Study On Hydrogeochemical Characteristics Of Groundwater And Estimates Of Carbon Sinks In Karst Forest

Ecological environment of China is quite vulnerable with a forest coverage of 18.21%which is less than 2/3 of the world average level. There are 1740000 km2 desertification land, more than 90%of the deteriorated grassland in the territory. Furthermore, according to the data of the national key monitoring cross-sections,26%of surface water is worse than that of V level and 62%cannot achieve III standard level of water environment. However, Maolan Karst Forest is the largest and most original karst forests in subtropics area. It has a unique feature of dualistic water bearing structure with whcih upper layer of karst fissure water and under layer of karst water coexists. In the karst area of complex hydrological conditions with high permeability of underground water and severe leakage of surface water, studies on the hydrological effects of forests is quite important and useful for correctly understanding of karst dynamic system, as well as scientifically managing and solving resource and environment problems in karst districts.In this study, the Maolan underground river basin which is in the original forest is taken as example. It covers three aspects:First, discussion of the chemistry changes in space and time in a non-forest conditions and the forest conditions. Second, study on the migration of the element in the rainwater through a system study of rainfall, through fall, stemflow, soil water and epikarst springs. Third, comparison of two different methods for analyzing the virgin forest carbon sinks.Take Banzhai Underground River System of Maolan natural reservation's primeval forest areas as example, Banzhai Underground River basin could be divided into three different kart water subsystems:karst underground river subsystem (subsystemⅠ), Banwangxi karst water subsystem (subsystemⅡ) and impure carbonated rock of karst fissure water subsystem (subsystem III). According to the sampling and analyzing of the soil and rocks hydrochemistry and geochemistry characteristics of the entire basin, results show as follows:(1) The kart water hydrochemical types were manly HCO3-Ca·Mg type and HCO3-Ca type in the basin, which was controlled by the content of carbonated rocks and hydrogeological conditions for the most part expect for Banzhai spring of HCO3·SO4-Ca type sometimes. Banzhai Underground River basin (subsystemⅠ) was in the core area of the primeval forest areas with high plant covering rate, less residents and less soil covering area. The chemical variation range of the ions in the water was small in the time and space, but the CO2 partial pressure was vital in comparison of the outer area. The variation of cation concentration was related with the dilution effect of the rainfall, however, the variation of anion was with the biological activity of the surface forest strongly. Moreover, the TOC also changed seasonally in the karst water.(2) Precipitation in the original rain forest area was 60%of the non-forest rainfall zone. Stable isotope geochemistry of the study area characteristics of the seasonal hydrological dynamics was obvious. By the analysis of the precipitation-Discharge curve, a larger attenuation coefficient, indicating that the area faster cycle rate, aquifer storage capacity was weak. Forest vegetation on water cycle delay was not obvious. Even after the rain canopy could intercept most of the water, but its mainly used for forest ecological water requirement of their own consumption, or the formation of evapotranspiration to the atmosphere within a small range of secondary water cycle. Overall, the hydrogen and oxygen stable isotopes of carbon as the continuous rainy season, had shown lighter trend began to rebound in September; exceptional drought conditions in the groundwater chemistry has undergone great changes due to evaporation, groundwater higher levels of chemical raw, different rainfall conditions, water chemistry response intensity was different.(3) HCO3-,K+,Na+,Ca2+,Mg2+and TOC increased in the migration process of the all kinds of the rainfall, runoff and penetrating rain, soil water, epikarst sping water, which indicated that soil layer could collect nutrients of all kinds of rains and litter decomposition, and it supplied the vegetations in the kart ecological system to grow and develop. Erosion test pieces of the forest areas was much more dissolved than that of the non-forest areas, and the erosion degree extended to the direction of the karstification.(4)The Al3+, the whole Fe, all Mn content was below the detection line. Rain forest in the core area was of only 85%of forests on rainfall, greatly reducing the erode ability of surface ground, the forest can effectively prevent soil erosion.(5)According to the comparing of the soil erosion rate in different land use types, the erosion rate was different in different depths in the same land use types, as well as the seasonal soil erosion rate in the same sample land. The annual rate of soil erosion was larger in the forestland which could promote the soil formation rate of carbonated rocks, and be in favor of the soil continued thickening of the karst area. Furthermore, the forest land was with good water conservation features. It was of great significance to improve coverage rate of forests in the karst area.(6) The detected sink of CO2 caused by the weathering of the carbonated rocks was nearly the same between the result by using the synchronal Erosion test and the hydrochemical-flow method, from which sink of CO2 was 328tC/a by the Erosion test and 397tC/a by the hydrochemical-flow method. It indicated that the Erosion test method was available to estimate the carbon sink. The erosion amount of the limestone pile under 20cm depth of the soil was equivalent as under 50cm.