Pesticides Research, 86 – Fate of Pyrethroids in Farmland Ponds

Fate of Pyrethroids in Farmland Ponds

PART I : Fate of pyrethroids in farmland ponds. Field studies

4 Introduction, part I

Many pesticides, especially insecticides, are toxic to aquatic life. During application of pesticides they may accidentally reach streams and ponds e.g. by wind drift. To assess potential harm from pesticides to aquatic organisms in farmland ponds it is essential to know the fate of the pesticides in such ponds.

Pyrethroid insecticides have been used widespread in Danish agriculture and they are very toxic to fish, crustacea and aquatic insects. For risk assessment purposes it is therefore important to know the distribution of these compounds in an aquatic ecosystem and their residence time in different parts of the system.

The aim of this study is to investigate the fate of pyrethroid insecticides in small farmland ponds after the pesticides have been applied to the surface of the pond. This includes distribution in the pond, disappearance and bioavailability. The data generated are used to validate a distribution model.

Since pyrethroids are used widespread in Danish agriculture and are relatively persistent in the sediment, the use of natural ponds of the farmland was not considered appropriate. Also since the biological community would be targeted during the experiment it was chosen to establish artificial experimental ponds.

Four ponds of size comparable to smaller ponds in a Danish moraine landscape were excavated in a clay layer. Sediment was established by introducing a layer of topsoil on the bottom of the ponds and on the top of that a layer of sediment from a natural pond. From the natural pond was further transferred some macrophytes. The ponds filled with water during a winter season. From the natural sediment and from immigration a community of various species of plants and animals developed to form an ecosystem.

During the experimental period we measured a series of water quality parameters in order to make sure that the condition of the ponds was close to the condition in natural ponds.

An ecosystem is a complex system built up by biotic and non-biotic elements. To model a system it is necessary to divide it into compartments. This distribution model should include all compartments that are important either because they contribute significantly to the mass balance and/or because they are important as residence for sensitive aquatic organisms.

The most important compartments of pond ecosystems were a priori selected to be:

The surface microlayer of the water phase where spray drifts will enter the pond ecosystem,
The water phase where most of the effected biota reside, and
The sediment phase where pyrethroids are expected to accumulate due to their physical-chemical properties.

Passive sampling of pyrethroids was carried out by aid of semipermeable membrane devices (SPMDs). SPMDs are layflat polyethylene tubes surrounding a neutral lipid (triolein). Uptake of lipophilic compounds into the membranes mimics uptake in aquatic organisms and the uptake may also give a possibility to estimate the concentration of dissolved pesticide in the water phase.

The investigation is reported in two parts. The first part includes field studies while the second part focuses on the distribution model.