Analysis of diazinon agricultural use in regions of frequent surface water detections in California, USA.

For five agricultural regions in California, USA, detection frequency of diazinon in surface water and several aspects of its use were determined from recent data (2005-2010): application method, product formulation and primary crops. Diazinon detection frequencies ranged from 10% to 91%. Application method and product formulations used were similar in all regions. The primary crops treated varied from lettuce (77%) in the regions with highest detections frequencies to tree crops (53%) in those with the lowest. The results suggest that the variation in diazinon detection frequencies likely was not due to the application method or formulation type.

Detections of eleven organophosphorus insecticides and one herbicide threatening Pacific salmonids, Oncorhynchus spp., in California, 1991-2010.

California's surface water monitoring results from 1991 through 2010 were analyzed to determine whether 12 organophosphorus insecticides and herbicides (i.e., azinphos methyl, bensulide, dimethoate, disulfoton, ethoprop, fenamiphos, methamidophos, methidathion, methyl parathion, naled, phorate, and phosmet) and their degradates have been detected above maximum concentration limits (MCLs) in Pacific salmonid habitats. Methidathion, methyl parathion, phorate, phosmet, and the oxygen analogue of naled (DDVP) detections exceeded MCLs. Methyl parathion detections may be accounted for by monthly use trends, while methidathion detections may be explained by yearly use trends. There were inadequate phorate, phosmet, or DDVP data to evaluate for correlations with use.

Pesticide washoff from concrete surfaces: literature review and a new modeling approach.

Use of pesticides over impervious surfaces like concrete and subsequent washoff and offsite transport significantly contribute to pesticide detection and aquatic toxicity in urban watersheds. This paper presents a comprehensive study on pesticide washoff from concrete surfaces, including reviews of reported experiments and existing models, development of a new model, and its application to controlled experimental conditions. The existing modeling approaches, mainly the exponential function and power-law function, have limitations in explaining pesticide washoff processes characterized from experimental data. Here we develop a mathematical and conceptual framework for pesticide washoff from concrete surfaces. The new modeling approach was designed to characterize pesticide buildup and washoff processes on concrete surfaces, including the time-dependence of the washoff potential after application and the dynamics in pesticide washoff during a runoff event. One benefit is the ability to integrate and quantify multiple processes that influence pesticide washoff over concrete surfaces, including product formulation, aging effects, multiple applications, and rainfall duration and intensity. The model was applied to experimental configurations in two independent studies, and satisfactorily simulated the measured temporal variations of pesticide washoff loads from concrete surfaces for the five selected pyrethroids in 15 runoff events. Results suggested that, with appropriate parameterization and modeling scenarios, the model can be used to predict washoff potentials of pesticide products from concrete surfaces, and support pesticide risk assessments in urban environmental settings.

Modeling complexity in simulating pesticide fate in a rice paddy.

Modeling approaches for pesticide regulation are required to provide generic and conservative evaluations on pesticide fate and exposure based on limited data. This study investigates the modeling approach for pesticide simulation in a rice paddy, by developing a component-based modeling system and characterizing the dependence of pesticide concentrations on individual fate processes. The developed system covers the modeling complexity from a "base model" which considers only the essential processes of water management, water-sediment exchange, and aquatic dissipation, to a "full model" for all commonly simulated processes. Model capability and performance were demonstrated by case studies with 5 pesticides in 13 rice fields of the California's Sacramento Valley. With registrant-submitted dissipation half-lives, the base model conservatively estimated dissolved pesticide concentrations within one order of magnitude of measured data. The full model simulations were calibrated to characterize the key model parameters and processes varying with chemical properties and field conditions. Metabolism in water was identified as an important process in predicting pesticide fate in all tested rice fields. Relative contributions of metabolism, hydrolysis, direct aquatic photolysis, and volatilization to the overall pesticide dissipation were significantly correlated to the model sensitivities to the corresponding physicochemical properties and half-lives. While modeling results were sensitive to metabolism half-lives in water for all fields, significances of metabolism in sediment and water-sediment exchange were only observed for pesticides with pre-flooding applications or with rapid dissipation in sediment. Results suggest that, in addition to the development of regional modeling scenarios for rice production, the registrant-submitted maximum values for the aquatic dissipation half-lives could be used for evaluating pesticide for regulatory purposes.

Use-exposure relationships of pesticides for aquatic risk assessment.

Field-scale environmental models have been widely used in aquatic exposure assessments of pesticides. Those models usually require a large set of input parameters and separate simulations for each pesticide in evaluation. In this study, a simple use-exposure relationship is developed based on regression analysis of stochastic simulation results generated from the Pesticide Root-Zone Model (PRZM). The developed mathematical relationship estimates edge-of-field peak concentrations of pesticides from aerobic soil metabolism half-life (AERO), organic carbon-normalized soil sorption coefficient (KOC), and application rate (RATE). In a case study of California crop scenarios, the relationships explained 90-95% of the variances in the peak concentrations of dissolved pesticides as predicted by PRZM simulations for a 30-year period. KOC was identified as the governing parameter in determining the relative magnitudes of pesticide exposures in a given crop scenario. The results of model application also indicated that the effects of chemical fate processes such as partitioning and degradation on pesticide exposure were similar among crop scenarios, while the cross-scenario variations were mainly associated with the landscape characteristics, such as organic carbon contents and curve numbers. With a minimum set of input data, the use-exposure relationships proposed in this study could be used in screening procedures for potential water quality impacts from the off-site movement of pesticides.

Vegetated ditches as a management practice in irrigated alfalfa.

The organophosphate (OP) insecticides diazinon and chlorpyrifos have been frequently detected in the San Joaquin River, California, USA. Irrigation tail waters are a significant source of OP pesticides in the watershed. This study tested several management practices for reducing offsite movement of chlorpyrifos to surface water from flood irrigated alfalfa. Management practices evaluated include (1) a constructed, vegetated irrigation tailwater return ditch and (2) increased lag time between chlorpyrifos application and start of flood irrigation. Chlorpyrifos concentrations in whole-water samples of irrigation runoff were variable and ranged from 0.22 microg/l to a maximum of 1.67 microg/l. The median concentration reduction at the end of a 200 m vegetated ditch was 38% compared to 1% in an adjacent conventional tail water ditch. Runoff data collected represented first flush runoff from sets that were irrigated between 48 and 144 h after chlorpyrifos application. There was no consistent effect of irrigation lag time on chlorpyrifos concentration in tailwater for lag times of up to 144 h. Consequently these data indicate that delayed irrigation is not an effective management practice for reducing chlorpyrifos off-site movement to surface water in California flood irrigated alfalfa.