喀什平原区地下水同位素年龄特征及更新速率分析

为分析喀什平原区300 m以浅地下水年龄和更新速率,准确划分地下水流系统,采用了3H和14C放射性同位素测试方法,对采集的120组地下水样品中3H和14C同位素组成进行了分析,研究了喀什平原区不同区域地下水的年龄特征。结果表明,研究区大部分潜水氚值为12.2~39.6 TU,地下水年龄从山前冲洪积砾质平原至细土平原区逐渐增大,为10~50年;在中部库木塔格背斜隆起一带,潜水氚值小于10 TU,地下水年龄大于500年,形成地下水年龄分水岭;区内地下水14C年龄约为2 000~13 000年,反映了地下水补给时段差异较大。从山前冲洪积砾质平原上游至冲积细土平原中部地区,地下水更新速率约为2%~7%,属积极交替带;冲积细土平原中下游地区,地下水更新速率约为1%~2%,属较积极交替带;在冲积细土平原下游地区,地下水更新速率均小于1%,属弱交替带。     

北京昌平地区地下水地球化学

利用在北京昌平地区采集的地下水样品的同位素和水化学特征,对昌平地区的地下水补给特征进行了分析。通过对样品的分组和讨论认为:第三组样品为当地大气降水垂向入渗水体的特征;第一组样品为下覆基岩向上覆第四系含水层的顶托补给,这股基岩水可能出自碳酸盐岩含水层;第二组样品存在几种可能的补给特征;第四组样品中3件为大气降水垂直入渗补给,另1件为混合样品。研究表明,昌平地区第四系含水层地下水分布及其补给方式情况各异,实际上说明了半干旱地区山前倾斜平原地区地下水分布的复杂性。     

山西晋中地区地下水高碘的地球化学特征研究

研究区位于山西省太原盆地西南部的冲积平原内。通过对研究区内950个样品中碘含量的分析测定可知,浅层水(50m)、中层水(50～200m)和深层水(200m)碘含量超标率分别为76.8%、70.3%和85.2%,其中,浅层水碘含量最高可达4117μg/L,是国家饮用水标准值(GB/T19380-2003,150μg/L)的27倍。地下水无机碘主要以I-存在,IO3-含量极少;约60%的样品中存在有机碘。研究结果表明,特殊的地形地貌及地下含水层结构特点是造成该地区地下水高碘的主要原因。此外,当地普遍采用中层或深层地下水进行农业灌溉的耕作方式及低洼地区的地表盐化都大大加剧了浅层地下水碘的积蓄。地下水碘的来源与研究区的海相、湖相和河相沉积有着不可分割的联系。本研究将为中国高碘地区的改水工程提供科学依据。     

黄河冲积扇平原浅层地下水中氮循环对砷迁移富集的影响

黄河冲积扇平原浅层地下水砷含量超标情况严重,豫北平原的主体是黄河冲洪积扇平原.全面了解豫北平原浅层地下水氮循环驱动下砷的富集模式,对地下水资源的可持续利用和居民健康至关重要.本文采集豫北平原513组浅层地下水样品,采用原子荧光光谱法测定砷含量,原子吸收光谱和离子色谱等方法进行全分析及微量元素分析,对该地区高砷地下水的水...     

河南平原浅层地下水年龄

利用³H法和CFCs法对河南平原第四系浅层地下水年龄进行计算,为河南平原浅层地下水可更新能力评价和水循环的研究提供依据。结果表明：2种方法计算出的年龄拟合误差较小（2 a）,均可代表河南平原浅层地下水年龄。总体上,河南平原浅层地下水主要为近50 a以来补给的现代水。太行山、伏牛山、大别山山前地区以及开封西部的黄河两岸等地区浅层地下水年龄均小于30 a,并且顺着地下水流向年龄逐渐增大。从山前地区和黄河两岸至平原区,浅层地下水开采潜力逐渐减小。总体上:平原北部地下水系统地下水年龄较小,地下水循环交替较快;平原南部地下水系统次之;平原中部地下水系统地下水年龄最大,地下水循环交替最慢。     

考虑水体面积变化的鄱阳湖平原区地表-地下水相互作用模拟

湖泊的水情变化会影响其与地下水之间的物理水文过程和生态行为,鄱阳湖独特的“河湖相”转换特征使得该地区地表-地下水交换过程更加复杂。采用Visual MODFLOW构建三维非稳定流地下水流数值模型,利用LAK3子程序模块,通过输入五河入湖以及鄱阳湖流入长江的水量,实现湖水面积的动态模拟。结果表明,2019年湖水位模拟值与实测值的均方根误差为0.225 m,地下水水位模拟值与实测值的均方根误差为0.571 m;模型模拟鄱阳湖水面积环比变幅-41%～83%,与遥感影像结论吻合。该模型减少了湖泊作为边界条件的约束,可以有效刻画鄱阳湖频繁变化的湖水位和水体面积,准确模拟地下水流场和地表-地下水相互作用关系对湖泊水体高度动态变化的响应。枯水期主要由地下水补给湖水,交换量为2.03×107～10.58×107 m3/mon;丰水期湖水补给地下水,交换量为2.04×107～16.53×107 m3/mon,湖区及周边地下水水位相比枯水期平均抬升2～3 m,地下水由湖区流向周边地区。本研究为地表水体剧烈变化地区提供了有效的数值模拟方法,研究结果可为鄱阳湖平原区未来水资源管理和环境评价提供基础。     

张家口坝下地区高氟地下水成因分析与健康风险评价

为了明晰张家口坝下地区高氟地下水的成因,探究其对当地居民饮用水安全的潜在影响,采集了391组潜水样品(井深≤100 m),通过水化学分析法、图解法、离子比例法、饱和指数计算法等对高氟地下水的分布与成因进行了分析,并利用美国EPA非致癌健康风险评价模型对四类受体人群进行健康风险评价.结果 表明,研究区高氟地下水(ρ(F)...     

河南平原浅层地下水总溶解固体和水化学类型的分布特征

利用75件地下水样品的水化学数据(丰水期38件,枯水期37件)分析了丰、枯水期河南平原第四系浅层地下水总溶解固体(TDS)和水化学类型的分布特征。结果表明:河南平原浅层地下水淡水(TDS<1g/L)发育。枯水期TDS>1g/L的地下水主要分布在新乡-开封-太康县一线以东地区、项城以及太行山东麓安阳市境内;丰水期TDS>1g/L的地下水分布在新乡以东的部分地区、兰考县城东镇以及太康县以东地区。枯水期地下水主要以HCO3型水为主,丰水期河南平原的北部、南部主要以HCO3型水为主,而中部主要以HCO3-SO4型水为主。丰、枯水期地下水水化学特征的差异是在气候(降水、蒸发)及人类活动等因素的影响下形成的。     

长江中游故道区高碘地下水分布与形成机理

高碘地下水(碘浓度大于100μg/L)广泛分布于我国沿海地区和干旱内陆盆地,威胁近千万人口的饮水安全,但目前对湿润区河湖平原地下水中碘的分布与成因机制的认识还十分薄弱.通过采集长江中游故道区75组浅层地下水样品和7组地表水样品进行了水化学分析,查明了地下水中碘的空间分布特征,并运用主成分分析识别了碘富集的水环境要素和水...     

乌鲁木齐河流域北部平原地下水可持续开采方案分析

乌鲁木齐河流域北部平原局部地区出现了地下水水位下降和生态环境退化等问题。为了实现地下水可持续开发利用,结合《乌鲁木齐市水资源综合规划报告》和《米东新区水资源规划报告》设计了现状开采方案、增加补给量方案、减少开采量方案和增加补给量与减少开采量联合方案。运用北部平原地下水非稳定流模型对这四个地下水开发情景模拟方案进行了模拟,模拟的时间段为2007～2050年。对预测期间地下水水位的动态变化、地下水水位降深及水均衡进行了分析,确定了增加补给量与减少开采量联合方案是乌鲁木齐河流域北部平原地下水的可持续开采方案。实施该方案应从北水南调引0.7×108m3/a地表水用于北部倾斜平原的农业灌溉,同时要减少地下水超采地区的地下水开采量0.50×108 m3/a。     

Flow of river water into a karstic limestone aquifer—2. Dating the young fraction in groundwater mixtures in the Upper Floridan aquifer near Valdosta, Georgia

Tritium/helium-3 (³H/³He) and chlorofluorocarbon (CFCs, CFC–11, CFC–12, CFC–113) data are used to date the young fraction in groundwater mixtures from a karstic limestone aquifer near Valdosta, Georgia, where regional paleowater in the Upper Floridan aquifer receives recharge from two young sources—the flow of Withlacoochee River water through sinkholes in the river bed, and leakage of infiltration water through post-Eocene semi-confining beds above the Upper Floridan aquifer. In dating the young fraction of mixtures using CFCs, it is necessary to reconstruct the CFC concentration that was in the young fraction prior to mixing. The ³H/³He age is independent of the extent of dilution with older (³H-free and ³He_trit-free) water. The groundwater mixtures are designated as Type-1 for mixtures of regional paleowater and regional infiltration water and Type-2 for mixtures containing more than approximately 4% of river water. The fractions of regional paleowater, regional infiltration water, and Withlacoochee River water in the groundwater mixtures were determined from Cl⁻ and δ¹⁸O data for water from the Upper Floridan aquifer at Valdosta, Georgia.The chlorofluorocarbons CFC–11 and CFC–113 are removed by microbial degradation and/or sorption processes in most anaerobic (Type-2) groundwater at Valdosta, but are present in some aerobic Type-1 water. CFC–12 persists in both SO₄-reducing and methanogenic water. The very low detection limits for CFCs (approximately 0.3 pg kg⁻¹) permitted CFC–11 and CFC–12 dating of the fraction of regional infiltration water in Type-1 mixtures, and CFC-12 dating of the river-water fraction in Type-2 mixtures. Overall, approximately 50% of the 85 water samples obtained from the Upper Floridan aquifer have CFC–12-based ages of the young fraction that are consistent with the ³H concentration of the groundwater. Because of uncertainties associated with very low ³H and ³He content in dilute mixtures, ³H/³He dating is limited to the river-water fraction in Type-2 mixtures containing more than about 10% river water. Of the 41 water samples measured for ³H/³He dating, dilution of ³H and low ³He concentration limited ³H/³He dating to 16 mixtures in which ³H/³He ages are defined with errors ranging from ±2 to ±7.5 a (1 σ). After correction for dilution with (assumed) CFC-free regional infiltration water and regional paleowater in the Upper Floridan aquifer, adjusted CFC–12 ages agree with ³H/³He ages within 5 a or less in 7 of the 9 co-dated Type-2 mixtures.Tritium data and dating based on both CFC–11 and CFC–12 in Type-1 mixtures indicate that travel times of infiltration water through the overlying Post-Eocene semi-confining beds exceed 35 a. The CFC and ³H/³He dating indicate that the river fraction in most groundwater entered the groundwater reservoir in the past 20 to 30 a. Few domestic and municipal supply wells sampled intercept water younger than 5 a. Calculated velocities of river water in the Upper Floridan aquifer downgradient of the sinkhole area range from 0.4 to 8.2 m/d. Radiocarbon data indicate that ages of the regional paleowater are on the 10 000-a time scale. An average lag time of approximately 10 to 25 a is determined for discharge of groundwater from the surficial and intermediate aquifers above the Upper Floridan aquifer to the Withlacoochee River.     

Determination of groundwater recharge regime and flowpath in the Lower Heihe River basin in an arid area of Northwest China by using environmental tracers: Implications for vegetation degradation in the Ejina Oasis

Environmental tracers (CFCs, stable isotopes ¹⁸O, ²H, and ³H) and major ions were employed to study river infiltration and groundwater recharge in the aquifer system in the basin of the Lower Heihe River, Northwest China. Three groups of waters have been recognized: (1) young groundwater, connected to the river, with large variation of CFC apparent ages ranging from <10 a to 40 a, and δ¹⁸O and δ²H values which are similar to the river water; (2) regional background water, unaffected by the river, having CFC apparent ages >40 a, and being depleted in ¹⁸O and ²H compared with the river water; and (3) groundwater in Gurinai, a grassland located about 100 km from the river, in which the predominant discharge is from the Badain Jaran desert, with CFC apparent ages ranging from 25 to >50 a and being enriched in ¹⁸O and ²H compared to the river water. The groundwater along the river contains CFCs and ³H down to depths of about 120 m, and the shallow groundwater exhibits CFC apparent ages in a wide range which are not dependent on the well depth. Groundwaters along the river show a similar trend of enrichment in ¹⁸O and ²H as the river water whereas groundwaters in depression cones are depleted in heavier isotopes, and have low CFC and ³H concentrations. The CFC apparent age of the groundwater increases with increasing distance downstream, indicating that the dominant part of the groundwater is from infiltration of river water in the upper reaches. Modifications of groundwater recharge are reflected in variations of stable isotope compositions, as well as CFC and ³H concentrations in the groundwater that was recharged from the river over the last decades. Despite recharging from river water, groundwater abstraction has induced a water balance deficit. The riparian ecosystem in the Ejina Oasis is constrained by both decreased river flow and increased groundwater abstraction. The vegetation degradation in the Ejina Oasis is controlled not only by natural aridification but also worsened by heavy groundwater abstraction and decreased river flow.     

Identifying interactions between river water and groundwater in the North China Plain using multiple tracers

Interactions between river water and groundwater have been used to help understand the movement of water and to evaluate water quality in the semi-arid area of the North China Plain (NCP). Stable isotopes, chlorofluorocarbons (CFCs) and hydrochemistry were used to study the influence of surface water from the Xiao River on regional groundwater. Using a mass balance approach based on chloride concentrations, hydrogen and oxygen isotope ratios, the average fraction of surface water recharging to groundwater was 50–60 %. CFC results indicated that the groundwater recharge age varied from 22.5 to 39.5 years. The vertical flow velocity of groundwater was estimated at about 1.8–3.5 m year^?1. Nitrate concentrations in groundwater varied from 9.42 to 156.62 mg L^?1, and exceeded 50 mg L^?1 in most aquifers shallower than 80 m bordering the Xiao River. The δ ¹⁵N-NO₃ data indicate that the major sources of nitrogen in groundwater are human sewage and animal excreta. Because groundwater is the main source of drinking water, there should be concern about public health related to the elevated nitrate concentrations in the NCP.     

Ambient vertical flow in long-screen wells: a case study in the Fontainebleau Sands Aquifer (France)

A tritium (³H) profile was constructed in a long-screened well (LSW) of the Fontainebleau Sands Aquifer (France), and the data were combined with temperature logs to gain insight into the potential effects of the ambient vertical flow (AVF) of water through the well on the natural aquifer stratification. AVF is commonly taken into account in wells located in fracture aquifers or intercepting two different aquifers with distinct hydraulic heads. However, due to the vertical hydraulic gradient of the flow lines intercepted by wells, AVF of groundwater is a common process within any type of aquifer. The detection of ³H in the deeper parts of the studied well (approximate depth 50 m), where ³H-free groundwater is expected, indicates that shallow young water is being transported downwards through the well itself. The temperature logs show a nearly zero gradient with depth, far below the mean geothermal gradient in sedimentary basins. The results show that the age distribution of groundwater samples might be biased in relation to the age distribution in the surroundings of the well. The use of environmental tracers to investigate aquifer properties, particularly in LSWs, is then limited by the effects of the AVF of water that naturally occurs through the well.     

Isotopic and noble gas study of Chalk groundwater in the London Basin, England

The chemical and isotopic composition of groundwater from 52 sites in the London (U.K.) area was determined as part of a project aimed at assessing the spatial variation in the age of Chalk groundwater, and in determining the relationship between fracture and matrix groundwater in this dual porosity system.Systematic changes in groundwater chemistry take place in the downgradient direction in response to several chemical processes. These processes include early concentration by evaporation and congruent dissolution of calcite followed by widespread incongruent dissolution and ion exchange in addition to local oxidation-reduction reactions, gypsum dissolution and saline intrusion. As a result of the above processes, Chalk groundwater follows an evolutionary path from Ca bicarbonate type to Na bicarbonate type.The age of Chalk groundwater was modelled using¹⁴C, δ¹³C,³H, δ²H and δ¹⁸0. There is a general increase in the groundwater age in a downgradient direction with the oldest water found in N central areas of the basin. Groundwater in the unconfined zones and in areas S of the Greenwich fault is almost entirely of unevolved, modem composition. Carbon-14 modelling suggests that Chalk groundwater in the S basin is generally less than 10000 a old while that in the north is generally between 10000 and 25000 a old. The presence of³H in concentrations of up to 7 TU in groundwater which yields ages of several 1000 a, however, indicates that mechanisms exist for the rapid introduction of recent groundwater to the confined aquifer. Results of palaeorecharge temperature determinations using δ²H, δ¹⁸0 and noble gas analytical results suggest that significant Devensian recharge did indeed occur in the aquifer.A model of the development of the Chalk recognizes that it is a classic dual porosity aquifer in which groundwater flow occurs predominantly in the fracture system. The upper 50 m of the aquifer was flushed with fresh water during the 2–3 × 10⁶ a of the Quaternary and therefore meteoric water largely replaced the Tertiary and Cretaceous marine water that previously saturated the system. Most processes which control the chemistry of the groundwater occur in the matrix where the surface area is exceptionally high. Although fracture flow dominates the flow regime, diffusion from the matrix into the fracture porosity controls the chemistry of Chalk groundwater.     

Using environmental tracers to assess the extent of river–groundwater interaction in a quarried area of the English Chalk

The Swanscombe area of Kent, SE England represents a typical example of a heavily quarried Chalk area currently undergoing re-development. Because the Chalk is also an important aquifer, a good understanding of groundwater movement is required if environmental impacts are to be minimised and the water resource maximised. In particular, the nature of the relationship between the River Darent and groundwater in the Swanscombe Chalk Block requires better characterisation. Here, ‘environmental tracers’ in the form of ambient concentrations of stable isotopes, chlorofluorocarbons (CFCs), sulphur hexafluoride (SF₆) and tritium (³H) are used to investigate this and other aspects of groundwater movement in the vicinity of the quarries. Stable isotopic contrasts indicate little evidence for widespread river infiltration to the regional Chalk aquifer, although stable isotope and ³H data suggest that 20–35% of the abstraction by river-valley public water supply boreholes may be derived from the river. The CFCs, while present at above-modern concentrations in almost all groundwaters, can be used as tracers, indicating basically S–N flowpaths in the area south of the quarries, though sub-karstic conduits associated with areas of Palaeogene cover add a level of uncertainty at the local scale. Simple piston flow residence times based on SF₆ range from 1 to 17 a, but the data are probably better interpreted in terms of mixing between varying amounts of modern recharge derived from the south and deeper stored groundwater. The information gained from environmental tracers can therefore contribute to effective resource management.     

Comparison of age distributions estimated from environmental tracers by using binary-dilution and numerical models of fractured and folded karst: Shenandoah Valley of Virginia and West Virginia, USA

Measured concentrations of environmental tracers in spring discharge from a karst aquifer in the Shenandoah Valley, USA, were used to refine a numerical groundwater flow model. The karst aquifer is folded and faulted carbonate bedrock dominated by diffuse flow along fractures. The numerical model represented bedrock structure and discrete features (fault zones and springs). Concentrations of ³H, ³He, ⁴He, and CFC-113 in spring discharge were interpreted as binary dilutions of young (0–8  years) water and old (tracer-free) water. Simulated mixtures of groundwater are derived from young water flowing along shallow paths, with the addition of old water flowing along deeper paths through the model domain that discharge to springs along fault zones. The simulated median age of young water discharged from springs (5.7  years) is slightly older than the median age estimated from ³H/³He data (4.4  years). The numerical model predicted a fraction of old water in spring discharge (0.07) that was half that determined by the binary-dilution model using the ³H/³He apparent age and ³H and CFC-113 data (0.14). This difference suggests that faults and lineaments are more numerous or extensive than those mapped and included in the numerical model.     

The practicalities of using CFCs and SF6 for groundwater dating and tracing

Knowledge of groundwater residence time is important in understanding key issues in the evolution of water quality, whether this occurs due to water–rock interaction or simply by mixing or contamination. The build-up in the atmosphere of the trace gases chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF₆) from the middle of the last century offers a convenient way of dating waters up to ∼60 a old. The gases are well-mixed in the atmosphere so their input functions are not area-specific as is the case with ³H. While any one of these trace gases can in principle provide a groundwater age, when two or more are measured on water samples the potential exists to distinguish between different modes of flow including piston flow, exponential flow and simple end-member mixing. As with all groundwater dating methods, caveats apply. Factors such as recharge temperature and elevation must be reasonably well-constrained. Primarily for SF₆, the phenomenon of ‘excess air’ also requires consideration. Primarily for the CFCs, local sources of contamination need to be considered, as do redox conditions. For both SF₆ and the CFCs, the nature and thickness of the unsaturated zone need to be factored into residence time calculations. However, as an inexpensive dating method, the trace gases can be applied to a wide range of groundwater problems where traditional age indicators might once have been used more sparingly. Examples include tracing flowlines, detecting small modern inputs in ‘old’ waters, and pollution risk assessment. In the future, with the main CFCs already declining in the atmosphere, new anthropogenic trace gases are likely to take their place.     

Flow of river water into a Karstic limestone aquifer. 1. Tracing the young fraction in groundwater mixtures in the Upper Floridan Aquifer near Valdosta, Georgia

The quality of water in the Upper Floridan aquifer near Valdosta, Georgia is affected locally by discharge of Withlacoochee River water through sinkholes in the river bed. Data on transient tracers and other dissolved substances, including Cl⁻, ³H, tritiogenic helium-3 (³He), chlorofluorocarbons (CFC-11, CFC-12, CFC-113), organic C (DOC), O₂ (DO), H₂S, CH₄, δ¹⁸O, δD, and ¹⁴C were investigated as tracers of Withlacoochee River water in the Upper Floridan aquifer. The concentrations of all tracers were affected by dilution and mixing. Dissolved Cl⁻, δ¹⁸O, δD, CFC-12, and the quantity (³H+³He) are stable in water from the Upper Floridan aquifer, whereas DOC, DO, H₂S, CH₄, ¹⁴C, CFC-11, and CFC-113 are affected by microbial degradation and other geochemical processes occurring within the aquifer. Groundwater mixing fractions were determined by using dissolved Cl⁻ and δ¹⁸O data, recognizing 3 end-member water types in the groundwater mixtures: (1) Withlacoochee River water (δ¹⁸O=−2.5±0.3‰, Cl⁻=12.2±2 mg/l), (2) regional infiltration water (δ¹⁸O=−4.2±0.1‰, Cl⁻=2.3±0.1 mg/l), and (3) regional paleowater resident in the Upper Floridan aquifer (δ¹⁸O=−3.4±0.1‰, Cl⁻=2.6±0.1 mg/l) (uncertainties are ±1σ). Error simulation procedures were used to define uncertainties in mixing fractions. Fractions of river water in groundwater range from 0 to 72% and average 10%. The influence of river-water discharge on the quality of water in the Upper Floridan aquifer was traced from the sinkhole area on the Withlacoochee River 25 km SE in the direction of regional groundwater flow. Infiltration of water is most significant to the N and NW of Valdosta, but becomes negligible to the S and SE in the direction of general thickening of post-Eocene confining beds overlying the Upper Floridan aquifer.     

Identifying groundwater recharge connections in the Moscow (USA) sub-basin using isotopic tracers and a soil moisture routing model

Globally, aquifers are suffering from large abstractions resulting in groundwater level declines. These declines can be caused by excessive abstraction for drinking water, irrigation purposes or industrial use. Basaltic aquifers also face these conflicts. A large flood basalt area (1.1?×?10⁵ km²) can be found in the Northwest of the USA. This Columbia River Basalt Group (CRBG) consists of a thick series of basalt flows of Miocene age. The two major hydrogeological units (Wanapum and Grand Ronde formations) are widely used for water abstraction. The mean decline over recent decades has been 0.6 m year^?1. At present day, abstraction wells are drying up, and base flow of rivers is reduced. At the eastern part of CRBG, the Moscow sub-basin on the Idaho/Washington State border can be found. Although a thick poorly permeable clay layer exists on top of the basalt aquifer, groundwater level dynamics suggest that groundwater recharge occurs at certain locations. A set of wells and springs has been monitored bi-weekly for 9 months for δ¹⁸O and δ²H. Large isotopic fluctuations and d-excess values close to the meteoric water line in some wells are indicating that recharge occurs at the granite/basalt interface through lateral flow paths in and below the clay. A soil moisture routing (SMR) model showed that most recharge occurs on the granitic mountains. The basaltic aquifer receives recharge from these sedimentary zones around the granite/basalt interface. The identification of these types of areas is of major importance for future managed-aquifer recharge solutions to solve problems of groundwater depletion.