The¹⁴C Age Correction Of The Groundwater In The Shule River Basin

Groundwater age determining has been a focus and difficult problem in the hydrogeology, because it is very difficult to obtain accurate hydrogeological parameters from the aquifer system. Environmental isotopes extensively exist in various kinds of water in nature, and they can change into as part of the water molecule itself or dissolved constituents in the water system.14C, which is used widely in the groundwater age study. However, it is extraordinary hard to get the initial value of the14C, because it may influence by many processes and reactions. As an issue of concern about the difficulty in the14C age correction, this thesis attempts to carry out some works in the Shule River Basin. Firstly, the groundwater hydrochemical evolution would be determined by using relationship between the dissolved ions, trace element and87Sr/86Sr, Secondly, the groundwater flow system will be characterized by the stable isotope and chemical indices. Then, the groundwater14C age will be corrected using different method (experiential model and reverse hydrogeochemical model) which can quantify the initial radioisotopic14C along the groundwater evolution path in the different zones. The main conclusions are as follows:The chemical type of the groundwater in the Shule River Basin changed from no-dominated to Na++K+for cations and HCO3-to no-dominated to SO42-for anions moving along the possible flow path from the piedmont of each basin to the fine soil plain areas and then downstream to the desert regions. The advantage of the alkaline earth metal ions replaced by the alkali metal cations, while the weak acid ions were permuted by the strong acid ions. The variance contribution were58.159%and14.621%calculated from the principal component analysis, which revealed that the groundwater chemistry is influenced mainly by evaporation and dissolution of evaporite minerals, as well as the alkalinity environment and common ion effect.. The dissolution of the Glauber’s salt, halite, gypsum and the carbonate minerals play significant role in the groundwater’evolution. It is also found that the reverse cation exchange widely exist in the aquifers.The87Sr/86Sr value was between0.711041to0.714029and the relationship between Sr2+and Ca2+and SO42-were good in the groundwater indicating the reaction of the evaporite rocks. In addition, recharge source of the surface water and the human activities influenced the phreatic water in some regions.There is no distinct distribution of the stable isotope values with the depth showing that the groundwater system has close connection between phreatic groundwater confined groundwater in the Yumen-tashi Basin. The stable isotope of the Guazhou groundwater presents a different variation with increasing depth, and showing different character in unconfined and pressurized water. In the phreatic groundwater, the values increase above the depth of60m indicating the influence from the evaporation. However, it decreases below the60m denoting the groundwater was recharged by various stages. In the confined groundwater, the stable isotope was quite steady with low values indicating a steady-going flow system. These indicate that the vertical hydraulic connection between the phreatic groups and the confined layers are weak, groundwater mainly moves as horizontal flow from the east to the west. It has similar distribution in confined groundwater in the Dunhuang Basin, however, in the southwest of the basin, the overlapping aquifer may have close connection with the under layers.It has found the dissolution of halite and Glauber’s salt, as well as the reverse cation exchange were dominated process along the flow path of GZ11-GZ10and GZ04-X01in the Guazhou Basin and the path of DH01-DH03in the Dunhuang Basin. Evaporation acts on the flow path of GZ11-GZ10in the phreatic groundwater, the evaporation factor was1.177. The groundwater14C concentration elevated owing the dissolvtion of carbon dioxide along the flow path, and the time of transmission is about740a. On the path of GZ04-X01, the dissolution of aragonite and cation exchange of the Mg/Na and Mg/Na are the main process which changed the concentration of the carbon element, this reaction led to the groundwater14C of the starting point was diluted by nearly three times, and the groundwater flows through nearly10281a from the starting point of GZ04to the ending point of X01. Along the DH01-DH03flow of the Dunhuang basin, both of the calcite and aragonite were involved during the reaction path, but has little effect on concentration of the groundwater radiocarbon, the transmission time was approximately12851a.The corrected14C age of the groundwater is between modern and16ka, indicating that the groundwater recharge mainly occurred during the Holocene and late Pleistocene. Most of the groundwater was recharged in the middle Holocene with the warm and humid climate. The circulation rate of the phreatic groundwater is between16.2-17.5m/a in the Guazhou Basin, indicating a well circulation condition, the confined groundwater has very slow circulation rate of2.4-3.7m/a in the whole basin, showing the groundwater resources are non-renewable.