Two-dimensional reactive transport model as a new approach for identifying the origins and contribution of arsenic in a soil and water system

Abstract

Distribution of arsenic (As) in the environment can occur through various paths, depending on the interrelationship between soil and surface water, and groundwater. This study aimed to identify the origins and pathways of As among various potential origins in soil and water systems using two-dimensional reactive transport models (2D RTMs) and to evaluate the contribution of each origin to the geochemical distribution of As in the study area, which included a reservoir, two streams, and two abandoned mines. The 2D RTMs were prepared by combining hydraulic and geochemical reactive models. The MODFLOW-2005 package was used to simulate the water flow in the study area. The geochemical reaction and distribution of As were simulated using Geochemist's Workbench. The concentration of As in the reservoir was ∼50 μg/L, while that in the soils collected around the mine X and Y was 15.0–1853.2 mg/kg (median: 126.1 mg/kg) and 3.6–1629.2 mg/kg (median: 60.9 mg/kg), respectively. Hydraulic and geochemical input data were derived from field surveys and laboratory experiments. Based on the geochemical and hydrogeological input data, 2D RTMs determined that 32.0 kg of As accumulated into the reservoir over 18 years through surface water, which is from mines X (2.7 kg) and Y (29.3 kg). Hence, mine Y would be the primary origin of As to the reservoir. Based on 2D RTM, 2.4 kg of As entered the reservoir via groundwater (DO <0.1 mg/L). Also, precipitation of sulfide minerals (e.g., orpiment; As2S3) influences As dispersion and contribution under subsurface conditions. These modeling results suggest that 2D RTMs could be a new approach for identifying and assessing the contribution of potential As origins in soil and water system.