Southeast Sulawesi province is an area in Indonesia with nickel reserves of 97.4 billion Wmt, spread over an area of 480 thousand hectares. This potential causes the number of mining companies in Southeast Sulawesi to grow rapidly, especially in recent years. The nickel mining system applied in Indonesia is an open-pit mine. The irregularities of nickel resource mining in Indonesia cause many serious environmental problems. Piles of leftover rocks in nickel mining waste dumps have the potential to be a source of heavy metal pollutants such as Pb, Cd, Cr, Fe, Cu, Zn, and Ni (Ngkoimani & Chaerul, 2017). Among these elements, cr has the strongest capacity to migrate under certain climatic conditions. Overburden containing high cr2O3 and Fe compounds can be a source of hexavalent chromium and iron seepage in surface water and groundwater (Tiwary et al., 2005).
Hexavalent chromium is soluble and highly mobile rather than the trivalent form. Cr6+ is highly carcinogenic and is known to be 100 – 1000 times more toxic than Cr3+ (trivalent chromium) (Linos et al., 2011). Cr6+ is known to cause cancer of the lungs, nasal cavity, paranasal sinuses, and stomach, as well as the larynx (Linos et al., 2011). Cr6+ is produced from the natural oxidation process of the cr3+ carrier mineral. Cr species are affected by pH and Eh water (Henderson, 1994). The release of Cr6+ into laterite groundwater is related to the oxidative reaction of Cr3+ with Mn oxide (Equeenuddin & Pattnaik, 2020; Fandeur et al., 2009), while the iron is a microelement that is essential for the body that is needed in the formation of blood, namely in hemoglobin (Hb), but if receiving too much Fe intake will cause various health problems such as poisoning, where vomiting, diarrhea, intestinal damage, hemochromatosis, cirrhosis, liver cancer, diabetes, heart failure, arthritis, impotence, infertility, hypothyroidism, and chronic fatigue occurs (Agustina et al., 2021).
Analysis of dissolved Cr6+ content of liquid waste resulting from the remaining nickel mining of PT. Vale Tbk, found cr6+ concentration exceeded water quality standards (Marzuki, 2016). Water quality analysis results at nickel mines in New Caledonia show high concentrations of Cr6+ and are strongly affected by rainfall (Gunkel-Grillon et al., 2014). A study conducted by (Tiwary et al., 2018) in Sukinda Valley found high concentrations of Cr6+ in mine water (inlet mine water and outlet mine water), surface water, and groundwater. Then research (Okto et al., 2019) found high concentrations of Fe in river water around a nickel mining site in South Konawe, Southeast Sulawesi. Mine water with high concentrations of cr6+ and fe, which are not treated properly can be infallible into aquifers, leading to cr6+ and fe pollution in the land. (Dhakate et al., 2008; Tiwary et al., 2005) estimated the possibility of migration of Cr6+ into the aquifer using a transport model. (X. Wang et al., 2020) found cr6+ can diffuse vertically as deep as 58 m from ground level. Cr6+ contained in wastewater in eastern China can migrate 1,450 m within 10 years (Zhou et al., 2014).
Identifying the concentration of groundwater contaminant sources is essential for managing and protecting water from anthropogenic pressures (Soltani et al., 2017). Toxicity Characteristic Leaching Procedure (TCLP) is widely used to assess the concentration in waste (Adhikari & Mal, 2021; Lim et al., 2009; Sun et al., 2006; P. Wang et al., 2019). According to (Xie et al., 2021), many numerical simulation methods have been applied in the modeling of groundwater systems including FDM (Finite Difference Method), FEM (Finite Element Method), BEM (Boundary Element Method) and FAM (Finite Analytical Method). At this time with the development of data processing methods, Visual Modflow has been developed by Waterloo Hydrogeologic, Canada, which adopts different methods up to and combined with modern visualization technology. Visual Modflow software has the ability to simulate more complex conditions, can provide more realistic results, and is widely used to simulate groundwater flow, evaluate groundwater resources, and predict the spread of contaminants with MT3DMS packages (Adhikari & Mal, 2021; Ma et al., 2012; Rahman et al., 2018). Along with the development of technology, Visual Modflow can also be combined with a genetic algorithm (GA) in solving groundwater problems (Widodo et al., 2018). Identifying the concentration of groundwater contaminant sources is essential for managing and protecting groundwater from anthropogenic pressures (Soltani et al., 2017). Toxicity Characteristic Leaching Procedure (TCLP) is widely used to assess the concentration in waste (Adhikari & Mal, 2021; Lim et al., 2009; Sun et al., 2006; P. Wang et al., 2019). According to (Xie et al., 2021), many numerical simulation methods have been applied in the modeling of groundwater systems including FDM (Finite Difference Method), FEM (Finite Element Method), BEM (Boundary Element Method), and FAM (Finite Analytical Method). At this time with the development of data processing methods, Visual Modflow has been developed by Waterloo Hydrogeologic, Canada, which adopts different methods up to and combined with modern visualization technology. Visual Modflow software has the ability to simulate more complex conditions, can provide more realistic results, and is widely used to simulate groundwater flow, evaluate groundwater resources, and predict the spread of contaminants with MT3DMS packages (Adhikari & Mal, 2021; Ma et al., 2012; Rahman et al., 2018). Along with the development of technology, Visual Modflow can also be combined with a genetic algorithm (GA) in solving groundwater problems (Widodo et al., 2018).
The aim of the study was to assess the concentrations of Cr6+ and Fe that can be escaped from leftover rocks in nickel mining waste dumps and conduct groundwater flow simulations using Visual Modflow as well as MT3DMS codes used to evaluate Cr6+ and Fe migrations in groundwater over 20 years across a variety of scenarios. This is the first time that water flow modeling and contaminant transport (Cr6+ and Fe) have been conducted at nickel mining areas in Southeast Sulawesi. The results of this study may have implications for planning the prevention or mitigation of groundwater pollution around nickel mining areas and provide input to the government on the determination of regulations in the design and layout of good waste dumps in nickel mining.