Ambient mercury concentrations at industrially influenced sites on the Highveld

Abstract

Mercury (Hg) is a toxic atmospheric pollutant that has the ability to bio-accumulate in the aquatic food chain, leading to animal and human exposure. During this study, assessments of ambient Total Gaseous Mercury (TGM) concentrations at industrially influenced sites in the interior of South Africa were conducted. Continuous TGM and ancillary data, i.e. other pollutant species and meteorological parameters, were measured at the Eco-Park (EP), Elandsfontein (EL) and Marapong (MP) monitoring stations. Mean TGM concentrations during the monitoring period was 3.95 ± 2.97 ng.m-3 for EP, which is substantially higher than at other South African and most international first world urban sites. The mean TGM measured at EL (2.49 ± 2.06 ng.m-3) was comparable to levels reported for urban South African and American cities, yet elevated above international background levels. Mean TGM levels at MP (1.61 ± 1.42 ng.m-3) was comparatively lower than the other two monitoring sites considered, but fell within ranges reported at suburban South African and Northern Hemisphere background sites. TGM spikes, which were associated with pollution events and diurnal cycles, were frequently observed and contributed to the relatively high afore-mentioned standard deviations. Multiple linear regression (MLR) was applied on the data as a receptor model approach. The derived site-specific optimum MLR equations allowed for relatively accurate calculation of TGM concentrations when correlated with measured concentrations, with the exception of very high or low concentrations. Combining information derived from the MLR analyses, diurnal plots, pollution roses and polar plots enabled identification of the dominant TGM source(s) for each site. A nearby coal-fired power station was identified as the most significant source at EP, while household combustion also made a discernible contribution. The dominant source contributing to elevated levels measured at EL was high stack emissions from various coal-fired power plants distributed across the region. At MP, household combustion in the surrounding semi- and informal settlements dominated TGM contributions. At all the monitoring sites, anthropogenic sources were much stronger than natural re-emission processes. Possible chemical processes that are likely to be important for atmospheric Hg in the South African interior were identified by information derived from the analytical tools utilised