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

Abstract. A dual stable water isotope (δ 2 H and δ 18 O) study was conducted in the developed (managed) landscape of the Schwingbach catchment (Germany). The 2-year weekly to biweekly measurements of precipitation, stream, and groundwater isotopes revealed that surface and groundwater are isotopically disconnected from the annual precipitation cycle but showed bidirectional interactions between each other. Apparently, snowmelt played a fundamental role for groundwater recharge explaining the observed differences to precipitation δ values.A spatially distributed snapshot sampling of soil water isotopes at two soil depths at 52 sampling points across different land uses (arable land, forest, and grassland) revealed that topsoil isotopic signatures were similar to the precipitation input signal. Preferential water flow paths occurred under forested soils, explaining the isotopic similarities between top-and subsoil isotopic signatures. Due to human-impacted agricultural land use (tilling and compression) of arable and grassland soils, water delivery to the deeper soil layers was reduced, resulting in significant different isotopic signatures. However, the land use influence became less pronounced with depth and soil water approached groundwater δ values. Seasonally tracing stable water isotopes through soil profiles showed that the influence of new percolating soil water decreased with depth as no remarkable seasonality in soil isotopic signatures was obvious at depths > 0.9 m and constant values were observed through space and time. Since classic isotope evaluation methods such as transfer-function-based mean transit time calculations did not provide a good fit between the observed and calculated data, we established a hydrological model to estimate spatially distributed groundwater ages and flow directions within the Vollnkirchener Bach subcatchment. Our model revealed that complex age dynamics exist within the subcatchment and that much of the runoff must has been stored for much longer than event water (average water age is 16 years). Tracing stable water isotopes through the water cycle in combination with our hydrological model was valuable for determining interactions between different water cycle components and unravelling age dynamics within the study area. This knowledge can further improve catchment-specific process understanding of developed, human-impacted landscapes.

Section: Isotopes Of Soil Water 421 Spatial Variabilitycontrasting

confidence: 50%

Section: Isotopes Of Soil Water 421 Spatial Variabilitymentioning

confidence: 99%

Exploring water cycle dynamics by sampling multiple stable water isotope pools in a developed landscape in Germany

Orlowski

Kraft

Pferdmenges

2016

“…These differences between the summer and winter profiles at the background site differ from the normally observed sawtooth pattern of evaporated (enriched) summer values and unevaporated (depleted) winter values along the vadose zone (DePaolo et al, 2004). Unlike in a sawtooth pattern where the differences in the δ-values can be used to estimate yearly recharge (Garvelmann et al, 2012), here the isotopic profile from the end of the winter represents mixing between the ambient enriched water (from the end of the summer) and the fresh depleted rainwater that preferentially propagates through the desiccation cracks into deeper parts of the vadose zone. Presenting the relation between δ 2 H and δ 18 O (in a δ 2 H-δ 18 O plane) of all the sediment samples from the background site, during the winter and summer, indicates a clear shift from the average rainwater values on the local meteoric water line (slope of 7.06) to subsurface values on the vadose zone water line (slope of 4.45) (Fig.…”

Section: Resultsmentioning

confidence: 95%

“…Oxygen and hydrogen isotopes (vapor) equilibration and the tunable laser-spectrometry methodology (detailed description of the method can be found in Garvelmann et al, 2012). A two-point calibration for the δ 18 O and δ 2 H values in the samples was preformed with standards in clay matrix from the same site, in loam and in sand.…”

Section: Monitoring Sampling and Analysismentioning

confidence: 99%

Desiccation-crack-induced salinization in deep clay sediment

Baram

Ronen

Kurtzman

et al. 2013

Self Cite

Abstract.A study on water infiltration and solute transport in a clayey vadose zone underlying a dairy farm waste source was conducted to assess the impact of desiccation cracks on subsurface evaporation and salinization. The study is based on five years of continuous measurements of the temporal variation in the vadose zone water content and on the chemical and isotopic composition of the sediment and pore water in it. The isotopic composition of water stable isotopes (δ 18 O and δ 2 H) in water and sediment samples, from the area where desiccation crack networks prevail, indicated subsurface evaporation down to ∼ 3.5 m below land surface, and vertical and lateral preferential transport of water, following erratic preferential infiltration events. Chloride (Cl − ) concentrations in the vadose zone pore water substantially increased with depth, evidence of deep subsurface evaporation and down flushing of concentrated solutions from the evaporation zones during preferential infiltration events. These observations led to development of a desiccation-crack-induced salinization (DCIS) conceptual model. DCIS suggests that thermally driven convective air flow in the desiccation cracks induces evaporation and salinization in relatively deep sections of the subsurface. This conceptual model supports previous conceptual models on vadose zone and groundwater salinization in fractured rock in arid environments and extends its validity to clayey soils in semi-arid environments.

“…However, recent experimental studies indicated potential for fractionation during the cryogenic extraction (Orlowski et al, 2016), probably induced by watermineral interactions (Oerter et al, 2014;Gaj et al, 2017a, b). The direct-equilibration method -as a potential alternative analysis method -we applied in our study was so far usually limited to study percolation processes (e.g., Garvelmann et al, 2012;Mueller et al, 2014;Sprenger et al, 2016a). To our knowledge, only Bertrand et al (2012) used the directequilibration method to study the water use by plant water use and the influence of evaporation fractionation, but they bulked the soil data of the upper 20 cm.…”

Section: Evaporation Dynamics Within the Soil-plant-atmosphere Interfacementioning

confidence: 99%

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

Sprenger

Tetzlaff

Soulsby

2017

165

Abstract. Understanding the influence of vegetation on water storage and flux in the upper soil is crucial in assessing the consequences of climate and land use change. We sampled the upper 20 cm of podzolic soils at 5 cm intervals in four sites differing in their vegetation (Scots Pine (Pinus sylvestris) and heather (Calluna sp. and Erica Sp)) and aspect. The sites were located within the Bruntland Burn longterm experimental catchment in the Scottish Highlands, a low energy, wet environment. Sampling took place on 11 occasions between September 2015 and September 2016 to capture seasonal variability in isotope dynamics. The pore waters of soil samples were analyzed for their isotopic composition (δ 2 H and δ 18 O) with the direct-equilibration method. Our results show that the soil waters in the top soil are, despite the low potential evaporation rates in such northern latitudes, kinetically fractionated compared to the precipitation input throughout the year. This fractionation signal decreases within the upper 15 cm resulting in the top 5 cm being isotopically differentiated to the soil at 15-20 cm soil depth. There are significant differences in the fractionation signal between soils beneath heather and soils beneath Scots pine, with the latter being more pronounced. But again, this difference diminishes within the upper 15 cm of soil. The enrichment in heavy isotopes in the topsoil follows a seasonal hysteresis pattern, indicating a lag time between the fractionation signal in the soil and the increase/decrease of soil evaporation in spring/autumn. Based on the kinetic enrichment of the soil water isotopes, we estimated the soil evaporation losses to be about 5 and 10 % of the infiltrating water for soils beneath heather and Scots pine, respectively. The high sampling frequency in time (monthly) and depth (5 cm intervals) revealed high temporal and spatial variability of the isotopic composition of soil waters, which can be critical, when using stable isotopes as tracers to assess plant water uptake patterns within the critical zone or applying them to calibrate traceraided hydrological models either at the plot to the catchment scale.