Soil and groundwater contamination from heavy metals released from mining activities is a worldwide environmental problem. Heavy metals and other contaminants might release from mining sites, especially from waste dumps or tailing ponds (Alligui and Boutaleb, 2010; Liang-qi et al., 2010). So, the monitoring wells should be established around the nearby areas in order to monitor the groundwater pollutions. However, such monitoring is quite expensive and time consuming. Various mathematical simulation models have been developed for the assessment of groundwater vulnerability to groundwater contamination, water resources management and design of monitoring well systems. HYDRUS-2D software codes (Fellner and Brunner, 2010) have been developed to simulate water movement and solute transport. The program can numerically solve the Richard's equation for saturated-unsaturated water flow and the advection-dispersion equation for solute and heat transport. In this software, the solute transport module also considers equilibrium and non-equilibrium advection-dispersion in the liquid phase, non-linear equilibrium physical and chemical sorption. Furthermore, the transport of heavy metals leached from mine tailings to groundwater systems needs to be realistically predicted in order to be able to assess the risk (Sabahi et al., 2009; Saghravani et al., 2010) and consequently, to develop and select the most appropriate strategies in monitoring and remediating the contaminated site. In order to apply numerical models, the hydraulic properties of the soils and solute transport parameters, such as sorption isotherms or retardation factors, must be accurately estimated. Most studies have been applied sorption properties of solute from batch experiments (Alshaebi et al., 2009), but there are some limits to describe solute behaviors in real world condition. Moreover, column experiments have been introduced as an alternative to investigate the displacement of dissolved solutes through soil and may provide information that are not available using equilibrium batch experiments such as physical and chemical non-equilibrium sorption (Akyol et al., 2011).
However, few simulations have been used chemical non-equilibrium sorption incorporate with unsaturated-saturated hydraulic properties to predict solute transport. Recently, from previous dissertation (Chotpantarat, 2008), the chemical non-equilibrium two-site model was shown to describe heavy metal transport in lateritic soil better than the equilibrium convection-dispersion models. Moreover, one of the most important hydraulic parameters is the leachate flux from a tailing pond where the clay liner has become permeable, such as after cracking. So, the objective of this study was to simulate the movement of heavy metals leached from mine tailings through shallow groundwater systems with variable leachate fluxes and different sorption parameters. To achieve such objective, the parameters, obtained from unsaturated-saturated soil tests and the chemical non-equilibrium two-site model were applied in the field-scale metal transport simulations here. [Mn.sup.(2+)] was chosen as the representative heavy metal because it can leach from tailings at a higher level or rate than other metals. The HYDRUS-2D numerical model was used for the field-scale simulation in this study because it can evaluate the potential transport and contamination of heavy metals in groundwater.
MATERIAL AND METHODS
Study area: The manganese transport released from Tailing Storage Facility (TSF) in the Akara mining site (Fig. 1) was numerically simulated using HYDRUS-2D. This area located between 16[degrees]16'25'' and 16[degrees]17'41'' north latitude and 100[degrees]38'50'' and 100[degrees]40'15'' east longitude and about 280 km north of Bangkok in Phichit Province, central Thailand. It was selected for the study as it is an active gold mine with comparable geology and mining strategies to many other such mines. Thus, as well as being of national importance, any such study here is likely to be of broad applicability to many other mining systems in the tropics.
Methods: A Tailing Storage Facility (TSF) in the mine has been designed to safely store the mine tailings and is located at the southern portion of the mining site, covering an area of approximately 320,000 [m.sup.2]. Groundwater levels typically conform to the surface topography. A north-south orientated hydraulic groundwater divide is located through Khao Mo and Khao Pong. Natural groundwater flows from the ore bodies in the C-H mining pit in a southwest direction to the adjacent areas (Fig. 1). The shallow groundwater wells in the nearby villages were mainly dug into the aquifer of the lateritic layer at depths between 1.5 and 7 meters. The soil profile in the surrounding area consists of a 20-40 cm top soil layer over the whole area, a 1.5 to 7 meter thick Lateritic layer and a thick clay layer of between 4.3-11 m depth formed at the bottom of the soil sequence.
The parameters of the unsaturated-saturated hydraulic were indirectly derived by input percentage of sand, silt, clay and bulk density into pedotransfer function in HYDRUS-2D. Such sorption parameters derived from equilibrium models with linear and Langmuir isotherms and...