Pesticide and Nitrate Trends in Domestic Wells where Pesticide Use Is Regulated in Fresno and Tulare Counties, California.

The California Department of Pesticide Regulation initiated regulations on pesticide use in 1989 to mitigate groundwater contamination by atrazine [6-chloro--ethyl-'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and subsequently for simazine (6-chloro-,'-diethyl-1,3,5-triazine-2,4-diamine), diuron ['-(3,4-dichlorophenyl)-,-dimethylurea], bromacil [5-bromo-6-methyl-3-(1-methylpropyl)-2,4(1,3)-pyrimidinedione], and norflurazon [4-chloro-5-(methylamino)-2-[3-(trifluoromethyl)phenyl]-3(2)-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--triazine) and diamino chlorotriazine (DACT, 2,4-diamino-6-chloro--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 NO, 22 of 67 wells indicated linear decreases in concentration coinciding with decreases in pesticide residues in those wells. Concentration of ACET, DACT, diuron, and NO 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.

Environmental chemistry, ecotoxicity, and fate of lambda-cyhalothrin.

Lambda-cyhalothrin is a pyrethroid insecticide used for controlling pest insects in agriculture, public health, and in construction and households. Lambda-cyhalothrin is characterized by low vapor pressure and a low Henry's law constant but by a high octanol-water partition coefficient (K(ow)) and high water-solid-organic carbon partition coefficient (K(oc)) values. Lambda-cyhalothrin is quite stable in water at pH < 8, whereas it hydrolyzes to form HCN and aldehyde under alkaline conditions. Although lambda-cyhalothrin is relatively photostable under natural irradiation, with a half-life > 3 wk, its photolysis process is fast under UV irradiation, with a half-life < 10 min. The fate of lambda-cyhalothrin in aquatic ecosystems depends on the nature of system components such as suspended solids (mineral and organic particulates) and aquatic organisms (algae, macrophytes, or aquatic animals). Lambda-cyhalothrin residues dissolved in water decrease rapidly if suspended solids and/or aquatic organisms are present because lambda-cyhalothrin molecules are strongly adsorbed by particulates and plants. Adsorbed lambda-cyhalothrin molecules show decreased degradation rates because they are less accessible to breakdown than free molecules in the water column. On the other hand, lambda-cyhalothrin adsorbed to suspended solids or bottom sediments may provide a mechanism to mitigate its acute toxicity to aquatic organisms by reducing their short-term bioavailability in the water column. The widespread use of lambda-cyhalothrin has resulted in residues in sediment, which have been found to be toxic to aquatic organisms including fish and amphipods. Mitigation measures have been used to reduce the adverse impact of lambda-cyhalothrin contributed from agricultural or urban runoff. Mitigation may be achieved by reducing the quantity of runoff and suspended solid content in runoff through wetlands, detention ponds, or vegetated ditches.

Chemistry and fate of simazine.

Simazine, first introduced in 1956, is a popular agricultural herbicide used to inhibit photosynthesis in broadleaf weeds and grasses. It is a member of the triazine family, and according to its physicochemical properties, it is slightly soluble in water, relatively nonvolatile, capable of partitioning into organic phases, and susceptible to photolysis. Sorption and desorption studies on its behavior in soils indicate that simazine does not appreciably sorb to minerals and has the potential to leach in clay and sandy soils. The presence of organic matter in soils contributes to simazine retention but delays its degradation. The primary sorptive mechanism of simazine to OM has been proposed to be via partitioning and/or by the interaction with functional groups of the sorbent. Farming practices directly influence the movement of simazine in soils as well. Tilled fields lower the runoff of simazine when compared to untilled fields, but tilling can also contribute to its movement into groundwater. Planting cover crops on untilled land can significantly reduce simazine runoff. Such practices are important because simazine and its byproducts have been detected in groundwater in The Netherlands, Denmark, and parts of the U.S. (California, North Carolina, Illinois, and Wisconsin) at significant concentrations. Concentrations have also been detected in surface waters around the U.S. and United Kingdom. Although the physicochemical properties of simazine do not support volatilization, residues have been found in the atmosphere and correlate with its application. Although at low concentrations, simazine has also been detected in precipitation in Pennsylvania (U.S.), Greece, and Paris (France). Abiotically, simazine can be oxidized to several degradation products. Although hydrolysis does not contribute to the dissipation of simazine, photolysis does. Microbial degradation is the primary means of simazine dissipation, but the process is relatively slow and kinetically controlled. Some bacteria and fungal species capable of utilizing simazine as a sole carbon and nitrogen source at a fast rate under laboratory conditions have been identified. Metabolism of simazine in higher organisms is via cytochrome P-450-mediated oxidation and glutathione conjugation.

Movement of diuron and hexazinone in clay soil and infiltrated pond water.

Pre-emergence herbicide residues were detected in domestic wells sampled near Tracy, CA. This study sought to determine the source of contamination by comparing soil distribution of diuron [N'-(3,4-dichlorophenyl)-N,N-dimethylurea] and hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] in an agricultural field where the soil was a cracking clay to infiltration of residues in water captured by an adjacent holding pond. Diuron and hexazinone were applied in December to a 3-yr-old alfalfa (Medicago sativa L.) crop. Water content of soil taken after major rainfall but before irrigation at 106 d after application was elevated at the lowest depth sampled centered at 953 mm, indicating water was available for percolation. Herbicide residues (reporting limit 8 microg kg(-1)) were confined above the 152 mm soil depth, even after subsequent application of two border-check surface irrigations. The pattern of distribution and concentration of residues in the soil were similar to results obtained from the LEACHM model, suggesting that macropore flow was limited to a shallow depth of soil. Herbicide residues were measured in runoff water at the first irrigation at 20 microg L(-1) for diuron and 1 microg L(-1) for hexazinone. Runoff water captured in the pond rapidly infiltrated into the subsurface soil, causing a concomitant rise in ground water elevation near the pond. Herbicide residues were also detected in the sampled ground water. We concluded that the pond was the predominant source for movement to ground water. Since addition of a surfactant to the spray mixture did not reduce concentrations in runoff water, mitigation methods will focus on minimizing infiltration of water from the pond.