Simulating emissions of 1,3-dichloropropene after soil fumigation under field conditions.

Soil fumigation is an important agricultural practice used to produce many vegetable and fruit crops. However, fumigating soil can lead to atmospheric emissions which can increase risks to human and environmental health. A complete understanding of the transport, fate, and emissions of fumigants as impacted by soil and environmental processes is needed to mitigate atmospheric emissions. Five large-scale field experiments were conducted to measure emission rates for 1,3-dichloropropene (1,3-D), a soil fumigant commonly used in California. Numerical simulations of these experiments were conducted in predictive mode (i.e., no calibration) to determine if simulation could be used as a substitute for field experimentation to obtain information needed by regulators. The results show that the magnitude of the volatilization rate and the total emissions could be adequately predicted for these experiments, with the exception of a scenario where the field was periodically irrigated after fumigation. In addition, the timing of the daily peak 1,3-D emissions was not accurately predicted for these experiments due to the peak emission rates occurring during the night or early-morning hours. This study revealed that more comprehensive mathematical models (or adjustments to existing models) are needed to fully describe emissions of soil fumigants from field soils under typical agronomic conditions.

Effect of surface application of ammonium thiosulfate on field-scale emissions of 1,3-dichloropropene.

Soil fumigation is important for food production but has the potential to discharge toxic chemicals into the environment, which may adversely affect human and ecosystem health. A field experiment was conducted to evaluate the effect of applying ammonium thiosulfate fertilizer to the soil surface prior to fumigating with 1,3-dichloropropene (1,3-D). The ammonium thiosulfate solution was applied as a spray with minimal water to minimize the effect on emissions from saturating (e.g. sealing) the soil pores with water. Two independent data sets were collected for determining the emission rate. One data set was used with three micrometeorological approaches: aerodynamic, integrated horizontal flux and theoretical profile shape; the other dataset with two indirect, back calculation methods that used the CALPUFF and ISCST3 dispersion models. Using the five methodologies, the 1,3-D emission rate was obtained for 16days. The maximum emission rates ranged from 7 to 20µgm-2s-1, the maximum 24-hour averaged emission rates ranged from 5 to 13µgm-2s-1, and the total 1,3-D emissions ranged from 12 to 26%. Comparing to fumigation without ammonium thiosulfate spray revealed that emissions were reduced from 3% (CALPUFF) to 29% (ADM). Using a simulation model, ammonium thiosulfate spray would be expected to reduce emissions by almost 21%. These data provide evidence that emissions of 1,3-D can be reduced by spraying ammonium thiosulfate fertilizer on the soil surface prior to soil fumigation, and provides another emission-reduction strategy to those recently reported (e.g., deep injection, water seals and organic amendments).

Irrigation, organic matter addition, and tarping as methods of reducing emissions of methyl iodide from agricultural soil.

Methyl iodide (MeI) is increasingly being used as a highly effective alternative to the soil fumigant methyl bromide. Due to its volatile and toxic nature, MeI draws wide attention on its potential atmospheric emission following field fumigation treatment. Using soil columns that make it possible to determine emissions and gas phase distribution of soil fumigants, we studied MeI behavior in two soils differing in organic matter content. Additionally, the effectiveness of surface irrigation and tarping with virtually impermeable film (VIF) was assessed. In the lower organic matter, bare soil (control), emissions of MeI were rapid and high (83% of total). Although the peak emission flux was reduced by irrigation, the total loss was very similar to the control (82%). Tarping with VIF dramatically reduced emissions (0.04% total emissions). In the higher organic matter soil, degradation rate of MeI was increased around 4-fold, leading to a significant reduction in emissions (63% total emissions). The work suggests that surface tarping with VIF would be highly effective as an emissions reduction strategy and would also result in the maintenance of high soil gas concentrations (important for pest control). Ripping of the tarp after two weeks led to an immediate spike release of MeI, but, even so, the flux rate at this time was almost 20 times lower than the peak flux rate in the control. Even with tarp ripping, the total emission loss from the VIF treatment remained low (6%).

1,3-dichloropropene and chloropicrin emissions following simulated drip irrigation to raised beds under plastic films.

Using laboratory soil chambers a nonscaled representation of an agricultural raised bed was constructed. For a sandy loam soil, 1,3-dichloropropene (1,3-D) and chloropicrin (CP) were applied at 5 cm depth with an excess of water (simulated drip irrigation). Application was made under both high density polyethylene (HDPE) and virtually impermeable film (VIF) covering the soil bed (the furrow was left uncovered). Soil gas distribution of the fumigants, together with emissions into the headspace above the bed, sidewall and furrow were determined over time. Total emissions from the HDPE treatment were cis 1,3-D 28%, trans 1,3-D 24%, and CP 8%. Due to its lower permeability, the values for VIF were 13%, 7%, and 1.5%, respectively. With HDPE, the majority (86-93%) of the emissions occurred from the bed, while for VIF the majority (92-99%) of the emissions was from the furrow. Compared to a range of literature values for shank injection, the use of drip application appears to offer a benefit in reducing 1,3-D and CP emissions. However, the most meaningful comparison is with our previous data for simulated shank injection where the same soil was covered (completely) with the same plastic films (1). In this comparison, only 1,3-D emissions under HDPE were lower with drip application; 1,3-D emissions under VIF and CP emissions under both films were greater with the drip application.

Laboratory assessment of emission reduction strategies for the agricultural fumigants 1,3-dichloropropene and chloropicrin.

With the increased use of the agricultural fumigants 1,3-dichloropropene (1,3-D) and chloropicrin (CP), it is important that strategies to reduce emissions of these fumigant from soil to the air are assessed to protect air quality. Using an established soil column approach, the following emission reduction strategies were compared to a control: (1) spray application of ammonium thiosulfate to the soil surface; (2) deep injection at 46 cm depth; (3) high density polyethylene sealed over the soil surface; (4) virtually impermeable film sealed over the soil surface; and (5) irrigation with ammonium thiosulfate solution. Relative to the control, 1,3-D emissions were reduced by 26.1, 1.0, 0.01, 94.2, and 42.5%, for treatments 1 through 5, respectively. For CP the reductions were 41.6, 23.3, 94.6, 99.9, and 87.5% for treatments 1 through 5, respectively. Virtually impermeable film gave the greatest reductions for both fumigants, while HDPE was very effective only for CP. Despite offering less significant emission reductions, the lower cost alternatives to tarping, particularly irrigation with ATS solution, may offer substantial benefitwhere tarping is not economically viable.

Effects of soil type, moisture content, redox potential and methyl bromide fumigation on Kd values of radio-selenium in soil.

Understanding the processes that determine the solid-liquid partitioning (K(d) value) of Se is of fundamental importance in assessing the risk associated with the disposal of radio-selenium-containing waste. Using a mini-column (rather than batch) approach, K(d) values for (75)Se were determined over time in relation to soil moisture content (field capacity or saturated), redox potential and methyl bromide fumigation (used to disrupt the soil microbial population) in three contrasting soil types: clay loam, organic and sandy loam. The K(d) values were generally in the range 50-500 L kg(-1), with mean soil K(d) increasing with increasing organic matter content. Saturation with water lowered the measured redox potentials in the soils. However, only in the sandy loam soil did redox potential become negative, and this led to an increase in (75)Se K(d) value in this soil. Comparison of the data with the Eh-pH stability diagram for Se suggested that such strong reduction may have been consistent with the formation of the insoluble Se species, selenide. These findings, coupled with the fact that methyl bromide fumigation had no discernible effect on (75)Se K(d) value in the sandy loam soil, suggest that geochemical, rather than microbial, processes controlled (75)Se partitioning. The inter-relations between soil moisture content, redox potential and Se speciation should be considered in the modelling and assessment of radioactive Se fate and transport in the environment.

Surface irrigation reduces the emission of volatile 1,3-dichloropropene from agricultural soils.

Low-cost, practicable techniques are required to limit the release of volatile organic compound-containing fumigants such as 1,3-D to the atmosphere. In this study, we aimed to quantify 1,3-D diffusion and emission from laboratory soil columns maintained under realistic conditions and thereby assess the efficacy of soil irrigation as a technique for reducing emissions. In two soils (one relatively high, and one relatively low, in organic matter), irrigation led to a limiting of upward diffusion of the fumigant and to the maintenance of higher soil gas concentrations. Therefore, rather than being emitted from the column, the 1,3-D was maintained in the soil where it was ultimately degraded. As a consequence, emission of 1,3-D from the irrigated columns was around half of thatfrom the nonirrigated columns. It is concluded that surface irrigation represents an effective, low-cost, and readily practicable approach to lessening the environmental impact of 1,3-D fumigant use. In addition, the higher organic matter soil exhibited emissions of around one-fifth of the lower organic matter soil in both irrigated and nonirrigated treatments, due to markedly enhanced degradation of the fumigant. Organic matter amendment of soils may, therefore, also represent an extremely effective, relatively low-cost approach to reducing 1,3-D emissions.

Soil migration, plant uptake and volatilisation of radio-selenium from a contaminated water table.

The properties of (79)Se make it of likely potential importance in safety studies for geological disposal of radioactive wastes. Despite a substantial literature on toxic and nutritional aspects of selenium in the environment little consideration has been given to the behaviour of radioactive selenium and its potential transfer from a radioactive waste repository to the biosphere. Column experiments (15 x 50 cm), using a sandy loam soil, indicated that the upwards migration of (75)Se (as a surrogate for (79)Se) from a contaminated water table was dependent upon the redox status of the soil. Low redox conditions within the water table strongly limited upwards (75)Se soil migration, presumably due to the immobilisation of reduced Se species. Under natural conditions, (79)Se from a radioactive waste repository is therefore likely to accumulate at considerable depth. As a consequence, its absence from the rooting zone is likely to limit its transfer into plants. Nevertheless, the column experiments indicated that when an overlap between roots and soil contamination occurs, uptake into the plant is observed. Quantification of (75)Se volatilisation from the column surfaces suggested that this is a significant pathway by which (79)Se may move either directly from soil to the atmosphere, or from soil to plants and then to the atmosphere.

A comparison of the soil migration and plant uptake of radioactive chlorine and iodine from contaminated groundwater.

A 6-month soil column experiment was conducted to compare the upward migration and plant uptake of radiochlorine and radioiodine from shallow, near-surface contaminated water tables. Both fixed and fluctuating water tables were studied. After 6 months, (36)Cl activity concentrations were relatively uniform throughout the soil profile apart from an accumulation at the soil surface, which was especially marked under a fluctuating water table scenario. In contrast, (125)I (a surrogate for (129)I) tended to accumulate at the boundary between the anoxic conditions at the base of the column and the oxic conditions above, due to its redox-dependent sorption behaviour. The uptake of (36)Cl by perennial ryegrass was much greater than that of (125)I due to its greater migration into the rooting zone and its ready availability in soil solution. In the context of radioactive waste disposal, where these radionuclides may potentially be released into groundwater, (36)Cl would be expected to present a greater potential for contamination of the biosphere than (129)I.

Effects of moisture content and redox potential on in situ Kd values for radioiodine in soil.

The soil solid-liquid distribution coefficient (Kd) value is of great significance in understanding and modelling the environmental behaviour of soil contaminants. For many years, the batch sorption technique has been used for the determination of such values. Here, we propose an alternative 'mini-column' approach in which somewhat more realistic soil conditions are maintained. In particular, this approach allows for determination of radionuclide Kd values under realistic soil moisture contents and in a system in which time-dependent processes such as changes in redox potential can take place. Data obtained for radioactive iodine (a key radionuclide in the consideration of radioactive waste disposal) are presented and indicate that soil moisture content, particularly in conjunction with soil redox potential (through water-logging of the soil), has a marked effect on measured Kd values. The results indicate the advantages and potential usefulness of the mini-column approach in assessing the environmental behaviour of radioactive, and other, soil contaminants.

Soil migration and plant uptake of technetium from a fluctuating water table.

Soil columns (50x15 cm) were used to determine the potential for 95mTc (as a surrogate for 99Tc which is an important component of some radioactive waste) to migrate from a contaminated, fluctuating water table, through sandy loam soil and into perennial ryegrass. Upward migration was significantly retarded with, generally, only the bottom few centimetres of soil becoming contaminated over the 6 months of the experiment. This is thought to have been due to the presence of anoxic conditions within the water table leading to the reduction of pertechnetate to Tc(IV) species which are relatively insoluble. However, some evidence of very slow upward migration over time was found. Only a small and inconsistent transfer of activity into the perennial ryegrass was observed. Whilst these observations would suggest that 99Tc is less important than radionuclides such as 129I and 36Cl in terms of the risk associated with radioactive waste disposal, the potential for a slow upward migration, and/or a pulse-release following the re-oxidation of reduced soil in which 99Tc has accumulated should not be overlooked.

Soil mobility of sewage sludge-derived dissolved organic matter, copper, nickel and zinc.

A soil (sandy loam) column leaching study aimed to determine the extent of mobility and co-mobility of Cu, Ni, Zn and dissolved organic matter (DOM) released from a surface-application (equivalent to 50 t ds ha(-1) of anaerobically-digested sewage sludge. Leaching of DOM through the soil column was found to be almost un-retarded. Decidedly similar behaviour was exhibited by Ni suggesting that it migrated as organic complexes. Whilst Cu was also found to be leached, significant retardation was evident. However, the importance of DOM in promoting the mobility of both Cu and Ni was evidenced by their lack of mobility when added to the soil column as inorganic forms. The presence of DOM did not prevent Zn from becoming completely adsorbed by the soil solid phase. In relation to WHO drinking water guidelines, only Ni concentrations showed potential environmental significance, due to the relatively poor retention of Ni by the sludge solid phase.

Soil transport and plant uptake of radio-iodine from near-surface groundwater.

This paper describes a 12-month experiment designed to study the extent of upward migration of (125)I (as a surrogate for (129)I) from near-surface groundwater, through a 50-cm column of soil and into perennial ryegrass. The water table was established at a depth of 45 cm below the soil surface. By 3 months, (125)I had migrated about half way up the soil column. After this, it tended to accumulate just above this mid-point, with only very small amounts being transported to the upper 20 cm of soil. This behaviour seemed to be explained well by soil moisture and redox conditions. The experiment indicated that (125)I was mobile only within the saturated/low redox zone at the base of the soil column and accumulated in the zone of transition between anoxic and oxic soil conditions. Uptake of (125)I by the ryegrass was found to be low.

Deletion of pgi alters tryptophan biosynthesis in a genetically engineered strain of Escherichia coli.

Deletion of the structural gene for phosphoglucose isomerase (pgi) of Escherichia coli dramatically alters the path of glucose catabolism by diverting carbon into the hexose monophosphate shunt. The effect of this genetic alteration on the conversion of glucose to tryptophan by strains optimized for the biosynthesis of this amino acid was determined by using 13C-nuclear magnetic resonance spectroscopy in vivo. Pgi- strains converted glucose to tryptophan almost twice as efficiently as did their Pgi+ counterparts.

Characterization of Acetate and Pyruvate Metabolism in Suspension Cultures of Zea mays by C NMR Spectroscopy.

Carbon-13 nuclear magnetic resonance (NMR) spectroscopy has been applied to the direct observation of acetate and pyruvate metabolism in suspension cultures of Zea mays (var Black Mexican Sweet). Growth of the corn cells in the presence of 2 millimolar [2-(13)C]acetate resulted in a rapid uptake of the substrate from the medium and initial labeling (0-4 hours) of primarily the intracellular glutamate and malate pools. Further metabolism of these intermediates resulted in labeling of glutamine, aspartate, and alanine. With [1-(13)C]acetate as the substrate very little incorporation into intermediary metabolites was observed in the (13)C NMR spectra due to loss of the label as (13)CO(2). Uptake of [3-(13)C]pyruvate by the cells was considerably slower than with [2-(13)C]acetate; however, the labelling patterns were similar with the exception of increased [3-(13)C] alanine generation with pyruvate as the substrate. Growth of the cells for up to 96 hours with 2 millimolar [3-(13)C]pyruvate ultimately resulted in labeling of valine, leucine, isoleucine, threonine, and the polyamine putrescine.

Modification of calf thymus DNA by methyl methanesulfonate. Quantitative determination of 7-methyldeoxyguanosine by mass spectrometry.

Quantitation of 7-methyldeoxyguanosine (m7dG) produced in the in vitro methyl methanesulfonate (MeMS) action on calf-thymus DNA is achieved by enzymatic degradation, liquid chromatographic separation and chemical ionization mass spectrometry. The total degree of methylation, measured by uptake of [14C]MeMS was 0.35%. Mass spectral analysis shows that m7dG constitutes 84% of the total methylated product. It is also shown that tandem mass spectrometry allows detection of m7dG, as the protonated base, down to 1 pmol level, suggesting that MS/MS analysis can be the method of choice in quantitation of the adducts of in vivo DNA modifications.

Chemical modification of nucleic acids. Methylation of calf thymus DNA investigated by mass spectrometry and liquid chromatography.

Mass spectrometry provides an extremely sensitive method for the identification and quantification of modified nucleosides and hence for determining chemical modifications of nucleic acids. When mass spectrometry is used in conjunction with a new high-performance liquid chromatographic system capable of separating 15 methylated and naturally occurring nucleosides, this allows the quantification of products of in vitro DNA methylation. With synthetic (2H3)methyl-labeled methylnucleosides as internal references, the distribution of methylated products formed when calf thymus DNA was reacted with N-methyl-N-nitrosourea(MeNU) was determined. Five modified products, 1-methyldeoxyadenosine(m1dA), 3-methyldeoxycytidine(m3dC), 7-methyldeoxyguanosine(m7dG), 3-methylthymidine(m3T) and O4-methylthymidine(m4T) were detected and the relative distributions were measured. The ability of mass spectrometry/mass spectrometry (tandem mass spectrometry) to increase specificity and sensitivity in this determination is demonstrated and its application to in vivo studies is suggested.