GEOCHEMICALLY FINGERPRINTING COMPLEX CONTAMINATION TO TAIHU LAKE, EASTERN CHINA

In May 2007, a severe cyanobacterial bloom and associated toxins on Lake Tai forced the water treatment facility of the Chinese city of Wuxi to close. From June 16, 2007 to August 11, 2007, we synoptically sampled water from the Lake and at the mouths of major inflowing and out-flowing streams to characterize concentrations of nutrients, major solutes and selected trace metals, and the stable isotopes of water during low flow conditions. We used the data from this survey to place the geochemistry of the lake system into a broad geochemical context, and determine if major contaminant sources in the watershed could be geochemically fingerprinted. Our results show that Taihu Lake watershed waters heterogeneously evolve from a Ca-Mg-HCO3 type to a Mixed Anion-Cation water chemistry. In the lake, higher concentrations of Na and Cl and enriched stable isotopic ratios of water, compared to that in inflowing streams, show the geochemical effects of summer evaporation. Covariance between nitrate and fluoride, and the ratios of Na/Cl, Cl/F and Cl/Br fingerprint complicated distributions and multiple sources of anthropogenic contamination to the lake. These probably include a combination of septic discharge, pesticides and fertilizers, and perhaps industrial sources. The extent to which cyanobacteria remove nutrients can be assessed by a simple mass calculation. Concentrations of ~10 mg/L NO3 and 0.7 mg/L PO4 in inflowing river water decreased by more than 50% and 80%, respectively, in the lake. We coupled these data to discharge measurements of stream flow, and found that in-situ lake processes, largely biological, remove on the order of 200,000 kg/d NO3 and 8,000 kg/d PO4 during times of low flow.