inverse and forward hydrogeochemical modeling of acid mine drainage

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This paper demonstrates the practical application of geochemical speciation- solubility, inverse ... interaction exists in the center of system. ... Chemical reactions in Equation 2 and 3 may be accelerated by the bacteria like .... thermodynamic, kinetic and surface .... (total cation+total anion)) equivalent ..... Energy Science and.
INVERSE AND FORWARD HYDROGEOCHEMICAL MODELING OF ACID MINE DRAINAGE Cihan GÜNE , Sevgi TOKGÖZ GÜNE Dokuz Eylül University Engineering Faculty Environmental Engineering Department, zmir, Turkey ([email protected])

Abstract: In almost all environmental problems, there is a need for knowledge or predictions of the solute concentrations in space and time. Through the whole cycle of a mining project, from the exploration and feasibility studies, permitting, active mining, to remediation, reclamation and closure of a mining site, there is a wide spectrum of environmental issues in which geochemical modeling can play a significant role. For example accurate prediction of acidic drainage from proposed mines is recognized by both industry and government as a critical requirement of mine permitting and long term operation. This paper demonstrates the practical application of geochemical speciationsolubility, inverse mass balance and reaction path models to describe water chemistry in abandoned Ala ehir (Manisa-Turkey) Hg mine. 1. INTRODUCTION The process which arises out upon the reaction of sulfide minerals with surface or groundwater under aerobic atmospheric conditions and occurs over the buffer capacity of all kinds of geological materials with the available water sources is called as acid mine drainage (AMD). However water quality hasn’t always an acidic character, it may be neutral or basic when minerals buffering acidic water are dominant. AMD leaking from the mines having been determined to be operated in Europe in 467b.c. in the era of Roman Empire has been still active today (CSS, 2002; Jennings et al., 2008). Mines constitute one of the industries generating most sulphide waste materials in the world (ICOLD, 1996). Worldwide out-of-date mining activities (coal, gold, copper, nickel, uranium, diamond etc.) have left a considerable pollution heritage (Nordstrom and Alpers, 1999). Balya PbZn Mine (Balikesir) and Alasehir (Manisa) Hg mines may be shown as an example for the areas having left such kind of historical pollution heritage.

Generally studies defining acid-base identity, hydraulic characteristics, quantity, quality and movement of surface and groundwater with observations, analyses and tests for the characterization of solids and waters in the medium, tests for potential mine wastes, comparisons of the estimations and observations by the use of the similarities with the mines in other areas and the detailed hydrogeochemical models for the estimation water quality in the medium and accuracy, validity and activity analyses and assessments have been performed in all stages of mining (Maest et al., 2005). Although a very large-scale study is necessary in order to avoid unintended environmental problems which may arise out in the medium, water-mineral and gas interaction exists in the center of system. AMD occurs as a result of sulphide minerals exposed to atmospheric conditions like pyrite and marcasite (FeS2), chalcopyrite (CuFeS2), Covellite (CuS) and Arsenopyrite (FeAsS). Dissolution of sulphide minerals especially in the absence of alkali materials and in high-oxidation

conditions (raining being rich in oxygen, surface and groundwater) creates a water type being rich in sulphate at a high acid level. Abundant of acidity and metal concentrations depend on many variables like the species, structure and quantity of sulphide minerals, distribution, quantity and structural characteristics of alkaline materials in the area, content and current quantity of underground and surface water, chemical and structural characteristics of geological units and coexistence of the field-specific components. Furthermore sulphide is associated basically up to the structure of pyrite and oxidation rate affects acid formation based on many reasons like reactive surface area of pyrite (Singer and Stumm, 1968), solution pH (Smith and Shumate, 1970), pyritic sulphide, catalysis agents and washing frequency (Caruccio et al.,1988) and the existence of Thiobacillus bacterium (U.S. EPA, 1971; in Skousen et al., 1998). 2. FORMATION WATER

OF

ACIDIC

Most important contribution to the formation of acidic waters is caused by the reactions occurring upon the dissolution of the sulphide minerals like pyrite in water. Dissolution reactions and the use of oxygen (Equation 1) and hydroxides (Equation 2) are the most dominant processes increasing the concentration of H+ species in the medium. 2FeS2(k) + 7O2 + 2H2O => 2Fe+2 + 4SO4-2 + 4H+ (1)

2Fe+3 + 6H2O 2Fe(OH)3(k) + 6H+ +3

14Fe 16H+

+ FeS2(k) +8H2O => 2SO4

-2

(2) + 15Fe+2 + (3)

When oxygen dissolved in water is over or connection with oxygen is over, Equation 3 is completed and may cause Fe+2 concentrations at a high level (Younger et al., 2002). Chemical reactions in Equation 2 and 3 may be accelerated by the bacteria like

Ferroplasma Acidarmanus (McGuire et al., 2001) ve Thiobacillus Ferrooxidans which may survive in acidic medium (Jennings, 2008). Hydrolysis reactions occurring when mixed up in acidic waters containing many dissolved metals other than iron, clean surface or groundwaters may cause the increase in the activities of the metal hydroxide species, precipitations and high H+ ion concentration in water (Equation 4 and 5). Al+3 + 3H2O Al(OH)3(k) + 3H+ +2

Mn + 0,25O2 + 2,5H2O Mn(OH)3(k) + 2H

(4) +

(5)

Estimation and management of an acceptable acid drainage for mine operation is a very critical necessity for both mine industry and control authorities. Estimation of the acid forming potential of any geological formation starts with correct estimation of existence and quantity of both geological formation and neutralization minerals and the minerals with acid forming potential. Methods used in determining the characterization of the acid forming minerals have started to be used since 1970 and continued in sitespecific methods today (Smith et al., 1974). If NP/AP rate between acid forming potential (AP) and neutralization potential (NP) of geological material is 2, it is estimated to form alkaline water (Skousen et al., 2002). But this is not a fully safety method, because AMD has occurred and unexpected results have been obtained although the rate at issue is over 2 in 11% of 56 mines (Kuipers et al., 2006). Furthermore encountering with unexpected geological material and/or fact (mineral, aquifer, groundwater flows etc.), inconsistencies in the conditions of laboratory and field may affect the results of the method directly.

Physical and chemical conditions like crystal forms, sizes and identities of the minerals creating the said effect in both groups in NP and AP determination may cause different effects in the velocity of the reactions occurring in both groups. Neutralization minerals with slow reaction rate lead to the formation of acidic waters. Reaction rates of sulphide carbonate and silicate minerals in atmospheric conditions may be different than the rates in laboratory environment and sulphides have faster reaction rates than carbonates and silicates (Sherlock et al., 1995). Many similar studies have introduced that laboratory tests are valid in limited conditions (Lawrence and Scheske, 1997; Paktunc, 1999; Weber et al., 2004). There is a considerable uncertainty in long-term estimations of AD made up of geological material in the fields related to the mines. For instance in a study where environmental impact assessment reports and operation conditions of 25 mines, surface water quality standards have been determined to be over permissible values in terms of pH and metal concentrations in 15 mines (Kuipers et al., 2006). Similarly fluctuations showing a change of 50% in acid forming rates in the kinetical humidity cell tests used as constructed between 3 and 7 years have been observed within 1 year (Morin and Hutt, 2000). 3. HYDROGEOCHEMICAL MODEL According to Derry (1999) models include characteristics and relations required for us to comprehend a real system with the aspects in which we are interested. Furthermore principle in model use requires being able to understand the scenarios answering the questions like why and what, sensitivity analyses, test assumptions, simulation results and their inconsistencies with the data observed (Oreskes et al., 1994). According to Nordstrom (2004) models

have essential and testable characteristics containing best answers of question and questions, usable results, new concepts which haven’t been comprehended in the beginning in the coherence of the existing portrait with the portrait created by intuitive power and conceptual limits. Hydrogeochemical model deals with mass transfer as well as minerals, ion interaction and aqueous species and takes place with detailed descriptions in the literature (Langmuir, 1997; Zhu and Anderson, 2002; Appelo and Postma, 2005; Merkel and Planer-Friedrich, 2008; Bethke, 2008; Glynn and Brown, 2011). Most general computer codes are PHREEQCI v.2 (Parkhurst and Appelo, 1999), WATEQ4Fv.2 (Ball and Nordstrom, 1991), Geochemist’s Workbench (Bethke, 1994), MINTEQ (Allison vd., 1991) and EQ3/6 (Wolery and Daveler, 1992) and used intensively in the applications of geochemical speciation, mass transfer and acid drainage models (Blowes et al., 2003). Hydrochemical model requires determinative information describing related geological system, existence of significant chemical reactions and design of formation and knowledge about thermodynamic, kinetic and surface characteristics for private chemical system (Zhu and Anderson, 2002). Previously a conceptual behavior trying to comprehend water-rock interaction in the field, what kind of model will be developed and which detailed data will be needed should be determined. In the beginning stage of model a sensitive, complete, representative and recognized measurement parameter and sampling planning are necessary. Whatever the final targets hydraulic and hydrogeological basic directional information are necessary as well as the characterization and discrimination of water-rock interaction in the area. Direction of the flow of groundwater

determines the contact order of water with the different minerals in aquifer and is of great importance in the application of inverse mass balance modeling. Hydrogeochemical modeling may enable the special calculations of the effect of the acidic water to occur in a mine on the studied area on underground and surface water with field-specific effects, accident action-reaction scenarios and risk assessments, approximate results which may be obtained in reactional barriers, many studies like the ones on which chemical components may bear greater field-specific contamination risks with first data taken from the site even before operating the mine. Development in personal computers in recent years has caused a quick development in hydrogeochemical model studies. Even in case of the little differences in the concentrations of various species, changes created in mass balance calculations may be calculated in seconds. While being in balance with certain minerals, water in certain physical and chemical composition may make significant contributions to the solutions for the problems regarding dissolutionprecipitation, sorption (adsorptiondesorption), surface complexation, ion exchange (cation and anion exchange), mixture with different waters, titration (alkalinity, acidity), determination of potential dissolved species, pH buffering, inverse mass balance and forward modeling. 3.1. Application of Hydrogeochemical Models According to Nordstrom (2004) dominant chemical composition in many surface and groundwaters create almost a dozen of hydrogeochemical processes. These processes are calcite and gypsum dissolution-precipitation, dissolution of silicate minerals (feldspar, mica, chlorite, amphibole, olivine and pyroxene) and

formation of clay minerals, dissolution of dolomite and precipitation of calcite, dolomite formation, pyrite oxidation and formation of aqueous iron oxyhydroxides, sulphate reduction and pyrite formation, silica precipitation, evaporation and cation exchange. Mass transport models containing one or few of these processes try to describe water flow in an area and chemical species and balance stabilities within the flow in one, two or three dimensions in terms of space and time. Simulations may be made with single and more complex inverse mass balance like speciation-solubility, mix and adsorption (ion exchange, surface complexation) and forward modeling by the use of PHREEQCI code. Model types which may be used in the calculations related especially to acid drainage from these modeling are discussed herein. PHREEQCI v2.18 and 3.1 code and WATEQ4F data base were used in the calculations. 3.2. Speciation Calculation

and

Solubility

Saturation indexes (SI) based on mineral solubility and chemical species (speciation-solubility simulation) in sea water were calculated first by Garrels and Thompson (1962) in hydrogeochemistry. Activities of potential dominant species may be calculated easily upon the description of measurement and analysis data in the entries in the requested sensitiveness, detail and necessary representation efficacy of physical, chemical and biological (limited) components of the medium from which water sample is taken. Obtained result ensures the distribution of the chemical mass (ion and molecular species) between gas, mineral and solution in an instant image of a dynamic system (admitted) in case of stable balance and potential mineral saturation in accordance with calculated species.

In the calculations of speciation measurement of temperature, pH and redox potential (pe, Eh, dissolved oxygen, oxygen partial pressure, redoxsensitive species…) chemical analysis of water and thermodynamic data required for considered elements are of vital importance (Plummer et al., 1983). According to data obtained from the measurements and analyses regarding the content of solution, saturation condition calculated up to appropriate mineral phases (SI=logIAP/K, IAP: ion activities of product, K: equilibrium constant) mineral equilibrium in the medium (saturated-unsaturated) and change of trends based on evolution through a flow path may offer very important information about groundwater systems. It is even possible to estimate the condition of minerals and redox encountered by water through flow path or to audit the appropriateness of physical and chemical conditions (Plummer et al., 1983). Development of anthropogenic point or expanded pollution loads differently than natural evolution, their behavior in the medium and possibility to describe the effects created by them on pH-Eh values and first and foremost possibility to obtain necessary evidences to separate them from natural contamination may offer very important advantages in order for us to solve the related problems. Since they don’t reflect the balance condition of waters in many cases, activities and mineral saturations of calculated species are considered suspiciously. But they constitute an excellent means as a result of the power of projection ensured by them. Possibility to calculate the effects created by the change in pH-Eh on species play a key role in the solution of many environmental problems. Especially possibility to describe groundwaters in accordance with the characteristics of aquifer is very useful for the possibility to

describe the mixture and distribution of waters with different characters. During the sampling of groundwaters, high sensitiveness in measurements and analyses of field parameters (pH, Eh, dissolved oxygen, temperature, alkanity..), devices with appropriate calibrations, parameters measured in the flow cell closed to atmosphere if possible and transfer in an appropriate time for analysis. Analysis charge balance ((% error= 100 x (total cation-total anion) / (total cation+total anion)) equivalent gram/liter) should be within permissible limits (