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Stable isotope values of the groundwater as function of the soil properties
Abstract
The stable isotopes oxygen-18 (18 O) and deuterium (2 H) are naturally abundant in precipitation. Due to fractionation effects, a seasonally variable distribution of water isotopes occurs in the precipitation of temperate and continental regions. Water isotopes, being conservative tracers, are ideal for investigating subsurface flow processes. They reveal information about soil water fluxes like evaporation, transpiration, downward infiltration and others that are difficult to determine by other techniques. We are using the observations from meteorological station and monthly average values of stable isotopes (δ 18 O and δ 2 H) in precipitation in Riga, Latvia to model the isotopic composition of soil water and groundwater recharge. A simplified soil root-zone model as fully mixed reservoir is used. It is assumed that all the precipitation is infiltrated immediately, and water is lost from the soil due to evapotranspiration as calculated with Penman-Monteith equation and any water in excess of field capacity is exported to the groundwater. We have found that the range of the average isotopic composition of groundwater recharge in different soils types is similar to the uncertainty of the groundwater isoscape values in the region. The approach can be applied to refine the existing isoscape maps by considering the relationship between soil properties, evapotranspiration and groundwater recharge and infer groundwater recharge processes.
... Stable isotope ratios of groundwater, especially phreatic groundwater, are expected to reflect values of modern precipitation input (Clark and Fritz, 1997;Darling et al., 2003;Filippini et al., 2015;Ma et al., 2017;Rozanski, 1985) because it is the main source of water in aquifers. Groundwater recharge and variations of δ 18 O and δ 2 H are controlled by factors such as soil type, vegetation coverage and type, precipitation type and seasonality (Barbecot et al., 2018;Birkel et al., 2018;Crosbie et al., 2015;Jasechko et al., 2014;Kalvāns et al., 2018;Matiatos and Wassenaar, 2019;Raidla et al., 2016;Sánchez-Murillo and Birkel, 2016;Wassenaar et al., 2009). ...
Stable isotopes are used to decipher hydrological processes in watershed research. A two-year monthly monitoring of hydrogen and oxygen stable isotope ratios (δ2H and δ18O) in a temperate catchment in Norther Europa, Latvia was undertaken. Isotope ratios in common water types – raised bog, confined groundwater, unconfined groundwater and surface water – were measured. We found characteristic signatures of isotope ratios for each of these four water types. The average isotope ratios of different water types ranged from −80.8 to −68.3‰ for δ2H and −11.46 to −8.76‰ for δ18O, with standard deviations from 18 to 25‰ and 0.10 to 1.59‰, respectively. The isotope ratios of the stream base flow were consistent with the groundwater isoscape and seasonally enriched by evaporation. Most enriched water are found in raised bogs and large lakes. The most depleted water is found in a spring discharging phreatic groundwater in a forest site with sandy soil. The most enriched water was associated with significant short-lived precipitation events, resulting in impoundment of water on the land surface and its enrichment due to evaporation. The novelty of this study is that this enriched isotope signal is propagated throughout the hydrological system, temporarily albeit significantly shifting isotope ratios of phreatic groundwater and surface runoff. Further case studies are needed to affirm if this is a regionally significant mechanism controlling isotope ratios of surface and subsurface water. The observed difference between the average δ18O of phreatic groundwater at two locations was 0.9‰. We suggest that the differences are due to different land use and soil conditions.
This paper presents first results of monthly water stable isotope monitoring programme covering the most important surface and groundwater types in the Salaca River basin. The aim is to characterise the isotopic values of different water types in the Salaca River basin, and test if their contribution can be identified in the Salaca river runoff. A monthly groundwater and surface water stable isotope monitoring programme was initiated in August 2015 covering the most of the important surface and groundwater types in the study region-groundwater and surface water in the raised bogs, free-surface groundwater including artificially drained agricultural lands, water emerging from the Lake Burtnieks as well as Burtnieks and Arulika confined aquifers. Preliminary results show that stable isotopes are useful tool to identify distinct water components and their evolution. However, it is needed to continue monitoring programme to draw significant conclusions.
A range of continental-scale soil datasets exists in Europe with different spatial representation and based on different principles. We developed comprehensive pedotransfer functions (PTFs) for applications principally on spatial datasets with continental coverage. The PTF development included the prediction of soil water retention at various matric potentials and prediction of parameters to characterize soil moisture retention and the hydraulic conductivity curve (MRC and HCC) of European soils. We developed PTFs with a hierarchical approach, determined by the input requirements. The PTFs were derived by using three statistical methods: (i) linear regression where there were quantitative input variables, (ii) a regression tree for qualitative, quantitative and mixed types of information and (iii) mean statistics of developer-defined soil groups (class PTF) when only qualitative input parameters were available. Data of the recently established European Hydropedological Data Inventory (EU-HYDI), which holds the most comprehensive geographical and thematic coverage of hydro-pedological data in Europe, were used to train and test the PTFs. The applied modelling techniques and the EU-HYDI allowed the development of hydraulic PTFs that are more reliable and applicable for a greater variety of input parameters than those previously available for Europe. Therefore the new set of PTFs offers tailored advanced tools for a wide range of applications in the continent.
We use data from Global Network of Isotopes in Precipitation (GNIP) database to explore how the atmosphere's meridional circulation cells control the latitudinal and seasonal distribution of δ18O and d-excess in precipitation. We demonstrate that the atmospheric general circulation (AGC) cells determine variations of zonally averaged isotopic composition of meteoric water; the local isotopic minimum near the equator coincides with the intertropical convergence (ITC), and two maxima on either side of the ITC coincide with the subtropical highs (STHs). Both the ITC and STHs migrate cum sole, as part of the systematic annual migration of the meridional cells. This migratory circulation pattern controls the phase of the annual oscillation of the precipitation δ18O. At latitudes equatorward of the STHs, δ18O reaches its maximum in the winter of the respective hemisphere and at higher latitudes in the summer. From the monthly latitudinal distribution of the vertical velocity at the 500-hPa level, we obtain the seasonal variations of the latitudinal positions of the subtropical moisture source regions and their climates. The sea surface temperature and relative humidity at the moisture source regions are used to predict seasonal changes of the d-excess of water vapor evaporated from the source regions. The GNIP data is consistent with the predicted phase of the d-excess. However, the observed magnitude of the seasonal oscillation is greater than the predicted values. This work provides a baseline for understanding the influence of subtropical moisture source regions and other climatological factors on the d-excess.
Subsurface-water flow pathways in three different land-use areas (non-irrigated grassland, poplar forest, and irrigated arable land) in the central North China Plain were investigated using oxygen (¹⁸O) and hydrogen (²H) isotopes in samples of precipitation, soils, and groundwater. Soil water in the top 10 cm was significantly affected by both evaporation and infiltration. Water at 10–40 cm depth in the grassland and arable land, and 10–60 cm in poplar forest, showed a relatively short residence time, as a substantial proportion of antecedent soil water was mixed with a 92-mm storm infiltration event, whereas below those depths (down to 150 cm), depleted δ¹⁸O spikes suggested that some storm water bypassed the shallow soil layers. Significant differences, in soil-water content and δ¹⁸O values, within a small area, suggested that the proportion of immobile soil water and water flowing in subsurface pathways varies depending on local vegetation cover, soil characteristics and irrigation applications. Soil-water δ¹⁸O values revealed that preferential flow and diffuse flow coexist. Preferential flow was active within the root zone, independent of antecedent soil-water content, in both poplar forest and arable land, whereas diffuse flow was observed in grassland. The depleted δ¹⁸O spikes at 20–50 cm depth in the arable land suggested the infiltration of irrigation water during the dry season. Temporal isotopic variations in precipitation were subdued in the shallow groundwater, suggesting more complete mixing of different input waters in the unsaturated zone before reaching the shallow groundwater.
The study presents a shallow groundwater isoscape for the Baltic region, which covers the major part of the Baltic Artesian Basin (BAB). BAB is an important palaeogroundwater reservoir, but prior to this study, little has been known about the spatial variability of δD and δ¹⁸O values in modern precipitation input across the region. To overcome this limitation, we hypothesized that the isotopic composition of shallow groundwater in the BAB could be used as a proxy for the mean weighted annual isotopic composition of local precipitation. However, the results of the study reveal many clear discrepancies between the isotopic composition of precipitation and shallow groundwater in the area. The isotopic composition of shallow groundwater is mostly biased towards isotopically depleted wintertime precipitation. We propose that the formation of shallow groundwater in the BAB area could be largely affected by variations in soil structure and land cover. The derived isoscape based on δ¹⁸O values of shallow groundwater in the BAB area is characterized by high spatial resolution and can therefore serve as a fairly reliable reference basis for further hydrogeological, ecological and forensic applications.
Intra-annual variation in the stable isotope ratios of precipitation imparts a time-dependent isotopic signal to continental hydrological and ecological systems. This variation is forced by seasonal climate cycles, with temperature and precipitation intensity each driving isotopic seasonality in different parts of the globe. In order to better characterize and understand the isotopic seasonality of precipitation and to advance applications such as the inversion of isotope/climate relationships for paleoclimate seasonality reconstruction, these relationships are analyzed using a database of precipitation isotope ratio observations and gridded maps. The results confirm the overwhelming dominance of temperature- and precipitation amount–related isotopic seasonality within specific, well-defined geographic zones and indicate that these zones encompass most of Earth's continental land surface. Analysis of global maps of precipitation isotopic seasonality, however, indicates spatially patterned variation in isotope/climate relationships, including patterns related to continentality in temperature-dominated regions and meridional trends in amount-dominated areas. These features reflect more complex climatological controls on isotope seasonality and suggest limitations and opportunities for paleoclimate reconstruction using isotopic archives.