Characterization And Simulation Of Base Cations Loss Due To Acid Deposition In Taihu Lake Watershed

The acid deposition in Taihu Lake Basin was one of the most serious environment problem in China,which has a significant effect on the loss of base cations.The release characteristics of representative carbonate rocks and their weathered products and sandy loam soil in the basin to acid solutions with different pH were studied by laboratory simulation tests.Meanwhile,the MAGIC model and simulation experiments were used to provide parameters to quantitatively describe the effects of acid deposition on the loss of base cations in the Taihu Lake Basin,and to determine the linkage between acid deposition and trends of major ions in soil and surface water by combining long-term monitoring data of acid deposition and lake water chemistry in the basin.The results indicated that:(1)The release of base cations from carbonate rocks was significantly and negatively correlated with the pH of the acid solution and the rock grain size(P<0.01).In addition,the release characteristics of each base cations in limestone and dolomite at the same pH acid solution were significantly different,i.e.,the release of Ca2+ in limestone was significantly higher than that in dolomite,while the release of K+,Na+ and Mg2+ was significantly lower than that in dolomite,which was determined by the content of the cations in the rock itself.(2)The sensitivity of base cations in the weathering products of carbonate rocks and sandy loam soil to simulated acid solution were Ca2+>Mg2+>K+.The amount of base cations released were not only related to pH of simulated acid solution,but also depended on the characteristics of the total amount of cations in the soil itself.In addition,the Ca2+content of limestone was significantly higher than that of dolomite,but the release of Ca2+ in the weathering products of limestone was significantly lower than that of dolomite at the same pH acid solution.The analysis suggested that the long-term acid deposition leaching and the variability of soil-forming parent material environment in the basin led to the lower content of exchange state Ca2+in the weathering products of limestone,which affected the release of Ca2+.(3)The buffer capacity of different media for simulated acid solution in Taihu Lake Basin was significant distinction,that was,carbonate rocks>the weathering products of dolomite>the weathering products of limestone>sandy loam soil.It was considered that a large number of carbonates would neutralize protons rapidly,which was the key mechanism for carbonate rocks and their weathered products to have strong buffering capacity.Furthermore,based on the average pH of rainfall in the current basin,the equivalent ratio of[Ca2++Mg2+/[HCO3-]in carbonate rock leachate under the action of pH=4.5 acid solution was calculated to be between 1?2,which was consistent with the relationship of the equivalent ratio of[Ca2++Mg2+]/[HCO3-](>1)in the current water column of Taihu Lake,indicating that acid deposition involved in accelerated dissolution of carbonate rocks was still the main factor leading to the change of base cations in the water column of Taihu Lake.(4)The results showed that the concentration of ions in the lake water observed from 1985 to 2019 showed a significant upward trend,and the calculation results of magic model were in good agreement with this upward trend.The simulation results of Ca2+,Mg2+and K+in the lake were relatively consistent with the measured values.When simulated Na+,Cl-and SO42-which were greatly affected by point source pollution,the content of sewage/wastewater discharge correction considered in the model.The results of model predictions of future trends in water chemistry in Taihu Lake under different pollutant discharge scenarios show that future changes in SO42-and base cations content in atmospheric deposition will affect changes in the concentration of major ions in the lake:if the decreasing trend of base cations content in rainfall observed in 2005 continued in the basin based on the national reduction policy,Ca2+,pH and ANC of lake water and salinity saturation(BS)of soil would be affected,and it might interfere with the expected recovery rate of acidic pollutant discharge reduction;Although sulfur emissions began to decline from 2006,simulated lake water such as Ca2+ and ANC did not develop as predicted by the declining results,because anthropogenically accelerated weathering has significantly altered the natural processes and equilibrium in the Taihu Lake Basin,and the increase of nitrogen deposition in the regional atmosphere and the application of nitrogen fertilizer strengthened the contribution to the increase of potential local soil acidification rate,resulted in large amounts of Ca2+entered the lake.The prediction results showed that although there would be no obvious acidification problem in Taihu Lake Basin in the next 30 years(i.e.2050),the possible impact of acidification on the hydrochemistry and ecosystem of Taihu Lake Basin still needed to be paid attention to.