The pyrethroids are all strongly hydrophobic and therefore are found associated primarily with bed sediment after entry into water bodies. Therefore, their persistence and phase distribution in sediment greatly influences their fate and effects. This chapter provides an up-to-date review of data on the persistence and partitioning of pyrethroids in sediment. Information from recent studies is summarized, and half-lives (T, 1/2 ), K OC and K DOC values are tabulated. Pyrethroids display differing persistence in sediment, with bifenthrin being more persistent than the other compounds. However, the bioavailable concentrations of pyrethroids decrease quickly in sediment due to the aging effect, with bioavailable T,1/2 < 2 months, suggesting diminishing toxicity over time. K OC S from earlier literature may have been underestimated due to incomplete phase separation. Recent studies using selective methods such as solid phase microextraction show that K OC S and K DOC S of pyrethroids are in the 10 6 range, with K DOC S a few times smaller than K OC . We also identify information gaps that may serve as topics for future research.

The freely dissolved concentration (C-free) in porewater can be used to improve prediction of sediment toxicity by pyrethroids. We used solid-phase microextraction (SPME) to analyze C-free of eight pyrethroids in sediment porewater. External calibration was applied to obtain C-free of chemicals, whereas internal calibration with C-13-cis-permethrin was used to determine total concentration (C-w). Total porewater concentration measured by using the isotopic-SPME method was well correlated with data obtained by exhaustive liquid-liquid extraction (LLE). Method detection limits (MDLs) of the SPME methods were lower than the 10(th) percentile of the reported LC50s for aquatic invertebrates, with relative standard deviation < 20%. The SPME method was further used on field contaminated samples. Measuring C-free by SPME may represent a good alternative to the estimation of total or OC normalized sediment concentrations for predicting sediment toxicity from pyrethroid contamination.

Pyrethroids are widely used insecticides in both agricultural and urban environments, and their potential movement to surface streams and toxicity to susceptible aquatic species is an emerging concern. Natural surface waters usually contain low levels of dissolved organic matter (DOM). Limited data have shown that DOM preparations can significantly alter the bioavailability and toxicity effects of pyrethroids. However, the importance of these effects in natural waters has not been investigated. In this study we measured uptake and acute toxicity of permethrin and cyfluthrin by Daphnia species in 15 surface water samples. Low levels of DOM (3-20 mg L(-1)) inhibited cyfluthrin uptake by Daphnia magna and acute toxicity to Ceriodaphnia dubia in most samples. For permethrin, the effects of DOM on bioavailability and toxicity were generally not significant. The effects of DOM on bioavailability of cyfluthrin could not be explained from the DOC concentration alone, suggesting that properties of DOM were also important in regulating bioavailability. Regression of K DOC with selected DOM properties revealed significant dependence of K DOC on the carboxylic acid content of DOM. Moreover, concentrations sensed by solid-phase microextraction (SPME) fibers were well correlated with the observed changes in bioaccumulation by D. magna and acute toxicity to C. dubia. Therefore, selective sampling methods such as SPME may be used for measuring the bioavailable concentrations of pyrethroids in waters with naturally occurring DOM levels and predicting the actual toxicity effects.

A semi-empirical probabilistic transport model was developed to simulate simazine and diuron well water concentrations in an agriculturally intensive coarse soil region of Fresno County, California. Model inputs included five random variables: the organic carbon normalized soil adsorption coefficient, root zone degradation, application rate, depth to ground water, and ground water recharge age. Transport was simulated in two phases: initial transport through the root zone using the mechanistic model LEACHM, and a second empirically-based phase that simulated transport from the bottom of the root zone to the water table, and ultimately to domestic water wells. Best-fit calibration estimates for combined deep vadose/shallow ground water degradation half-lives were 330 d and 455 d for simazine and diuron, respectively. Simulations based on these fitted half-lives yielded coefficients of determination and root mean square errors of 0.973 and 0.993, and 0.367 and 0.364 for observed vs predicted simazine and diuron concentration percentiles, respectively. The calibrated model output also described the observed relationships of decreasing detection frequency and decreasing concentration with increasing depth to ground water. A novel contribution is the calculation of regional mass budgets for the herbicides. In modeling results, 9–54% and 2–16% (10th–90th percentiles) of applied simazine and diuron, respectively, leached out of the 1.5 m root zone, while the upper 10th percentile of well water concentrations corresponded to approximately 1–2% of application for both pesticides.

Synthetic pyrethroids have been detected in recent California surface water monitoring. Filtration is avoided during sample workup because pyrethroids are extremely hydrophobic, tending to sorb to most surfaces. The resultant analytical pyrethroid concentrations reflect both dissolved pyrethroid and pyrethroid associated with suspended sediment in the water column. Such “whole-water” analytical data are not directly comparable to aquatic acute toxicity effect concentrations measured in laboratory sediment-free water. Consequently, any potential aquatic toxicity risk is indeterminate. In this study a simple probabilistic model was developed to allow a screening-level assessment of pyrethroid whole-water monitoring data. The results suggest that water column toxicity of pyrethroids is possible in California's agriculturally-dominated tributaries, and indicate that additional monitoring to better characterize pyrethroid water-column concentrations are warranted. Model refinement will depend on future work that more firmly establishes the relationship between pyrethroid partitioning and bioavailability, and that addresses the potential influence of dissolved organic carbon on pyrethroid sorption and bioavailability.

Transport of pesticides by surface runoff during rainfall events is a major process contributing to pesticide contamination in rivers. This study presents an empirical regression model that describes pesticide loading over time in the Sacramento and San Joaquin Rivers. The empirical model is physically based, but uses highly aggregated parameters, allowing the prediction of pesticide loading with the knowledge of precipitation and pesticide use only. The model was applied to analyze pesticide monitoring data obtained from the two California rivers during various 1991-2000 winter storm seasons, and closely simulated loading dynamics of the pesticides for six out of seven cases studied, which involved four pesticides identified based on historical sampling results with a detection frequency of ≥10%. The coefficients of determination for regression ranged from 0.167 to 0.907, all were significant at <0.001. The unresolved discrepancy between the model and data may be attributable to a number of sources including limitations related to sampling, laboratory analysis, pesticide use reporting as well as model formulation etc. The accuracy of the model predictions, however, are well within the limits of the expected model performance, given the time and spatial scales of the data analyzed. The results of this study provide strong evidence that precipitation and pesticide use are the two major environmental variables dictating the dynamics of pesticide transport into surface water in these watersheds. The capability of the statistical model to provide time-series estimates on pesticide loading in rivers is unique and may be useful for Total Maximum Daily Load (TMDL) assessments.

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.

Simazine, diuron, and bromacil are the most frequently detected currently registered pesticides in California groundwater. These herbicides have been used for several decades in Fresno and Tulare counties, California; however, previous data are inadequate to determine whether the detections are a result of recent or historical applications (i.e., within the last decade, or 20-30 yr ago). Chlorofluorocarbon (CFC) groundwater age-dating was used in conjunction,vith one-dimensional transport modeling to address this question. The estimated times between herbicide application and subsequent detection in groundwater samples from 18 domestic wells ranged from 3 to 33 yr; the aggregate data indicate that more than half of the detections are associated with applications in the last decade. The data also suggest that changes in groundwater quality arising from modified management practices will probably not be discernible for at least a decade. A secondary objective of this study was to evaluate the contribution of simazine degradates deethylsimazine (DES; 2-amino-4-rhloro-6- ethylamino-s-triazine) and diaminochlorotriazine (DACT; 2,4-diamino-6-chloro-s-triazine) to total triazine concentrations (defined here as simazine + DES + DACT) in 30 domestic wells. The N-dealkylated s-chlorotriazine degradates DES and DACT substantially contribute to total triazine concentrations in Fresno and Tulare County groundwater, composing 24 to 100% of the total triazines, with a median of 82%, If s-chlorotriazines display a common mode of toxicological action, the prevalence of triazine degradates in water samples found in this and other studies indicate that drinking water standards based on total s-chlorotriazine concentrations may be most appropriate.

One goal of mandated well monitoring in California is to search for residues of active ingredients previously undetected in the state's groundwater. The realization that pesticide residues move into groundwater via several different pathways has led 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 preemergence 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 (DGW) 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, California, 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.