Modelling transport and transformation of mercury fractions in heavily contaminated mountain streams by coupling a GIS-based hydrological model with a mercury chemistry model.
Many heavily polluted areas are located in remote regions that lack routine hydrologic monitoring. A modelling method that can produce scenarios of water chemistry trends for regions that lack hydrological data is therefore needed. The Wanshan mining area, in Guizhou province in south-western China, is such a region, as it is heavily polluted with mercury (Hg). In order to model Hg transport in a stream draining the Wanshan mining area, a Geographic Information System (GIS) hydrologic model (HEC-HMS) was coupled with a simulation model for Hg fractions in water (WASP Hg). Hydrological variations in the stream flow can thereby be simulated based on readily available precipitation data. The WASP 7 MERC Hg model was used for simulating variations in total Hg, dissolved Hg and methyl-Hg concentrations. The results of HEC-HMS modelling of flow show clear seasonal variation. Winter (Oct-Dec) constitutes the dry season with low flow, while the summer season (Jun-Aug) is rainy with high flow. 48% of total annual precipitation happens in the three summer months. The stream flows at the high flow events were several times higher than normal flow. The modelled total suspended solids and Hg concentrations were tested against monitoring data from two sampling campaigns conducted in September 2007 and August 2008. The model produced reasonable simulations for TSS, THg, DHg and MeHg, with relative errors generally around 10% for the modelled parameters. High flow events are the main contributors for release of both suspended particles and Hg. The three high flow events account for about 50% of annual discharge of THg. The annual total discharge of Hg was 8.8 kg Hg high up in the stream and 2.6 kg where the stream meets a large river 20 km downstream of the pollution source. Hence, about 70% of Hg is retained in the stream through sedimentation.