| Ordos basin is the important energy source region of China, which plays an important role in the realization of Western Developing Strategy. Due to the arid and semiarid climate, shortage of water resource has been the uppermost obstacle to the construction of energy source base. Because the deep confined groundwater is the principle source of the groundwater in Ordos basin, it's significant to evaluate the deep groundwater scientifically and develop it reasonably. Obviously the acquiring of the renewal velocity quantitatively with the help of modern technology, is not only the key object of groundwater circulation research, but also is the first step to select the important indexes to judge the renewal capability of groundwater. To fulfill this purpose, the most important work is to acquire the accurate age of groundwater. The present data of 14C age of groundwater in Ordos basin show that the deep groundwater was mainly recharged about 20,000 or much less years ago, which is in the range of 20000 to 2000 years B.P. determined by 14C dating method. Therefore this thesis try to apply 14C dating method to determine the age of deep confined groundwater in Ordos Artesian Basin. The groundwater 14C dating method is actually to regard the dissolved inorganic carbon (DIC) in groundwater as the tracer according to the law of radio nuclide decaying. Therefore the determined age only represents the age of DIC after the recharge, which will be different from the true age of groundwater obviously and the groundwater 14C age should be corrected. Commonly the dissolved inorganic carbon in groundwater is originated from multi sources, each of which would have different resource and different 14C initial activity. As the result of addition of various carbon sources to the groundwater, the uncorrected 14C age will be different from the actual age of groundwater. Because the DIC distribution in groundwater is solely affected by the hydrogeochemical reaction path during the evolution of groundwater from the initial point to the final point in the same flow path, the keystone of 14C age correction is to determine the influential extent of 14C activity which is incurred by the path of hydrogeochemical reaction. According to the ideas above, and on the basis of well acknowledgement to the physical geography, geological and hydrogeological conditions of this area, this paper first analyzed the groundwater cycle character and geochemical distribution law and its evolutive mechanism on the target aquifers—Huanhe aquifer and Luohe aquifer. And qualitatively analyzed the key factors which affect the initial concentration of 14C, then the inverse hydrogeological modeling techniques was used to quantitatively identify the main reacting pathes affecting the 14C concentration. At last, as the evaluation targets, the hydraulic conductivety and storitivity, recharge conditions of the aquifer and groundwater flow velocity was used to evaluate the renewability of the deep groundwater—Huanhe aquifer groups and Luohe groups in the Cretaceous. The study area of this paper can be divided into two hydrogeological units by the middle tectonic ridgy belt located in the north of Baiyu Mountain which stretches nearly from east to west. One is the northern desert plateau groundwater systerm (I), the other is the southern Loess Plateau groundwater system(II). According to the characteristics of the deposition gyration and the aquifer media, the stratum can be divided into Cenozoic aquiferous groups, Luohandong aquiferous groups, Huanhe aquiferous groups and Luohe aquiferousgroups. Huanhe and Luohe aquiferous group are the target aquiferous groups in this paper. The main achievements of this paper are as follows: 1.The water cycle characteristics of groundwater was analyzed by the isotopic method in the Ordos Cretaceous Artesian Basin. The values of δ18O and δD of groundwater in all of the aquiferous groups distribute along the local meteoric line, this indicates that the groundwater in each Cretaceous aquifer groups and the Cenozoic aquifer in the basin are recharged by precipitation. The differences of δ18O and δD values of groundwater between the northern and southern aquifers are increasing with the increasing of groundwater level depth, and also, the δ18O and δD values of the southern aquifers are all lower than those of the northern aquifers, which indicats that the groundwater in the southern and northern aquifers respectively have different groundwater cycling intensity. In the southern area, the δ18O and δD values of groundwater in each aquifer cover obviously different scope,which shows the delaminating characteristics of the 18O and D isotopes in some degree, and the vertical hydraulic contact is weak, and the lateral flow is the dominate runoff method. The distributing range of δ18O and δD values of groundwater in each aquiferous groups has little overlap areas, but as a whole, the δ18O and δD values of groundwater are increasing gradually from the bottom up; but in the northern area, they have obvious overlaps, and they have no obvious delaminating characteristics, which indicates the strong vertical hydraulic contact. The characteristics of the 18O and D isotopes mentioned above have been verified by the typically hierarchical bores B13 and B15. The δ18O and δD values of surface water distributed strictly along the meteoric line, and the slope of the correlative line is almost in consisted with the meteoric line, which indicate that the surface water in the southern area is mainly confluxed by the rain water. The δ18O and δD of surface water in the north form a high relative line, the slope of which is obviously smaller than the evaporation line, indicating that the water in this area is intensely influenced by the evaporation; the surface water evaporation line alwaysintersect with the relative line of the δ18O and δD in groundwater, which indicates that the groundwater in the Cretaceous aquifers recharge the surface water somehow. In conclusion, groundwater in study area is recharged mainly by the meteoric precipitation; in the northern area, the vertical flow is obvious, and the groundwater discharge towards the regional surface water; in the southern area, things are different, there is no obvious groundwater runoff in the vertical, the groundwater flow is dominated by the lateral flow. 2. The distribution law of the groundwater chemistry and its mechanisms in Huanhe aquifer and Luohe aquifer are analyzed qualitatively. The values of TDS,Cl-,SO42-,K++Na+,Ca2+ and Mg2+ in Huanhe aquifer and Luohe aquifer are generally high in the west while low in the east, and high in the south while low in the north. Divided by Dongsheng-Sishililiang, the surface watershed, the concentrations of TDS and other five kinds of ions in northern Luohe aquifer distributes with the characters of high in the west while low in the east; and divided by Malianhe River, the concentrations of TDS and other five kinds of ions in southern Luohe aquifer distributes with the characteristics that the ones closer to the Mlianhe River from the east and west with the higher values and the ones near the boundaries in the east and west with low values. Meantime, the concentration of HCO3-distributes with the characteristics of high in the east and low in the west. The groundwater types in the southern and northern regions are different with each other, for the groundwater types in northern Luohe aquifer distribute with obvious horizontal characteristics that change in the order HCO3 type water→HCO3·SO4 one→HCO3·SO4·Cl one→SO4·Cl one from the east to the west; while the groundwater types in the southern Luohe aquifer are characterized with the Malianhe River as the axis. The groundwater types in the east of the axis change from the east to the west in the order HCO3 type water →HCO3·SO4·Cl one→SO4·Cl one, and they change from the west to the east in the order HCO3 type water→HCO3·SO4 one→HCO3·SO4·Cl one→SO4·Cl one.The distributing area of groundwater types in southern Huanhe aquifer is divided by the axis (Huan County -Qingyang County -Xunyi County). In the east side, the chemical facies change in the order HCO3 type water →HCO3·SO4 one→HCO3·SO4·Cl one, while the horizontal characters in the west side are dimness, and the general trend of groundwater types changes in the order HCO3 type water→HCO3·SO4 one→HCO3·SO4·Cl one and SO4 one→SO4·Cl one from the west to the east. Generally, it can be concluded from the vertical distribution law of chemical facies that the concentrations of most ions of Luohe aquifer are greater than in Huanhe aquifer in most of the northern region, and they inverse in most area of the southern region on the south of Baiyushan Mountain. The differences of vertical distribution law of most ions in groundwater of B13 and B15 bores also proved that the hydraulic conductivity between aquifer in the northern region is greater than that in the southern region obviously, and the evaporation effect in the northern region is greater than that in the southern region too. According to the horizontal and vertical distribution law of ion concentrations and some minerals'saturation indexes in Huanhe aquifer and Luohe aquifer, the main actions effecting on chemical field are concluded qualitatively including leaching action,cation-exchange action,evaporation and concentrating action and mixing action, and these actions also effect on the initial concentration of 14C in groundwater. 3. Utilizing the NETPATH code to simulate respectively the inverse geochemical reactions along the LuoHe aquifers flow paths in both southern region(GG13-J003) and northern region(G18-G21), HuanHe aquifers flow paths which are from well 348 to QiJi 2-II in the southern region and from G7 to ZK199 in the northern region. The simulated results show that the main hydrogeochemical actions are leaching and cation exchange in the HuanHe and LuoHe aquifers of the southern region of Baiyu Mountain, and in the northern region the actions still include the obvious mixing action except the above twoactions. Calcite and dolomite have different evolution mode in different aquifers. In the Luohe aquifer (in southern region), the action is different extent dissolution with dolomite dissolved and calcite deposited. While in the LuoHe aquifers there is a different action which is different extent dissolution with calcite dissolved and dolomite deposited. The dissolution of calcite of LuoHe aquifers in south region is larger than that in the north region. The phenomenon indicates that the leaching effect of groundwater in south region is more intense than that in the north region along the flow path. Gypsum dissolved in both Luohe and Huanhe aquifers in south region. And the dissolution in Huanhe aquifers is more than that in Luohe aquifers. But it didn't attend the actions in the north region. The exchange action of Ca-Na ions happened in both the Huanhe aquifer and Luohe aquifers of southern region and Luohe aquifers of northern region. There're two regulations as follows. One is that the count of exchange in the south region is larger than that in the north region. The other is the count of exchange in Huanhe aquifers is larger than that in Luohe aquifers. 4. On the base of simulating hydrogeochemical reactions along the flow path, this paper corrected the 14C age of groundwater. The correction of 14C concentration of groundwater in the start point is through the traditional model. Along the flow path which is from start point to end point, the 14C concentration is revised with the quality equation function and the transmission model of isotope quality which can be used to confer the input and output of geochemical action. The conclusion is that the 14C age of Luohe aquifers at the spot of the end flow path in the southern region and the northern region are 21196~ 22969a and 3967a respectively. And the 14C age of Huanhe aquifers in the south region is 6205a. All the above show that the groundwater cycle condition is better in the north area than that in the south area. And with the increasing depth of groundwater from Huanhe aquifers to Luohe aquifers in the south region, the age of groundwater is older gradually which also display that the hydraulic link between aquifers is weak. (5) Using the permeability and storitivity, recharge condition and...
|
PDF Full Text Request