Impact On Chemical Composition Of River Water By River Damming And Isotopic Implication

Currently, most rivers worldwide have been intensively impounded. Riverdamming becomes a big problem, not only in inducing the physical obstructionbetween upstream and downstream, but also in destroying the natural continuity ofriver. The discontinuity of water quality was often neglected, which presents achallenge to traditional river geochemistry research. To understand the changes inbasic chemistry of the river water after impounding, we investigated theWujiang-Miaotiao River reservoirs in series in the Wujiang River Basin, and theXinanjiang Basin. In this study, we analized the sources of organic matter in surfacesediments by means of carbon and nitrogen isotopic composition, and used the siliconisotope technique to undersatnd”Si effect” in karstic reservoirs. The following is themajor conclusions.1. Water chemical characteristics of Wujiang basin is mainly controlled by thecarbonate rocks weathering, and the Xinanjiang basin is mainly controlled by thesilicate rocks weathering. Our results showed that, thermal stratification is easilydeveloped in the studied reservoirs, in warm seasons, and consequently DO andtemperature showed a trend decreasing along the water column. Longitudinally,dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) content had adecreasing trend from upstream to downstream in the Xinanjiang Basin, but δ13CDIChad a contrary trend against the DIC and DOC.2. Results of δ13Corg, δ15N and C/N showed that, organic matters in the sedimentof the Xinanjiang are mainly from phytoplankton, and only a small part fromterrigenous organic matter. The content of TOC and TON in sediment had an upwardtrend from the upstream to the central reservoir area; while TOC and TON showed ashrap decrease downstream the dam, indicating a significant retention effect. The dataof dissolved silicon content and silicon isotopic composition of the Maotiao riverrevealed that the silicon content in carbonate rock area are more susceptible influenced by human factors.3. Chemical weathering rates were calculated using the water chemistry data ofthe reservoir surface and downstream of the dam, in each reservoir, respectively. Theresults showed that the difference between the chemical weathering rates calculatedfrom reservoir surface water and water downstream of the dam was greater inreservoirs with a longer water retention time. In Hongjiadu Reservoir with the longestwater retention time among the studied reservoirs, this difference reaches9.02%.4. Total phosphorus (TP) of the sediment tended to increase along theXinanjiang River, while the total nitrogen (TN) was decreasing. Due to the impacts ofthe dam, nitrogen and phosphorus concentrations in the reservoir and downstreamshowed a decreasing trend. Organic phosphorus (OP) is the main form of TP, and ON isthe main form of TN, accounting for more than90%of the TN. The content of Ca-Pand Fe-P ranked the second and third, in the forms of phosphorus, respectively. Due tothe impacts of the dam, Fe-P content is lower than Ca-P before the dam, but it becomehigher than Ca-P in sediment downstrem the dam. This result showed that thepresence of dam can change the composition of the phosphorus forms. Meanwhile,the percentage of the content of NH4-in TN has increasing tendency downwardly.