Sample locations and descriptions are given in Table1. Water samples consisted of organic free (A) and four natural waters (B), (C), (D) and (E). Water samples were collected between late April and early May, during a period when OP applications were not scheduled and at a time after which the spring 2000 freshet had passed. Field blanks in this study had frequent but low ng/l observations of dimethoate, and occasional low ng/l observations of azinphos-methyl and diazinon. Spiked levels were corrected for natural amounts prior to the calculation of recovery. Method blanks had no detections for the measured OPs.