GW10B: Ground Water Protection List Monitoring for Oryzalin
Abstract
Oryzalin, an active ingredient on the Ground Water Protection List (GWPL), was selected for well monitoring because of its threat to ground water as predicted by a prioritization scheme that is based on use intensity and modeling results simulating movement of pesticides to ground water. Well selection was focused on the heaviest oryzalin use in sections of land that were considered vulnerable to offsite movement of pesticide residues. From December 2010 to April 2011, the Department of Pesticide Regulation (DPR) sampled 41 wells in Fresno, Kern, Madera, Merced, San Joaquin, and Tulare counties for oryzalin and several other herbicides previously found in California ground water by DPR (hexazinone, tebuthiuron, simazine, bromacil, prometon, atrazine, norflurazon, and diuron). DPR did not detect oryzalin in the sampled wells, yet several of the other monitored pesticides were detected in 18 wells. Oryzalin use in most of the sections containing these 18 wells and in the sections containing the remaining 23 wells was substantially heavier than the use of the pesticides that were detected. Furthermore, compared to the detected pesticides, there were no unique oryzalin use patterns that could account for its lack of detection in ground water. This suggested that it was oryzalin’s physical/chemical properties preventing its movement to ground water. Compared to the detected pesticides oryzalin has a lower aqueous solubility, higher potential for soil adsorption, and faster dissipation in soil under both aerobic and anaerobic conditions. From the results of this current study and the lack of oryzalin detections in previous ground water monitoring studies conducted by DPR, it is unlikely that oryzalin is a threat to California ground water under its current labeled use directions and use patterns.
The GWPL prioritization scheme overestimated oryzalin’s threat to ground water, most likely from over-weighting its use intensity component, which for oryzalin has historically been relatively high. Partitioning of greater weighting to the scheme’s modeling component, which relies largely on the chemical’s physical/chemical properties would have reduced oryzalin’s predicted threat to ground water. Further ground water monitoring of other pesticides and investigations into reweighting of the various ranking components in the prioritization scheme will yield more insight into improving the scheme’s predictive capabilities.