appelo, 1999) • Low-temp aqueous geochemical model • Calculations performed: • Speciation and S.I. calculations • Batch reaction and 1-D transport calculations • Inverse modeling • PHREEQCI

18. Reactive Transport Code Example • MT3D • USEPA, 3-D solute transport (Zheng, 1990) • MOC, MMOC, HMOC • Runs with MODFLOW, or similar, flow output • MT3D96, MT3D99 (proprietary) • RT3D • PNNL, multi-species transport (Clement, 1997) • Based on MT3D; includes 8 rxn modules

19. Other Models • Surface Water Models • OTIS (USGS, 1998) simulates 1-D fate and transport of water-borne solutes in streams and rivers • EFDC (Hamerick, 1999) simulates 3-D flow, transport, and biogeochemical processes in surface water systems

20. Other Models (continued) • Watershed Models • BASINS (USEPA, 1998) multipurpose environmental analysis system for performing watershed- and water-quality-based studies • Dynamic System Models • STELLA (HPS, 1994) universal simulation tool to build understanding of dynamic systems and interrelationships

21. USEPA Modeling Resources • Center for Subsurface Modeling Support (CSMoS) • NRMRL/SPRD – Ada, OK • www.epa.gov/ada/csmos.html • On-Line Model Database: www.epa.gov/ada/mdb_form.html

22. USEPA Modeling Resources • Center for Exposure Assessment Modeling (CEAM) • NERL/ERD – Athens, GA • www.epa.gov/ceampubl/ceamhome.htm • EPA Scientific Model Database: athord1.ath.epa.gov:9876/Models.nsf

23. Other Modeling Resources • USGS Water Resources Software Page • water.usgs.gov/software • Richard B. Winston’s Home Page • www.mindspring.com/~rbwinston/rbwinsto.htm • Geotech & Geoenviron Software Directory • www.ggsd.com • Int’l Ground Water Modeling Center • www.mines.edu/igwmc

24. “The fascinating impressiveness of rigorous mathematical analysis, with its atmosphere of precision and elegance, should not blind us to the defects of the premises that condition the whole process.” - T.C. Chamberlin “Everything should be made as simple as possible, but not simpler.” - Albert Einstein