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A porewater-based stable isotope approach for the investigation of subsurface hydrological processes

DOI:10.5194/hess-16-631-2012
Authors:
Jakob Garvelmann at _boden & grundwasser~ Allgäu GmbH
Jakob Garvelmann
  • _boden & grundwasser~ Allgäu GmbH
Christoph J. Kuells at University of Applied Sciences, Lübeck, Germany
Christoph J. Kuells
  • University of Applied Sciences, Lübeck, Germany
Markus Weiler at University of Freiburg
Markus Weiler
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Abstract and Figures

Predicting and understanding subsurface flowpaths is still a crucial issue in hydrological research. We present an experimental approach to reveal present and past subsurface flowpaths of water in the unsaturated and saturated zone. Two hillslopes in a humid mountainous catchment have been investigated. The H<sub>2</sub>O<sub>(liquid)</sub> – H<sub>2</sub>O<sub>(vapor)</sub> equilibration laser spectroscopy method was used to obtain high resolution δ<sup>2</sup>H vertical depth profiles of pore water at various points along two fall lines of a pasture hillslope in the southern Black Forest, Germany. The Porewater-based Stable Isotope Profile (PSIP) approach was developed to use the integrated information of several vertical depth profiles of deuterium along transects at the hillslope. Different shapes of depth profiles were observed in relation to hillslope position. The statistical variability (inter-quartile range and standard deviation) of each profile was used to characterize different types of depth profiles. The profiles upslope or with a weak affinity for saturation as indicated by a low topographic wetness index preserve the isotopic input signal by precipitation with a distinct seasonal variability. These observations indicate mainly vertical movement of soil water in the upper part of the hillslope before sampling. The profiles downslope or at locations with a strong affinity for saturation do not show a similar seasonal isotopic signal. The input signal is erased in the foothills and a large proportion of pore water samples are close to the isotopic values of δ<sup>2</sup>H in streamwater during base flow conditions indicating the importance of the groundwater component in the catchment. Near the stream indications for efficient mixing of water from lateral subsurface flow paths with vertical percolation are found.
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A porewater-based stable isotope approach for the investigation of subsurface hydrological process
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Garvelmann et al hess-16-631-2012
Data
September 2015
Jakob Garvelmann · Christoph J. Kuells · Markus Weiler
... Unlike, for example, suction cups or mechanical squeezing, DVE-LS is assumed to provide isotope data that are not tension-specific but represent the bulk water of a given sample (Sprenger et al., 2015a). On the hillslope scale, the DVE-LS method has been used to reveal present and past subsurface water flow paths in the unsaturated and saturated zone of humid (Garvelmann et al., 2012) or alpine regions (Mueller et al., 2014). On a similar scale, it has been used to obtain high-resolution water isotope depth profiles for the investigation of spatial and temporal dynamics of water flow and solute transport in a heterogeneous glacial till (Stumpp and Hendry, 2012). ...
... Nonetheless, we further scrutinized not only Al-laminated bags but also transparent freezer bags. Similar to the Allaminated bags, they allow for easy handling and have therefore been used previously by our and other research groups (e.g., Garvelmann et al., 2012). Transparent freezer bags had been tested for the same purpose before with weight losses of only 0.06 g in the first 10 d (Hendry et al., 2015). ...
Technical note: Unresolved aspects of the direct vapor equilibration method for stable isotope analysis ( δ <sup>18</sup>O, δ <sup>2</sup>H) of matrix-bound water: unifying protocols through empirical and mathematical scrutiny
Article
Full-text available
The direct vapor equilibration laser spectrometry (DVE-LS) method has been developed for obtaining matrix-bound water stable isotope data in soils, the critical zone, and bedrock, deriving therefrom subsurface water flow and transport processes and, ultimately, characterizing, for example, groundwater recharge and vulnerability. Recently, DVE-LS has been increasingly adopted due to its possible high sample throughput, relative simplicity, and cost-efficiency. However, this has come at the cost of a non-unified standard operation protocol (SOP), and several contradictory suggestions regarding protocol details do exist which have not been resolved to date. Particularly, sample container material and equilibration times have not yet been agreed upon. Beside practical constraints, this often limits DVE-LS applicability to interpreting relative isotope dynamics instead of absolute values. It also prevents data comparability among studies or laboratories, and several previous comparisons of DVE-LS with other, more traditional approaches of water extraction and subsequent stable isotope analysis yielded significant discrepancies for various sample matrices and physical states. In a series of empirical tests, we scrutinized the controversial DVE-LS protocol details. Specifically, we tested 10 different easily available and cost-efficient inflatable bags previously employed or potentially suitable for DVE-LS sample collection and equilibration. In storage tests similar to the DVE-LS equilibration process but lasting several weeks, we quickly found heat-sealed bags made of laminated aluminum (Al) sheets to be superior by several orders of magnitude over more frequently used freezer bags in terms of evaporation safety and accompanying adverse isotope effects. For the first time, Al-laminated bags allow the applied equilibration time to be adapted exclusively to sample requirements instead of accepting reduced data quality in a trade-off with material shortcomings. Based on detailed physical considerations, we further describe how to calculate the minimum available container headspace and sample-contained liquid water volume and how their ratio affects analytical precision and accuracy. We are confident that these guidelines will expand DVE-LS applicability and improve data quality and comparability among studies and laboratories by contributing to a more unified, physically well-founded SOP based on more appropriate components.
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... Instead, the isotopic composition of the vapor contained in a gaseous sample can directly be analyzed in the lab and even in the field. Firstly, this leads to the development of equilibration-based lab methods which allow for indirect water isotope measurements from samples without the need for the extraction of liquid water from destructive samples (Wassenaar et al., 2008;Garvelmann et al., 2012). Subsequently, different approaches for in situ sampling of stable water isotopes have been developed. ...
... Following the equilibration bag method after Wassenaar et al. (2008) and Garvelmann et al. (2012), the sample bags were filled with dehumidified air in the lab and permanently sealed with sealing tongs (Weber Packaging GmbH). After 24 h of equilibrium at constant temperature, the sample bags were punctured by a hollow needle connected to the inlet port of a cavity ring-down spectrometer (CRDS) stable water isotope analyzer (L2120-I, Picarro, Santa Clara, USA). ...
Temporal dynamics of tree xylem water isotopes: in situ monitoring and modeling
Article
Full-text available
  • Aug 2021
  • BG
We developed a setup for a fully automated, high-frequency in situ monitoring system of the stable water isotope deuterium and 18O in soil water and tree xylem. The setup was tested for 12 weeks within an isotopic labeling experiment during a large artificial sprinkling experiment including three mature European beech (Fagus sylvatica) trees. Our setup allowed for one measurement every 12–20 min, enabling us to obtain about seven measurements per day for each of our 15 in situ probes in the soil and tree xylem. While the labeling induced an abrupt step pulse in the soil water isotopic signature, it took 7 to 10 d until the isotopic signatures at the trees' stem bases reached their peak label concentrations and it took about 14 d until the isotopic signatures at 8 m stem height leveled off around the same values. During the experiment, we observed the effects of several rain events and dry periods on the xylem water isotopic signatures, which fluctuated between the measured isotopic signatures observed in the upper and lower soil horizons. In order to explain our observations, we combined an already existing root water uptake (RWU) model with a newly developed approach to simulate the propagation of isotopic signatures from the root tips to the stem base and further up along the stem. The key to a proper simulation of the observed short-term dynamics of xylem water isotopes was accounting for sap flow velocities and the flow path length distribution within the root and stem xylem. Our modeling framework allowed us to identify parameter values that relate to root depth, horizontal root distribution and wilting point. The insights gained from this study can help to improve the representation of stable water isotopes in trees within ecohydrological models and the prediction of transit time distribution and water age of transpiration fluxes.
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... In recent decades, many efforts have been made to describe the subsurface hydrological process (Garvelmann et al., 2012;Stieglitz et al., 2003), while the determination of subsurface saturated flow remains a challenging task due to its unobservability and high spatiotemporal variability. Hillslope trenches (van Verseveld et al., 2009), soil moisture measurements (Han et al., 2020;Nanda et al., 2019), groundwater level monitoring (Ocampo et al., 2006), plot-scale tracer experiments (McGuire and McDonnell, 2010;Weiler and Naef, 2003), and streamflow and chemistry measurements (Burns et al., 2016) have been applied in different contexts to determine subsurface flow or connectivity conditions. ...
Physical structure and rainfall controls on subsurface hydrological connectivity in hillslope-riparian-stream continuums
Article
The hydrological connectivity of hillslope-riparian-stream (HRS) continuums is crucial for runoff generation and solute transport. The achievement of water resource protection and water quality improvement requires a systematic understanding of the structure and rainfall controls on HRS connectivity. Herein, two HRS continuums with different soil depths and slopes (HRS-1: thin soil depth and steep slope; HRS-2: thick soil depth and gentle slope) were established. We monitored the soil moisture from the surface to the soil-bedrock interface at 15 min intervals from March to June 2021. The HRS connectivity was analyzed based on soil saturation conditions, and partial least squares regression (PLSR) was used to reveal the relationships between rainfall and HRS connectivity. The results showed that the time required to establish hydrological connectivity in HRS-1 was shorter than that in HRS-2, which indicated that the contribution to runoff of the HRS continuum with a thin soil depth and steep slope was dominant during the early stage of rainstorm. As rainfall intensity increased, the required time was shortened exponentially due to the changes in hydrological connectivity patterns. In addition, the higher connectivity strength (i.e., the magnitude of HRS connectivity) was observed in the HRS-2 than that in the HRS-1 during heavy rainfall events. The PLSR analysis showed that rainfall amount, 30 min maximum rainfall intensity, 15 min maximum rainfall intensity, and rainfall duration were important controls affecting connectivity strength. Rainfall amount and peak rainfall intensity exerted more important effects than did antecedent soil moisture on the connectivity strength. Furthermore, there was a clear rainfall threshold for HRS connectivity, from 14.8 mm in HRS-1 to 21.1 mm in HRS-2. The increased soil depth and reduced slope enhanced the rainfall threshold of HRS connectivity. Our results indicate that the physical structure of the HRS continuum exerts a primary control on the rainfall threshold.
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... Numerous studies have compared the single isotope (δ 18 O or δ 2 H) values of different water fractions and the distributions of water fractions within dualisotope (δ 18 O and δ 2 H) space, and applied mixing models based on the mass conservation of stable isotopes. The results enable the characterization of water movement and mixing between different pools (Orlowski et al., 2016;Scheihing et al., 2018;Sprenger et al., 2018), the major flows paths from hillslopes to basins (Garvelmann et al., 2012;Liu et al., 2004), and the fractions of different water sources in runoff (Gou et al., 2019;Li et al., 2019;Welsh et al., 2018). ...
Stable isotopes of surface water and groundwater in a typical subtropical basin in south‐central China: Insights into the young water fraction and its seasonal origin
Article
Full-text available
  • Apr 2022
The young water fraction (F yw ) can be reliably determined in heterogeneous river basins, and the seasonal origin index (SOI) can be used to determine the relative contributions of winter versus summer precipitation to runoff. F yw and SOI are also important descriptors of how a basin controls runoff generation and the transport of contaminants and nutrients. In this study, high‐frequency sampling of precipitation, river water, creek water, and shallow groundwater was conducted in the Xiangjiang River basin, in south‐central China, from January 2010 to December 2012. The samples were subjected to stable isotope measurements to determine the SOI and F yw of surface water and shallow groundwater. The principal findings are: (1) The δ ² H variations of the river water, creek water, and shallow groundwater were driven by the input of precipitation but were attenuated during runoff confluence processes. Also, the low lc‐excess values of the river water indicated a strong evaporative effect. (2) 21.4% and 19.7% of the river water and shallow groundwater were younger than 1.5 and 1.7 months, respectively, indicating a quicker renewal cycle than for the creek water, which had a fraction of 8.0% younger than 2.6 months, which originated from the adjacent mountains. (3) Winter precipitation was more prevalent in the river water and creek water, indicating the substantial inter‐seasonal carryover of precipitation that supplied the runoff. The average SOI value of the shallow groundwater was near zero, indicating the mixing of the precipitation in different seasons that supplies the shallow groundwater. Our findings emphasize the importance of vegetation and soil conservation in maintaining the inter‐seasonal carryover of water storage held by soil, plants, and rock fractures, and the necessity to assess the risks of streamflow contamination because of the rapid renewal cycle of river water in this flat and intensively cultivated landscape.
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... Previous studies used equilibration times ranging from hours to several days, and no standardized protocol for different soil or sediment samples exists yet. Garvelmann et al. (2012) Sprenger et al. (2015). Hendry et al. (2013) obtained an accurate porewater isotopic composition of fine-textured soil samples by equilibrating Cretaceous sediments for seven days. ...
Influence of equilibration time, soil texture, and saturation on the accuracy of porewater water isotope assays using the direct H2O(liquid)–H2O(vapor) equilibration method
Article
The hydrogen and oxygen stable isotopes of water (δ²H and δ¹⁸O) are powerful tracers for studying subsurface water flow processes, but the extraction of porewater from unsaturated soil is complex and laborious. The direct liquid–vapor equilibration method (DLVE) for isotope analysis overcomes this challenge by analyzing headspace vapor in isotopic equilibrium with the porewater under closed system conditions. However, the effect of the equilibration time, soil texture, and porewater saturation on isotopic results is not fully understood yet. We tested three differently textured, disaggregated soils (sandy, silty loam and clay) to assess how the equilibration time is impacted by (i) porewater saturation (100%, 80%, 60%, and 40%), and (ii) soil surface area. For all tests, the water loss through diffusion was negligible (<0.3%). The experiments showed that for disaggregated sandy soil, 24 h was a sufficient isotopic equilibration time regardless of water saturation level. Similarly, 24 h was sufficient for kaolinite samples with 100% saturation exhibiting little isotopic variance even after 168 h of equilibration. For saturated silty loam with 2% organic carbon content, 96 h was the optimal equilibration time, whereas saturated silty loam with 4% organic carbon content did not reach isotopic equilibrium by 168 h. The optimal equilibration time increased depending on whether soil samples were disaggregated or kept in a core cutter. Inconclusive results for silty soils with organic carbon revealed the need to further investigate the possible influence of organic carbon on the DLVE method.
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... The H1, H2 and M1 hydrothermal kaolin quarries were further evolved by interactions with meteoric water during a postmineralization period. Isotopic re-equilibrium reflects a lower temperature resulting in kaolinite being slightly enriched in δ 18 O and highly enriched in δD (Garvelmann et al., 2012). The formation temperatures of kaolinites in the EFSSR deposits were calculated by using the isotopic fractionation factor equation of Sheppard and Gilg (1996) and was determined to be 63º to 64ºC for the Hamamtepe site and 46º to 58ºC for the Muratlar site (assuming meteoric water was δ 18 O = -8 ‰ during deposition; this value is taken from Baba and Gündüz (2010)). ...
Characteristics and evolution of the Etili silica sinter epithermal deposits, Çanakkale – Turkey: Relation to alkali chloride vs acid-sulfate fluids
Article
Full-text available
The Oligo-Miocene Etili epithermal deposits are a well-preserved fossil geothermal system in the Çanakkale Region and is one of the largest fossil siliceous hot spring deposits of Turkey. Main E-W and NE-SW trending faults systems occur with minor NW-SE fracture systems perpendicular to the main faults. Silica-rich deposits are commonly observed in or on the calc-alkaline tuffs, ashes and pyroclastic rocks that were the products of the magmatism controlled by the extensional tectonic regime. The Etili epithermal system was examined at two representative locations including the Hamamtepe and Muratlar sites. Morphologies of the proximal apron were defined by lithofacies that included silica infiltrate, spring conduits, nodular and finely laminated geyserite, sinter clast breccia, silicified volcanic rocks, and epithermal veins. Microfossils were detected in mat structures developed in the proximal apron. The silica deposits have low abundances of altered mineral assemblages (e.g., kaolinite, halloysite, and alunite), which are otherwise commonly observed in the region. The origin of the Etili Fossil Silica Sinter Region (EFSSR) was constrained by using geochemical and isotopic data. δ¹⁸O and δD isotopic values of kaolinites ranged +9.4 / +9.6 ‰; to 84 / -74 ‰ respectively. δ¹⁸O isotopic values from siliceous and silicified samples ranged from 8.3 to 18.4 ‰, which shows two different formation types for the Etili Fossil Silica Sinter Region. We hypothesized hypogene origins for the kaolins and a mixed hypo/supergene origin for siliceous and silicified samples. The calculated model formation temperature from the δ¹⁸O values of the silica and siliceous samples is in the range of 63° to 140°C. δ³⁴S isotopic values of alunite ranged from -19.6 to +16.6 ‰, which also indicated two different sources that include magmatic hydrothermal and meteoric waters affected by bacterial activity. The timing of acid-sulphate alteration can be grouped into three periods by ⁴⁰Ar/³⁹Ar dating of alunites. These ages are: (A) 32.4 ± 1.2 to 22.6 ± 0.22 Ma in the Muratlar silica site, (B) 12.3 ± 0.3 to 15.2 ± 0.3 Ma in the northern part of the Hamamtepe site and (C) 5± 0.18 to 7 ± 0.3 Ma in the southern part of the Hamamtepe silica site. These consistent findings show that the emplacement periods of the Evciler pluton and the ages of alteration deposits are similar to each other. Silica sinters and geyser mounds represent very late stage near-neutral pH alkali chloride geothermal activities. The epithermal deposits in the EFSSR comprises a complex history of three different hydrothermal processes, each of which have different compositional and temporal emplacement periods related to the Evciler pluton.
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... The SWC measurement in the unsaturated zone is a common method for quantifying soil water recharge (Ochoa et al., 2009), providing useful information on effects of land use changes and estimating infiltration process and deep percolation (Dahan et al., 2009;Dahan et al., 2007;Gutiérrez-Jurado et al., 2017;Ochoa et al., 2007). Additionally, isotopes tracers can extend our understanding of evaporation rates (Allison and Barnes, 1983;Barnes and Allison, 1988;Mahendrappa et al., 1966;Robertson and Gazis, 2006), the extent of soil water recharge (Garvelmann et al., 2012;Sprenger et al., 2018), residence times of water along a soil profile (Gazis and Feng, 2004), the infiltration and groundwater recharge (Bengtsson et al., 1987;Song et al., 2009;Tan et al., 2017;Wang et al., 2012), root water uptake patterns (Dawson and Ehleringer, 1991;Ma and Song, 2016;Rothfuss and Javaux, 2017;Wang et al., 2010;Zhao et al., 2018). In summary, the combined physical and isotopic techniques have been proven useful in investigating vertical water movement in the unsaturated zone and identifying the recharge mechanism to groundwater. ...
Evaluating performance of soil water movement and groundwater recharge in an irrigated agricultural area of Yinchuan Plain, China
Preprint
  • Oct 2021
Surface irrigation has been predominantly used for field crops in agriculture area to boost agricultural yields and outputs, however, this may also raise groundwater tables, salinize soils and reduce water quality due to poor irrigation management. Therefore, it is essential for requiring a better understanding of the hydrologic mechanisms related to soil water fluxes (e.g., evaporation, transpiration, infiltration, deep percolation and groundwater capillary rise) by surface irrigation. This study investigated the impact of surface irrigation on soil water movement and recharge to groundwater in the Yellow River irrigation area of Yinchuan Plain, China. Combining comprehensive filed observation and stable isotopic techniques, we described the soil water mechanism under two land covers (bare ground or maize) in 2019 and 2020. The soil depths affected by precipitation infiltration and evaporation were mainly 0-50 cm, while the soil influenced by irrigation was the entire profile in the mode of piston flow. According to soil water potential variation from 70 to 100 cm, we conclude that the maize root took up the soil water up to the depth of 100 cm during the tasseling period. The infiltration and capillary rise in 2020 were similar with those in 2019. However, the total deep percolation was 156.6 mm in 2020 which was smaller than that in 2019 because of the maize root water uptake. The leakage of ditch was the major recharge resource of groundwater for the fast water table rise. This study is critical for agricultural water management to improve irrigation efficiency and water use efficiency in arid regions.
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... They found a good correlation between the two approaches (root mean square error -RMSE -equal to 0.6 ‰ for δ 18 O and within 1.7 ‰-3.1 ‰ for δ 2 H). Volkmann and Weiler (2014) tested their own design of a water vapour probe under field conditions and could show that it produced δ liq soil values in agreement with those following destructive sampling and isotopic analysis with the direct equilibration method (Garvelmann et al., 2012). The inter-method (destructive vs. non-destructive) RMSE values were comparable to the intra-method variability of soil water δ values. ...
Reviews and syntheses: Gaining insights into evapotranspiration partitioning with novel isotopic monitoring methods
Article
Full-text available
  • Jun 2021
  • BG
Disentangling ecosystem evapotranspiration (ET) into evaporation (E) and transpiration (T) is of high relevance for a wide range of applications, from land surface modelling to policymaking. Identifying and analysing the determinants of the ratio of T to ET (T/ET) for various land covers and uses, especially in view of climate change with an increased frequency of extreme events (e.g. heatwaves and floods), is prerequisite for forecasting the hydroclimate of the future and tackling present issues, such as agricultural and irrigation practices. One partitioning method consists of determining the water stable isotopic compositions of ET, E, and T (δET, δE, and δE, respectively) from the water retrieved from the atmosphere, the soil, and the plant vascular tissues. The present work emphasizes the challenges this particular method faces (e.g. the spatial and temporal representativeness of the T/ET estimates, the limitations of the models used, and the sensitivities to their driving parameters) and the progress that needs to be made in light of the recent methodological developments. As our review is intended for a broader audience beyond the isotopic ecohydrological and micrometeorological communities, it also attempts to provide a thorough review of the ensemble of techniques used for determining δET, δE, and δE and solving the partitioning equation for T/ET. From the current state of research, we conclude that the most promising way forward to ET partitioning and capturing the subdaily dynamics of T/ET is by making use of non-destructive online monitoring techniques of the stable isotopic composition of soil and xylem water. Effort should continue towards the application of the eddy covariance technique for high-frequency determination of δET at the field scale as well as the concomitant determination of δET, δE, and δE at high vertical resolution with field-deployable lift systems.
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Soil Water Movement and Groundwater Recharge Under Different Land Uses in a Flood‐Irrigated Area
Article
The water shortage in agriculture area in China requires to reduce the consumption of excessive water in flood irrigation. Therefore, the dynamics of soil water regime is needed to investigate and water-saving irrigation is necessary to alleviate water shortage. This study investigated the impact of flood irrigation on soil water movement and recharge to groundwater in the Yellow River irrigation area of Yinchuan Plain, China. Combining comprehensive field observation, stable isotopic techniques and water balance simulation, we described the soil water mechanism in vadose zone covered with bare soil in 2019 and planted with maize in 2020. The soil layers affected by precipitation infiltration and evaporation were mainly 0-50 cm, while the soil influenced by irrigation was the entire profile in the mode of piston flow. The maize root took up the soil water up to the depth of 100 cm during the tasseling period. The infiltration and capillary rise in 2020 were similar with those in 2019. However, the total deep percolation was 156.5 mm in 2020 which was about 50% of that in 2019 because of the maize root water uptake. The leakage of ditch water was the major recharge resource of groundwater for the fast water table rise. Precise irrigation is required to minimize deep percolation and leakage of ditch water and reduce excessive unproductive evapotranspiration. Therefore, understanding the soil water movement and groundwater recharge is critical for agricultural water management to improve irrigation efficiency and water use efficiency in arid regions. This article is protected by copyright. All rights reserved.
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Seasonal recharge mechanism of the upper shallow groundwater in a long-term wastewater leakage and irrigation region of a river alluvium aquifer
Preprint
Full-text available
  • Jan 2023
Understanding the mechanisms that control seasonal groundwater recharge at local and intermediate scales is critical for understanding contaminant transport. Preferential flow accompanied with intensive and seasonal recharge allows contaminants to migrate rapidly through the unsaturated zones to underlying aquifers. In this study, we investigated the recharge mechanism from multiple water sources to the upper shallow groundwater of alluvial aquifers at the Tanghe Wastewater Reservoir (TWR) with a length of 17.5 km in the Xiong'an New Area, North China Plain. To do so, we sampled 30 m deep soil porewater profiles and groundwater boreholes perpendicular to the TWR. We traced the recharge processes and related pollutants transport pathways based on stable water isotopes (2H and 18O). The stable isotopes in porewater of the soil profiles revealed vertical recharge rates ranging from 63 to 109 cm/year for the layered unsaturated zone with silt and silty clay loams. However, fast flow (i.e., preferential flow or lateral flow) occurred in sand aquifers with the mixing of slow translatory flow with multiple recharge sources (i.e., precipitation, irrigation water, and wastewater remaining in porewater). The distribution of δ2H and δ18O relationship of the upper shallow groundwater showed two hysteresis loops which reflected the impact of fast flow with seasonal variation and translatory flow with legacy water in soil: 1) groundwater in regions affected by the TWR wastewater leakage shows a narrow loop and a nearly straight line with end-members of precipitation, which recharged to groundwater by fast flow, and evaporated porewater crossing with the TWR evaporation line; and 2) groundwater in irrigated farmlands with low and high irrigation amounts and strong evaporation shows a concentrated loop overlapping with shallow porewater, suggesting the impact of porewater persists in shallow soil on groundwater. The recharge type of fast flow determines seasonal variation of groundwater SO42- and NO3-. The SO42- concentrations in groundwater were diluted after recharge of fast flow and then increased due to the contribution of slow flow in porewater. However, NO3- increased as the fast flow carried pollutants from shallow soils to the aquifer. Measures are needed to prevent contamination caused by preferential flow in river alluvium aquifers at local scales which can extend to regional scales.
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Isotopic exchange effects in soil water
Chapter
Full-text available
  • Dec 1998
Investigations into the processes affecting the hydrological response of catchments typically revolve around interpreting a time record of the flux of water leaving the catchment outlet (the hydrograph) in response to a time record of precipitation intensity. This chapter reviews the fundamentals of isotopic behavior in soil water, including processes of leading to variations in soil water concentrations. It provides a simple conceptual model for explaining soil water movement and presents suggestions for new research directions. It also discusses exchange process modifications to the isotopic composition of soil water and the dependence of these processes on physical parameters of the catchment. Factors affecting the hydrograph are complex and to some extent non-linear, and inevitably there is a confounding of the effects of different processes, that is, there is insufficient information in the hydrograph record to reconstruct the processes that gave rise to it. The degree to which the original processes can be reconstructed is greatly enhanced if additional independent information can be obtained.
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Effect of Catchment-Scale Subsurface Mixing on Stream Isotopic Response
Article
Full-text available
  • Dec 1991
A 3.8-ha watershed on the west coast of New Zealand was instrumented with suction lysimeters and automatic water samplers to determine the relationship between subsurface isotopic and chemical concentrations to those of rainfall and resulting streamflow. A t test showed that ±2‰ represented a significant difference between successive sample deuterium values. Eleven rainfall episodes were subdivided into two categories: (1) two events where stream isotopic composition did not deflect >2‰ from prestorm values, and (2) four events which demonstrated new water flushing. Detailed analysis of one 47-mm rainfall (9.8-mm runoff) event showed that old water dominated stream water exiting the watershed by 90% using a standard two-component hydrograph separation for deuterium (corroborated by Cl and electrical conductivity). Three-component hydrograph separation indicated that 12–16% was in the form of soil water, with
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Isotopes in Groundwater Hydrology
Chapter
  • Dec 1998
The concentration of stable isotopes in groundwater depends mainly on the origin of the water. The concentration of radioactive isotopes and dissolved compounds in groundwater depends on the initial concentration and residence time of groundwater in the aquifer. These two factors determine the amount of radioactive isotope(s) decayed, removed, or added during water-rock processes. As geochemical tools, stable and radioactive environmental isotopes provide information on the geochemical processes operating on groundwater and on the hydrogeological characteristics of aquifers. Information provided by environmental isotopes is also useful in modeling groundwater systems. This chapter discusses the scientific background of applications of environmental isotope techniques to groundwater hydrology. The stable isotope composition of groundwater reflects that of the precipitation in the recharge area that seeps through the soil and the unsaturated zone to reach the water table. Stable isotopes are often used to identify groundwater recharge by rivers and lakes. These water bodies frequently have isotopic compositions different from that of precipitation over the study area.
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Isotopes in the Water Cycle: Past, Present and Future of a Developing Science
Book
  • Jan 2005
Environmental isotope and nuclear techniques provide unmatched insights into the processes governing the water cycle and its variability under past and present climates. This monograph is recommended to advanced students and specialists and presents historical perspective, state of the art applications and new developments of isotopes in hydrology, environmental disciplines and climate change studies. The spectrum of isotope applications addressed in this monograph ranges from the assessment of groundwater resources in terms of recharge and flow regime, identification of palaeogroundwater, water balance of river basins and lakes, to studies of the past and present global environmental and climate changes.
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Seasonal Isotope Hydrology of Three Appalachian Forest Catchments
Article
Seasonal oxygen-18 variations in precipitation, throughfall, soil water, spring flow and stream baseflow were analysed to compare the hydrology of two forested basins in West Virginia (WV) (34 and 39 ha) and one in Pennsylvania (PA) (1134 ha). Precipitation and throughfall were measured with funnel/bottle samplers, soil water with ceramic-cup suction lysimeters and spring flow/baseflows by grab and automatic sampling during the period March 1989 to March 1990. Isotopic damping depths, or depths required to reduce the amplitude of subsurface oxygen-18 fluctuations to 37% of the surface amplitude, were generally similar for soil water on the larger PA basin, and baseflows and headwater spring flows on the smaller WV basins. Computed annual isotopic damping depths for these water sources averaged 49 cm using soil depth as the flow path length. The equivalent annual mean hydraulic diffusivity for the soil flow paths was 21 cm2 d-1. Mean transit times, based upon an assumed exponential distribution of transit times, ranged from 0·2 y for soil water at a depth of 30 cm on the larger catchment, to 1·1-1·3 y for most spring flows and 1·4-1·6 y for baseflows on the smaller catchments. Baseflow on the larger PA basin and flow of one spring on a small WV basin showed no detectable seasonal fluctuations in oxygen-18, indicating flow emanated from sources with mean transit times greater than about 5 y. Based upon this soil flow path approach, it was concluded that seasonal oxygen-18 variations can be used to infer mean annual isotopic damping depths and diffusivities for soil depths up to approximately 170 cm.
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