The effect of the runoff size on the pesticide concentration in runoff water and in FOCUS streams simulated by PRZM and TOXSWA.

Within the European Union the exposure of aquatic organisms to pesticides is assessed by simulations with the so-called FOCUS Surface Water Scenarios. Runoff plays an important role in these scenarios. As little is known about the effect of runoff size on the exposure, we investigated the effect of runoff size on the concentration in the runoff water and in streams simulated with the PRZM and TOXSWA models for two FOCUS runoff scenarios. For weakly sorbing pesticides (KF,oc<100Lkg-1) the pesticide concentration in the runoff water decreased exponentially with increasing daily runoff size. The runoff size hardly affected the pesticide concentration in the runoff water of strongly sorbing pesticides (KF,oc≥1000Lkg-1). For weakly sorbing pesticides the concentration in the FOCUS stream reached a maximum at runoff sizes of about 0.3 to 1mm. The concentration increased rapidly when the runoff size increased from 0 to 0.1mm and gradually decreased when runoff exceeded 1mm. For strongly sorbing pesticides the occurrence of the maximum concentration in the stream is clearly less pronounced and lies approximately between 1 and 20mm runoff. So, this work indicates that preventing small runoff events (e.g. by vegetated buffer strips) reduces exposure concentrations strongly for weakly sorbing pesticides. A simple metamodel was developed for the ratio between the concentrations in the stream and in the runoff water. This model predicted the ratios simulated by TOXSWA very well and it demonstrated that (in addition to runoff size and concentration in runoff) the size of the pesticide-free base flow and pesticide treatment ratio of the catchment determine the stream concentration to a large extent.

Monitoring and risk assessment of pesticides in irrigation systems in Debra Zeit, Ethiopia.

Since Ethiopia is going through a rapid transformation of its agricultural sector, we assessed the human health and environmental risks due to the past use of organochlorine pesticides (OCPs) as well as the risks of the current pesticide use by farmers. A monitoring programme and risk assessment was carried out for the Wedecha-Belbela irrigation system in the Debra Zeit area. The Wedecha and Belbela rivers and adjacent temporary ponds were sampled and examined for the presence of OCPs between August and October 2014, while data on the current pesticide use by small- and large-scale farmers was collected by interviews. The usage patterns were evaluated for risks of using the river or temporary ponds as source of drinking water and for risks for the aquatic ecosystems in the river and ponds with the aid of the PRIMET_Registration_Ethiopa_1.1 model. The samples were collected in five sampling periods, and results indicate that most of the 18 target OCPs were not detected above the detection limit, while g-chlordane may pose chronic risks when surface water is used as drinking water. Endosulfan and heptachlor pose risks to aquatic organisms at second-tier level, while for heptachlor-epoxide B, g-chlordane and b-BHC only risks could be determined at the first tier due to a lack of data. For all nine pesticides used by small-scale farmers the calculated acute risks to humans were low. Second tier risk assessment for the aquatic ecosystem indicated that lambda-cyhalothrin, endosulfan, profenofos, and diazinon may pose high risks.

Surface water risk assessment of pesticides in Ethiopia.

Scenarios for future use in the pesticide registration procedure in Ethiopia were designed for 3 separate Ethiopian locations, which are aimed to be protective for the whole of Ethiopia. The scenarios estimate concentrations in surface water resulting from agricultural use of pesticides for a small stream and for two types of small ponds. Seven selected pesticides were selected since they were estimated to bear the highest risk to humans on the basis of volume of use, application rate and acute and chronic human toxicity, assuming exposure as a result of the consumption of surface water. Potential ecotoxicological risks were not considered as a selection criterion at this stage. Estimates of exposure concentrations in surface water were established using modelling software also applied in the EU registration procedure (PRZM and TOXSWA). Input variables included physico-chemical properties, and data such as crop calendars, irrigation schedules, meteorological information and detailed application data which were specifically tailored to the Ethiopian situation. The results indicate that for all the pesticides investigated the acute human risk resulting from the consumption of surface water is low to negligible, whereas agricultural use of chlorothalonil, deltamethrin, endosulfan and malathion in some crops may result in medium to high risk to aquatic species. The predicted environmental concentration estimates are based on procedures similar to procedures used at the EU level and in the USA. Addition of aquatic macrophytes as an ecotoxicological endpoint may constitute a welcome future addition to the risk assessment procedure. Implementation of the methods used for risk characterization constitutes a good step forward in the pesticide registration procedure in Ethiopia.

Effects of linuron on a rooted aquatic macrophyte in sediment-dosed test systems.

Effects of linuron on the sediment-rooted aquatic macrophyte Myriophyllum spicatum L. were studied in sediment-dosed test systems following a proposed guideline with extended test duration. Sediment, pore water, overlying water and macrophyte shoots were sampled weekly for chemical analyses. Linuron was stable in the sediments. Sediment and pore water concentrations were in equilibrium after 48 h. Overlying water concentrations increased over time, but did not reach equilibrium with pore water concentrations and were 100 times lower. Mass balances showed a rapid uptake of linuron by macrophyte roots. Known pathways and the compound's properties support the conclusion that Myriophyllum takes up linuron from pore water directly through the roots. Hence, effects on macrophytes in this type of sediment toxicity test should be expressed in terms of pore water concentrations. Pore water concentration is the most relevant parameter for describing effects on macrophytes.

Estimating degradation rates in outdoor stagnant water by inverse modelling with TOXSWA: a case study with prosulfocarb.

BACKGROUND: The regulatory risk assessment of pesticides requires the assessment of exposure of aquatic ecosystems in small surface waters adjacent to agricultural fields. This exposure is predicted using simulation models, for which an important input parameter is the degradation rate in water. In regulatory dossiers, the decline rate in water from outdoor mesocosms is often available, but this rate encompasses more processes than degradation. Therefore, a procedure was designed for estimating the degradation rate in water that was suitable for mesocosm studies with limited datasets, e.g. datasets lacking site-specific sorption coefficients and relevant sediment properties. The procedure, based upon inverse modelling with TOXSWA, has been tested on a dataset for prosulfocarb in stagnant ditches. RESULTS: A degradation half-life in the ditch water of 2.9 days (20 °C) was found. This short half-life was to a significant extent accounted for by biodegradation rather than hydrolysis or photolysis. This half-life was considerably shorter than the half-life in the water layer of two laboratory water-sediment system experiments. CONCLUSION: The estimation procedure resulted in a unique half-life for the degradation rate in water. Such outdoor mesocosm studies seem to be better suited to assessing the degradation rate in water in ditches than the conventional water-sediment studies.