Kean S. Goh

Removal mechanisms and fate of insecticides in constructed wetlands

Constructed wetlands (CWs), along with other vegetative systems, are increasingly being promoted as a mitigation practice to treat non-point source runoff to reduce contaminants such as pesticides. However, studies so far have mostly focused on demonstrating contaminant removal efficiency. In this study, using two operational CWs located in the Central Valley of California, we explored the mechanisms underlying the removal of pyrethroids and chlorpyrifos from agricultural runoff water, and further evaluated the likelihood for the retained pesticides to accumulate within the CWs over time. In the runoff water passing through the CWs, pyrethroids were associated overwhelmingly with suspended solids >0.7 μm, and the sorbed fraction accounted for 38-100% of the total concentrations. The derived K(d) values for the suspended solids were in the order of 10(4)-10(5), substantially greater than those reported for bulk soils and sediments. Distribution of pyrethroids in the wetland sediments was found to mimic organic carbon distribution, and was enriched in large particles that were partially decomposed plant materials, and clay-size particles (<2 μm). Retention of suspended particles, especially the very large particles (>250 μm) and the very fine particles, is thus essential in removing pyrethroids and chlorpyrifos in CWs. Under flooded and anaerobic conditions, most pyrethroids and chlorpyrifos showed moderate persistence, with DT(50) values between 106-353 d. However, the retained pyrethroids were very stable in dry and aerobic sediments between irrigation seasons, suggesting a possibility for accumulation over time. Therefore, the long-term ecological risks of CWs should be further understood before their wide adoption.

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.

Environmental Fate and Toxicology of Carbaryl

Carbaryl is an agricultural and garden insecticide that controls a broad spectrum of insects. Although moderately water soluble, it neither vaporizes nor volatilizes readily. However, upon spray application the insecticide is susceptible to drift. It is unstable under alkaline conditions, thus easily hydrolyzed. Carbaryl has been detected in water at ppb concentrations but degradation is relatively rapid, with 1-naphthol identified as the major degradation product. Indirect and direct photolysis of carbaryl produces different naphthoquinones as well as some hydroxyl substituted naphthoquinones. Sorption of the insecticide to soil is kinetically rapid. However, although both the mineral and organic fractions contribute, because of its moderate water solubility it is only minimally sorbed. Also, sorption to soil minerals strongly depends on the presence of specific exchangeable cations and increases with organic matter aromaticity and age. Soil microbes (bacteria and fungi) are capable of degrading carbaryl; the process is more rapid in anoxic than aerobic systems and with increased temperature and moisture. Carbaryl presents a significant problem to pregnant dogs and their offspring, but some have questioned the applicability of these data to humans. In addition, for toxicokinetic and/or physiological reasons, it has been argued that dogs are more sensitive than humans to carbaryl-induced reproductive or developmental toxicity. However, these arguments are based on either older pharmacokinetic studies or on speculation about possible reproductive differences between dogs on the one hand and rats and humans on the other. In view of the wider evidence from both human epidemiological and laboratory animal studies, the question of the possible developmental and reproductive toxicity of carbaryl should be considered open and requiring further study.

Insecticide Washoff from Concrete Surfaces: Characterization and Prediction

Pesticide runoff from impervious surfaces is a significant cause of aquatic contamination and ecologic toxicity in urban waterways. Effective mitigation requires better understanding and prediction of off-site transport processes. Presented here is a comprehensive study on pesticide washoff from concrete surfaces, including washoff tests, experimental data analysis, model development, and application. Controlled rainfall experiments were conducted to characterize washoff loads of commercially formulated insecticides with eight different active ingredients. On the basis of the analysis of experimental results, a semimechanistic model was developed to predict pesticide buildup and washoff processes on concrete surfaces. Three pesticide product specific parameters and their time dependences were introduced with empirical functions to simulate the persistence, transferability, and exponential characteristics of the pesticide washoff mechanism. The parameters were incorporated using first-order kinetics and Ficks second law to describe pesticide buildup and washoff processes, respectively. The model was applied to data from 21 data sets collected during 38 rainfall events, with parameters calibrated to pesticide products and environmental conditions. The model satisfactorily captured pesticide mass loads and their temporal variations for pesticides with a wide range of chemical properties (log KOW = 0.6-6.9) under both single and repeated (1-7 times) rainfall events after varying set times (1.5 h∼238 days after application). Results of this study suggested that, in addition to commonly reported physicochemical properties for the active ingredient of a pesticide product, additional parameters determined from washoff experiments are required for risk assessments of pesticide applications on urban impervious surfaces.

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.

Diazinon—Chemistry and Environmental Fate: A California Perspective

Diazinon, first introduced in USA in 1956, is a broad-spectrum contact organophosphate pesticide that has been used as an insecticide, and nematicide. It has been ond of the most widely used insecticides in the USA for household and agricultural pest control. In 2004, residential use of diazinon was discontinued; as a result, the total amount applied has drastically decreased. [corrected]. Consequently, the amounts of diazinon applied have been drastically decreased. For example, in California, the amount of diazinon applied decreased from 501,784 kg in 2000 to 64,122 kg in 2010. Diazinon has a K(oc) value of 40-432 and is considered to be moderately mobile in soils. Diazinon residues have been detected in groundwater, drinking water wells, monitoring wells, and agricultural well. The highest detection frequencies and highest percentages of exceedance of the water quality criterion value of 0.1 μg/L have been reported from the top five agricultural counties n California that had the highest diazinon use. Diazinon is transported in air via atmospheric processes such as direct air movement and wet deposition in snow and rain, although concentrations decrease with distance and evaluation from the source. In the environment, diazinon undergoes degradation by several processes, the most important of which is microbial degradation in soils. The rate of diazinon degradation is affected by pH, soil type, organic amendments, soil moisture, and the concentration of diazinon in the soil, with soil pH being a major influencing factor in diazinon degradation rate. Studies indicate tha soil organic matter is the most important factor that influences diazinon sorption by soils, although clay content and soil ph also play an important role in diazinon sorption. Diazinon is very highly to moderately toxic aquatic arganisms, Diazinon inhibits the enzyme acetylcholinesterase, which hydrolyzes the neurotransmitter acetylcholine and leads to a suite of intermediate syndromes including anorexia, diarrhea, generalized weakness, muscle tremors, abnormal posturing and behavior, depression, and health. Differences in metabolism among species and exposure concentrations play a vital role in diazinons bioaccumulation among different aquatic organisms in a wide range of accumulating rates and efficiencies.

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.