Using environmental tracers to determine the relative importance of travel times in the unsaturated and saturated zones for the delay of nitrate reduction measures

Gerber, Christoph; Purtschert, Roland; Hunkeler, Daniel; Hug, R.; Sültenfuß, Jürgen

doi:10.1016/j.jhydrol.2018.03.043

Cited by 20 publications

(13 citation statements)

References 53 publications

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“…The thickness of the unsaturated zone required to store an annual precipitation affects the travel time in the unsaturated zone (Gerber et al, ), which is often neglected in dynamic catchment age studies. Vertical flow through the unsaturated zone can delay the arrival of precipitation to saturated zone, challenging concepts developed in “traditional” non‐Mediterranean catchments.…”

Section: Discussionmentioning

confidence: 99%

Cosmogenic Isotopes Unravel the Hydrochronology and Water Storage Dynamics of the Southern Sierra Critical Zone

Visser

Thaw

Deinhart

et al. 2019

Water Resources Research

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One of the principal questions in hydrology is how and when water leaves the critical zone storage as either stream flow or evapotranspiration. We investigated subsurface water storage and storage selection of the Southern Sierra Critical Zone Observatory (California, USA) within the age-ranked storage selection framework, constrained by a novel combination of cosmogenic radioactive and stable isotopes: tritium, sodium-22, sulfur-35, and oxygen-18. We found a significant positive correlation between tritium and stream flow rate and between sulfur-35 and stream flow rate, indicating that the age distribution of stream flow varies with stream flow rate. Storage selection functions that vary with stream flow rate are better able to reproduce tritium concentrations in stream flow than functions that are constant in time. For the Southern Sierra Critical Zone, there is a strong preference to discharge the oldest water in storage during dry conditions but only a weak preference for younger water during wet conditions. The preference of evapotranspiration for younger water, constrained by oxygen-18 in stream water, is essential to parameterize subsurface storage but needs to be confirmed by isotopic or other investigations of evapotranspiration. This is the first study to illustrate how a combination of cosmogenic radioactive isotopes reveals the hydrochronology and water storage dynamics of catchments, constrains the subsurface architecture of the critical zone, and provides insight into landscape evolution.Plain Language Summary Watersheds store water underground in soils and weathered bedrock.How long it takes for water to flow through the subsurface to feed streams is difficult to measure but important to understand how watersheds function. We used a novel combination of isotopic tracer methods to study the mixture of water ages in Providence Creek, a stream in the southern Sierra Nevada, California (USA). We studied naturally occurring radioactive isotopes of hydrogen (tritium), sodium-22, and sulfur-35. The abundance of these isotopes decreases because of radioactive decay as water spends more time underground. Each of these isotopes has a distinct half-life (12.3 years, 2.6 years, and 87 days, respectively). By using this combination of isotopes we were able to tease out the mixture of water ages in the stream. This level of detail helps us understand how the subsurface "selects" water from storage to generate stream flow. We find that Providence Creek has a strong preference to remove the oldest water in storage during dry conditions but shows only a slight preference to remove younger water during wet conditions. Based on the age mixtures of stream water, we estimate that the Providence Creek watershed stores 3 m of water in the subsurface.

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Section: Discussionmentioning

confidence: 99%

Cosmogenic Isotopes Unravel the Hydrochronology and Water Storage Dynamics of the Southern Sierra Critical Zone

Visser

Thaw

Deinhart

et al. 2019

Water Resources Research

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“… * Precipitation norm data from Wynau station for 1961–1980 (MeteoSuisse) ** Estimated direct groundwater recharge in the Gäu valley based on tracer experiments (Gerber et al, 2018 ) …”

Section: Discussionmentioning

confidence: 99%

“…** Estimated direct groundwater recharge in the Gäu valley based on tracer experiments (Gerber et al, 2018 )…”

Section: Discussionmentioning

confidence: 99%

Field-scale monitoring of nitrate leaching in agriculture: assessment of three methods

Wey

Hunkeler

Bischoff³

et al. 2021

Environ Monit Assess

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Deterioration of groundwater quality due to nitrate loss from intensive agricultural systems can only be mitigated if methods for in-situ monitoring of nitrate leaching under active farmers’ fields are available. In this study, three methods were used in parallel to evaluate their spatial and temporal differences, namely ion-exchange resin-based Self-Integrating Accumulators (SIA), soil coring for extraction of mineral N (Nmin) from 0 to 90 cm in Mid-October (pre-winter) and Mid-February (post-winter), and Suction Cups (SCs) complemented by a HYDRUS 1D model. The monitoring, conducted from 2017 to 2020 in the Gäu Valley in the Swiss Central Plateau, covered four agricultural fields. The crop rotations included grass-clover leys, canola, silage maize and winter cereals. The monthly resolution of SC samples allowed identifying a seasonal pattern, with a nitrate concentration build-up during autumn and peaks in winter, caused by elevated water percolation to deeper soil layers in this period. Using simulated water percolation values, SC concentrations were converted into fluxes. SCs sampled 30% less N-losses on average compared to SIA, which collect also the wide macropore and preferential flows. The difference between Nmin content in autumn and spring was greater than nitrate leaching measured with either SIA or SCs. This observation indicates that autumn Nmin was depleted not only by leaching but also by plant and microbial N uptake and gaseous losses. The positive correlation between autumn Nmin content and leaching fluxes determined by either SCs or SIA suggests autumn Nmin as a useful relative but not absolute indicator for nitrate leaching. In conclusion, all three monitoring techniques are suited to indicate N leaching but represent different transport and cycling processes and vary in spatio-temporal resolution. The choice of monitoring method mainly depends (1) on the project’s goals and financial budget and (2) on the soil conditions. Long-term data, and especially the combination of methods, increase process understanding and generate knowledge beyond a pure methodological comparison.

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“…Used within the right concept of flow structures, the points with dated groundwater are useful proxies of difficult‐to‐access groundwater dynamics. With more cost‐effective access to chemical analysis, age data are increasingly measured in well networks, thus giving information on spatial patterns and stratification of ages (Ayraud et al, 2008; Cook, Solomon, Plummer, Busenberg, & Schiff, 1995; Gerber, Purtschert, Hunkeler, Hug, & Sültenfuss, 2018; Visser, Broers, Heerdink, & Bierkens, 2009). Upslope, age stratifications give useful information on localized recharge and heterogeneity patterns (McMahon, Plummer, Böhlke, Shapiro, & Hinkle, 2011).…”

Section: Introductionmentioning

confidence: 99%

Lagged rejuvenation of groundwater indicates internal flow structures and hydrological connectivity

Kolbe

Marçais

Dreuzy

et al. 2020

Hydrological Processes

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Large proportions of rainwater and snowmelt infiltrate into the subsurface before contributing to stream flow and stream water quality. Subsurface flow dynamics steer the transport and transformation of contaminants, carbon, weathering products and other biogeochemistry. The distribution of groundwater ages with depth is a key feature of these flow dynamics. Predicting these ages are a strong test of hypotheses about subsurface structures and time-varying processes. Chlorofluorocarbon (CFC)based groundwater ages revealed an unexpected groundwater age stratification in a 0.47 km 2 forested catchment called Svartberget in northern Sweden. An overall groundwater age stratification, representative for the Svartberget site, was derived by measuring CFCs from nine different wells with depths of 2-18 m close to the stream network. Immediately below the water table, CFC-based groundwater ages of already 30 years that increased with depth were found. Using complementary groundwater flow models, we could reproduce the observed groundwater age stratification and show that the 30 year lag in rejuvenation comes from return flow of groundwater at a subsurface discharge zone that evolves along the interface between two soil types. By comparing the observed groundwater age stratification with a simple analytical approximation, we show that the observed lag in rejuvenation can be a powerful indicator of the extent and structure of the subsurface discharge zone, while the vertical gradient of the age-depth-relationship can still be used as a proxy of the overall aquifer recharge even when sampled in the discharge zone. The single age stratification profile measured in the discharge zone, close to the aquifer outlet, can reveal the main structure of the groundwater flow pattern from recharge to discharge. This groundwater flow pattern provides information on the participation of groundwater in the hydrological cycle and indicates the lower boundary of hydrological connectivity. K E Y W O R D SCFCs, groundwater age stratification, groundwater recharge, hillslope storage Boussinesq equations, Krycklan, subsurface discharge, subsurface hydrological connectivity

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Using environmental tracers to determine the relative importance of travel times in the unsaturated and saturated zones for the delay of nitrate reduction measures

Cited by 20 publications

References 53 publications

Cosmogenic Isotopes Unravel the Hydrochronology and Water Storage Dynamics of the Southern Sierra Critical Zone

Cosmogenic Isotopes Unravel the Hydrochronology and Water Storage Dynamics of the Southern Sierra Critical Zone

Field-scale monitoring of nitrate leaching in agriculture: assessment of three methods

Lagged rejuvenation of groundwater indicates internal flow structures and hydrological connectivity

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