Dutta et al. found similar re-aeration patterns in a 1 m column experiment in a sand dominated soil, with re-aeration occurring quickly once drying commenced. Even with the presence of a limiting layer, defined by lower pore gas velocities and higher carbon concentration, a sandy loam channel acted as a conduit of O2 into the deep vadose zone maintaining a relatively oxic state and thus decreasing the ability of the vadose zone to denitrify. In systems with higher DOC loadings to the subsurface, oxygen consumption may proceed at higher rates creating sub-oxic conditions in the recharge water and more readily create reducing conditions favorable to denitrification in the subsurface . We note here that microbial growth, which was not modeled in this study, could also affect the rates of O2 consumption and re-aeration, which could lead to underestimation of O2 consumption . Overall, denitrification capacity across different lithologies was shown to depend on the tight coupling between transport,square flower bucket biotic reactions as well as the cycling of Fe and S through chemolithoautotrophic pathways. Under large hydraulic loadings , overall denitrification was estimated to be the greatest as compared to the lower hydraulic loading scenarios .
The main reason for the higher denitrification capacity was the significant decline in O2 concentration estimated for this scenario, whereas such conditions could not be maintained below one meter with lower hydraulic loadings under scenarios S2 and S3. However, nitrate was also transported deeper into the column under S1 as compared to S2 or S3. Tomasek et al. found the reverse in a floodplain setting, where intermittent indundation with flood water, comparable to our S2 and S3 contexts, resulted in higher rates of denitrification in the zone that was always inundated, due to priming of the microbial community and pulse releases of substrates and electron donors. Future studies examining the impact of AgMAR on denitrification should include processes such as mineralization to see if the same behavior would be observed. It seems that there may exist a threshold hydraulic loading and frequency of application that could result in anoxic conditions and therefore promote denitrification within the vadose zone for different stratigraphic configurations, although this was not further explored in this study. In another study, Schmidt et al. found a threshold infiltration rate of 0.7 m d-1 for a three hectare recharge pond located in the Pajaro Valley of central coastal California, such that no denitrification occurred when this threshold was reached. For our simulations, we used a fixed, average infiltration rate of 0.17 cm hr-1 for our all-at-once and incremental AgMAR scenarios, however, application rates can be expected to be more varied under natural field settings. Our results further indicate that the all-at-once higher hydraulic loading, in addition to causing increased levels of saturation and decrease in O2, resulted in leaching of DOC to greater depths in comparison to lower, incremental hydraulic loading scenarios .
Akhavan et al. 2013 found similar results for an infiltration basin wherein 1.4% higher DOC levels were reported at depths down to 4 m when hydraulic loading was increased. Because organic carbon is typically limited to top 1 m in soils , leached DOC that has not been microbially processed could be an important source of electron donors for denitrification at depth. Systems that are already rich in DOC within the subsurface are likely to be more effective in denitrifying, and thus attenuating, NO3 – , such as floodplains, reactive barriers in MAR settings, or potentially, organically managed agroecosystems .This finding can also be exploited in agricultural soils by using cover crop and other management practices that increase soluble carbon at depth and therefore remove residual N from the vadose zone . While lower denitrification capacity was estimated for scenarios S2 and S3, an advantage of incremental application was that NO3 – concentration was not transported to greater depths. Thus, higher NO3 – concentration was confined to the root zone. If NO3 – under these scenarios stays closer to the surface, where microbial biomass is higher, and where roots, especially in deep rooted perennial systems such as almonds, can access it, it could ultimately lead to less NO3 – lost to groundwater. While there is potential for redistribution of this NO3 – via wetting and drying cycles, future modeling studies should explore multi-year AgMAR management strategies combined with root dynamics to understand N cycling and loading to groundwater under long-term AgMAR. Simulation results indicate that wetter antecedent moisture conditions promote water and NO3 – to move deeper into the domain compared to the drier base case simulation.
This finding has been noted previously in the literature, however, disagreement exists on the magnitude and extent to which antecedent moisture conditions affect water and solute movement and is highly dependent on vadose zone characteristics. For example, in systems dominated by macropore flow, higher antecedent soil moisture increased the depth to which water and solutes were transported . In a soil with textural contrast, where hydraulic conductivity between the topsoil and subsoil decreases sharply, drier antecedent moisture conditions caused water to move faster and deeper into the profile compared to wetter antecedent moisture conditions . In our system, where a low-permeability layer lies above a high permeability layer , the reverse trend was observed. Thus, a tight coupling of stratigraphic heterogeneity and antecedent moisture conditions interact to affect both NO3 – transport and cycling in the vadose zone, which should be considered while designing AgMAR management strategies to reduce NO3 – contamination of groundwater. Furthermore, dry and wet cycles affect other aspects of the N cycle that were not included in this study . Specifically, the effect of flood water application frequency on mineralization of organic N to inorganic forms should be investigated to assess the full N loading amount to groundwater under AgMAR. Pyrethroids are the most commonly utilized residential insecticides partly due to the generally held belief that they pose minimal risk to human health. In addition, there are numerous worldwide applications for pyrethroids in agriculture, horticulture, public health and textiles . They have insecticidal activity in their parent form and donot require metabolic activation to exert their neurotoxic effects, which are mediated by increased open time of voltage-gated sodium channels . The initial symptoms of acute occupational pyrethroid intoxication include parasthesia consisting of burning and itching sensations on the skin or dizziness that develops approximately 4–6 h after exposure,black flower bucket although dermal symptoms can manifest after minutes of application. Systemic symptoms can occur up to 48 h after acute exposure . Pyrethroids are classified as Type I or Type II pyrethroids. Type I pyrethroids are esters of primary or secondary alcohols, whereas Type II pyrethroids are esters of secondary alcohols with a cyano group at the -carbon of the alcohol component. The acid andalcohol moieties both contain chiral centers, leading to the possibility of several stereoisomers for each pyrethroid, which may exhibit isomer-specific insecticidal activity . The type II pyrethroid, CM, is derived from the 8- stereoisomers that comprise the pyrethroid cypermethrin, which is one of the most common pyrethroids in agricultural and residential use. CM is a racemate of two cypermethrin stereoisomers: – -cyano-3-phenoxybenzyl–cis-3–2,2-dimethylcyclopropane carboxylate, and – -cyano-3-phenoxybenzyl–cis-3– 2,2-dimethylcyclopropane carboxylate , which are considered the two most stable cis-isomers . The major detoxification pathway of CM is through hydrolysis by esterases and hydroxylation by cytochrome P450s . In vitro studies have shown that alcohol and aldehyde dehydrogenases can also contribute to the metabolism of pyrethroids . Dosing studies with CM and cypermethrin in 6 human volunteers indicate an elimination half-life range of 8–22 h for a single dermal exposure . The assessment of human exposure to insecticides such as pyrethroids is often based on quantification of metabolites excreted in urine . The major urinary metabolites of CM in humans are 3-phenoxybenzoic acid and cis-3–2,2-dimethylcyclopropane carboxylic acid , which are conjugated prior to being excreted in the urine .
3-PBA is a metabolite common to a large number of pyrethroid insecticides, while cisDCCA is a more specific metabolite and useful urinary biomarker of exposure for CM, permethrin and cyfluthrin . Thus, urinary concentrations of 3-PBA may serve as a general biomarker of pyrethroid exposure, while cisDCCA represents a more specific biomarker for human exposure to CM. Currently, there are no published studies specifically assessing the occupational exposure to CM. One pyrethroid study investigated occupational exposure in Chinese cotton workers spraying deltamethrin, fenvalerate and a deltamethrin methamidophos mixture . Hardt and Angerer evaluated occupational exposure in individual workers after applying a mixture of up to 7 synthetic pyrethroids, including CM. Another study described occupational exposure to permethrin and fenvalerate , and a number of other studies have documented general pyrethroid exposure in non-occupational settings utilizing 3-PBA as a general biomarker of pyrethroid exposure . The primary objective of the present study was to investigate occupational exposure to CM by quantitating the daily urinary levels of cis-DCCA and 3-PBA before, during, and after the application of CM in a cohort of Egyptian agriculture workers who were spraying CM on cotton fields daily for up to 10 consecutive days. A biomonitoring study on a subset ofthis Egyptian agriculture worker population determined that 94–96% of the dose was due to dermal exposure . While chlorpyrifos exposure has previously been characterized in these individuals , this is the first study to describe a longitudinal assessment of exposure to CM.A detailed description of the study setting was previously published , and a previous chlorpyrifos bio-monitoring study on a subset of this Egyptian agriculture worker population estimated that 94–96% of the dose was due to dermal exposure . Briefly, the present study was conducted in Menoufia, one of 29 governorates in Egypt, which is located in the Nile River Delta north of Cairo. Daily urine samples were collected from a cohort of 37 Ministry of Agriculture workers which were divided into three job categories: applicators who spray the insecticide on the cotton crop with a backpack mistblower sprayer; technicians who walk the fields with the applicator in order to point out particular areas which need attention; and engineers who direct the work mainly from the edge of the field. These workers were assigned to 3 regions where CM was sprayed daily in 3–5 h work shifts for up to 10 consecutive days in the summer of 2008. The workers provided daily spot urine samples before, during, and after the insecticide application cycle. The urine samples used for analysis were collected just prior to start of the work day, on 24 h intervals at approximately 3 pm. One technician was included in the demographic analysis, but did not provide any urine specimens during the pyrethroid application and was therefore excluded from the current study. Samples were placed on wet ice in a cooler and transported to Menoufia University , where they were stored at −20 ◦C until being shipped to the State University of New York at Buffalo on dry ice for analysis. Creatinine concentrations were measured using the Jaffe reaction ; urinary cis-DCCA and 3- PBA concentrations are expressed as micrograms or nanomoles per gram creatinine. All protocols and questionnaires were approved by Institutional Review Boards of Menoufia University and Oregon Health & Science University, the institute administering the parent grant that funded the field studies.The Type II pyrethroid CM is metabolized by hydrolytic cleavage of the ester bond to form the metabolite cis-DCCA, while the alcohol moiety is further metabolized to 3-PBA likely by oxidative enzymes such as cytochrome P450s and aldehyde dehydrogenase . Urinary levels of cis-DCCA and 3-PBA were measured in the Egyptian cotton field worker to estimate their exposure to CM over the course of the spray schedule . The results show great variability from worker to worker, even within the same job category. The 3 field stations demonstrate 3 distinct exposure scenarios, including different length of spray period and peak metabolite levels during the application. Accordingly, not all applicators were highly exposed, while several individual engineers and technicians were highly exposed. To our knowledge this is first study to conduct a longitudinal assessment of occupational human exposure specifically to CM. This cohort is unique because the workers were applying a single pesticide for up to 10 consecutive days. Similar to the previously reported occupational exposure to chlorpyrifos exposure in this cohort , applicators had the highest levels of urinary metabolites and thus greater exposure to the parent compound, while technicians and engineers had lower exposures.