Determination Of Hydraulic Characteristics In Constructed Wetlands Using Stable Isotopes

The constructed wetlands are on sustainable development in recent years due to their low cost,easy management and high removal efficiencies.The pollutants such as suspended material,nitrogen,phosphorus,organic matters,and metals are removed by the complex and abundant,reactions relying on physical,chemical and biological processes.The internal flow patterns and hydraulic characteristics in different regions are multiple.It is accepted that stagnant areas and preferential flow areas are common in surface flow constructed wetlands,which lead to the decrease of removal efficiencies.It is important to investigate the flow patterns inside constructed wetlands,and to remove the stagnant areas and preferential flow areas,which can help us to improve the treatment efficiencies for nutrients and prolong the operation life of constructed wetlands.At present,many researchers focus on the application of tracer,hydraulic models and velocity measurement to simulate hydraulic flow patterns in CWs.However,in large-scale CWs,adding tracers is not a good choice because of the high cost,potential of secondary pollution caused by adding external material into the CWs.As to hydraulicmodelits results can not reflect the hydraulic flow patterns synchronously as the parameter is always empirical.The above two technologies concentrate on the simulation of process parameters,and can not reflect the flow patterns real-timely.Finally direct velocity measurement inside the CWs needs expensive apparatus,not to mention that field measurement is considered a labor-consuming job.In liquid water,isotopes molecules’ vapor pressure is inversely proportional with its molecular weight.The abundance of heavy water isotopes would be higher(higher 8D and δ180 values)with a longer hydraulic retention time(HRT).If an accurate survey was done on the distribution of water isotopic composition,the hydraulic flow patterns would be known inside CWs.In present study,the annual isotopic composition of Xiaomei river constructed wetland were investigated,and discussed the feasibility of the isotope technology to characterize hydraulic flow patterns.The flow patterns of Zhaoniu river constructed wetland and Tuhai river constructed wetland were determined based on isotopic technology,and influence factors on flow patterns were analyzed.Meanwhile,the influence of flow patterns on the nutrients spatial distribution inside constructed wetlands were studied as well.The main conclusions are as follows:The results showed that δD and δ18O values in the three constructed wetland were under the eastern China meteoric water line(ECMWL).Linear correlation betweenδ18O and δ2H in Xiaomei river constructed wetland was δD = δ6.6718O-5.87(Rsquare was 0.86).The isotopes in small region constructed wetland were showed regional difference.The distribution map of δ18O could determine the location of SAs and PFAs accurately.The determination of PFAs in winter had much errors,but was exact in the other three seasons.The flow patterns determined by isotopic technology was consistent with the velocity distribution,and isotopic technology was effective in the determination of hydraulic patterns in small region constructed wetland.Linear correlation between δ18O and 8D in both wetlands were 8D = 6.45δ18O-9.3(Rsquare was 0.89)for Zhaoniu river constructed wetland and 82H = 4.38δ18O-26.63(Rsquare was 0.81)for Tuhai river constructed wetland.The linear relation in both wetlands were deviated from ECWML,which indicated that evaporation had obvious effect on isotopic composition.The side and "T" type inflow patterns led to PFAs and SAs easily,and concave side of the bend also could led to SAs.In the cross section of the watercourse channel,different plant species and density led to the different resistance to the flow,and SAs formed in the higher resistance area while PFAs formed in the lower resistance area.In the SAs,ammonia concentration was in the lowest level and the declined from inlet to outlet outside the SAs.High concentration ammonia was difficult to flow into the SAs due to the week exchange ability in SAs,which led to the low area utilization ratio.The distribution of TP was similar with ammonia during the stage of plant growth,and could be influenced by the release of decaying plants and bottom substrates.