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Searching for the source: using shallow groundwater chemistry to determine the sources for streamflow in a pre-alpine catchment


Kiewiet, Leonie. Searching for the source: using shallow groundwater chemistry to determine the sources for streamflow in a pre-alpine catchment. 2020, University of Zurich, Faculty of Science.

Abstract

Streamflow in undisturbed catchments in humid temperate climates consists predominantly of groundwater, even during large rainfall events. Groundwater contributions to streamflow sustain stream baseflow, influence stream water temperature, and affect stream water chemistry. Despite the obvious importance of groundwater for streamflow and watermanagement, only few studies have focused on the spatial variability in its isotopic and chemical composition. Although we might expect that groundwater chemistry differs from location to location due to differences in subsurface pathways and residence times, groundwater chemistry is often assumed to be relatively uniform, and to be well-represented bystream baseflow.

There are at least two problems with this status quo. First, we missout on learning about flow pathways within catchments, and whichareas contribute to streamflow during different stages of wetness. Second, assumptions of homogeneity can be problematic for source-area analyses if the spatial variability leads to temporal variability in the composition of the groundwater that actually contributes to streamflow due to the expansion of the hydrologically connected and contributing area during rainfall events. If the composition of transiently connected groundwater sources is significantly different from the permanently connected groundwater, changes in stormflow chemistry might be (partially) caused by contributions from previously disconnected groundwater sources, rather than the inferred contributions from soil wateror rain water. This can lead to large uncertainties in the results ofsource area analyses, and might go unnoticed if we are not aware of the spatial variability in the groundwater chemistry and isotopic composition.

Therefore, this thesis aims to enhance our knowledge of the spatial variability in shallow groundwater composition in a small pre-alpine headwater catchment by quantifying the variability and investigating which processes cause this variability. This information was then used to identify the source areas for baseflow and stormflow, and to estimate the uncertainty in hydrograph separation analyses due to the spatial variability in the groundwater composition. The analyses are based on spatial sampling of groundwater, soil water and streamwater during nine baseflow snapshot campaigns, and additional sampling of streamwater and rainwater during four rainfall events in the pre-alpine Studibach catchment (Alptal), using a measurement network of 51 groundwaterwells, 18 suction lysimeters, seven stream gauges and three rain gauges.

The results of the groundwater sampling showed that the spatial variability in shallow groundwater chemistry was large, and for most parameters larger than the temporal variability. Differences from the catchment average concentrations were used to distinguish four shallow groundwater types, of which three are related to landscape elements: Type I: riparian-like areas, characterized by high concentrations of iron, manganese and cobalt; Type II: hillslopes, characterized by high concentrations of copper, zinc and nickel; and Type III: ’deep’ groundwater, which had a depleted isotopic signature compared to the other types, and high concentrations of strontium. Groundwater Type IV was influencedby bedrock with a different composition, and had high concentrations of sulfate and magnesium. The groundwater compositions and types were only weakly related to topographic and hydrodynamic site attributes. However, soil and bedrock leachates, and subsurface topographic data(obtained from geophysical profiles) confirmed the spatial distribution and chemical composition characteristics for each groundwater type.

Baseflow was not an equal mixture of the different groundwater types .For the majority of the sampling campaigns baseflow chemistry moststrongly resembled riparian-like groundwater (I) for all but one sub-catchment. However, similarity to the hillslope-type groundwater (II) was larger shortly after snowmelt, reflecting differences in hydrologic connectivity. Three-component end-member mixing analyses indicated that groundwater dominated stormflow, and that soil water fractions were minimal for three of the four events. However, the large variability in the soil and groundwater composition compared to the changes in stormflow composition led to large uncertainties. Stormflow was not a conservative mixture of rainwater and baseflow, which indicates that other sources (soil water or other groundwater sources) induced changes in streamflow composition during events. Streamwater chemistry changed gradually during events, which likely reflects a gradual increase in the hydrologically connected and contributing area rather than a threshold type behaviour.

The hydrograph separation results depended highly on the choice ofwhich and how many samples were used to characterize the pre-eventwater composition. Generally, including more samples yielded more robust results. The number of groundwater samples needed to characterize the average groundwater composition and its variability was much larger than is common in small-scale hydrograph separation studies. However, by taking a few more samples than is typical, one can already obtain anestimate of the variability. Analyses that do not include the variabilityin pre-event water composition (for instance by taking only a baseflow sample) likely underestimate the real uncertainty.

Overall, this thesis demonstrates that an improved understanding and representation of shallow groundwater chemistry is useful for studies in small headwater atchments. Therefore, it is recommended that the hydrologic community uses more information on the variability in thegroundwater composition and subsurface topography in hydrological analyses. Increasing the number of groundwater samples is important for robust analyses. This should inspire us to go out and measure. This message is especially important in the age of declining field research and for climatic regions for which we currently have limited data. Lastly, the results of fundamental analyses, like hydrograph separation, shape our conceptual understanding, and thereby influence the way we develop futureresearch or models to investigate hydrological processes at larger scales. As such, getting the fundamentals about groundwater and its variabilityright is paramount.

Abstract

Streamflow in undisturbed catchments in humid temperate climates consists predominantly of groundwater, even during large rainfall events. Groundwater contributions to streamflow sustain stream baseflow, influence stream water temperature, and affect stream water chemistry. Despite the obvious importance of groundwater for streamflow and watermanagement, only few studies have focused on the spatial variability in its isotopic and chemical composition. Although we might expect that groundwater chemistry differs from location to location due to differences in subsurface pathways and residence times, groundwater chemistry is often assumed to be relatively uniform, and to be well-represented bystream baseflow.

There are at least two problems with this status quo. First, we missout on learning about flow pathways within catchments, and whichareas contribute to streamflow during different stages of wetness. Second, assumptions of homogeneity can be problematic for source-area analyses if the spatial variability leads to temporal variability in the composition of the groundwater that actually contributes to streamflow due to the expansion of the hydrologically connected and contributing area during rainfall events. If the composition of transiently connected groundwater sources is significantly different from the permanently connected groundwater, changes in stormflow chemistry might be (partially) caused by contributions from previously disconnected groundwater sources, rather than the inferred contributions from soil wateror rain water. This can lead to large uncertainties in the results ofsource area analyses, and might go unnoticed if we are not aware of the spatial variability in the groundwater chemistry and isotopic composition.

Therefore, this thesis aims to enhance our knowledge of the spatial variability in shallow groundwater composition in a small pre-alpine headwater catchment by quantifying the variability and investigating which processes cause this variability. This information was then used to identify the source areas for baseflow and stormflow, and to estimate the uncertainty in hydrograph separation analyses due to the spatial variability in the groundwater composition. The analyses are based on spatial sampling of groundwater, soil water and streamwater during nine baseflow snapshot campaigns, and additional sampling of streamwater and rainwater during four rainfall events in the pre-alpine Studibach catchment (Alptal), using a measurement network of 51 groundwaterwells, 18 suction lysimeters, seven stream gauges and three rain gauges.

The results of the groundwater sampling showed that the spatial variability in shallow groundwater chemistry was large, and for most parameters larger than the temporal variability. Differences from the catchment average concentrations were used to distinguish four shallow groundwater types, of which three are related to landscape elements: Type I: riparian-like areas, characterized by high concentrations of iron, manganese and cobalt; Type II: hillslopes, characterized by high concentrations of copper, zinc and nickel; and Type III: ’deep’ groundwater, which had a depleted isotopic signature compared to the other types, and high concentrations of strontium. Groundwater Type IV was influencedby bedrock with a different composition, and had high concentrations of sulfate and magnesium. The groundwater compositions and types were only weakly related to topographic and hydrodynamic site attributes. However, soil and bedrock leachates, and subsurface topographic data(obtained from geophysical profiles) confirmed the spatial distribution and chemical composition characteristics for each groundwater type.

Baseflow was not an equal mixture of the different groundwater types .For the majority of the sampling campaigns baseflow chemistry moststrongly resembled riparian-like groundwater (I) for all but one sub-catchment. However, similarity to the hillslope-type groundwater (II) was larger shortly after snowmelt, reflecting differences in hydrologic connectivity. Three-component end-member mixing analyses indicated that groundwater dominated stormflow, and that soil water fractions were minimal for three of the four events. However, the large variability in the soil and groundwater composition compared to the changes in stormflow composition led to large uncertainties. Stormflow was not a conservative mixture of rainwater and baseflow, which indicates that other sources (soil water or other groundwater sources) induced changes in streamflow composition during events. Streamwater chemistry changed gradually during events, which likely reflects a gradual increase in the hydrologically connected and contributing area rather than a threshold type behaviour.

The hydrograph separation results depended highly on the choice ofwhich and how many samples were used to characterize the pre-eventwater composition. Generally, including more samples yielded more robust results. The number of groundwater samples needed to characterize the average groundwater composition and its variability was much larger than is common in small-scale hydrograph separation studies. However, by taking a few more samples than is typical, one can already obtain anestimate of the variability. Analyses that do not include the variabilityin pre-event water composition (for instance by taking only a baseflow sample) likely underestimate the real uncertainty.

Overall, this thesis demonstrates that an improved understanding and representation of shallow groundwater chemistry is useful for studies in small headwater atchments. Therefore, it is recommended that the hydrologic community uses more information on the variability in thegroundwater composition and subsurface topography in hydrological analyses. Increasing the number of groundwater samples is important for robust analyses. This should inspire us to go out and measure. This message is especially important in the age of declining field research and for climatic regions for which we currently have limited data. Lastly, the results of fundamental analyses, like hydrograph separation, shape our conceptual understanding, and thereby influence the way we develop futureresearch or models to investigate hydrological processes at larger scales. As such, getting the fundamentals about groundwater and its variabilityright is paramount.

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Additional indexing

Item Type:Dissertation (monographical)
Referees:Seibert Jan, van Meerveld H J, Egli Markus, Stähli Manfred
Communities & Collections:07 Faculty of Science > Institute of Geography
UZH Dissertations
Dewey Decimal Classification:910 Geography & travel
Language:English
Place of Publication:Zürich
Date:2020
Deposited On:07 Oct 2020 10:09
Last Modified:07 Oct 2020 10:09
Number of Pages:91
OA Status:Green

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