Carbon and nitrogen isotope analysis of atrazine and desethylatrazine at sub-microgram per liter concentrations in groundwater

Abstract

Environmental degradation of organic micropollutants is difficult to monitor due to their diffuse and ubiquitous input. Current approaches-concentration measurements over time, or daughter-to-parent compound ratios-may fall short, because they do not consider dilution, compound-specific sorption characteristics or alternative degradation pathways. Compound-specific isotope analysis (CSIA) offers an alternative approach based on evidence from isotope values. Until now, however, the relatively high limits for precise isotope analysis by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) have impeded CSIA of sub-microgram-per-liter scale micropollutant concentrations in field samples. This study presents the first measurements of C and N isotope ratios of the herbicide atrazine and its metabolite desethylatrazine at concentrations of 100 to 1,000 ng/L in natural groundwater samples. Solid-phase extraction and preparative HPLC were tested and validated for preconcentration and cleanup of groundwater samples of up to 10 L without bias by isotope effects. Matrix interferences after solid-phase extraction could be greatly reduced by a preparative HPLC cleanup step prior to GC-IRMS analysis. Sensitivity was increased by a factor of 6 to 8 by changing the injection method from large-volume to cold-on-column injection on the GC-IRMS system. Carbon and nitrogen isotope values of field samples showed no obvious correlation with concentrations or desethylatrazine-to-atrazine ratios. Contrary to expectations, however, δ (13) C values of desethylatrazine were consistently less negative than those of atrazine from the same sites. Potentially, this line of evidence may contain information about further desethylatrazine degradation. In such a case, the common practice of using desethylatrazine-to-atrazine ratios would underestimate natural atrazine degradation.

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

“…However, concentration data do not provide clear evidence of degradation processes, since a reduction in concentration might, for example, indicate degradation of the contaminant, changes in the application pattern and amount, or dilution (Battaglin and Goolsby, 1999;Schreglmann et al, 2013). In contrast to degradation, non-destructive processes such as dilution do not reduce the contaminant loads to subsurface and surface water.…”

“…However, except for the identification of nitrate sources, it has so far not been used to examine diffuse pollution at the catchment scale. Similarly, although analytical methods for CSIA of different elements have been developed and applied for several pesticides and pesticide metabolites (Badea et al, 2009;Hartenbach et al, 2008;Meyer et al, 2008;Milosevic et al, 2013;Penning and Elsner, 2007;Reinnicke et al, 2011;Schreglmann et al, 2013), isotopes of pesticides have not yet been continuously monitored and analyzed in rivers.…”

“…Initial soil water concentrations of up to a few milligrams per liter have been reported for pesticides at realistic application rates (Liu et al, 2012), while carbon isotopes of organic contaminants, including the pesticide atrazine and its metabolite desethylatrazine, could be measured at concentrations of around 100 ng L −1 (Jochmann et al, 2006;Schreglmann et al, 2013). Consequently, the difference between pore water concentrations and the detection limit for carbon isotope analysis can be four orders of magnitude.…”

Pesticide fate on catchment scale: conceptual modelling of stream CSIA data

“…Overall, this study demonstrates the benefit of combining monitoring and conceptual modelling of concentration and CSIA data and advocates the use of traveltime distributions for assessing pesticide fate and transport on catchment scale. Goolsby, 1999) or the extent of dilution by pristine water (Schreglmann et al, 2013). Hence, concentration data alone cannot conclusively allow distinction between destructive (i.e.…”

“…In the context of diffuse agricultural pollution, CSIA has mainly been used to distinguish natural from anthropogenic nitrate sources and discern denitrification (Divers et al, 2014;Johannsen et al, 2008;Kellman and HillaireMarcel, 2003;Voss et al, 2006;Wexler et al, 2014). Although CSIA may provide evidence of pesticide degradation (Fenner et al, 2013), CSIA data of pesticides remain restricted to the analysis of isotope fractionation under laboratory conditions (Hartenbach et al, 2008;Meyer and Elsner, 2013;Meyer et al, 2009;Penning et al, 2010;Reinnicke et al, 2011;Wu et al, 2014) and grab samples of groundwater and streamwater (Milosevic et al, 2013;Schreglmann et al, 2013). Degradation of chloroacetanilide herbicides and associated isotope fractionation have been recently studied in lab-scale wetlands (Elsayed et al, 2014), but CSIA of herbicides has not yet been applied on catchment scale to evaluate in situ degradation of pesticides.…”