| Karst rocky desertification is one of the three major ecological problems in China,also the primary ecological problem in karst areas,having huge negative impact on the environment,economy and society of the region.From 2006 to 2015,China carried out the first stage project of rocky desertification control and achieved remarkable results.The trend of the overall expansion of land rocky desertification has been initially curbed,changing from ctontinuous expansion in the past to net reduction.The ecological situation in karst areas shows a benign development trend.At present,the state is promoting the second stage project of rocky desertification control,required to summarize the effective experience of the first stage project and also to analyze the problems.This paper takes the typical low-lying wing-type karst trough-Longtan trough as an example in the key county of rocky desertification comprehensive control and national ecological protection and construction planning,Youyang,to analyze the hydro-ecological environment effects of the rocky desertification control and the difference of rocky desertification degree between the progresive-reverse slopes in trough valley.Taking the groundwater and surface flow along the progresive-reverse slopes in the pepper planting area of Ganxi Town in the northern part of the trough valley as the research object,January 2017 to December 2018,besides collecting water samples at each monitoring point regularly,the surface flow and groundwater samples along the progresive-reverse slopes were collected during the heavy rainfall period from July 4,2018 to July 6,2018,the water environmental indicators,anions and cations,carbon isotopes of the water were measured,to analyzed the effects of the rocky desertification control and the difference of rocky desertification degree between the progresive-reverse slopes in trough valley to the hydrogeochemical characteristics of surface flow and groundwater,and the ecological environmental effects of the regional rock desertification control are discussed.The results show that:?1?The groundwater chemical types in the trough are Ca-Mg-HCO3,Ca-HCO3 and Mg-HCO3,reflecting the control of groundwater hydrochemical characteristics by medium-thick layered dolomitic limestone and dolomite.The chemical composition of the groundwater in progresive-reverse slopes is sensitive to the difference and treatment of the rocky desertification,and has obvious temporal and spatial variation characteristics.The pH value is low in the rainy season,and the change range of reverse slope is larger than the progressive slope.The conductivity,Ca2+,Mg2+,and HCO3-during the rainy season and“Huaxi Autumn Rain”are higher than the dry season,and the reverse slope is higher than the progressive slope;The content of NO3-,SO42-is higher than other periods in rainy season and fertilization period,the reverse slope of the valley is higher than that of the progressive slope,the content of Laoquan and Heiyadong has interannual variations.The content of K+and Na+has a significant upward process in rainy seasons,due to the influence of the fertilization periods;the highest value occurs in the dry season,the content of it is higher than that of the reverse slope,which is the most prominent in the Malutan.?2?There is a significant difference in the intensity of rocky desertification in the progresive-reverse slopes,and the hydrogeochemical characteristics of the overland flow and groundwater during heavy rainfall are also significantly different.The flow of the progressive slope is delayed about 2.5 hours,the water volume is large,the pH value and conductivity,Ca2+,Mg2+,HCO3-concentration are high;K+,total Fe,Al3+content is low,and it has surface karst flow characteristics.The flow of reverse slope delayed about 3.5 hours,the water volume is small,the pH value and conductivity,Ca2+,Mg2+,HCO3-concentration are low;K+,total Fe,Al3+content is high,and it has Saturated surface runoff characteristics.During heavy rainfall,there is a significant difference in water chemistry between the Laoquanand the Malutan.Before the rainfall,the pH value and conductivity,HCO3-and Ca2+concentrations of Laoquan were higher than that of Malutan;during the rainfall process,the content of Malutan is higher than that of Laoquan.?3?In the seasonal scale,The?13CDIC ranges of the Laoquan,Heiyadong,Huolongdong,Malutan and Longdongtang are-4.94‰-11.62‰,-5.55‰-12.82‰,-5.26‰-13.94‰,-4.43‰-11.45‰and-3.99‰-11.93‰,respectively,which is positively biased compared with the theoretical?13CDIC value?-14‰?formed by the CO2 dissolved carbonate rock.During the period of heavy rainfall,the value of?13CDICIC of slope surface flow along the progressive slope is negative compared with that of the reverse slope,while the phenomenon of?13CDIC value of groundwater on the corresponding slope is opposite.The molar ratio of water body(Ca2++Mg2+)/HCO3-at the monitoring point is mainly between 0.5 and 1,From the relationship between(Ca2++Mg2+)/HCO3-and?13CDIC,it is inferred that the weathering of carbonate rocks in the study area is controlled by both carbonate and exogenous acid,the involvement of exogenous acid in karstification is an important cause of?13CDIC bias.The content of NO3-,SO42-in the water body of Laoquan and Heiyadong along the reverse slope is higher,and the weathering of carbonate rocks tends to H2SO4 and HNO3 end elements;the content of NO3-,SO42-in the monitoring points along the progressive slope is lower as a whole,and the weathering of carbonate rocks tends to H2CO3 end elements.?4?The karstification process of each monitoring point is affected by exogenous acid.Among them,the contribution rate of it to Laoquan and Heiyadong is higher than others,?H2SO4+HNO3?to(Ca2++Mg2+)is between 10%70%,8%87%,and to HCO3-is between 5%53%,4%77%,respectively.According to the contribution rate of H2CO3,H2SO4 and HNO3 to DIC in water body,the theoretical range of the value of?13CDIC in groundwater is calculated.The actual value of?13CDIC measured is positive compared with the theoretical value.The possible reason is the occurrence of regional rocky desertification.After the occurrence of rocky desertification,the vegetation coverage decreases,soil loss is serious,and rock bareness,biogenic soil CO2 decreases and atmospheric CO2 increases,resulting in the?13CDIC of groundwater positively biased.In the process of heavy rainfall,the enrichment effect of?13CDIC between surface flow and groundwater transformation is significantly different,which reflects the difference of rocky desertification degree along the slope of trough and valley.?5?Calculated that the DIC loss of each point:Laoquan?DIC range is0.53mmol/L1.32mmol/L;Heiyadong?DIC range is 0.59mmol/L1.98mmol/L;Huolongdong?DIC range is 0.17mmol/L1mmol/L;Malutan?DIC range is0.14mmol/L0.78mmol/L;Longdongtang?DIC range is 0.27mmol/L1.15mmol/L.It indicates that there is a certain amount of carbon loss in the research area,which reflects that the effect of karst carbon sink is still weak after the treatment of rocky desertification. |
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