Tracing nitrate in water from the forest to the aquifer using stable isotopes of oxygen and nitrogen
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Restored stretch of the river Thur.
Picture: Graf Pannatier (WSL).
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Habitats from gravel bar to alluvial forest at river Thur.
Picture: Graf Pannatier (WSL).
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Alluvial forest close to the restored stretch of the river Thur.
Picture: Graf Pannatier (WSL).
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The sources of nitrate in a riparian zone
along a restored corridor of the river Thur in Switzerland are investigated using
oxygen and nitrogen isotope ratios in nitrate. These data contribute to a
better understanding of nitrogen transformation in hydrologic flow paths from
upland agricultural fields to the river passing through different succession
stages of vegetation. The effects of river restoration on water quality in
alluvial and dry hardwood forests are assessed.
Tracing nitrate from the forest to the aquifer
Forests usually supply drinking water of
good quality thanks to the filtering of water through the soil. In Switzerland,
about 42% of groundwater protection zones are located in forested areas.
However, the continuous input of atmospheric nitrogen since the 1970s can have
led to the saturation of forest ecosystems and resulted in an increased release
of nitrate to groundwater. Nitrate originating from agriculture lands can also
contribute to increased nitrate leaching into groundwater, even if the
catchment is largely forested. A good understanding of flow paths and nitrate
sources in catchments is therefore important to assess the forest influence on
groundwater quality and to select the most effective practices of land
management (forest, agriculture, river) aiming at a sustainable water supply of
good quality. Numerous studies have investigated either the chemistry of
percolation waters in the forest root zone or the groundwater quality. However,
very few have assessed the biochemical transformation of the water composition
along its flow path from the forest soil to the well and the mixing in the
aquifer with water from other sources within the catchment.
Identification
of nitrate sources
This project aims at identifying the
different sources of nitrate (atmospheric deposition, nitrification in forest
soils and N-fertilizers) in the saturated and unsaturated zones and at a better
understanding of the N transformations along the flow paths from the forest to
the aquifer by analyzing the oxygen and nitrogen isotopes in nitrate.
The study is performed in a riparian zone
including different succession stages (gravel bars, river banks, grassland,
alluvial and climax forests) along a restored corridor of the river Thur
(Canton Thurgau, Switzerland). The relative contribution of the different
nitrate sources in water is assessed. The seasonal variations of nitrate concentrations
and of oxygen and nitrogen isotope ratios in nitrate are determined in three
transects across the riparian zone from the riverbank to the upland
agricultural fields, passing through the different succession stages of
vegetation.
Methods
The experimental site is located along a
restored corridor of the river Thur where a monitoring infrastructure (piezometers,
wells) has been installed in the framework of the RECORD project (see below).
Water samples for isotopic analysis are collected in the river Thur and along
three terrestrial transects in the saturated and unsaturated zones. Laboratory
experiments are performed to investigate:
- the effect of soil evaporation and soil
respiration on the d18O
values of newly-formed nitrate,
- the effect of source mixing and
subsequent denitrification on the d18O values in nitrate.
The denitrifier method is used to analyze
oxygen and nitrogen isotopes in nitrate.
A
national and international project
This project is financially supported by
the Swiss State Secretariat for Education and Research within the framework of
the European Cost Action FP601 "Forest Management and the Water Cycle”
(FORMAN).
It is also integrated in the
interdisciplinary RECORD project launched in April 2007 within the Competence
Center of Environment and Sustainability (CCES) of the ETH domain. An important
question to be addressed is whether river revitalization influences the quality
of drinking water pumped from alluvial ecosystems that are hydraulically linked
to the river and the runoff of neighboring ecosystems.
Links
Contact
Stefano Bernasconi (ETH Zürich, Geologisches
Institut)
| Keywords: |
nitrate, nitrate sources, stable isotope, soil solution, water quality, nitrification, denitrification, riparian forest, river revitalisation, groundwater |
