Tracing the Distribution of Heavy Metals in Sediments of the Pearl River Estuary

Abstract

The Pearl River Delta is the socio-economic hub of southern China. The region has experienced rapid industrialisation, particularly since the start of the Chinese Reform Programme in 1979. This rapid industrial development has been coupled with increased pressures on the natural environment environment from pollution and waste associated with this development. These pollutants are transported from within the Pearl River catchment and deposited in sediments of the Pearl River Estuary (PRE).

Geospatial and principal component analysis of the heavy metals Al, Cd, Co, Cr, Cu, Fe, Ni, Pb, Ti and Zn in surface sediments identifies four patterns of spatial distribution that are attributed to three separate heavy metal sources. After normalisation for the effects of the fine particle size affinity, the same spatial distribution patterns remain, but with much less localised variability. Anticlockwise estuarine circulation and the interaction of marine and river water, where distributaries of the North and West Rivers enter the PRE, account for higher concentrations of all metals except Al in the west of the estuary. Pb and Zn are identified as the heavy metals displaying significant enrichment from non-natural sources. Their concentrations which peak at 168 and 699.9mg/Kg respectively along the North River, are found to be associated with higher baseline concentrations resulting from the erosion of metal sulphide bearing carbonates within the catchment. These higher baseline concentrations have then been further enriched by Galena (PbS) and
Sphalerite (ZnS) mining operations, atmospheric deposition from fossil fuel burning and some point sources associated with urban and industrial waste.

Temporal analysis of heavy metals and stable Pb isotope variability again
highlights Pb and Zn as being significantly enriched from anthropogenic sources, with maximum enrichment factors of 2.6 and 1.8 respectively. The enrichment of these two metals is shown to begin in the 1950s and acceleration of their enrichment in the early 1990s. This second phase of enrichment in the 1990s is also associated with slight enrichment of Cr, Cu and Ni that is also attributed to anthropogenic sources.