GEOCHEMICAL ANALYSIS OF TRIBUTARY WATERS AND POTENTIAL NUTRIENT SOURCES TO LAKE TAIHU, CHINA

Nutrients and other pollutants from intensive urbanization and agriculture have contaminated Lake Taihu China, which provides drinking water for millions of people. At times, the lake becomes hypereutrophic from nutrient loading. Before an effective nutrient remediation plan can be developed, the tributary sources of the nutrients and other contaminants need to be determined. We hypothesize that different sources of nutrient contamination should be associated with other contaminants in tributaries in the watershed, and that the transport and mixing of contaminants in the lake can be tracked.

To test this hypothesis, in summer 2007, we synoptically collected 16 samples from the lake near tributary inlets and outlets throughout the lake watershed, and 35 samples from the mouths of inflowing and outflowing tributaries 2007. In 2010, we refined our sampling to collect 32 water samples from throughout the lake and 119 samples throughout the northwestern inflowing tributary network. In each case, we analyzed the samples for field parameters (e.g. pH, specific conductance, DO, TDS) and major and minor solutes useful to fingerprint solute sources and chemical reactions controlling them.

Concentrations of base cations, alkalinity and pH show that carbonate dissolution naturally controls the major ion chemistry of the lake. Stochiometric correlation between sodium and chloride concentrations show a mixing trend suggest halite from a yet unknown source contributes chloride to the lake. A break in linearity in a scatter plot of nitrate against chloride indicates localized denitrification within both tributary streams and the lake. Scatter plots of ratios of the conservative halogens Cl, Br and F suggest distinct mixtures of different contaminant sources that could be related to different nutrient outputs. Future work will focus on spatially characterizing where major kinds of water quality occur in the watershed, isotopically characterizing the extent to which evaporation controls water chemistry, and modeling chemical controls over solutes in the lake.