This report summarizes well sampling protocols, data collection procedures, and analytical results for the presence of pesticides in ground water developed by the California Department of Pesticide Regulation (DPR). Specific well sampling protocols were developed to meet regulatory mandates of the Pesticide Contamination Prevention Act (PCPA) of 1986 and to provide further understanding of the agronomic, chemical, and geographic factors that contribute to movement of residues to ground water. The well sampling data have formed the basis for the DPR's regulatory decisions. For example, a sampling protocol, the Four-Section Survey, was developed to determine if reported detections were caused by nonpoint-source agricultural applications, a determination that can initiate formal review and subsequent regulation of a pesticide. Selection of sampling sites, which are primarily rural domestic wells, was initially based on pesticide use and cropping patterns. Recently, soil and depth-to-ground water data have been added to identify areas where a higher frequency of detection is expected. In accordance with the PCPA, the DPR maintains a database for all pesticide well sampling in California with submission required by all state agencies and with invitations for submission extended to all local and federal agencies or other entities. To date, residues for 16 active ingredients and breakdown products have been detected in California ground water as a result of legal agricultural use. Regulations have been adopted for all detected parent active ingredients, and they have been developed regardless of the level of detection.
One goal of mandated well monitoring in California, U.S.A, is to search for residues of active ingredients previously undetected in the state's groundwater. The realization that pesticide residues move into groundwater via a number of different pathways has lead us to develop an empirical approach to delineate vulnerable areas; major climatic and edaphic features of areas where pesticides residues have been detected in well water have been identified on a geographic basis. The objective of this study was to evaluate the use of our empirical model in a retrospective well sampling study for norflurazon, a pre-emergence herbicide with physical-chemical properties that indicated potential to move offsite with water. In our modeling approach, sections of land, which are 2.59 km 2 areas, were identified as having a greater potential for contamination based on soil and depth-to-groundwater data. Wells were sampled from a subset of these sections where use of norflurazon was historically the greatest. Norflurazon residue was detected in 8 of 43 wells sampled in Fresno County, CA., and in concentrations ranging from 0.07 to 0.69 µg L-1. This result was considered highly successful because residues had not been detected in 18 previous California groundwater studies for other active ingredients, some of which had been detected in other state and federal sampling programs. Location of sampling sites in these previous 18 California studies was based only on pesticide use data. The detections of norflurazon in this study indicated that, even though using an empirical modeling approach appeared to be unorthodox, it enabled us to effectively identify vulnerable areas.
ABSTRACTresult in runoff of residues so improved incorporation of pre-emergence herbicides into the soil is recomPre-emergence herbicide residues were detected in domestic wells mended to reduce concentrations in runoff water. fornia's ground water (Troiano et al., 2000). Pre-emeragricultural field where the soil was a cracking clay to infiltration of gence herbicide residues were detected in seven wells residues in water captured by an adjacent holding pond. Diuron and sampled within a 1554-ha area located near the town hexazinone were applied in December to a 3-yr-old alfalfa (Medicago of Tracy, CA: atrazine was detected in five wells at 0.16 sativa L.) crop. Water content of soil taken after major rainfall but to 2.8 g L Ϫ1, diuron in one well at 0.06 g L Ϫ1, hexazibefore irrigation at 106 d after application was elevated at the lowest none in three wells at 0.051 to 0.11 g L Ϫ1 , and simazine depth sampled centered at 953 mm, indicating water was available (6-chloro-N,NЈ-diethyl-1,3,5-triazine-2,4-diamine) in one for percolation. Herbicide residues (reporting limit 8 g kg Ϫ1 ) were well at 0.098 g L Ϫ1.Tracy is centrally located on the confined above the 152 mm soil depth, even after subsequent applicawestern side of the Central Valley of California (Fig. 1). tion of two border-check surface irrigations. The pattern of distribu-The predominant cropping pattern was a rotation of tion and concentration of residues in the soil were similar to results obtained from the LEACHM model, suggesting that macropore flow alfalfa with corn (Zea mays L.) and bean (Phaseolus was limited to a shallow depth of soil. Herbicide residues were meavulgaris L.). The residues were related to agricultural sured in runoff water at the first irrigation at 20 g L Ϫ1 for diuron applications, especially since the only reported pesti- ment of atrazine in cracking-clay soils in another area of California had been investigated (Graham et al., 1992). That study indicated potential movement of herbicides M ovement of pesticide residues from agricultural into cracks with some residues detected below the plow applications to ground water has been well doculayer, but movement to shallow ground water was not mented (Hallberg, 1989). In California, the approach to confirmed. regulation of pesticides detected in ground water is to Another potential pathway observed during the surallow continued use if management practices can be vey was water-holding ponds located within or near identified that mitigate the threat of contamination. This the cropped fields. The ponds collected runoff water course of action balances economic considerations with generated from rainfall or irrigation events. Through environmental protection. The effectiveness of this apinterviews, the ground water in this area was determined proach relies on elucidating the pathways for movement to be shallow at around 4500 mm. Since the ponds were of residues to ground water with concomitant developbetween 2400 and 3000 mm deep, excavation of the ment of farm management practices that...
The California Department of Pesticide Regulation initiated regulations on pesticide use in 1989 to mitigate groundwater contamination by atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine] and subsequently for simazine (6‐chloro‐N,N′‐diethyl‐1,3,5‐triazine‐2,4‐diamine), diuron [N′‐(3,4‐dichlorophenyl)‐N,N‐dimethylurea], bromacil [5‐bromo‐6‐methyl‐3‐(1‐methylpropyl)‐2,4(1H,3H)‐pyrimidinedione], and norflurazon [4‐chloro‐5‐(methylamino)‐2‐[3‐(trifluoromethyl)phenyl]‐3(2H)‐pyridazinone]. Annual water samples from 2000 to 2012 were obtained from domestic wells in Fresno and Tulare counties in regulated areas designated either as leaching groundwater protection areas (GWPAs), where residues move downward in percolating water, or runoff GWPAs, where residues move offsite in rain or irrigation runoff water to sensitive sites such drainage wells. Concentrations decreased below the reporting limit, so maximum likelihood estimation methodology for left‐censored data was used. Decreasing trends in concentration were measured in both GWPA designations for simazine, its breakdown products desisopropyl atrazine (ACET, 2‐amino‐4‐chloro‐6‐ethylamino‐s‐triazine) and diamino chlorotriazine (DACT, 2,4‐diamino‐6‐chloro‐s‐triazine), and diuron. Bromacil crop use was predominant in runoff GWPAs, where decreases over time were also measured. In contrast, increased trends were observed for norflurazon and its breakdown product desmethyl norflurazon [DMN, 4‐chloro‐5(amino)‐2‐(α,α,α trifluorometa‐tolyl] in runoff GWPAs. Use of simazine, diuron, and bromacil was regulated before norflurazon, so patterns of detection represent a shift to use of unregulated products. For NO3, 22 of 67 wells indicated linear decreases in concentration coinciding with decreases in pesticide residues in those wells. Concentration of ACET, DACT, diuron, and NO3 in well water was two to five times greater when located in runoff GWPAs. Greater amounts of herbicide were applied to crops grown in runoff GWPAs, but high concentrations in runoff water entering ponds or drainage wells could also be a factor for increased well water concentration. Initial regulatory measures appear to have been effective in reducing groundwater concentrations, but continued monitoring is needed to evaluate changes made to the regulatory approach in 2004.
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