Modeling effectiveness of two runoff mitigation measures in the Netherlands.

Rainfall that exceeds the soil's maximum infiltration rate is prone to runoff, and the excess rainfall will flow toward open water systems. Nutrients, pesticides or other contaminants may be transported along with this overland flow, thus contaminating surface waters. There are various measures that can be implemented to prevent or reduce runoff, which involve either improving the soil's infiltration capacity or temporarily storing more water at the field scale. The aim of this study was to determine the effectiveness of two mitigation measures, i.e., micro-dams and edge-of-field trenches, in reducing the total number of runoff events and the runoff volume for specific rainfall events. For this purpose, numerical simulations were performed with a deterministic soil-water-atmosphere-plant model for reference situations and for situations involving either of the two mitigation measures. The mitigation measures are implemented as a change in the ponding threshold height above which the model predicts runoff. For this purpose, we considered several soil / groundwater level / crop / intrinsic field soil surface storage situations that are common in the Netherlands. For ridge-furrow cropping systems, micro-dams are more effective than edge-of-field trenches. Depending on the soil type (excluding sand), the minimum effectiveness is 70% and may be >90% in specific situations. For the edge-of-field trench, the reduction in runoff events was mostly in the 24-35% range, while the effectiveness for the runoff volume for a rainfall event that typically occurs once per year was in the 13-48% range (excluding sand). Due to the relatively high hydraulic conductivity at saturation for the sandy soils, runoff was simulated in only a few cases for these soils. The effectiveness was evidently dependent on intrinsic field soil surface storage and soil types, varied slightly between crop types and was very similar across the groundwater level classes considered.

Field test of the TOXSWA pesticide fate model: Comparison of simulated and observed chlorpyrifos in water, sediment and macrophytes in four stagnant ditches.

TOXSWA is a numerical model describing pesticide behavior in an edge-of-field waterbody. It is widely used to predict exposure in regulatory risk assessment for aquatic ecosystems. Exposure concentrations are predicted based upon pesticide process parameters obtained in standardized laboratory experiments. However, few tests of the model performance based on field data have been carried out. We compare simulated concentrations to observations from a field experiment with four shallow stagnant ditches over sprayed with chlorpyrifos, a moderately volatile pesticide with a significant sorption capacity. Input parameters describing the four ditches, such as dimensions, water depth, sediment and macrophyte characteristics were measured in detail. Additionally, laboratory experiments were carried out to determine site-specific values for parameters describing chlorpyrifos degradation in water and sediment, as well as sorption to the two dominant macrophyte species. Based upon these estimated parameters, the correspondence between simulated and measured concentrations in water, sediment and macrophytes is poor. We attribute this discrepancy to a lack of site-specific input for the processes of volatilization and sorption to sediment, which both are important processes for chlorpyrifos. Therefore, we calibrated TOXSWA using the optimization tool PEST. The transfer coefficient for volatilization and the coefficient for sorption to sediment were optimized based on the observed concentrations in water and sediment. This resulted in a substantial improvement of correspondence. Optimized values of the transfer coefficient for volatilization and the coefficient for sorption to sediment are substantially higher than their initial estimates (4-8-fold and 2-4-fold increase, respectively), but can be well explained. The optimized coefficients vary less than a factor 2 between the four ditches. We conclude that TOXSWA can adequately predict chlorpyrifos behavior in the four ditches, provided that reliable site-specific parameter estimates are available. Field tests for other pesticides, waterbodies and agro-environmental conditions are warranted.

Development of surface water exposure scenarios for risk assessment of pesticides in Korea.

Surface water exposure scenarios used in the risk assessment of Korea's aquatic ecosystems, were developed to represent the 90th percentile pesticide exposure situation as a part of the country's pesticide registration procedure. The scenarios are used to estimate the pesticide concentration in the water of a rice paddy and small streams for three protection goals: (i) mudfish in rice paddies, (ii) the aquatic ecosystem of small streams located near rice paddies, and (iii) the aquatic ecosystem of small streams located near fruit orchards. The scenarios were derived taking into account major exposure routes, such as spray drift, runoff, and drainage. The scenarios were parameterized for appropriate models including the pesticide root zone model (PRZM) and the toxic substances in surface waters model (TOXSWA). A total of 17 pesticide compounds and 28 formulated products were selected to test the risk assessment using the developed scenarios. The simulated predicted environmental concentrations (PECs) fully reflected a) the exposure routes for each protection goal b) the use patterns of the products c) physicochemical properties of the pesticides, and d) meteorological conditions of Korea. However, while assessing the risks for aquatic organisms we observed that for most of the selected pesticides the calculated exposure concentrations were higher than the regulatory acceptable concentration (RAC). To implement the exposure scenarios and models for pesticide authorization in Korea, further research on the RACs is needed. We also recommend studies to develop a higher-tier model and risk-mitigation measures that can be applied to the Korean situation.

Dynamics and recovery of a sediment-exposed Chironomus riparius population: A modelling approach.

Models can be used to assess long-term risks of sediment-bound contaminants at the population level. However, these models usually lack the coupling between chemical fate in the sediment, toxicokinetic-toxicodynamic processes in individuals and propagation of individual-level effects to the population. We developed a population model that includes all these processes, and used it to assess the importance of chemical uptake routes on a Chironomus riparius population after pulsed exposure to the pesticide chlorpyrifos. We show that particle ingestion is an important additional exposure pathway affecting C. riparius population dynamics and recovery. Models ignoring particle ingestion underestimate the impact and the required recovery times, which implies that they underestimate risks of sediment-bound chemicals. Additional scenario studies showed the importance of selecting the biologically relevant sediment layer and showed population effects in the long term.

Fate of the insecticide lambda-cyhalothrin in ditch enclosures differing in vegetation density.

Use of the insecticide lambda-cyhalothrin in agriculture may result in the contamination of water bodies, for example by spray drift. Therefore, the possible exposure of aquatic organisms to this insecticide needs to be evaluated. The exposure of the organisms may be reduced by the strong sorption of the insecticide to organic materials and its susceptibility to hydrolysis at the high pH values in the natural range. In experiments done in May and August, formulated lambda-cyhalothrin was mixed with the water body of enclosures in experimental ditches containing a bottom layer and macrophytes (at different densities) or phytoplankton. Concentrations of lambda-cyhalothrin in the water body and in the sediment layer, and contents in the plant compartment, were measured by gas-liquid chromatography at various times up to 1 week after application. Various water quality parameters were also measured. Concentrations of lambda-cyhalothrin decreased rapidly in the water column: 1 day after application, 24-40% of the dose remained in the water, and by 3 days it had declined to 1.8-6.5%. At the highest plant density, lambda-cyhalothrin residue in the plant compartment reached a maximum of 50% of the dose after 1 day; at intermediate and low plant densities, this maximum was only 3-11% of the dose (after 1-2 days). The percentage of the insecticide in the ditch sediment was 12% or less of the dose and tended to be lower at higher plant densities. Alkaline hydrolysis in the water near the surface of macrophytes and phytoplankton is considered to be the main dissipation process for lambda-cyhalothrin.