How a complete pesticide screening changes the assessment of surface water quality.

A comprehensive assessment of pesticides in surface waters is challenging due to the large number of potential contaminants. Most scientific studies and routine monitoring programs include only 15-40 pesticides, which leads to error-prone interpretations. In the present study, an extensive analytical screening was carried out using liquid chromatography-high-resolution mass spectrometry, covering 86% of all polar organic pesticides sold in Switzerland and applied to agricultural or urban land (in total 249 compounds), plus 134 transformation products; each of which could be quantified in the low ng/L range. Five medium-sized rivers, containing large areas of diverse crops and urban settlements within the respective catchments, were sampled between March and July 2012. More than 100 parent compounds and 40 transformation products were detected in total, between 30 and 50 parent compounds in each two-week composite sample in concentrations up to 1500 ng/L. The sum of pesticide concentrations was above 1000 ng/L in 78% of samples. The chronic environmental quality standard was exceeded for 19 single substances; using a mixture toxicity approach, exceedances occurred over the whole measurement period in all rivers. With scenario calculations including only 30-40 frequently measured pesticides, the number of detected substances and the mixture toxicity would be underestimated on average by a factor of 2. Thus, selecting a subset of substances to assess the surface water quality may be sufficient, but a comprehensive screening yields substantially more confidence.

Estimating catchment vulnerability to diffuse herbicide losses from hydrograph statistics.

The prediction of diffuse herbicide losses to surface waters using process-based models is data and time demanding. There is a need for simpler and more efficient catchment screening tools. We developed and tested a new proxy for screening catchments for their vulnerability to diffuse herbicide losses relying on widely available river flow data only. The proxy combines the fast flow index (FFI) (i.e., the long-term average proportion of fast flow to total discharge) and the fast flow volume (FFVs) (i.e., the stormflow component of the hydrograph during the spring flush period). We tested the proxy and the underlying hypotheses by regression analyses of existing high-frequency, multiannual monitoring datasets of atrazine, metolachlor, alachlor, and acetochlor concentrations from six US and European catchments. The percentages of applied amounts of three of the four herbicides lost to surface water were positively correlated with the FFV, and the slope of the correlation was positively correlated with the FFI. Based on the empirical data, we derived quantitative proxies to estimate atrazine and metolachlor losses based on the measured FFI and FFV values. The application to 65 European catchments revealed that many of them had a lower vulnerability than the six test catchments. The observed FFI dependency of the slope seems reasonable because with increasing FFI less rain is needed to trigger fast flow and more herbicide is available in the topsoil. The proposed FFI-FFVs proxy can guide researchers and authorities in selecting monitoring areas, setting monitoring results into context, and prioritizing mitigation strategies according to catchment vulnerability.

Comparison of atrazine losses in three small headwater catchments.

Understanding the processes causing herbicide transport to surface waters is crucial to determine mitigation options to reduce these losses. To this end, we investigated the atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) transport in three agricultural catchments (1.1-2.1 km2) in the watershed of Lake "Greifensee" (Switzerland). In 1999, atrazine application data were recorded for all three catchments. Time proportional samples were taken at a high temporal resolution at the catchment outlets. Extremely wet conditions caused large relative losses from the catchments, ranging between 0.6 and 3.5% of the amount applied. Most of the atrazine load was due to event-driven diffuse losses from the fields. Farmyard runoff contributed less but caused the highest concentrations (up to 31 microg L(-1)) in the brooks. The maximum concentrations due to diffuse losses varied between 1.2 and 8.2 microg L(-1) among the catchments. Despite different absolute concentration levels, the concentration time-series were very similar. It seems that the travel-times within the catchments were mainly controlled by the rainfall pattern with little influence of the catchment properties. These properties, however, caused the relative losses to vary by a factor of 6 between the catchments. This variability could be partly explained by differences in the connectivity of the fields to the brooks and by their hydrological soil properties. A comparison of the losses from the three catchments with those from the entire watershed of Lake Greifensee demonstrated that they were representative for the larger area. Hence, the study results provide a good data set to evaluate distributed models predicting herbicide losses.

Competitive sorption of protons and metal cations onto kaolinite: experiments and modeling.

Competitive sorption of protons, Cu, and Pb onto kaolinite (KGa-2) was investigated over wide concentration ranges and quantitatively described using three different models based on surface complexation and cation exchange reactions. In all models, two types of binding sites were assumed for kaolinite: edge sites (SOH0.5-) with pH-dependent charge and face sites (X-) with permanent negative charge. In a first step, proton sorption was measured by potentiometric acid-base titrations of kaolinite dispersed in 0.01, 0.03, and 0.1 M NaNO3 electrolyte solutions. The acid-base titration data were fitted to obtain site densities and protonation constants for the edge and face sites, respectively. In a second step, the sorption of Cu and Pb onto kaolinite was investigated at fixed pH values by metal titration using ion-selective electrodes for Cu2+ and Pb2+, respectively, and by independent batch sorption experiments. Our metal sorption data cover a range of pH 4-8 for Cu and pH 4-6 for Pb, three different ionic strengths (0.01, 0.03, and 0.1 M NaNO3), and up to eight orders of magnitude in free metal ion activity. An additional experiment was conducted to explore the sorption competition between Cu and Pb. In all three models, sorption of protons and metal cations to the edge sites of kaolinite was described with a 1-pK basic Stern (BS) approach. The three models differed only in the description of cation sorption to the face sites. In the first model (BS/GT), we used a Gaines-Thomas (GT) cation exchange equation for the face sites. This model yielded a satisfactory description of Cu sorption, but failed to describe Pb sorption isotherms at pH 4, 5, and 6. In the second model (BS/BS), we replaced the Gaines-Thomas equation by a basic Stern surface complexation formulation, thereby introducing electrostatic terms for sorption to face sites and allowing for free binding sites X-. This did not improve the fits of Cu or Pb sorption to kaolinite, however. In the third model (BS/BS(ext)), we extended the BS/BS-model by introducing additional monodentate sorption complexes at face sites (XCu+ and XPbNO3). This model described both Cu and Pb sorption very well over the entire range in metal concentrations and pH. It also correctly predicted the competitive effect of Pb on sorption of Cu. Model calculations with all three models suggested that Cu and Pb were sorbed mainly to face sites at low pH, while sorption to edge sites dominated at high pH values.

Simultaneous assessment of sources, processes, and factors influencing herbicide losses to surface waters in a small agricultural catchment.

To take appropriate measures to minimize agricultural herbicide inputs into surface waters, detailed knowledge is required about all the factors that control the losses of a given compound from point sources (i.e., farmyards) as well as from the diffuse sources (i.e., the fields) within a given catchment. In this and in a companion paper, we present the results of a comprehensive field study, in which the temporal and spatial variability of the losses of three herbicides (atrazine, dimethenamid, and metolachlor) into the surface waters within a small catchment (2.1 km2) were investigated on different scales (i.e., field scale to whole catchment) after a controlled application of the compounds. In this paper, we discuss the loss dynamics of the three herbicides (and some of their metabolites) from the whole catchment over a period of 67 d after application. An identical mixture of the three herbicides was applied on 13 cornfields within 12 h, allowing for a comparison of their losses under identical meteorological conditions. Thanks to a high temporal sampling resolution, it was possible to distinguish between losses from a farmyard and losses from the fields. Farmyard losses contributed less than 20% to the total loads but caused the highest concentrations. The major herbicide losses from the agricultural fields occurred during the first two rain events after application that led to significant surface runoff and preferential flow into tile drains. In the soils of all fields, dimethenamid declined somewhat faster than atrazine and metolachlor, whereas atrazine was mobilized most effectively to runoff water. Relative losses of the three compounds did not vary by more than a factor of 3 (0.82, 0.27, and 0.41% of the mass applied for atrazine, dimethenamid, and metolachlor, respectively). Highest peak concentrations at the outlet of the catchment were found for atrazine (i.e., approximately 8 microg L(-1) for a short period (<2 h) due to point source losses and between 1 and 3.5 microg L(-1) during more than 24 h due to diffuse losses).

Variability of herbicide losses from 13 fields to surface water within a small catchment after a controlled herbicide application.

Diffuse losses from agricultural fields are a major input source for herbicides in surface waters. In this and in a companion paper, we present the results of a comprehensive field study aimed at assessing the overall loss dynamics of three model herbicides (i.e., atrazine, dimethenamid, and metolachlor) from a small agricultural catchment (2.1 km2) and evaluating the relative contributions of various fields having different soil and topographical characteristics. An identical mixture of the three model herbicides as well as an additional pesticide for identification of a given field were applied within 12 h on 13 cornfields (total area approximately 12 ha), thus ensuring that the herbicides were exposed to identical meteorological conditions. After the simultaneous application, the concentrations of the compounds were monitored in the soils and at the outlets of three subcatchments containing between 4 and 5 cornfields each. Particular emphasis was placed on the two rain events that led to the major losses of the herbicides. The rank orders of herbicide dissipation in the soils and of the compound-specific mobilization into runoff were the same in all three subcatchments and were independent of the field characteristics. In contrast, the field properties caused the relative losses from two subcatchments to differ by up to a factor of 56 during the most important event, whereas compound-specific differences of the three neutral herbicides caused the losses to vary only by a factor of 2 during the same event. The enormous spatial variability was mainly caused by factors influencing the fraction of rain that was lost to surface water by fast transport mechanisms. Thus, the key factors determining the spatially variable herbicide losses were the permeability of the soils, the topography, and the location of subsurface drainage systems. These results illustrate the large potential to reduce herbicide losses by avoiding application on risk areas.