Case-study Wark: Pesticide immission situation

Motivation for the choice of the catchment

Model choice and implementation

Monitoring campaign

Synoptic view of monitoring campaigns for the Wark case-study

Motivation for the choice of the catchment


Pesticide immission situation is expected to be most dynamic in catchments which are dominated by surface runoff. Peak concentrations can be expected during precipitation events in application periods. Hence the choice of a case-study should orient towards a catchment with low-infiltrating soils and hillslopes with farming activity on them. Another Luxembourgish specificity is the need to find a catchment that is located entirely on Luxembourg state territory, which allows getting all relevant agricultural data like crop rotations and pesticide use statistics. The Wark catchment in the North of Luxembourg fulfilled all these requirements and has a medium-scale size of 82 km2 that guarantees an averaging of agricultural practices in this type of landscape.  

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Model choice and implementation


M3 uses the distributed hydrological model SWAT with a full description of pesticide fate to simulate emissions from farming plots and routing through the catchment. The hydrological calibration will be performed with the discharge data from the official gauge at the outlet of the catchment. The amount of pesticides used in the catchment will have to be iteratively determined via crop rotation data from plots, general statistics on agricultural pesticide use and passive sampler data. Uncertainties in pesticide modelling will lie mainly in amounts used but also in the pertinence of environmental properties of the compounds in the local soils.

The impact of pesticides will be assessed with the ecotoxicological model Aquatox which will be implemented on different (isolated) segments in the catchment. Pesticide inputs will be provided by SWAT. The model describes biodynamics of key members of the food web and can simulate the acute and chronic effects of pesticides on biota. There is no possibility to verify the effects of pesticides in the field. The simulation results will nevertheless be cross-checked with other evaluation schemes like the SPEAR index and the toxic unit approach.

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Monitoring campaign

Monitoring campaigns have been chosen to address the main aspects of pesticide dynamics in the Wark catchment. A triggered autosampler will monitor flood waves in order to capture surface runoff from the catchment. The autosampler will be located at the catchment outlet and permit the calculation of mass balances. A spatial differentiation of pesticide emissions will be reached through the continuous exposure of passive samplers at key locations (tributaries, confluences) in the catchment. Although passive samplers only provide concentration averages over 14-day exposure time, they will allow to validate pesticide application figures and compound environmental properties (susceptibility for wash out). Base-flow loads have two main sources: Wastewater Treatment Plant (WWTP) emissions or groundwater contributions. WWTP outlets are generally accessible to monitoring with POCIS. Nevertheless it is not possible to cover some of the WWTPs in the catchment because of too low water levels in the outlet which prevent POCIS immersion. Instead the pursued strategy is to get a more general assessment of pesticide emissions in agricultural settings. This will allow applying the data in country-wide applications. For this purpose 5 WWTP outlets in agricultural settings have been selected to be monitored continuously. One of these WWTPs is located in the Wark catchment.

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Synoptic view of monitoring campaigns for the Wark case study

Campaign

Aim

Validation strategy

Location(s)

Parameters

Event-triggered autosampler

Capture pesticide runoff in flood waves

Verify pesticide use and runoff behaviour in the catchment on 12 events

Catchment outlet

Pesticides, Anions, o-PO4, NH4, POC, DOC, part. P

Passive Samplers (POCIS)

River

Continuous monitoring of pesticide immission concentration

Two-week exposure of POCIS provide mean concentrations to verify mass balances

6 sampling locations in the Wark and tributaries

Pesticides

Passive Samplers (POCIS)

WWTPs

Continuous monitoring of pesticide emissions

Mean emission concentrations provide base-flow contribution of WWTPs

5 WWTPs in agricultural settings

Pesticides

River metabolism

Characterize functioning of river segments; provide references for Aquatox

Verify evolution of metabolism metrics (biomass evolution) during the season

2 segments in the lower Wark

Oxygen, nutrients

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