Scott R. Yates

Physical factors affecting the transport and fate of colloids in saturated porous media

[1] Saturated soil column experiments were conducted to explore the influence of colloid size and soil grain size distribution characteristics on the transport and fate of colloid particles in saturated porous media. Stable monodispersed colloids and porous media that are negatively charged were employed in these studies. Effluent colloid concentration curves and the final spatial distribution of retained colloids by the porous media were found to be highly dependent on the colloid size and soil grain size distribution. Relative peak effluent concentrations decreased and surface mass removal by the soil increased when the colloid size increased and the soil median grain size decreased. These observations were attributed to increased straining of the colloids; i.e., blocked pores act as dead ends for the colloids. When the colloid size is small relative to the soil pore sizes, straining becomes a less significant mechanism of colloid removal and attachment becomes more important. Mathematical modeling of the colloid transport experiments using traditional colloid attachment theory was conducted to highlight differences in colloid attachment and straining behavior and to identify parameter ranges that are applicable for attachment models. Simulated colloid effluent curves using fitted first-order attachment and detachment parameters were able to describe much of the effluent concentration data. The model was, however, less adequate at describing systems which exhibited a gradual approach to the peak effluent concentration and the spatial distribution of colloids when significant mass was retained in the soil. Current colloid xfiltration theory did not adequately predict the fitted first-order attachment coefficients, presumably due to straining in these systems. INDEX TERMS: 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport

Impact of Fumigants on Soil Microbial Communities

ABSTRACT Agricultural soils are typically fumigated to provide effective control of nematodes, soilborne pathogens, and weeds in preparation for planting of high-value cash crops. The ability of soil microbial communities to recover after treatment with fumigants was examined using culture-dependent (Biolog) and culture-independent (phospholipid fatty acid [PLFA] analysis and denaturing gradient gel electrophoresis [DGGE] of 16S ribosomal DNA [rDNA] fragments amplified directly from soil DNA) approaches. Changes in soil microbial community structure were examined in a microcosm experiment following the application of methyl bromide (MeBr), methyl isothiocyanate, 1,3-dichloropropene (1,3-D), and chloropicrin. Variations among Biolog fingerprints showed that the effect of MeBr on heterotrophic microbial activities was most severe in the first week and that thereafter the effects of MeBr and the other fumigants were expressed at much lower levels. The results of PLFA analysis demonstrated a community shift in all treatments to a community dominated by gram-positive bacterial biomass. Different 16S rDNA profiles from fumigated soils were quantified by analyzing the DGGE band patterns. The Shannon-Weaver index of diversity,H, was calculated for each fumigated soil sample. High diversity indices were maintained between the control soil and the fumigant-treated soils, except for MeBr (H decreased from 1.14 to 0.13). After 12 weeks of incubation, Hincreased to 0.73 in the MeBr-treated samples. Sequence analysis of clones generated from unique bands showed the presence of taxonomically unique clones that had emerged from the MeBr-treated samples and were dominated by clones closely related to Bacillus spp. andHeliothrix oregonensis. Variations in the data were much higher in the Biolog assay than in the PLFA and DGGE assays, suggesting a high sensitivity of PLFA analysis and DGGE in monitoring the effects of fumigants on soil community composition and structure. Our results indicate that MeBr has the greatest impact on soil microbial communities and that 1,3-D has the least impact.

Emission of pesticides into the air

During and after the application of a pesticide in agriculture, a substantial fraction of the dosage may enter the atmosphere and be transported over varying distances downwind of the target. The rate and extent of the emission during application, predominantly as spray particle drift, depends primarily on the application method (equipment and technique), the formulation and environmental conditions, whereas the emission after application depends primarily on the properties of the pesticide, soils, crops and environmental conditions. The fraction of the dosage that misses the target area may be high in some cases and more experimental data on this loss term are needed for various application types and weather conditions. Such data are necessary to test spray drift models, and for further model development and verification as well. Following application, the emission of soil fumigants and soil incorporated pesticides into the air can be measured and computed with reasonable accuracy, but further model development is needed to improve the reliability of the model predictions. For soil surface applied pesticides reliable measurement methods are available, but there is not yet a reliable model. Further model development is required which must be verified by field experiments. Few data are available on pesticide volatilization from plants and more field experiments are also needed to study the fate processes on the plants. Once this information is available, a model needs to be developed to predict the volatilization of pesticides from plants, which, again, should be verified with field measurements. For regional emission estimates, a link between data on the temporal and spatial pesticide use and a geographical information system for crops and soils with their characteristics is needed.

Modeling microbial fate in the subsurface environment

As groundwater is relied upon more and more as a source of drinking water, the practice of recharging groundwater with treated wastewater has become a more attractive means of replenishing depleted groundwater reserves. However, even extensively treated wastewater may contain pathogenic microorganisms, particularly enteric viruses. Once in the soil, these microorganisms, especially the viruses, are capable of migrating considerable distances to reach groundwater, where they pose a potential health hazard to persons consuming the water. Over one half of the waterborne disease outbreaks in the U.S. are due to the use of contaminated groundwater, and microorganisms are the cause of illness in the majority of these outbreaks. Several models have been developed to predict the behavior of microorganisms in the subsurface environment. These models range from conventional, theoretical models of contaminant transport to empirically based models using laboratory and field data. The ultimate goal of the models is to...

Production of methyl bromide by terrestrial higher plants

Methyl bromide (CH3Br) is considered to play an important role in stratospheric ozone depletion, but its sources are currently not well defined. We have observed that Brassica plants can take up Br− from soil, produce CH3Br and release it into the air. Emission of CH3Br was detected from plants grown in natural soils containing 0.06–0.31 mg/kg Br−; the emission increases with increasing soil Br− level. We estimate that cabbage produces 0.4±0.2 Gg/yr, and rapeseed plants 6.6±1.8 Gg/yr. Given the ubiquitous distribution of Br− in soil, CH3Br production by terrestrial higher plants is likely a large source for atmospheric CH3Br.

An analytical solution for one-dimensional transport in porous media with an exponential dispersion function

An analytical solution describing the transport of dissolved substances in heterogeneous porous media with an asymptotic distance-dependent dispersion relationship has been developed. The solution has a dispersion function which is linear near the origin (i.e., for short travel distances) and approaches an asymptotic value as the travel distance becomes infinite. This solution can be used to characterize differences in the transport process relative to both the classical convection-dispersion equation which assumes that the hydrodynamic dispersion in the porous medium remains constant and a dispersion solution which has a strictly linear dispersion function. The form of the hydrodynamic dispersion function used in the analytical solution is , where α(x) = a L[1 − e−bx/L] and is the average pore water velocity. The proposed model may provide an alternate means for obtaining a description of the transport of solutes in heterogeneous porous media, when the scale dependence of the dispersion relationship follows the behavior given by α(x). The overall behavior of the model is illustrated by several examples for constant concentration and flux boundary conditions.

Laboratory study of oxytetracycline degradation kinetics in animal manure and soil.

Oxytetracycline (OTC) is a major member of tetracyclines, which are widely administered to animals in confined feeding operations. To diminish the contamination of OTC in the environment, which results from the application of OTC-containing manure as fertilizer in agricultural lands, OTC degradation kinetics in manure and soil under laboratory aerobic conditions was investigated. OTC degradation kinetics was found to be described well by the previously developed availability-adjusted first-order model at all moistures and low temperatures (<or=25 degrees C). OTC degradation increased with increasing moisture from 60 to 100%. However, OTC became very persistent in water-saturated manure. Increasing temperature greatly accelerated OTC degradation, and thermal degradation became noticeable at high temperatures (>or=35 degrees C) in manure. At 25 degrees C, OTC half-life was determined to be 8.1 days in manure with moisture at 80%, 33 days in manure-amended soil (amendment ratio at 5%), and 56 days in non-amended soil with both moistures at 20%, demonstrating that OTC may become persistent in the environment once it is released from manure into soil. No pronounced effect of coexistent antibiotics on OTC degradation in manure was observed.

Effect of propargyl bromide and 1,3-dichloropropene on microbial communities in an organically amended soil

Abstract In this study we investigated the response of microbial communities in unamended and manure-amended soil treated with the fumigants propargyl bromide (PBr) and 1,3-dichloropropene (1,3-D). The soil fumigants were applied at a rate of 10, 100, and 500 mg kg(-1). After treatment of the soils, the metabolic activity was assessed by monitoring the dehydrogenase activity (DHA). PBr and 1,3-D initially inhibited the DHA when applied at 500 mg kg(-1); however, after 8 weeks, recovery of the DHA only occurred in amended soil. Bacterial community level changes were monitored over a 12-week period after fumigation using denaturing gradient gel electrophoresis of polymerase chain reaction-amplified 16S rDNA fragments. Band numbers were drastically reduced upon application of the fumigants, but reestablished more rapidly in the amended soil. To determine changes in the community diversity, the Shannon-Weaver index of diversity, H, was calculated for all treatments. In unamended and amended soil, the community diversity decreased with increasing fumigant concentration. In the PBr-treated soils, the diversity was higher in amended soil at all concentrations throughout the study, while in the 1,3-D treatments, the results were mixed. At 1, 4, 8, and 12 weeks after fumigation, major bands were excised from the gels and the DNA was cloned for sequence analysis. The bacterial communities in the fumigated amended soils were dominated by Streptomyces spp., other genera of actinomycetales, including Frankia, Cytophagales, Actinomadura, and Geodermatophilus, and a number of unidentified bacteria. Our results suggest that it may be feasible to reduce the impact of fumigant pesticides on soil microbial populations by stimulating microbial community structure, diversity and activity through the addition of organic amendments.

Hydrolysis and photolysis of oxytetracycline in aqueous solution

Oxytetracycline ((2Z,4S,4aR,5S,5aR,6S,12aS)-2-(amino-hydroxy-methylidene)-4-dimethylamino-5,6,10,11,12a-pentahydroxy-6-methyl-4,4a,5,5a-tetrahydrotetracene-1,3,12-trione) is a member of tetracycline antibiotics family and is widely administered to farm animals for the purpose of therapeutical treatment and health protection. Increasing attention has been paid to the environmental fate of oxytetracycline and other veterinary antibiotics with the occurrence of these antibiotics in the environment. The hydrolysis and photolysis degradation of oxytetracycline was investigated in this study. Oxytetracycline hydrolysis was found to obey the first-order model and similar rate constant values ranging from 0.094 ± 0.001 to 0.106 ± 0.003 day− 1 were obtained at different initial concentration ranging from 10 to 230 μ M. Solution pH and temperature were shown to have remarked effects on oxytetracycline hydrolysis. The hydrolysis in pH neutral solution appeared to be much faster than in both acidic and alkaline solutions. Oxytetracycline half-life decreased from 1.2 × 102 to 0.15 day with the increasing temperature from 4 ± 0.8 to 60 ± 1°C. The presence of Ca2 + made oxytetracycline hydrolytic degradation kinetics deviate from the simple first-order model to the availability-adjusted first-order model and greatly slowed down the hydrolysis. Oxytetracycline photolysis was found to be very fast with a degradation rate constant at 3.61 ± 0.06 day− 1, which is comparable to that of hydrolysis at 60°C. The presence of Ca2 + accelerated oxytetracycline photolysis, implying that oxytetracycline become more vulnerable to sunlight irradiation after chelating with Ca2 +. The photolysis may be the dominant degradation pathway of oxytetracycline in shallow transparent water environment.

Accelerated degradation of methyl isothiocyanate in soil

Methyl isothiocyanate (MITC, CH3NCS) is the primary breakdown product of metam-sodium, and a potential replacement fumigant pesticide for methyl bromide. Methyl isothiocyanate is toxic and has a high potential for volatilization, therefore, minimizing its atmospheric emission is of the utmost importance. One method to reduce fumigant emissions is to enhance their degradation by incorporating organic amendments into the soil surface. In this study we determined the combined effect of temperature and chicken manure application rate on the degradation of MITC. The degradation of MITC was significantly accelerated by both increasing temperature and amendment rate. Differences between sterile and nonsterile degradation kinetics in unamended and organically amended soil indicate that MITC degradation is equally controlled by chemical and biological processes. The amelioration of soil with organic amendments should be further considered when designing fumigation practices that allow for reduced emissions.