Results for Study GW08: Ground Water Protection List Monitoring for Tebuthiuron
Abstract
Tebuthiuron is a broad-spectrum herbicide with a high potential to leach through soil to ground water based on its physical-chemical properties. It is used primarily on rights-of-way and compacted soil surfaces prior to paving (pre-paving). Use tends to be concentrated in Southern California and, from 1996 to 2005, appears to be increasing in this area. The Department of Pesticide Regulation’s (DPR’s) Environmental Monitoring Branch (EMB) staff sampled wells from April to June 2008 to determine whether tebuthiuron has migrated to ground water in California counties with high reported use. Tebuthiuron was detected at quantifiable levels in four of the 59 wells sampled. Concentrations ranged from 0.052 to 0.142 µg/L. Two of these detections were located in adjacent sections of Los Angeles County. The other two detections were located in Solano and San Diego Counties. Four additional wells had trace detections of tebuthiuron. The U.S. Environmental Protection Agency (U.S. EPA) has established a lifetime health advisory (HAL) of 500 µg/L for tebuthiuron in drinking water.
The two detections of tebuthiuron within a one square mile area in Los Angeles County meets the minimum criteria established by DPR to consider these detections the result of legal agricultural applications. However, the reported use of tebuthiuron confounds this assessment and does not completely support the source as agricultural. Tebuthiuron is only used as a pesticide in California; it has no other uses and is not a breakdown product of any other chemical. Although it is clear that use of this registered pesticide was reported in the counties where ground water detections were found, under the current reporting system it is not possible to determine whether rights-of-way, pre-paving applications, or both caused the detections. Pesticide applications to railroad and utility rights-of-way are considered nonproduction agricultural use whereas pre-paving applications are not considered agricultural use. The two detections in adjacent sections of Los Angeles have railroad tracks on each side within a third of a mile and there is a history of legal tebuthiuron applications to Los Angeles railroad rights-of-way, so it is possible that these detections are from legal agricultural use. In areas with both agricultural and nonagricultural use of tebuthiuron it will always be a struggle to definitively determine which use pattern contributed to the detections. The potential contributions from pre-pavement applications of persistent herbicides, including tebuthiuron, should be studied more carefully. Regardless of the source, if tebuthiuron use continues to increase, it is probable that additional tebuthiuron residues will be detected in ground water. Therefore DPR staff will continue to monitor for tebuthiuron in ground water to better assess the source of any future detections.