5:15 PM - 7:15 PM
[AGE34-P13] Numerical modeling of heavy metal migration through unsaturated soil from surface AMD to groundwater in Myanmar -Assessment under present and future climate scenarios-
Keywords:AMD, Heavy metals, Groundwater contamination, HYDRUS-1D, Climate change
Mining plays a crucial role in supporting Myanmar’s national economy. However, excessive mining and improper mineral management have caused serious environmental problems. One by-product of mining is acid mine drainage (AMD), which contains heavy metals such as lead, cadmium, arsenic, copper, and zinc. Improper management of AMD from mines has resulted in soil and groundwater contamination, posing significant environmental risks.
The Nweyon area, known for gold exploration and extraction, faces similar challenges. Local residents rely heavily on groundwater for their daily needs, making it essential to assess the risk of groundwater contamination from AMD. This risk can be evaluated by analyzing the transport of heavy metals from surface AMD through unsaturated soil to groundwater. In this study, numerical simulations were conducted to model heavy metal transport in unsaturated soil under the climatic conditions of the Nweyon area. Furthermore, considering the growing impact of climate change, the study also evaluated heavy metal transport under future climatic conditions, in addition to the current climate.
We calculated water and heavy metal transport using HYDRUS-1D, a one-dimensional soil heat and mass transport modeling software. The simulation domain extended from the surface to the groundwater table at a depth of 3 m, with nodes placed at 3 cm intervals. The Mualem-van Genuchten model, estimated using ROSETTA based on the local soil texture, was applied for the soil hydraulic properties. Adsorption characteristics of heavy metals (Cu, Hg, Zn, As, Cr(VI), Pb, Cd) were defined using partitioning coefficients obtained from the literature, while diffusion coefficients and other solute transport properties were substituted with chloride ion values from previous studies.
Present and future climatic conditions for the Nweyon area were obtained from d4PDF (Database for Policy Decision-Making for Future Climate Change). Precipitation and evaporation rates for 2001-2010 (assumed as current conditions) and 2101-2110 (assumed as future conditions) were retrieved. The surface boundary condition was set using the daily difference between precipitation and evaporation rates. The initial soil moisture content was set at 0.18 m3 m-3, and each heavy metal was introduced at a concentration of 1000 mmol m-3 in the top 0-9 cm soil layer. Simulations were performed over 10-year periods for both present and future climate scenarios.
Results showed that while evaporation rates in the Nweyon area remained similar between current and future scenarios, precipitation increased from 1231 cm to 1570 cm over the 10-year period, indicating that heavy metal transport through soil is expected to accelerate in the future, leading to a higher pollution risk. The transport rates of heavy metals varied depending on their distribution coefficients. Zn exhibited the fastest movement, reaching the groundwater table within approximately six months from the start of the simulation. Cr (VI) displayed a similar transport rate, reaching the groundwater table in about 250 days. Transport rates decreased in the order of Cd, Cu, As, Pb, and Hg, with arsenic reaching the groundwater within 10 years, while lead advanced to a depth of only 150-200 cm, and mercury showed minimal movement from the surface. Notably, slower-moving heavy metals demonstrated a greater increase in transport rates under future climatic conditions.
These findings have provided important insights into the assessment of AMD-related groundwater contamination risks in Myanmar, which could be valuable for future groundwater conservation efforts.
The Nweyon area, known for gold exploration and extraction, faces similar challenges. Local residents rely heavily on groundwater for their daily needs, making it essential to assess the risk of groundwater contamination from AMD. This risk can be evaluated by analyzing the transport of heavy metals from surface AMD through unsaturated soil to groundwater. In this study, numerical simulations were conducted to model heavy metal transport in unsaturated soil under the climatic conditions of the Nweyon area. Furthermore, considering the growing impact of climate change, the study also evaluated heavy metal transport under future climatic conditions, in addition to the current climate.
We calculated water and heavy metal transport using HYDRUS-1D, a one-dimensional soil heat and mass transport modeling software. The simulation domain extended from the surface to the groundwater table at a depth of 3 m, with nodes placed at 3 cm intervals. The Mualem-van Genuchten model, estimated using ROSETTA based on the local soil texture, was applied for the soil hydraulic properties. Adsorption characteristics of heavy metals (Cu, Hg, Zn, As, Cr(VI), Pb, Cd) were defined using partitioning coefficients obtained from the literature, while diffusion coefficients and other solute transport properties were substituted with chloride ion values from previous studies.
Present and future climatic conditions for the Nweyon area were obtained from d4PDF (Database for Policy Decision-Making for Future Climate Change). Precipitation and evaporation rates for 2001-2010 (assumed as current conditions) and 2101-2110 (assumed as future conditions) were retrieved. The surface boundary condition was set using the daily difference between precipitation and evaporation rates. The initial soil moisture content was set at 0.18 m3 m-3, and each heavy metal was introduced at a concentration of 1000 mmol m-3 in the top 0-9 cm soil layer. Simulations were performed over 10-year periods for both present and future climate scenarios.
Results showed that while evaporation rates in the Nweyon area remained similar between current and future scenarios, precipitation increased from 1231 cm to 1570 cm over the 10-year period, indicating that heavy metal transport through soil is expected to accelerate in the future, leading to a higher pollution risk. The transport rates of heavy metals varied depending on their distribution coefficients. Zn exhibited the fastest movement, reaching the groundwater table within approximately six months from the start of the simulation. Cr (VI) displayed a similar transport rate, reaching the groundwater table in about 250 days. Transport rates decreased in the order of Cd, Cu, As, Pb, and Hg, with arsenic reaching the groundwater within 10 years, while lead advanced to a depth of only 150-200 cm, and mercury showed minimal movement from the surface. Notably, slower-moving heavy metals demonstrated a greater increase in transport rates under future climatic conditions.
These findings have provided important insights into the assessment of AMD-related groundwater contamination risks in Myanmar, which could be valuable for future groundwater conservation efforts.