A model-based assessment of the potential use of compound-specific stable isotope analysis in river monitoring of diffuse pesticide pollution

摘要

Compound-specific stable isotope analysis (CSIA) has, in combination withmodel-assisted interpretation, proven to be a valuable approach to quantifythe extent of organic contaminant degradation in groundwater systems. CSIAdata may also provide insights into the origin and transformation of diffusepollutants, such as pesticides and nitrate, at the catchment scale. WhileCSIA methods for pesticides have increasingly become available, they havenot yet been deployed to interpret isotope data of pesticides in surfacewater. We applied a coupled subsurface-surface reactive transport model(HydroGeoSphere) at the hillslope scale to investigate the usefulness ofCSIA in the assessment of pesticide degradation. We simulated the transportand transformation of a pesticide in a hypothetical but realistictwo-dimensional hillslope transect. The steady-state model resultsillustrate a strong increase of isotope ratios at the hillslope outlet,which resulted from degradation and long travel times through the hillslopeduring average hydrological conditions. In contrast, following an extremerainfall event that induced overland flow, the simulated isotope ratiosdropped to the values of soil water in the pesticide application area. Theseresults suggest that CSIA can help to identify rainfall-runoff events thatentail significant pesticide transport to the stream via surface runoff.Simulations with daily rainfall and evapotranspiration data and onepesticide application per year resulted in small seasonal variations ofconcentrations and isotope ratios at the hillslope outlet, which fell withinthe uncertainty range of current CSIA methods. This implies a goodreliability of in-stream isotope data in the absence of transport viasurface runoff or other fast transport routes, since the time of measurementappears to be of minor importance for the assessment of pesticidedegradation. The analysis of simulated isotope ratios also allowedquantification of the contribution of two different reaction pathways(aerobic and anaerobic) to overall degradation, which gave further insightinto the transport routes in the modelled system. The simulations supportedthe use of the commonly applied Rayleigh equation for the interpretation ofCSIA data, since this led to an underestimation of the real extent ofdegradation of less than 12% at the hillslope outlet. Overall, thisstudy emphasizes the applicability and usefulness of CSIA in the assessmentof diffuse river pollution, and represents a first step towards atheoretical framework for the interpretation of CSIA data in agriculturalcatchments.