The Analysis Of Water Quality Trends And Carbon Sinks Flux In Zhaidiunderground River Systems

On the basis of the hydrogeological survey, flow rate and water chemistrydynamic monitoring of Zhaidi underground river system, the characteristics of Zhaidiunderground river system was analyzed in this paper, furthermore, water qualityevolution trends of two periods (1983 to 1984 and 2008 to 2011)was summarizedusing water quality evaluation method and analytical approach of water qualityvariation. The source of nitrate-N of underground river was discussed and karstcarbon sink amount and flux of the two periods also were estimated respectively.Zhaidi underground river system is characterized by multi-block and multi-layerstructure, aquifer media is highly heterogeneous, recharging by inpouring, percolatingand infiltration. It is still in the state of natural flow field without uniformhydrodynamic field. However, it is featured as a highly non-continuous flow systemwith high-frequency transferring rate of"three water", large variation range of waterflow, Ca (Mg)-rich alkaline environment.The groundwater chemical type is HCO₃-Catype with pH ranging from 7.05 to 7.86. Cations and HCO₃-mainly were controlled bydilution effect and present low values in rainy season, high values in dry season, andirreproducible interannual variation. The contents of SO42-and Cl-are low with littlechange and has close relationship with human activities.Comprehensive analysis results of water quality evaluation show that theindicators of affecting the underground river water quality mainly include Pb, NO2-,and Ni, mostly occurredin 1984 and 2008, the water quality in 2009 to 2010 is almostall in class-I water. Years of trend analysis indicated that the water quality of Zhaidiunderground river system shows improvement gradually from class II or III to class I.Difference method and ratio method was employed to analyse the water quality trendsand good results was obtained, in which only Mg²⁺content decreases while six ofother general indicators increase at small range, including K+, Na+, Ca²⁺, Cl-, SO42-,HCO₃-; detection rates for Cu, Zn, Ni, Mn is high, the rest of trace elements are occasionally been detected, the difference value between the two periods is less thanzero, the ratio is smaller than one; and the difference value for Cu, Cd, As is largerthan or equal to zaro, and the ratio is greater than or equal to one.NO₃^-, NO₂^-and NH4+present an increasing trend. The mean value difference ofNO₃-is 2.28mg/l with 2.6 timesdifference between two periods is; the mean valuedifference of NO2-is 0.0645mg/l with 22.5 times difference between two periods,maxima up to 105 times. Nitrate source of Zhaidi underground river wass determinedbased on the analysis of groundwater nitrate nitrogen and oxygen isotope and land usepatterns, it mainly comes from manure pollution(animal waste).δ15N values rangefrom 10.44‰to 18.52‰.The carbon sink fluxes of the Zhaidi underground river system in years of2008-2010 and 1984 were estimated to be 68.82 t/km2·a (normal year) and51.56t/km2·a (dry year) respectively. The size of karst carbon sink intensity isinfluenced by bothunderground river flow and [HCO₃-]concentration.The [HCO₃-]concentration and the carbon sink intensity showed a negative correlation trend(non-linear relationship). Karst carbon sink intensity has a good linear correlationrelationship with water flow.