Pesticide fate on catchment scale: conceptual modelling of stream CSIA data
Compound-specific stable isotope analysis (CSIA) has proven beneficial in the characterization of contaminant degradation in groundwater, but it has never been used to assess pesticide transformation on catchment scale. This study presents concentration and carbon CSIA data of the herbicides S-me...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-10-01
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| Series: | Hydrology and Earth System Sciences |
| Online Access: | https://www.hydrol-earth-syst-sci.net/21/5243/2017/hess-21-5243-2017.pdf |
| Summary: | Compound-specific stable isotope analysis (CSIA) has proven beneficial in the
characterization of contaminant degradation in groundwater, but it has never
been used to assess pesticide transformation on catchment scale. This study
presents concentration and carbon CSIA data of the herbicides S-metolachlor
and acetochlor from three locations (plot, drain, and catchment outlets) in
a 47 ha agricultural catchment (Bas-Rhin, France). Herbicide
concentrations at the catchment outlet were highest
(62 µg L<sup>−1</sup>) in response to an intense rainfall event
following herbicide application. Increasing <i>δ</i><sup>13</sup>C values of
S-metolachlor and acetochlor by more than 2 ‰ during the study
period indicated herbicide degradation. To assist the interpretation of these
data, discharge, concentrations, and <i>δ</i><sup>13</sup>C values of
S-metolachlor were modelled with a conceptual mathematical model using the
transport formulation by travel-time distributions. Testing of different
model setups supported the assumption that degradation half-lives (DT50)
increase with increasing soil depth, which can be straightforwardly
implemented in conceptual models using travel-time distributions. Moreover,
model calibration yielded an estimate of a field-integrated isotopic
enrichment factor as opposed to laboratory-based assessments of enrichment
factors in closed systems. Thirdly, the Rayleigh equation commonly applied in
groundwater studies was tested by our model for its potential to quantify
degradation on catchment scale. It provided conservative estimates on the
extent of degradation as occurred in stream samples. However, largely
exceeding the simulated degradation within the entire catchment, these
estimates were not representative of overall degradation on catchment scale.
The conceptual modelling approach thus enabled us to upscale sample-based
CSIA information on degradation to the catchment scale. Overall, this study
demonstrates the benefit of combining monitoring and conceptual modelling of
concentration and CSIA data and advocates the use of travel-time
distributions for assessing pesticide fate and transport on catchment scale. |
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| ISSN: | 1027-5606 1607-7938 |