An attempt was made to detect pesticides remaining in river water and
raw water for tap water supply, located in an agricultural area. As a
result, 11 kinds of pesticides were detected. The residual pesticides
in water were also degraded to by-products, since chlorine is a strong
oxidant that does not exist in natural conditions. Organophosphate pe
sticides containing P = S bonds were easily degraded and produced oxon
s (P = O) as a primary by-product. Thiobencarb was also degraded by ch
lorination, producing chlorobenzyl alcohol, chlorotoluene, chlorobenzy
l chloride, chlorobenzoic acid and chlorobenzyl aldehyde as chlorinati
on by-products. Also, when thiobencarb was detected in raw water, the
above by-products were detected in the filtered water in a purificatio
n plant for water supply. Therefore, the management and control of pes
ticides in drinking water and ambient water quality management should
include testing for chlorination by-products.