The risk assessments for the use of wood preservatives proposed by the OECD and used under the Biocidal Products Directive (BPD) (98/8/EC) require the derivation of leaching rates for active substances. These rates are to be used as input data in to agreed exposure scenarios. A comparison of the leaching of active substances from wood treated with a metal free preservative formulation with and without water repellent is presented. The water repellent was selected to have good beading properties rather than conferring dimensional stability. The wood is exposed horizontally in a semi-field test based on the principles of NT Build 509. The volumes of leachates recovered after rain events are analysed to inform a discussion of the action of the water repellent. The leachate analysis data indicate that initially the leaching by natural rainfall is unaffected by the water repellent. However, after approximately 3 weeks (~ 45 mm rain) the leaching of active substances from the wood treated with water repellent is suppressed. After approximately 871mm of natural rainfall, active A shows 25% reduction in leaching and active B shows 24% reduction in leaching. The leaching of active substances from wood in service is an important consideration in environmental risk assessments. These results indicate that the environmental impact of wood preservative active substances can be reduced by the inclusion of water repellents in the formulation.

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