Study GW 09: Ground Water Protection List Monitoring for Metolachlor and Alachlor
Abstract
Metolachlor, primarily used on beans, corn, cotton, and processing tomatoes, is a preemergent herbicide on the Department of Pesticide Regulation’s (DPR’s) Ground Water Protection List (GWPL). Pesticides on the GWPL have the potential to contaminate ground water based on their physical-chemical properties and application methods. DPR annually monitors for pesticides on the GWPL to determine if they have migrated to ground water; metolachlor’s increased, heavy use on processing tomatoes since 2000, in the form of s-metolachlor, made it a candidate for monitoring. Alachlor was also included for monitoring due to previous detections of its degradates in Californian ground water.
From April to December 2009, DPR sampled sixty-eight wells in Kings, Sacramento, San Joaquin, Solano, Stanislaus, and Yolo counties for metolachlor, alachlor, and their respective ethanesulfonic acid and oxanilic acid degradates. Metolachlor and alachlor were not detected in any sample. Metolachlor ethanesulfonic acid (MESA) and metolachlor oxanilic acid (MOXA) were detected in 33 wells (49%) and 12 wells (18%), respectively. MESA concentrations ranged from 0.05 to 2.84 parts per billion (ppb), and MOXA concentrations ranged from 0.05 to 0.53 ppb. Alachlor ethanesulfonic acid (AESA) and alachlor oxanilic acid (AOXA) were detected in 16 wells (23%) and 1 well (2%). Most of the AESA detections occurred in the same wells as the MESA detections. Concentrations of AESA ranged from 0.05 to 1.04 ppb, and the one AOXA detection was 0.06 ppb. While detections of the alachlor and metolachlor degradates continue, the amounts detected are low and, based on current information, do not constitute a significant human health risk from drinking water intake.
Metolachlor and alachlor are mainly used in crop production; the subsequent detection of their degradates in ground water are likely from agricultural use. Metolachlor use shifted from corn, cotton, and beans to processing tomatoes when it was supplanted by s-metolachlor, a resolved isomer mix of metolachlor. Alachlor use has decreased due to the increased planting of Roundup Ready™ corn and large decreases in planted bean acreage. Downward movement of metolachlor and alachlor compounds to ground water might be mitigated by future changes in use patterns and irrigation methods.
As part of our continuous evaluation of registered pesticide products, ground water samples were also analyzed for atrazine, bromacil, diuron, hexazinone, norflurazon, prometon, simazine, tebuthiuron, and several of their degradates. Continued monitoring for these pesticides allows DPR to assess the effectiveness of our ground water protection program, identify new ground water protection areas (GWPAs), and determine if further regulatory action is needed. These pesticides or their degradates were found in 14 wells (20%), with concentrations ranging from 0.052 ppb to 0.302 ppb. Residues of atrazine, simazine, diuron, hexazinone, tebuthiuron metabolite 104, desmethylnorflurazon (DSMN), deethylatrazine (DEA), deisopropylatrazine (ACET), and didealkylated triazine (DACT) were detected. Eight of the fourteen wells were located outside of a GWPA; DPR is evaluating this data and is considering adding these sections to the list of GWPAs.