Contents of nitrogen, phosphorus and organic materials in sediments and theirs distribution along depth at Xixi Wetland.

A special sampler was made to take continous specimens of sediment (up to 2m long) from Xixi Wetland. The contents of total nitrogen (TN),total phosphorous (TP) and organic matter (OM) were measured on the sediment specimens taken from different depths,and their distributions along depth in this wetland were determined. Concentrations of TN,TP and OM in the pore-water of sediments and the upper water body were also measured. On the base of the measurements,the eutrophication pollution degree in the sediments was evaluated by using the organic index method. It was found that the active zone of pollution was extended to a depth of 60cm below the river bed. Sediments in the active zone were slightly-moderately polluted,and no pollution was observed at the depth greater than 60cm (i.e.,relatively stable zone). The distributions of TN,TP and OM along depth showed a consistent trend. The amounts of TN and TP in the pore water of shallow sediments were found to be much more than those in the above water body. So nitrogen and phosphorous in the pore water was likely released into the above water due to the concentration gradient. The test results indicate that the water quality in the wetland could not be completely recovered only by replacing the polluted water with clean water. It is also needed to dredge the contaminated sediments in the wetland. The depth of active zone (i.e.,60cm) provides an implication for a proper dredging depth.