The Effects Of Sediment Capping On Water Quality In An Iris Wilsonii Microcosm

There are many researches and engineering have been carried out to solve the problems of eutrophication and sediment pollution.The control and prevention meth-ods also ranged from control of external source pollution,discharge control and sewage diversion,to control inner source pollution.And Many works for the internal repair of contaminated water and sediments have been proposed,such as sediment dredging,ar-tificial aeration,sediment capping bioremediation,chemical control and so on.The technology of sediment capping is mainly used to isolate sediment and water by covering one or more layers of clean covering on the sediment surface.The covering material layer can inhibit the contamination of migration of the sediment to the water to some extent.However,some studies have shown that sediment capping may have negative effects on water self-purification capacity,benthos and aquatic plants.So far,the responses of sediments to the biogeochemical cycle after being covered,and the changes of redox environment caused by the sediment capping are still unclear.The study of the change process and regularity of the main water quality indices after the input of high concentration of nutrient in the wetland ecosystem is important for the ecological restoration.In this study,an indoor simulation experiment was conducted.We established a microcosm,which included water,Iris wilsonii and sediment,and used clean river sand?thickness 1 cm?to covered the sediment.We controlled for nutrients at 3 concentration level?C₀,C₁,C₂?in microcosm with sediment capping?S?,and uncapping?N?,respec-tively;we then compared the changes of main water quality indices,which included NH₄⁺-N,NO₃^--N,TN,PO₄^3--P,TP,DO?dissolved oxygen?,pH,T?water temperature?.The experiment last 21 days.The results showed that:?1?After adding high concentrated solutions,the water quality indices of differ-ent groups decreased rapidly after they reached their own maximum value though dif-ferent indices showed different trends of variations.The concentration of NH₄⁺-N took the shortest time to drop to a low level within 5-9 days,then fluctuated slightly in the subsequent stage.NO₃^--N keep increasing until NH₄⁺-N reached some low level,and then decreased slightly.TN decreased rapidly in the initial stage of the experiment,and changed similarly with NO₃^--N in the subsequent stage.TP and PO₄^3--P went down rapidly during the initial stage,then declined continuously in a similar pattern but with a little time-delay.Generally,there were differences among difference water quality indices.The concentration of NO₃^--N decreased until NH₄⁺-N reached a low level,be-fore that NO₃^--N has been rising all the time.PO₄^3--P and TP monotonically decrease after their concentrations reached max value at the initial stage of experiment.?2?The maximum and average concentrations of water quality indices in the capped group were higher than those in the uncapped group,and the water quality in-dices in the capped group had shorter time to fall to a relatively lower level than the uncapped group.The results showed that,after capped sediment,the decreasing rate of water quality indices were slowed down,and the presence of nutrients had been pro-longed.?3?In the no-nutrient-treated group,the concentration of PO₄^3--P and TP in the capped group and uncapped group were kept at very low levels.And the PO₄^3--P and TP in the capped group were slightly higher than that in the uncapped group,while the concentration of NH₄⁺-N,NO₃^--N and TN in capped group were significantly higher than that in the uncapped group,which suggested that the phosphorus indices was less affected by the overlay treatment in this study.?4?The comparison of the correlation between water quality indices in the initial stage and subsequent stage of the experiment revealed that there was a positive corre-lation between NH₄⁺-N and PO₄^3--P in the initial stage but there was a negative corre-lation in the subsequent stage.In the capped group the correlation between NH₄⁺-N and NO₃^--N was negative in the initial stage and subsequent stage,but in the un-capped group there was a negative correlation between NH₄⁺-N and NO₃^--N in the initial stage,and positive correlation in the subsequent stage.The correlation between other indices was not obvious in the initial stage and subsequent stage.Correlation between water quality indices of comparison capped group and uncapped group,there was a signifi-cant positive correlation between NO₃^--N and PO₄^3--P and TP in the capped group,while NO₃^--N was not significantly correlated with PO₄^3--P and TP in uncapped treat-ment.In the uncapped group,DO had a significant negative correlation with PO₄^3--P and TP,but there was no significant correlation between DO and TP and PO₄^3--P in the capped group.There was no significant difference in the relationship among other in-dices between the capped group and the uncapped group.?5?Comparison of incidence relation between water quality indices between capped group and the uncapped group,compared to the uncapped group,the incidence relation between DO and NH₄⁺-N in capped group was higher,and the incidence rela-tion between pH and TP in capped group is higher than that in uncapped group.The grey relation degree between TN and NO₃^--N was highest,while pH and DO had the highest grey relation degree with NO₃^--N.The grey relation degree between pH and TN indices in capped group decreases with increasing concentration,but it was increases with the increase of concentration.There was no significant difference in the incidence relation between other indices among different concentration groups.On the whole,the results showed that with the input of high concentrated nutrient solutions in Iris wilsonii microcosms,the water quality indices in capped groups were higher and the decrease ratios of indices were smaller comparably,while in groups without nutrient input,the concentrations of PO₄^3--P,TP in capped and uncapped groups kept almost the same low concentration,but for nitrogenous indices,capped groups were significantly higher than that in uncapped groups.Sediment capping changed the correlation between NO₃^--N and NH₄⁺-N in the subsequent stage.of ex-periment,and increase the correlation between NO₃^--N and phosphorus indices and re-ducing the correlation between DO and phosphorus indices,and increase the incidence relation between pH and DO.Generally,the sediment capping treatment might reduce the self-purification capacity in water ecosystems to some extent,especially for nitrog-enous indices.