Using Hydrochemical And Nitrogen Stable Isotopes To Assess Water Quality, Evaluate Nitrogen Sources And Estimate Pollutant Fluxes For West Lake

Eutrophication was a worldwide problem. West Lake, as the soul of tourism, had strong accumulation of history and culture, so the water quality situation was concerned by many people. However, eutrophication declined the tourism value and the sustainable development potential. This thesis was devoted to evaluating eutrophication levels, tracking the nitrate pollution sources by N isotope and estimating pollutant fluxes. The results presented herein would be useful to control of water quality in West Lake and effective utilization of water resources throughout China.The main conclusions were as follows:(1) The water quality of West Lake was greatly improved since transferred fresh water from Qiantang River, but the TN concentration remained high, over the IV category of regional water environment domain standards. Single factor index evaluation method showed that water quality variables(except TN) spatial and temporal variations were both between Ⅱand Ⅳ category, TN varied in Ⅳ category to worse than Ⅴ category. Comprehensive identification index evaluation method showed the eutrophication level of West Lake was mid-nutrient status.(2) NO3--N was the main N form in lake water, transfer water and runoff water, and also accounted for much in rain water. It was showed that significant positive correlations existed among NO3--N, Cl-and SO42-. It indicated that the source of NO3--N, Cl-and SO42-might be chemical fertilizers and manure. The δ15N value of West Lake basin was varied between+2.6‰and+12.0‰. The results showed that chemical fertilizer, manure, soil nitrogen and precipitation could be the NO3--N in the West Lake, as well as denitrification in sediment and the exchange between ground water and lake water may lead to the high value of δ15N.(3) The result estimated by interpolation methods showed that in sampling period, the pollutant fluxes of COD, TN, NO3--N, BOD5, NH4+-N and TP were1595.3t,576.0t,360.3t,255.8t,84.2t and8.2t, respectively. The transfer water contributed the most pollutant influx, accounting for about62%,63%,50%,62%,61%and54%of the COD, BOD5, TP, TN, NO3--N and NH4+-N influx, respectively. Runoff was the second greatest pollution source, contributing about36%,33%,47%,33%,37%and26%of the COD, BOD5, TP, TN, NO3--N, and NH4+-N influx, respectively. Rainfall was also an important pollution source, accounting for2%,4%, 3%,5%,2%and20%of the COD, BOD5, TP, TN, NO3--N, and NH4+-N influx, respectively.