Flowpath identification in a Swiss mountain catchment using natural solutes and stable water isotopes

Overview

Climate and land use changes lead to increasing pressures on our rivers, and pollution from diffuse anthropogenic sources affects river water quality and thus poses a major threat to aquatic ecosystems. In 2018, only 32% of the UK rivers were considered to be at Good Ecological Status (DEFRA, 2019), and water quality is likely to deteriorate even more in the coming years due to an intensification of agriculture and further land use changes.
To prevent and mitigate the impact of point and diffuse sources of contamination on aquatic ecosystems, we have to improve our understanding of how, when, and where solutes and contaminants are mobilized in the landscape. We furthermore have to increase our ability to predict along which pathways water and contaminants travel through the landscape, and how these flowpaths change with land properties and catchment characteristics.
To understand general patterns of fast and slow flow processes, we can use stable water isotopes (deuterium and oxygen-18) as natural tracers to track the path of water inputs through the landscape, from precipitation to streamflow (e.g., Kirchner, 2019). The analysis of stable water isotopes provides information on the “travel time” of water, the time water takes to travel through the catchment from entering it as rainfall to leaving it as streamflow. Using stable water isotopes, we can therefore separate “recent” (rainwater) from “older” groundwater contributions to streamflow. This is relevant for process understanding of how catchments store and release water, but also has substantial implications for solute and contaminant transport, because longer travel times allow more time for attenuation processes.
However, stable water isotopes only provide limited information on the flowpaths of water through the landscape, and cannot identify the catchment areas that contribute predominantly to streamflow at any given time. But different parts of the catchment (i.e. bedrock vs. soils, riparian areas vs. hillslopes) are often made up of different minerals and materials, and water will pick up the (natural) chemical signature of its surroundings. Consequently, streamwater chemistry differs depending on the flowpaths water takes through the catchment. The magnitude and variation of natural and anthropogenic solutes in streamwater can therefore be considered a “fingerprint” of catchment transport, mixing, and reaction processes. Hydrochemical signatures of streamflow can thus be used to infer transport pathways through the landscape (Knapp et al., 2020; Li et al., under review).
The aim of this project is to develop a framework on how changing concentrations of natural solutes and their fluxes during rain events and baseflow periods can identify from which sources (i.e. which catchment areas) the streamwater originates, and along which pathways the water has travelled through the landscape. The underlying hypotheses are:
-Different catchment areas will be “activated” during different rain events, depending on antecedent conditions and event characteristics
-Depending on flowpaths, interactions with reactive material will alter nutrient, contaminant and solute concentrations to a different degree.
These hypotheses will be tested with data from the Erlenbach, a small catchment in the Swiss pre-Alps characterized by interchanging slopes and plateaus. Because the composition of the soils differs substantially between the forested slopes and the wetland-like plateaus, in addition to highly heterogeneous geology across the catchment, the Erlenbach catchment is an ideal location to test the above hypotheses.
With this project we aim to develop a concise framework of water and solute transport at the catchment scale, which can later be applied in settings with stronger anthropogenic impact. In consequence, the project constitutes an important stepping-stone in mitigating anthropogenic impacts on river water quality.

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Image Captions

Pic1 – “The meteorological station at the Erlenbach catchment”
Pic2 – “A groundwater well in the catchment”

Methodology

In this project, the student will work on quantitative approaches to relate streamwater chemistry to sources and flowpaths of water through a catchment. To investigate the hypotheses outlined above, the student will use a suite of different methodologies, including the analysis of water quality data and the collection and analysis of further soil and streamwater samples.
In a first step, the student will analyze an existing streamwater timeseries, to identify differences and similarities in solute mobilization across solutes on the scale of individual rain events. This high-frequency timeseries consists of isotopes and solutes measured over 4 years in the Erlenbach catchment at sub-hourly frequency (von Freyberg et al., 2017; Knapp et al., 2020). The dataset is unique with respect to both its range of different solutes as well as the sampling frequency of the data, and provides a unique opportunity for a detailed investigation of solute mobilization at the hydrologic event scale.
Based on results from the event classification and existing spatial information on geology and landcover, a detailed field campaign will be planned for the collection of spatially distributed soil water samples. The field sampling will provide spatial information on soil water chemistry, and how the soil water composition changes over time as function of catchment conditions.
The field site is part of a long-term monitoring set-up of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) as well as the Swiss National River Monitoring and Survey Programme (NADUF; Zobrist et al., 2018). This approach allows placing results obtained from this study into the context of long-term data sets and understanding the implications of the findings from this study for larger-scale systems.

Project Timeline

Year 1

Thorough literature review; analysis of existing solute and isotope time series data from the Erlenbach; planning and preparation of field work; visit to Switzerland

Year 2

Several weeks of field work in Switzerland:
-spatially-distributed sampling during different stages of catchment wetness to test hypothesis 1
-event-based sampling in small sub-catchments to test hypothesis 2
Sample analysis

Year 3

Data analysis; present results at an international conference (e.g., AGU); preparation of manuscripts for publication

Year 3.5

Finalize data analysis; writing up of thesis and preparation of manuscripts for publication

Training
& Skills

Field methods: installation of soil water lysimeters, shallow groundwater wells; water sample collection and handling
Laboratory skills: water sample handling and preparation, training in chemical analysis of water samples: IC (ion chromatography), ICP-MS (inductively coupled plasma mass spectrometry), and stable water isotope analysis
Data analysis: statistical methods; handling of big data
Training in writing skills through detailed feedback on manuscripts and thesis drafts
Presentation skills: presentation of research at national and international conferences
Project management skills: how to plan, initiate and execute a project efficiently and successfully

References & further reading

Kirchner, J. W. (2019). Quantifying new water fractions and transit time distributions using ensemble hydrograph separation: theory and benchmark tests. Hydrology and Earth System Sciences, 23(1), 303-349. https://hess.copernicus.org/articles/23/303/2019/

Knapp, J. L. A., von Freyberg, J., Studer, B., Kiewiet, L., & Kirchner, J. W. (2020). Concentration-discharge relationships vary among hydrological events, reflecting differences in event characteristics. Hydrology and Earth System Sciences, 24(5), 2561-2561. https://hess.copernicus.org/articles/24/2561/2020/hess-24-2561-2020.html

Li, L., Sullivan, P., Bishop, K., Cirpka, O. A., Kirchner, J. W., Knapp, J. L. A. , van Meerveld, H. I., Rinaldo, A., Seibert, J., Wen, H. (in press). Crossing boundaries: toward integrated hydrological and biogeochemical theories at the catchment scale. WIREs Water

von Freyberg, J., Studer, B., & Kirchner, J. W. (2017). A lab in the field: high-frequency analysis of water quality and stable isotopes in stream water and precipitation. Hydrology and Earth System Sciences, 21(3), 1721-1739. https://hess.copernicus.org/articles/21/1721/2017/hess-21-1721-2017.html

Zobrist, J., Schoenenberger, U., Figura, S., & Hug, S. J. (2018). Long-term trends in Swiss rivers sampled continuously over 39 years reflect changes in geochemical processes and pollution. Environmental Science and Pollution Research, 25(17), 16788-16809. https://link.springer.com/article/10.1007/s11356-018-1679-x

Further Information

For further information on the project please contact Julia Knapp at julia.knapp@usys.ethz.ch

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