Groundwater recharge pathway according to the environmental isotope: the case of Changwu area, Yangtze River Delta Region of China

Groundwater recharge is an important factor affecting water circulation. As groundwater has slow seepage, directly observing the seepage velocity and recharge path of groundwater in the aquifer is difficult. Environmental isotope technology has become an important means to clarify the mechanism of groundwater movement and the mechanism by which groundwater recharges from the micro and macro perspectives. The Changwu area of Jiangsu Province was taken as an example to identify the recharge sources of groundwater and the recharge paths of groundwater and surface water by using the measured data of isotopes D, 18O, 34S, and T. The results indicated that the shallow aquifer and the I confined aquifer in the Changwu area are mainly recharged by precipitation and surface lake water. The II confined aquifer along the Yangtze River is recharged by modern precipitation. Moreover, the II confined aquifer in the Henglin area was recharged by the ancient Yangtze River before 4,000 years ago, and no recharge relationship exists now. the recharge condition of the II confined aquifer around the northwest of Gehu Lake is in the climate environment of 8,000 years ago and was caused by the surface depression lake water at that time. Additionally, the concealed limestone aquifer is primarily supplied by the II confined aquifer, while the concealed sandstone aquifer supplies the II confined aquifer. Hence, to find out the recharge conditions of groundwater aquifers based on the environmental isotope is conducive to scientific and reasonable evaluation of groundwater resources and to ensure the sustainable development and utilization of groundwater resources.

Tracing δ18O and δ2H in Source Waters and Recharge Pathways of a Fractured-Basalt and Interbedded-Sediment Aquifer, Columbia River Flood Basalt Province

The heterogeneity and anisotropy of fractured-rock aquifers, such as those in the Columbia River Basalt Province, present challenges for determining groundwater recharge. The entrance of recharge to the fractured-basalt and interbedded-sediment aquifer in the Palouse region of north-central Idaho is not well understood because of successive basalt flows that act as restrictive barriers. It was hypothesized that a primary recharge zone exists along the basin’s eastern margin at a mountain-front interface where eroded sediments form a more conductive zone for recharge. Potential source waters and groundwater were analyzed for δ18O and δ2H to discriminate recharge sources and pathways. Snowpack values ranged from −22 to −12‰ for δ18O and from −160 to −90‰ for δ2H and produced spring-time snowmelt ranging from −16.5 to −12‰ for δ18O and from −120 to −90‰ for δ2H. With the transition of snowmelt to spring-time ephemeral creeks, the isotope values compressed to −16 and −14‰ for δ18O and −110 and −105‰ for δ2H. A greater range of values was present for a perennial creek (−18 to −13.5‰ for δ18O and −125 to −98‰ for δ2H) and groundwater (−17.5 to −13‰ for δ18O and −132 to −105‰ for δ2H), which reflect a mixing of seasonal signals and the varying influence of vapor sources and sublimation/evaporation. Inverse modeling and the evaluation of matrix characteristics indicate conductive pathways associated with paleochannels and deeper pathways along the mountain-front interface. Depleted isotope signals indicate quicker infiltration and recharge pathways that were separate from, or had limited mixing with, more evaporated water that infiltrated after greater time/travel at the surface.

Application of Hydrochemical and Isotopic Data to Determine The Origin and Circulation Conditions of Karst Groundwater in An Alpine and Gorge Region in The Qinghai–Xizang Plateau: A Case Study Of Genie Mountain

Understanding of the recharge origin, runoff channels, and discharge characteristics of karst groundwater is very important for construction of underground projects and identification of water supply targets. Complex structural systems, lithological differences, and extreme heterogeneity of aquifers combine to create a complex karst aquifer structure in alpine and gorge areas; however, because of the topography, direct investigation of aquifer structure is difficult. In this study, field survey, hydrochemical, and isotopic data are analyzed to reveal the development of karst groundwater and to describe the karst water cycle in Genie Mountain, Qinghai–Xizang Plateau. The results show that atmospheric precipitation and melting ice and snow are the groundwater recharge sources, and groundwater circulation is shallow, with groundwater ages generally no more than 60 yr. The groundwater cycle can be divided into three levels: epikarst water circulation; mid to deep karst water circulation; and deep geothermal water circulation. The karst springs located in the outlet of the Huolong gully contain markedly higher levels of Na + and SO 4 2 − than other karst springs because of the leaching effect of groundwater on mirabilite. The presence of evaporites also indicates that the groundwater of Huolong gully is influenced by evaporation. The runoff of thermal springs undergoes deep circulation and is controlled by faults. This water mainly dissolves carbonate rock, with little influence of evaporation. This study shows that hydrochemical and isotopic methods can be used to discriminate different water types, and can be applied to study the characteristics of complex groundwater runoff in alpine and gorge areas.

Recharge Sources and Genetic Model of Geothermal Water in Tangquan, Nanjing, China

This paper introduces a method to study the origin of geothermal water by analysis of hydrochemistry and isotopes. In addition, the genetic mechanism of geothermal water (GTW) is revealed. The study of the origin of geothermal water is useful for the sustainability of geothermal use. As an example, Tangquan is abundant in GTW resources. Elucidating the recharge sources and formation mechanism of the GTW in this area is vitally important for its scientific development. In this study, the GTW in Tangquan was systematically investigated using hydrochemical and isotopic geochemical analysis methods. The results show the following. The GTW and shallow cold water in the study area differ significantly in their hydrochemical compositions. The geothermal reservoir has a temperature ranging from 63 to 75 °C. The GTW circulates at depths of 1.8–2.3 km. The GTW is recharged by the infiltration of meteoric water at elevations of 321–539 m and has a circulation period of approximately 2046–6474 years. The GTW becomes mixed with the shallow cold karst water at a ratio of approximately 4–26% (cold water) during the upwelling process. In terms of the cause of its formation, the geothermal system in the study area is, according to analysis, of the low-medium-temperature convective type. This geothermal system is predominantly recharged by precipitation that falls in the outcropping carbonate area within the Laoshan complex anticline and is heated by the terrestrial heat flow in the area. The geothermal reservoir is composed primarily of Upper Sinian dolomite formations, and its caprock is made up of Cambrian, Cretaceous, and Quaternary formations. Through deep circulation, the GTW migrates upward along channels formed from the convergence of northeast–east- and north–west-trending faults and is mixed with the shallow cold water, leading to geothermal anomalies in the area.

Natural and Anthropogenic Geochemical Tracers to Investigate Residence Times and Groundwater–Surface-Water Interactions in an Urban Alluvial Aquifer

A multi-component geochemical dataset was collected from groundwater and surface-water bodies associated with the urban Fountain Creek alluvial aquifer, Colorado, USA, to facilitate analysis of recharge sources, geochemical interactions, and groundwater-residence times. Results indicate that groundwater can be separated into three distinct geochemical zones based on location within the flow system and proximity to surface water, and these zones can be used to infer sources of recharge and groundwater movement through the aquifer. Rare-earth-element concentrations and detections of wastewater-indicator compounds indicate the presence of effluent from wastewater-treatment plants in both groundwater and surface water. Effluent presence in groundwater indicates that streams in the area lose to groundwater in some seasons and are a source of focused groundwater recharge. Distributions of pharmaceuticals and wastewater-indicator compounds also inform an understanding of groundwater–surface-water interactions. Noble-gas isotopes corroborate rare-earth-element data in indicating geochemical evolution within the aquifer from recharge area to discharge area and qualitatively indicate variable groundwater-residence times and mixing with pre-modern groundwater. Quantitative groundwater-residence times calculated from 3H/3He, SF6, and lumped-parameter modeling generally are less than 20 years, but the presence of mixing with older groundwater of an unknown age is also indicated at selected locations. Future investigations would benefit by including groundwater-age tracers suited to quantification of mixing for both young (years to decades) and old (centuries and millennia) groundwater. This multi-faceted analysis facilitated development of a conceptual model for the investigated groundwater-flow system and illustrates the application of an encompassing suite of analytes in exploring hydrologic and geochemical interactions in complex systems.

Using geochemistry and environmental tracers to study shallow unconfined aquifer recharge and mineralization processes in the Yinchuan Plain, arid Northwest China

Irrigation water extracted from the Yellow River plays a key role in water resource management in the Yinchuan Plain (YCP), arid Northwest China. Investigating the soluble matters (ion and gas) of groundwater provides information to explain the unconfined shallow aquifer recharge and groundwater mineralization processes after long-term flood irrigation activity. Environmental tracing with the elements, 2H, 18O, 3H, and CFCs, combining geochemistry using major ions and selected trace elements, was conducted for 43 water samples from September to October 2019 in the YCP. Evaporite and silicate weathering dominate the shallow unconfined groundwater geochemical compositions. Water–rock interactions control the mineralization characteristics regularly along the groundwater flow paths from the southwest toward the northeast. Stable isotopes suggest that Yellow River water and precipitation in winder and/or from Helan Mountainous area are the main recharge sources. The shallow unconfined aquifer mixed young (post-1940) and old (pre-1940) water with young water ratios from 53.1 to 73.5% inferred from the CFC concentrations and 3H activities. Water reinfiltrations extracted from the Yellow River and from the old groundwater are confirmed. Lateral flow recharge for the shallow unconfined aquifer is less indistinctive than that from the water re-infiltration in the plain areas.

Triple oxygen isotope systematics of evaporation and mixing processes in a dynamic desert lake system

This study investigates the combined hydrogen deuterium and triple oxygen isotope hydrology of the Salar del Huasco, an endorheic salt flat with shallow lakes at its centre that is located on the Altiplano Plateau, N Chile. This lacustrine system is hydrologically dynamic and complex because it receives inflow from multiple surface and groundwater sources. It undergoes seasonal flooding, followed by rapid shrinking of the water body at the prevailing arid climate with very high evaporation rates. At any given point in time, ponds, lakes, and recharge sources capture a large range of evaporation degrees. Samples taken between 2017 and 2019 show a range of δ18O between −13.3 ‰ and 14.5 ‰, d-excess between 7 ‰ and −100 ‰, and 17O-excess between 19 and −108 per meg. A pan evaporation experiment conducted on-site was used to derive the turbulence coefficient of the Craig–Gordon isotope evaporation model for the local wind regime. This, along with sampling of atmospheric vapour at the salar (-21.0±3.3 ‰ for δ18O, 34±6 ‰ for d-excess and 23±9 per meg for 17O-excess), enabled the accurate reproduction of measured ponds and lake isotope data by the Craig–Gordon model. In contrast to classic δ2H–δ18O studies, the 17O-excess data not only allow one to distinguish two different types of evaporation – evaporation with and without recharge – but also to identify mixing processes between evaporated lake water and fresh flood water. Multiple generations of infiltration events can also be inferred from the triple oxygen isotope composition of inflow water, indicating mixing of sources with different evaporation histories. These processes cannot be resolved using classic δ2H–δ18O data alone. Adding triple oxygen isotope measurements to isotope hydrology studies may therefore significantly improve the accuracy of a lake's hydrological balance – i.e. the evaporation-to-inflow ratio (E / I) – estimated by water isotope data and application of the Craig–Gordon isotope evaporation model.