DISTRIBUTION AND RISK ASSESSMENT OF LM6, DECHLORINATED METABOLITE OF TERBUTHYLAZINE, IN GROUNDWATER

Abstract

During routine monitoring of triazine herbicides in groundwater, used for drinking purposes in an area of Lombardy north of Milan, a new non-chlorinated metabolite of terbutylazine, known by the abbreviation LM6, has been discovered for the first time. Thanks to the development of an analytical method by high resolution HPLC-MS, in 2015 two campaigns devoted to monitor LM6 together with triazine herbicides on groundwaters, used as drinking water supply, and on supplied drinking waters, were carried out. Terbuthylazine and triazinic metabolites desethylterbuthylazine and deisopropylatrazine had always concentrations <0.050 µg L-1 in the raw water, with the 75th percentile ≤ 0.015 µg L-1. LM6 showed higher concentrations with median <0.050 µg L-1 and the 80th percentile <0.1 µg L-1. While the exceedance of 0.1 µg L-1 limit was measured in about 20% of the raw water, the percentage of water samples that exceed the limit after treatment is only 9%. The diffusion of LM6 metabolite in groundwater used for drinking purposes has made necessary to carry out a risk assessment. Since the toxicological and physico-chemical data available are very limited, two different approaches were followed. Following the guideline SANCO/221/2000 to state the relevance of pesticide metabolites, LM6 has been classified as relevant metabolite, for which Directive 98/83/EC on drinking water applies, providing a threshold value of 0.1 µg L-1. The classification as relevant metabolite is due to the fact that terbuthylazine, the parental compound, is classified as carcinogen of category 3, while there are no studies on the carcinogenicity of LM6. Since it is evident that the structure of this metabolite is very different from parental’s one, we applied an alternative methodology, approved by EFSA, called Threshold of Toxicological Concern, by which, even if we apply a conservative approach, a threshold value for drinking water of 3 µg L-1 for LM6 is derived.