The agricultural industry and urban pesticide users are
increasingly relying upon pyrethroid insecticides and shifting
to more potent members of the class, yet little information
is available on residues of these substances in aquatic
systems under conditions of actual use. Seventy sediment
samples were collected over a 10-county area in the
agriculture-dominated Central Valley of California, with
most sites located in irrigation canals and small creeks
dominated by agricultural effluent. The sediments were
analyzed for 26 pesticides including five pyrethroids, 20
organochlorines, and one organophosphate. Ten-day sediment
toxicity tests were conducted using the amphipod
Hyalella azteca and, for some samples, the midge
Chironomus tentans. Forty-two percent of the locations
sampled caused significant mortality to one test species
on at least one occasion. Fourteen percent of the sites (two
creeks and four irrigation canals) showed extreme
toxicity (>80% mortality) on at least one occasion. Pyrethroid
pesticides were detected in 75% of the sediment samples,
with permethrin detected most frequently, followed by
esfenvalerate > bifenthrin >lambda-cyhalothrin. Based
on a toxicity unit analysis, measured pyrethroid concentrations
were sufficiently high to have contributed to the toxicity
in 40% of samples toxic to C. tentans and nearly 70% of
samples toxic to H. azteca. Organochlorine compounds
(endrin, endosulfan) may have contributed to the toxicity
at a few other sites. This study provides one of the first
geographically broad assessments of pyrethroids in
areas highly affected by agriculture, and it suggests there
is a greater need to examine sediment-associated
pesticide residues and their potential for uptake by and
toxicity to benthic organisms.