Investigating groundwater–river interactions using environmental tracers
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Abstract:
Groundwater and surface water are hydraulically connected in many landscapes, and a better understanding of their connectivity is critical for effective management of water resources. Environmental tracers are a useful preliminary tool to study the interaction between groundwater and surface water and provide independent means for corroborating or refuting information based on more traditional investigations. This paper discusses the results of using major ions, stable isotopes (deuterium and oxygen-18) and a radioactive isotope (radon-222) as environmental tracers to better understand groundwater–surface water interactions in the Border Rivers catchment, Australia. In the upstream reaches of the catchment, shallow groundwater close to the river has a similar major-ion and stable-isotope chemistry to that of the river water, and is different to the groundwater distant from the river. The near-stream groundwater has an enriched isotopic signature (less negative) whereas groundwater far from the river has a depleted isotopic signature. Overall, the comparison of chloride concentrations with deuterium suggests that three types of groundwater occur in the Border Rivers catchment: (i) the near-stream groundwaters influenced by direct recharge from the river; (ii) the groundwaters marginal to the river that are more influenced by diffuse rainfall recharge; and (iii) saline groundwaters in the downstream part of the catchment which never (or rarely) receive recharge from surface water. River water samples obtained during the high-flow season show a very low variation in radon concentrations (0.11–0.39 Bq/L). The longitudinal transect of radon concentration measurements in river water during the high-flow season indicates that there is no groundwater contribution to stream flow. Radon concentrations are lower in groundwater close to the rivers and increase with distance from the river, in general coincidence with the salinity and chloride concentration. This indicates river water infiltration into nearby alluvial aquifers, rather than groundwater discharge to the river. The results of hydrochemical and environmental isotope sampling indicate that in the upper catchment area (upstream of Keetah) the river is connected to and actively recharges the near-stream shallow alluvial aquifer. Using the environmental isotope data, we have also demonstrated that recharge of the alluvial aquifers by surface water occurs by bank infiltration, with diffuse recharge during high-rainfall events more dominant further away from the river. This information would be useful for a better understanding of the nature and extent of hydrogeological processes at the river–aquifer interface and their links with biogeochemical processes and ultimately water allocation policies.Keywords:
Environmental isotopes
Isotopic signature
Groundwater discharge
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MODFLOW
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Characterization of a deep circulation groundwater flow system is a big challenge, because the flow field and aqueous chemistry of deep circulation groundwater is significantly influenced by the geothermal reservoir. In this field study, we employed a geochemical approach to recognize a deep circulation groundwater pattern by combined the geochemistry analysis with isotopic measurements. The water samples were collected from the outlet of the Reshui River Basin which has a hot spring with a temperature of 88 °C. Experimental results reveal a fault-controlled deep circulation geothermal groundwater flow system. The weathering crust of the granitic mountains on the south of the basin collects precipitation infiltration, which is the recharge area of the deep circulation groundwater system. Water infiltrates from the land surface to a depth of about 3.8–4.3 km where the groundwater is heated up to around 170 °C in the geothermal reservoir. A regional active normal fault acts as a pathway of groundwater. The geothermal groundwater is then obstructed by a thrust fault and recharged by the hot spring, which is forced by the water pressure of convection derived from the 800 m altitude difference between the recharge and the discharge areas. Some part of groundwater flow within a geothermal reservoir is mixed with cold shallow groundwater. The isotopic fraction is positively correlated with the seasonal water table depth of shallow groundwater. Basic mineral dissolutions at thermoneutral conditions, hydrolysis with the aid of carbonic acid produced by the reaction of carbon dioxide with the water, and hydrothermal alteration in the geothermal reservoir add some extra chemical components into the geothermal water. The alkaline deep circulation groundwater is chemically featured by high contents of sodium, sulfate, chloride, fluorine, silicate, and some trace elements, such as lithium, strontium, cesium, and rubidium. Our results suggest that groundwater deep circulation convection exists in mountain regions where water-conducting fault and water-blocking fault combined properly. A significant elevation difference of topography is the other key.
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Studies of groundwater flow systems are important to explore local groundwater resources and improve the degree of hydrogeological researches. Environmental isotope technology has widely been used in hydrogeology and can provid obvious help in the studies of groundwater recharge and circulation. Hydrogeology survey was carried out in the Zhangye basin in 2014 and a lot of water isotope samples was achieved.Combined with the research results of other researchers,the authors conducted an isotope analysis. The research results show that( 1) the stable isotope values in the Qilian mountain and the Longshou mountain are very different,leading to very different isotope values in the downstream groundwater.( 2) There are groundwater deep circulation zones from the upper stream to the downstream in the Zhangye basin. According to the isotope distribution,the authors divided the groundwater flow system in different areas and put forward a conceptual model for the groundwater flow system.
Environmental isotopes
Groundwater model
Groundwater discharge
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