Research NoteSpatial and temporal variation in DOC in the Yichun River, China1
Introduction
The reactive chemistry of dissolved organic carbon (DOC) means it is very important in affecting the behavior and fate of both inorganic and organic pollutants in a fresh water aquatic environment. The mobility and bioavailability of trace elements in an aquatic system are determined to a large extent by the abundance of DOC, especially humic substances (Decho and Luoma, 1994). The solubility of organic chemicals can also be raised by the presence of humic substances (Ding and Wu, 1993). In addition to its effects on the behavior of trace amount of pollutants, naturally occurring DOC also acts as one of the most important precursors of chlorinated by-products during disinfection (Cantor and Hoover, 1990; Tao et al., 1994; Tao, 1996).
For non-contaminated rivers, DOC in the water derives from natural sources and is relatively stable, even though it is not conservative material. DOC in water of most rivers (non-contaminated) ranges from 1 to 10 mg C/l. In China, elevated DOC levels were detected in northeast China during a nationwide survey, especially in the drainage basin of the Songhua River where 7.2 mg C/l humic substances (XAD column absorbed) was reported in the summer of 1986 (Tao et al., 1990). The spatial and temporal variations in DOC levels in many other rivers have been studied (Leclerc et al., 1994; Kullberg et al., 1993; Soulsby, 1995). Kullberg et al. (1993)found large spatial and temporal variations in the transport of DOC from a terrestrial environment to acidified water. Soulsby (1995)reported on the variations in DOC and other parameters in an acidic forested catchment in upland Wales as a response to storms.
The objectives of this study were to investigate the spatial and temporal variations in DOC in the Yichun River, to study the relationship between DOC level and flow rate, and to estimate the DOC flux in the river.
Located within Heilongjiang Province, China, the Yichun River is a major tributary of the Songhua River that in turn drains into the Amur River. The drainage area of the Yichun River is about 2500 km2. The climate of the region is basically humid temperate under the strong influence of monsoons with an annual precipitation of 500–650 mm. The drainage area consists mostly of forested mountains and hills dominated by pine forests with extensive wetlands along river channels. The soils in the region typically have high organic matter content.
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Methodology
Water samples were collected from the Yichun River at various locations from April 1993 to March 1994. To determine temporal variation, samples were collected daily during the ice-free period in addition to the first day of each month from December to April when the river was totally covered by ice at the Yichun Environmental Monitoring Station. In both winter and summer, samples were also collected along three sections of the river: Delu Creek, the Wuma River and downstream from the Yichun
Spatial variations in DOC in the Yichun River drainage basin
Water samples were collected along Delu Creek (a tributary of the Wuma River), the Wuma River (a tributary of the Yichun River) and the Yichun River mainstream (downstream from the Wuma River) at the end of November 1993 when the river was covered by ice. Sampling locations are shown in Fig. 1. A total of 32 samples were collected beneath the ice cover. The DOC variation of these samples was determined and is shown in Fig. 2.
Delu Creek is a small mountain stream. The DOC in the stream slowly
Conclusion
DOC in the Yichun River and its tributaries increased with distance downstream. In the winter season when the river was totally covered by ice, the DOC in the water was primarily from autochthonous sources and was negatively proportional to the flow rate. In the summer, however, an allochthonous source dominated the DOC supply and the DOC level fluctuated together with the flow rate. The relationship between DOC and the flow rate in the summer and the winter can be described by two equations,
Acknowledgements
The author is grateful to Dr Ernest E. Angino, Dr Robert W. McColl, and Dr David Shankman for their valuable comments.
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Funding was provided by National Excellent Young Scientist Foundation of China [49525102].