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In situ DRIFT spectroscopy is used in the current study to follow sorption/desorption of water vapor on several clay minerals that vary in surface charge and morphological character. Spectra of the dried sorbents indicate the significant effect of those factors on both liquid and vapor phase water remaining within the particles. Changes in the IR bands recorded during exposure to water vapor at 50% RH confirm the assignment of those bands to water. They also indicate that the dynamics of liquid phase sorption/desorption for all the sorbents investigated is consistent with the concept of capillary condensation within nanometer scale pores that was advanced in previous studies conducted with 1,2-dihaloethanes. The intraparticle accumulation of the vapor phase water, however, was found to be profoundly affected by both surface charge and particle morphology. These differences are attributed to the high polarity and hydration capabilities of water molecules. Band area changes documented during a second sorption/desorption cycle are nearly identical to those occurring during the first with regard to both timing and extent. They also confirm that some liquid phase water is retained by the sorbents even after five days of desorption and amounts of sorbed vapor phase water increase over the course of the experiments.

Dissolved organic matter (DOM) has been found to significantly affect the soil sorption/desorption of napropamide [2-(alpha-naphthoxy-N, N-diethylpropionamide] and to enhance its transport through soil columns. A method to qualitatively predict if DOM will enhance the transport of napropamide based on easily determined sorptive characteristics is investigated. Batch sorption experiments were specifically designed to simulate field application of napropamide. The formation of a napropamide-DOM complex was enhanced by pretreating soils with pesticide and allowing the carrier solvent to evaporate prior to preparing a water extract of the soil (K1). A second adsorption coefficient was measured by equilibrating K1 with untreated soil (K2). Column transport studies using the same soils as the batch equilibrium studies were performed. After allowing napropamide to dry, columns were leached. Initial leachate from all soils contained napropamide indicating that the collected napropamide had been transported through the column without adsorption. However, the amount leached was dependant on soil type and in general soils with higher organic matter content had higher concentrations of napropamide in the initial 30 mL of leachate when compared with lower organic matter (OM) soils. The ratio of the K1 and K2 solutions from the batch equilibrium studies were found to be positively correlated with the mass of napropamide in the initial leachate. Therefore the ratio of K1 to K2 provided a better qualitative predictor of the potential for facilitated transport of napropmamide than soil OM content.

DRIFT spectroscopic experiments are described here that build on our previous time-resolved in situ experiments using dihaloethanes as molecular probes. The earlier studies produced the first molecular level evidence for differences between clay minerals, humic substances, and lysimeter soils as sorbents for these chemicals. In the current study, spectroscopic experiments with several established field soils are combined with porosity characterizations derived from both N2 and CO2 isotherms. Density functional theory is used to generate porosity distributions from both types of isotherms which, in turn, are used here to define the microstructural character of the soil. Results from the combined techniques strongly suggest that sorption/desorption behavior of volatile organic chemicals (VOCs) can be understood in the context of soil particle architecture. The porosimetry provides evidence that the microstructural array in particles of three of the soils investigated is determined primarily by the platelet character of quasicrystalline clay minerals while that of another one of the soils, an Oxisol, appears to be dominated by its high iron oxide content. With all the soils, curve-fitting of the time-resolved DRIFT spectra indicates the presence of both a sorbed liquid and a persistently sorbed species that accumulate at different rates: band areas for the persistent species continue to increase during desorption. For the soils whose microstructure is dominated by clay minerals, rates of accumulation for both species increase and desorption of the sorbed liquid is facilitated after treatment with hypochlorite to partially remove organic matter. The treatment has little effect, however, on the sorption behavior observed with the Oxisol. The changes in sorption/desorption behavior correlate well with the differences in porosity distributions determined using N2. In contrast, CO2 determined microporosity (

Soil organic matter can be divided into solid (SOM) and water-dissolved (DOM) fractions; both of which can associate with herbicides. The objective was to examine the effects of DOM in applied solutions on atrazine adsorption in soil containing various SOM contents. A sandy loam with low (0.14%, Soil L), medium (1.2%, Soil M), or high (6.7%, Soil H) total organic matter, and artificial soil (Soil A) with no organic matter were used. These soils were treated with several different atrazine concentrations, dried, and extracted with water; the DOM concentrations in these extracts were 0, 10.1, 41.1, and 156.5 mg L -1 , respectively. These extracts and the untreated soils were used in batch experiment. The distribution coefficients (K d ) for atrazine of Soils L, M, and H under mixing with Soil A extract were 0.19, 0.52, 2.51, respectively, indicating that the higher the SOM content, the higher the atrazine affinity to the soil solid phase. The effects of the DOM concentration on atrazine adsorption were estimated from the ratios (Rd) between the K d for applied solution with no DOM and those for applied solutions containing DOM. R d > 1 indicates that the DOM decreases the herbicide adsorption. The R d values for Soils L, M, and H, each soil mixed with its extract, were 2.68,1.93, and 1.07, respectively. In contrast, in the case where the soil was mixed with a solution that contained a higher DOM concentration than the soil extract, an increase in the DOM concentration in the applied solution decreased the R 4 value. This was probably because of adsorption of DOM-atrazine complexes on the SOM.

The lysimeters of the San Dimas Experimental Forest were filled with fine sandy loam in 1937 and then planted with vegetation native to California. Extensive investigation of soils collected from the site in the last several years has revealed an array of soil development processes occurring there. The decomposition of plant litter, the flow of carbonaceous substances in the soil profiles, and the activity of earthworms have all contributed to significant differentiation of soils at the site as a function of depth and vegetation type. The impact of these processes on the microstructural character of soil particles is examined here. Soil samples collected from lysimeters planted with oak, pine, and chamise are included in this study. These samples are characterized with regard to their porosity using both N2 and CO2 as probe molecules. Porosity distributions are derived using density functional theory (DFT) that allows a uniform conceptual basis to be applied over the entire range of gaseous porosimetry. A third probe molecule, 1,2-dichloroethane (DCA), is used to determine whether the dynamics of transport for organic chemicals within soil particles can be related to differences in microstructural properties as defined by porosimetry. Time-resolved sorption/desorption processes for DCA on the various soil sorbents are characterized using in situ diffuse reflectance infrared spectroscopy (DRIFT). With all the sorbents investigated, the spectral sequences indicate the presence of three sorbed species: a sorbed liquid, a labile vapor, and a persistent vapor-phase species that increases in concentration even after prolonged desorption. N2 porosity distributions for these samples indicate that the accumulation of organic matter has significantly diminished the mesoporosity of the surface soils relative to the soils at further depths. The greatest reduction was observed for the soil sample with the highest measured organic C. Band areas resulting from curve-fitting the spectral data showed that levels of DCA sorbed as a liquid are similarly reduced. Microporosity as defined by the CO2-derived distributions exhibit the reverse trend with the highest levels for the surface mineral horizons. Differences in microporosity with depth reflect the activity of earthworms. No correlation was found between the CO2-measured microporosity of the soil samples and band areas for the persistently sorbed species. The presence and time-dependent behavior of this band is interpreted in the context of microstructural porosity and capillary flow. For both the sorbed liquid and persistent vapor, the major effect of C accumulation early in the soil development process appears to be the retardation of molecular transport into and out of soil particles.

The fumigant 1,3-dichloropropene (1,3-D), in combination with chloropicrin, is considered a major replacement to methyl bromide (MeBr). This study was conducted to better understand phase partitioning of 1,3-D and the role of organic matter in its adsorption to soil. Partition of 1,3-D between air and water (K(H)), and between soil and water (K(f)), was determined by quantifying the concentration in both phases upon equilibrium. At 20 degrees C, the K(H) values of (Z)- and (E)-1,3-D were 0.052 and 0.033, respectively. In four California and Florida soils, the K(f) values of 1,3-D isomers ranged from 0.39 to 8.55, and the K(oc) values ranged from 18 to 60. The relatively high K(H) and low K(f) imply that 1,3-D is highly mobile in most soils after subsurface application. Adsorption of 1,3-D in native soils and soils amended with manure compost increased with increasing soil organic matter content. This suggests that organic wastes may be applied to soil to increase 1,3-D adsorption, thus reducing its potential for offsite movement.

moves deeper into the soil profile than would be expected from results obtained from models. Soil-derived dissolved organic matter (DOM) has been shown to This rapid, sometimes unpredictable transport of orform stable complexes with the herbicide napropamide [2-(-naphthoxy-N,N-diethylpropionamide] capable of enhancing the transport ganic chemicals in soil is most often attributed to preferof napropamide through soil columns. Two soils, one containing sew- ential flow pathways. Jury et al. (1986) found that about age sludge–derived organic matter (SS) and the other having only 20% of the total mass of napropamide and prometryn natural organic matter (NoSS) were treated with napropamide and [2,4-bis (isopropylamino)-6-(methylthio)-s-triazine] apallowed to dry to promote complex formation. Soil columns were plied to the soil surface moved beyond the depth where prepared by packing a 10-cm layer of untreated, dry, sieved soil fol- existing chemical transport models predicted they would lowed by an overlying 5-cm layer of napropamide-treated soil. Col- reach by mass flow and adsorption reactions. In addiumns were irrigated and the effluent collected and placed in dialysis tion, it was found that under both conservation and no chambers. After equilibration napropamide concentrations were detill management an average of 18.8, 9.4, and 10.4% of termined on both sides of the membrane and complex and quantified the recovered atrazine [2-chloro-4-(ethylamino)-6-(isobased on the amount of napropamide unable to cross the membrane. It was found that for the SS soil 7% and for the NoSS 2.4% of the propylamino)-s-triazine], prometryn, and napropamide, applied napropamide underwent facilitated transport. In addition, respectively, were found in the 30- to 150-cm depth layer most of the complex transported through the columns had a molecular while they were expected to be retained in the top 20 weight between 500 and 1000 Daltons (Da). The solutions from the cm (Ghodrati and Jury, 1992). A review by Flury (1996) SS soil were also found to have formed at least two distinct complexes of the experimental literature on pesticide transport in that were resolved after passing through the untreated soil layer. The field soils revealed that there is still a lack of knowledge results obtained were in agreement with other published results and about the mechanisms responsible for rapid chemical the techniques used offer a way to separate and concentrate DOM transport. Mechanisms such as soil-induced changes in complexes from column effluents for further characterization. the sorptive behavior of the pesticide, due to complexation with DOM, can also lead to increased transport of a small portion of applied compound through soil in

The herbicide napropamide [N,N-diethyl-2-(1-naphthyloxy)propanamide] has been shown to complex with various sources of dissolved organic matter (DOM), potentially increasing the risk of ground water contamination by facilitating napropamide transport through soil. Two factors that may reduce the threat of facilitated transport of chemicals through soils are soil profile depth and soil horizon variability. Movement through the soil profile may cause herbicide complexed with DOM to break apart and readsorb to the soil. Differences in soil horizon organic matter content throughout the soil profile may also alter the potential for facilitated transport of herbicides. Napropamide was used in column studies to determine if organic matter differences in horizons and soil profile depth would reduce the risk of facilitated transport. A silt loam soil was used with and without sewage sludge amendment (SS and NoSS) to represent differences in organic matter content. Results showed that

Napropamide [2-(α-naphthoxy)-N,N-diethyl propionamide] has been shown to form soluble complexes with dissolved organic matter (DOM). Batch equilibrium and soil column leaching studies were performed to evaluate the effect of a drying event following herbicide application on napropamide-DOM complex formation. Napropamide was applied directly to the soil and allowed to dry and compared with studies where the herbicide was not allowed to dry. In batch studies the presence of napropamide-DOM complexes was inferred from reduced sorption coefficients measured on extracts of treated soils as compared with those from aqueous solutions. In column studies, drying was shown to induce herbicide movement with the wetting front even when preferential flow pathways were eliminated. The formation of napropamide-DOM complexes was verified by dialysis tubing techniques. Increased napropamide movement through soil was the result of stable napropamide-DOM complexes that lacked an adsorption affinity. Although less than 6% of the total herbicide applied moved by facilitated transport, this amount of rapidly mobile pesticide could significantly increase the potential for ground water contamination. Results suggest that preventing the applied herbicide solution from drying before irrigation could reduce or eliminate the threat of facilitated transport of napropamide by DOM. We suggest that a short irrigation period after herbicide application could allow for herbicide movement beyond the soil surface to deter drying while preventing deep movement of pesticide with the water front.

Contamination of groundwater by pesticide percolation is of great concern. Field observations have revealed that some pesticides move deeper into the soil profile than would be expected from predictions made by solute transport models. The discrepancies have been attributed to preferential flow of water carrying pesticides via macropores in field soils. The same phenomenon may also he explained by transport facilitated by a carrier such as dissolved organic matter (DOM). A laboratory study was conducted to evaluate the transport of napropamide [2-(a-naphthoxy-N,N-diethylpropionamide] through soil via DOM. Soils were sieved, packed in columns, and treated at the surface with 14 C-labeled napropamide. Water was applied to the columns by flooding and leachate was collected. It was found that 14 C-labeled napropamide was present in the first 0.22 cm of leachate. The 14 C and DOM concentrations were highest in the initial leachate and decreased with increasing leachate. Napropamide concentration fell below detection at some depth in all columns and recovery in the soil averaged 95% of the applied napropamide. Gas chromatographic analyses verified that 14 C activity in the leachate was associated with napropamide. A dialysis equilibrium technique determined that 17 to 56% of the napropamide in the leachate was retained inside a 500-Da dialysis membrane. The rapid leaching of a small fraction of napropamide was not a result of preferential flow in our experiments but is due to DOM-facilitated transport. Thus, under field conditions rapid pesticide leaching could be the combined effects of preferential flow and facilitated transport.

Dissolved organic matter (DOM) has been shown to form a stable complex with Napropamide [2(α-naphthoxy)-N,N-diethyl propionamide] and to facilitate its transport through soil columns. Liquid-liquid extraction of organics is a common method to transfer napropamide from water into an organic phase for gas chromatography analysis. A study was conducted to determine the effect of Aldrich humic acid, soil-derived dissolved organic matter, electrical conductivity, and hydrogen ion activity on the ability of hexane to extract napropamide from solutions and from soil extracts. The electrical conductivity of Aldrich humic acid solutions (0.0, 48, and 100 mg C L -1 ) were adjusted to 0.01, 0.97, and 1.69 dS m -1 by adding NaCl and CaCl 2 , and pH (8.2, 7.4, and 5.6) was adjusted using HCl and NaOH. Electrical conductivity had no effect on extraction efficiency. In the absence of DOM pH had no effect on extraction efficiency. Extraction efficiency decreased with increasing DOM concentration. Maximum reduction in extraction efficiency occurred in the presence of DOM when solution pH was near neutrality. A maximum extraction efficiency of 100% was observed in the absence of DOM and a minimum of 68% when napropamide was added to DOM solutions at pH 8.2 and then lowered to pH 5.6. Management practices such as liming and allowing napropamide to dry on the soil may increase environmental transport. Also quantification of napropamide in environmental samples can be affected by DOM.

Equilibrium dialysis is commonly used to measure the complex formed between dissolved organic matter (DOM) and pesticides such as napropamide [2(α-naphthoxy)-N,N-diethyl propionamide]. Currently the dialysis method most often used employs dialysis tubing that is clamped on both ends. Limitations of the dialysis tubing method include difficulty in sample addition to the dialysis tubing, dilution effects, and the need for large sample volumes. A new dialysis chamber constructed of Teflon and using flat dialysis membranes is described and characterized using napropamide as a model compound. Using a dialysis membrane with a molecular weight cut off of 500 Daltons the system reached equilibrium across the membrane within 120 h. Significant sorption of napropamide to the membrane was observed. Napropamide sorption was linear, reversible, and dependant on the membrane used. Sample addition and removal from the chamber is possible without destroying the integrity of the membrane. The dialysis chamber allowed for fast and easy quantitative addition and removal of sample for dialysis without wasting sample. The small chamber made possible dialysis of very small samples (as little as 1.5 mL) while maintaining a large surface area for diffusion. The chamber also provided dialysis with a maximum dilution factor of two since the volume of dialysate needed is equal to the volume of sample introduced.

Organic matter amendments to soil are usually thought to improve soil quality. Increased soil organic matter can lead to increased concentrations of dissolved organic matter (DOM). Complexation of DOM with pesticides can lead to increased aqueous phase concentration and therefore enhanced movement through the soil profile. Dissolved organic matter has been shown to form a stable complex with napropamide [2(α-naphthoxy)-N,N-diethyl propionamide] and facilitate its transport through soil columns. Equilibrium dialysis is a common method to determine napropamide DOM complexation. Napropamide was applied to a silt loam soil with (SS) and without (NoSS) sewage sludge application. Solutions from batch equilibrium experiments were analyzed using equilibrium dialysis with various molecular weight cutoffs to determine napropamide DOM complexation. A maximum of 16% of solution phase napropamide was found to be associated with a DOM complex having a molecular weight >500 Daltons. The maximum complex size had a molecular weight

Phenanthrene, a 3-ring aromatic hydrocarbon, was used as a model substrate to analyze the effect of sorption on the bioavailability of polycyclic aromatic hydrocarbons (PAHs). This study focused on sorption to mineral-associated humic acid (HA). Batch sorption analyses of phenanthrene to mineral-HA complexes were performed as integral part of the bioavailability experiments to account for the sorbed fraction under the various experimental conditions. A mixed culture that was enriched from a coal tar contaminated soil dominated by Pseudomonas sp . served as an inoculum. Phenanthrene mineralization was substantially enhanced upon sorption to mineral-HA complexes and the degree of enhancement was positively correlated with the fraction of sorbed phenanthrene. This stimulation is thought to be related to sorption of both the microorganisms and phenanthrene to the colloidal surfaces. This study suggests that when sorbed contaminants are still bioavailable, the presence of surfaces may stimulate mineralization.

The application of sewage sludge to agricultural soils is practiced to minimize landfill disposal. Organic matter amendments to soil are generally thought to improve soil quality, but pesticide application to these soils may lead to groundwater contamination problems. The complexation of pesticides with a water-soluble carrier such as dissolved organic matter (DOM) may facilitate chemical movement through soil. Sewage sludge amendments may lead to greater downward movement of organic chemicals if associated with DOM. Napropamide [2-α-napthoxy)-N,N-diethylpropionamide] was applied to a silt loam soil with (SS) and without (NoSS) sewage sludge application. Laboratory batch equilibrium and soil column studies were performed to determine the potential for herbicide complexation with DOM. Over 98% of the herbicide in soil columns followed typical adsorption and transport behavior as the center of mass of the lower organic matter soil (NoSS) moved twice the depth as that of SS. However, napropamide was detected in the initial leachate eluted from repacked soil columns with steps taken to prevent preferential flow. Napropamide concentrations in the initial leachate of SS were twice that from NoSS with

Phenanthrene, a three ring aromatic hydrocarbon, was used as a model substrate to distinguish between binding and sorption to dissolved and to mineral-associated humic acid (HA), with an emphasis on its implication to bioavailability. Binding was analyzed by a fluorescence quenching technique using Aldrich HA and HAs derived from compost, loamy and clay loamy soils. Measurements were made both in buffered water and in a mineral medium to be used later in a bioavailability study. Sorption of phenanthrene to mineral-associated HA was analyzed in batch experiments. Mineral–HA complexes were prepared with goethite, hematite, and Ca2+- and Cu2+-montmorillonite. Interaction of HA with clay minerals was greater at low pH and was dependent on the mineral type. Sorption of phenanthrene to these complexes was substantially higher at lower pH and higher when the HA was associated with montmorillonite rather than goethite or hematite. Binding coefficients (Kb(oc)) obtained for dissolved HA were at least an order of magnitude higher than sorption coefficients (Kp(oc)) obtained for mineral-associated HA. Both binding and sorption coefficient values were substantially higher for Aldrich HA. This work emphasizes the need for a detailed sorption study before conducting well controlled bioavailability experiments.

This represents the second phase in our efforts to develop a molecular level understanding of sorption/desorption processes at soil surfaces contributing to prolonged retention of nonpolar organic chemicals. Applying techniques developed with clay minerals, the processes are followed in situ from both kinetic and mechanistic perspectives using controlled environment diffuse reflectance infrared spectroscopy in conjunction with chemicals that exhibit isomerization properties. The rapid accumulation and ease of desorption of the first sorbed species detected as vapor phase chemical flows through humic sorbents is consistent with macroscopic partitioning behavior: its conformer populations are in accord with the nonpolar nature of humic substances. A band for a second species, detected only after several hours of sorption, increases in intensity during the sorption phase of the experiment and continues to increase even after days of desorption. Both of these species appear to be in the vapor state, consiste...

Critical parameters that need to be considered in a bioavailability study are reviewed and applied to a study on the sorption (binding) of phenanthrene to dissolved humic acid (HA). Sorption coefficients values ( K ∞ ) of phenanthrene to HA were measured for HA from several sources. These values were used to calculate the amount of HA that was needed to bind different fractions of solution-phase phenanthrene. Sorption linearity and complete reversibility were assumed. A mixed culture of phenanthrene degraders, dominated by Pseudomonas sp. was sensitive to changes in dissolved phenanthrene concentrations in the range that was used in this study. Therefore, it was expected that if bound phenanthrene was not available, as is commonly stated in the literature, sorption would affect mineralization rate by reducing the concentration of free phenanthrene. However, it was found that mineralization of phenanthrene was not affected by sorption even when 90% of the phenanthrene was in the bound phase. It was concluded that the organisms were able to use the phenanthrene directly from the bound phase and at the same rate as from the free phase.

In 1971, an experiment was conducted in a field containing high amounts of residual DDT (dichlorodiphenyltrichloroethane) to evaluate deep plowing, followed by flooding, with and without organic matter applications, as soil and water management tools to reduce total DDT residues and preferentially degrade the residual DDT to DDD [l,l-dichloro-2,2-bis (p-chlorophenyl) ethane]. The experimental site was revisited in 1994 to determine residual soil concentrations of DDT isomers and their metabolites in soil, soil dust, and the atmosphere. Also, volatilization flux measurements were made to evaluate rates of movement into the atmosphere. Soil concentrations of all DDT isomers and metabolites had decreased in all plots, with p, p'-DDE [1, 1-dichloro2,2-bis (p-chlorophenyl) ethylene] the major component of the total remaining residues (DDTR). The total DDTR residues in the surface 75 cm varied from 10 to 28% of their amounts in 1971. The highest concentrations were found in the deep plowed, unflooded plots with DDTR decreasing from 4 mg kg-’ at 0 to15 cm to 0.3 mg kg-’ at 60 to 75 cm. Deep plowing evidently increased DDT persistence by placing it deeper into the soil profile, which protected it from degradation and volatilization. Concentrations of all isomers were lower in the previously flooded plots. Degrading DDT under reducing conditions brought about by flooding lessened or prevented the formation of DDE in the soil thus ultimately reducing its redistribution into the environment. Significant concentrations of both o,p' and p,p'-DDE and DDT were detected in the atmosphere above the plots. Measurable volatilization fluxes were observed over 48-h periods in February and September. Irrigating the soil with 20 mm water dramatically increased the volatilization flux of all the DDT isomers and metabolites, particularly p,p'-DDE. The finding that DDT isomers continue to volatilize

Movement of pesticides from fields in runoff water and on sediment can degrade surface waters. Research typically found that because of the high water-to-sediment ratio, the total amount of pesticide in water greatly exceeded that associated with sediment. These observations suggested that erosion control would have little effect in reducing pesticide transport from the field. A laboratory study was conducted in a miniature furrow system to determine whether reduction in erosion would contribute to a reduction of napropamide [2-α-naphthoxy)-N,N-diethylpropionamide] transport. Variable erosion was created by polymer addition to the water combined with using variable water flow rates. Bromide was used as a nonsorbing independent tracer. Napropamide transport was linearly related to the amount of fine soil particulates eroded. Bromide transport was independent of the amount of erosion, and the percentage of the applied Br - removed in the runoff was less than that of napropamide. The results indicate that reduction in erosion of fine particulates will result in reduced transport of sorbed pesticides from the field.

Diffuse reflectance infrared spectra are collected in situ as a function of time during the sorption/desorption of 1,2-dichloroethane on montmorillonite and kaolinite. The spectra indicate that the sorptive becomes associated with the clay minerals in both a liquid and a labile vapor state, confirming previous results obtained with illite as the sorbent. They also provide evidence for an additional sorbed species that accumulates at a rate quite different from those of the other two. Features characteristic of this species persist in the spectra for periods of days after desorption is initiated. The results are interpreted in the context of sorbent porosity and the existence of a network of narrow pores that limits the rates of both sorption and desorption for the persistent species.

This study was conducted to reconcile an apparent inconsistency between the simazine laboratory sorption isotherm data and the field lysimeter transport experiment reported by Poletika et al. (this issue). In this investigation, linear and nonlinear one- and two-stage simazine sorption models were fitted to the sorption and desorption isotherm laboratory data to obtain parameter estimates for use in the transport model. Once obtained, the calibrated sorption model was combined with the parameterized outflow concentration record from a mobile Br tracer to represent rate-limited sorption and transport of the simazine added simultaneously with the Br. The calibrated model did an excellent job of representing the final simazine profile in the soil, particularly with the nonlinear model. This is in contrast to a single-stage adsorption model tested by Poletika et al. (this issue), which reached poor agreement with the field profile when laboratory-measured sorption parameters were used. The results demonstrate the compatibility of field and laboratory experiments on pesticide movement and also indicate that sorption isotherms may require substantially longer to reach equilibrium than is customarily allowed in current protocols.

In situ diffuse reflectance infrared spectroscopy is used in this study to gain insight into the molecular level mechanisms by which 1,2-dichloroethane (DCA) is retained on the surfaces of an illite specimen clay. The results support the conclusions presented earlier for 1,2-dibromoethane in which sorption is described as a process of condensation to form a liquid enriched in the gauche conformer due to the presence of a dielectric field. In addition, spectral evidence suggest the presence of a "sorbed" vapor phase, different in composition to unassociated vapor phase chemical, that desorbs more rapidly than the liquid. The behavior of DCA on clay surfaces is consistent with accepted concepts used to explain characteristic isotherm behavior observed for the sorption of vapor phase chemicals on porous media. This in turn supports the importance of sorbent porosity in the retention process.

Sorption/desorption processes for 1,2-dibromoethane (EDB) at clay surfaces were investigated in situ by means of diffuse reflectance infrared spectroscopy. Spectra were collected during the sorption phase wile N 2 saturated with EDB flowed through the clay samples. Desorption processes were similarly followed by flowing N 2 alone through the samples after the sorption period was completed. The spectral results indicated the presence of EDB in two conformations, anti and gauche, within a liquid phase

The fumigant 1,3-dichloropropene (1,3-D) has been identified as a partial replacement for methyl bromide (CH 3 Br) in soil fumigation. 1,3-Dichloropropene is formulated for soil fumigation as Telone II (Dow AgroSciences, Indianapolis, IN) for shank application and as an emulsifiable concentrate (EC) (Telone EC or InLine; Dow AgroSciences) for drip application. This study investigated the effect of formulation on the phase partitioning, transformation rate, and volatilization of 1,3-D isomers. Air-water partitioning coefficients (K H ) were slightly higher for Telone II than for Telone EC, presumably due to the higher apparent water solubility of the EC formulation. Sorption of 1,3-D isomers in two soils was not affected by formulation. Formulation had no significant effect on the rate of 1,3-D transformation in water or soil. In general, differences in the rate of 1,3-D transformation and phase partitioning due to formulation as Telone II or Telone EC were very small. Thus, the effect of formulation on 1,3-D fate may be ignored in transformation and phase partition of 1,3-D in water and soil. Packed soil columns without plastic tarp indicated that with relatively shallow subsurface (10 cm) drip application of Telone EC, emission of 1,3-D isomers was more rapid and produced greater maximum instantaneous flux than deeper (30 cm) shank injection of Telone II. Both application methods resulted in the same cumulative emissions for both isomers, 45% for (E)-1,3-D and approximately 50% for (Z)-1,3-D. These results suggest that for drip application of fumigants to be effective in reducing emissions, the fumigant must be applied at sufficient depths to prevent rapid volatilization from the soil surface if the water application rate does not sufficiently restrict vapor diffusion.

Polymer application to soil is a growing practice to improve soil physical properties and reduce soil erosion. Polymer addition can potentially influence herbicide and pesticide sorption in soil. The one-point distribution coefficient K d values of two herbicides in the absence and presence of each of 10 polymers (7 polyacrylamides and 3 polysaccharides) were determined by the batch equilibrium method. The results showed that nonionic napropamide [2-(alpha-naphthoxy)-N,N-diethyl propionamide] sorption was essentially unaffected by the presence of any of the polymers. The influence of polymers on anionic picloram (4-amino-3,5,6-trichloropicolinic acid) sorption depends on the charge characteristics of polymers and salt concentrations in the solution. Electrostatic interaction and competition for sorption sites are two primary underlying mechanisms for the polymer influence. At low salt concentration, the increased picloram sorption in the presence of both cationic and anionic polymers was attributed to different electrostatic interactions and polymer partitioning between soil and solution phases. At high salt levels, the presence of polymers had either no influence or a slightly negative influence on the picloram sorption, which was attributed to competition for sorption sites. In field conditions, it is more likely that polymers have no or a slightly negative influence on herbicide sorption due to the presence of salts.

Polyacrylamide (PAM) treatment of irrigation water is a growing conservation technology in irrigated agriculture in recent years. There is a concern regarding the environmental impact of PAM after its application. The effects of anionic PAM on the sorption characteristics of four widely used herbicides (metolachlor, atrazine, 2,4-D, and picloram) on two natural soils were assessed in batch equilibrium experiments. Results showed that PAM treatment kinetically reduced the sorption rate of all herbicides, possibly due to the slower diffusion of herbicide molecules into interior sorption sites of soil particles that were covered and/or cemented together by PAM. The equilibrium sorption and desorption amounts of nonionic herbicides (metolachlor and atrazine) were essentially unaffected by anionic PAM, even under a high PAM application rate, while the sorption amounts of anionic herbicides (2,4-D and picloram) were slightly decreased and their desorption amounts increased little. The impact mechanisms of PAM were related to the molecular characteristics of PAM and herbicides. The negative effects of PAM on the sorption of anionic herbicides are possibly caused by the enhancement of electrostatic repulsion by presorbed anionic PAM and competition for sorption sites. However, steric hindrance of the large PAM molecule weakens its influence on herbicide sorption on interior sorption sites of soil particles, which probably leads to the small interference on herbicide sorption, even under high application rates.

Soil-derived dissolved organic matter (DOM) has been shown to form stable complexes with the herbicide napropamide [2-(alpha-naphthoxy-N,N-diethylpropionamide] capable of enhancing the transport of napropamide through soil columns. Two soils, one containing sewage sludge-derived organic matter (SS) and the other having only natural organic matter (NoSS) were treated with napropamide and allowed to dry to promote complex formation. Soil columns were prepared by packing a 10-cm layer of untreated, dry, sieved soil followed by an overlying 5-cm layer of napropamide-treated soil. Columns were irrigated and the effluent collected and placed in dialysis chambers. After equilibration napropamide concentrations were determined on both sides of the membrane and complex and quantified based on the amount of napropamide unable to cross the membrane. it was found that for the SS soil 7% and for the NoSS 2.4% of the applied napropamide underwent facilitated transport. In addition, most of the complex transported through the columns had a molecular weight between 500 and 1000 Daltons (Da). The solutions from the SS soil were also found to have formed at least two distinct complexes that were resolved after passing through the untreated soil layer. The results obtained were in agreement with other published results and the techniques used offer a way to separate and concentrate DOM complexes from column effluents for further characterization.

Experimental evidence supporting the environmental screening model of Jury et al. (1983) is reviewed and discussed. For several laboratory studies of volatilization, initial and boundary conditions matched those used in our model and simulations were run. In all cases, good agreement was found between predicted and measured volatilization losses, with and without accompanying water evaporation. When five chemicals of widely differing volatility were exposed to identical experimental conditions, the model correctly predicted the relative loss behavior observed. The convective mobility predictions of the model were shown to be consistent with several laboratory studies of compound leaching, as long as the water flow rate was slow enough (< 0.01 cm/s) to ensure equilibrium between the solution and adsorbed phases. The Millington and Quirk tortuosity model used in our representation of the soil diffusion coefficient was found to give a good prediction of the water content dependence of the effective diffusion coefficient observed in several studies. The thickness of the stagnant boundary layer predicted from our similarity assumption was shown to be consistent with the apparent thickness inferred from several laboratory and field measurements of volatilization.

Le modele d'etude du transport de composes organiques separe les composes chimiques en categories de volatilisation et mobilite

Thin clay films prepared from aqueous suspensions of the potassium or sodium salt of picloram (4-amino-3,5,6-trichloropicolinic acid) with AI-, Fe-, or Cu-saturated montmorillonite or with montmoril- lonite coated with hydrous oxides of AI, Fe, or Cu were examined in an air-dry condition by infrared (IR) techniques to elucidate possible modes of interaction between picloram and the mineral surfaces. Deuter- ation was used to confirm band assignments of picloram and its salts prior to interaction with montmoril- lonite. Picloram interactions with AI- and Fe-saturated montmorillonite and with montmorillonite coated with hydrous oxides of A1 and Fe were concentration dependent. At high picloram concentrations simi- larities with the IR spectrum of potassium picloram indicated that much of the picloram was present in the salt form. As the concentration of picloram was reduced below that equivalent to 1 meq/g clay, the IR spectrum indicated the presence of the monomeric acid on the mineral surface. The spectrum of picloram on montmorillonite with a coating of copper hydrous oxide was similar to that of a Cu-picloram complex indicating coordination type bonding. The spectrum of picloram with Cu-saturated montmorillonite did not correspond to any of the other spectra of picloram examined. The interaction of picloram with montmo- rillonite coated with a hydrous oxide coating of Cu and with Cu-saturated montmorillonite were indepen- dent of picloram concentration in the range 2.20-0.44 meq/g clay.

The vapor pressure of hexachlorobenzene (HCB), measured with the gas saturation technique, was related to the temperature by the equation log₁₀p = 12.94 − (5279/T) giving HCB vapor pressure values from 0.420 × 10⁻⁵ to 22.4 × 10⁻⁵ mm Hg over the temperature range 15 to 45° C. The heat of vaporization of HCB was calculated to be 24.2 kcal/mol. The vapor pressure data was used to measure the vapor phase diffusion of HCB in soil using a vapor-diffusion cell. The steady-state vapor diffusion of HCB in soil was directly related to soil air-filled porosity and was therefore greatly reduced by increased soil bulk density and increased soil water content. The apparent steady-state HCB diffusion coefficient was linearly related to the soil porosity term, Pₐ¹⁰/³ / PT² where Pₐ is the soil air-filled porosity and PT is the total soil porosity. The calculated diffusion coefficient in air for HCB was 1.0 × 10⁴ cm²/day.

Equilibrium dialysis was used to measure the extent of interaction between dissolved organic matter (DOM) and five nonionic pesticides. The association-dissociation of DOM with the relatively polar, nonionic pesticide napropamide [2-(a-naphthoxy)-N,N-diethylpropionamide], was emphasized in this study. A negative linear correlation was found between the log of the solubility and the log of the association coefficient of these five pesticides with dissolved humic acid derived from peat (peat-DHA) (...)

Freshwater sediments were collected from eight different locations in California to study the influence of sediment properties on the adsorption and desorption of the model herbicide, diuron (3-(3,4-dichlorophenyl)-l,l-dimethylurea). The sediments were characterized to physical and chemical properties using the same methods used for soils. The sediments had a range of organic carbon of 0.91 to 19% and were high in silt and clay. The adsorption of diuron was described by the Freundlich equation. Adsorption at 25 °C expressed as the Freundlich coefficient, k, was positively correlated to the amount of organic carbon present. There was no correlation with total CEC. The desorption of diuron was evaluated by taking the difference between the slopes of the adsorption and desorption isotherms. As the difference between these slopes increased, less diuron was desorbed. These differences were positively correlated to k and organic carbon. Adsorption at temperatures of 5, 25, and 40°C on three sediments indicated that the adsorption of diuron decreased as the temperature increased. tern of a lake, then it would not only be important to study their chemical and physical properties, but also the potential retention or release of pesticides on these sediments. The capacity of lake sediments to adsorb or desorb pesticides is likely to be one of the most important factors which influence the concentration of a pesticide in the lake water. Since sediments are formed and exposed to conditions different from soils, it is important to know if the adsorption of a pesticide would vary with the same physical-chemical properties as for soils. The study was undertaken to determine the extent to which variation of several sediment characteristics, including organic matter, cation exchange capacity, and pH, affects sorption of the model herbicide, diuron (3(3,4-dichlorophenyl)-l,l-dimethylurea). Diuron is nonpolar herbicide of relatively low water solubility. It is used as both a preand postemergence herbicide. Additional Index Words: herbicides, pesticides, water quality, sediment characteristics, organic matter content, apparent heats of adsorption. Peck, D. E., D. L. Corwin, and W. J. Farmer. 1980. Adsorption-desorption of diuron by freshwater sediments. J. Environ. Qual. 9:101106. Although herbicide-soil interactions have been extensively studied, litte attention has been given to herbicide-sediment interactions. There have been some investigations of insecticide adsorption on lake sediment (Lotse et al., 1968; Vieth and Lee, 1971); however, there appears to be a lack of quantitative studies of the adsorption of herbicides on lake sediments. Poinke and Chesters (1973) have reviewed the literature on pesticide-sediment interactions. With the increasing use of herbicides in modern agriculture, it is likely that some will reach water bodies. Once in the water, it becomes important to understand the physical-chemical dynamics of the herbicide and sediment. It has been shown that sediments can play an important role in affecting the concentration of pesticides in a lake. Earlier investigations have shown (Bridges, 1961) that as the concentration of a given pesticide decreases in the water phase, there is a concomitant increase in the pesticide content of the sediment. As discussed by Chesters and Konrad (1971), these sediments may act as reservoirs, being able to recycle pesticides back into the aquatic environment. If lake sediments are considered as part of the life sys’ Contribution of the Dep. of Soil and Environ. Sci., Univ. of California, Riverside, CA 92521. The research leading to this report was supported by the Univ. of California, Water Resour. Center, as part of Water Resour. Center Project UCAL-WRC-W-485. Received 23 Sept. 1978. 2 Graduate Research Assistant, Graduate Research Assistant, and Associate Professor of Soil Science, Univ. of California, Riverside, respectively. Current address of senior author: Cooperative Extension Service, Univ. of Nevada, P.O. Box 651, Overton, NV 89040. Senior author is presently Soil and Water Scientist, Univ. of Nevada Agricultural Experiment Station, Logandale, NV 89021. MATERIALS AND METHODS The sediments were collected from eight different sites in California. Some of the chemical and physical properties of the sediments are listed in Table 1. The sediments were chosen to represent a number of differing environments and to be high in fine materials and organic matter. The sampling tool used to collect the sediment (Fig. 1) was modification of a tool designed by Barkley (1971). The sediment collection tube consisted of a 60-cm (2A-in) length of PVC pipe, 5 cm in) in diameter, with the circumference of one end beveled to aid penetration into the sediment. As the collection tube was pushed into the sediment, air and water were displaced upward through the tool stem. When the tube was filled with sediment, a rubber stopper was placed in the top of the tool. As the tool was withdrawn from the bottom, a vacuum was created holding the sediment in the tube. The success or failure of collecting a sediment sample depended upon the tool being completely air tight before removing the sediment from the bottom. Sediments were collected in water depths ranging from 0.5 to 4 m. Beyond a 4-m water depth it was difficult to maintain sufficient vacuum to hold the sediment in the tube. A composite sample consisting of 12 to 16 cores, 15 cm in depth, was collected at each site. The 0to 15-cm depth was obtained by sectioning the sediment core after removal from the collection tube. The sediment core slipped easily from the collection tube after releasing the vacuum. Intact 60-cm cores were retained in extra collection tubes for additional studies to be reported another time. Once collected, the sediments were immediately quickfrozen in dry ice. The frozen sediments were freeze-dried and each sampling site cornposited and ground to 60-mesh (0.25 mm). Preliminary investigations showed it necessary to reduce particle size to 60-mesh to provide a reproducible, homogeneous sample for the adsorption measurements. Preliminary screening through a 1-mm mesh removed fresh, undecomposed organic matter. Grinding may be expected to have an effect on the final particle size distribution in the sand fraction but would not be expected to significantly affect adsorption properties of the sediments. Most of the adsorption would be expected to be associated with the smaller size fractions. The chemical and physical properties of the sediments were determined using methods developed for soils. Organic carbon was determined using a programmed microcombustion apparatus in a pure oxygen atmosphere (Allison, 1965). In this method any sediments containing carbonates were treated with sulfurous acid before determining carbon. Total N was determined by the semimicro-Kjeldahl method (Bremner, 1965). The percent LOI (loss on ignition) was determined using a procedure developed for ash content (Sneddon et al., 1971) in which the sample was ignited 375°C for 16 hours. Cation exchange capacity of the sediment was determined using a modified BaCl2 method (Chapman and Pratt, 1961). J. Environ. Qual., Vol. 9, no. 1, 1980 101 Published January, 1980

Tests sur 35 composes chimiques selectionnes pour determiner la mobilite convective et diffusive, la susceptibilite a la volatilisation a la surface du sol et la persistance dans le sol en fonction de la profondeur d'incorporation

Vapor pressures of technical-grade triallate [S-(2,3,3-trichloroallyl)diisopropylthiocarbamate] were calculated from the vapor density measurements determined at seven different temperatures, using a gas-saturation technique and solid adsorbent traps. The equation Log10P = 11.05 – (4401/T) delineated the relationship between the apparent vapor pressure and temperature, giving the triallate vapor pressure values from 1.083 × 10−4to 16.371 × 10−4mm Hg over the temperature range of 20 to 45 C. The vapor flux of triallate from glass surfaces was of the order of 5.7 μg cm−2h−1at 25 C.

The physicochemical dynamics of a pesticide-sediment-water system were studied utilizing a one-dimensional numerical model of diffusion under saturated conditions, with nonlinear Freundlich/Langmuir adsorption-desorption. Specifically, the model was formulated to assess the pollution potential of pesticides to aquatic systems, based upon their ability to migrate within the aquatic environment. Initial physical and chemical characterization of eight freshwater sediments, and adsorption studies using two chemically dissimilar herbicides—bromacil and diquat—revealed insightful positive relationships between adsorption coefficients and sediment properties. Bromacil showed a high positive correlation between the Freundlich sorption partition coefficient and the organic carbon content. Diquat adsorption, as characterized by a Langmuir-type adsorption, showed a high positive correlation between the Langmuir affinity constant and the surface area, while the Langmuir adsorption maxima correlated highly with the cation exchange capacity of sediments treated for the removal of organic matter. Apparent heats of adsorption for bromacil at 5° and 25°C were low, indicating a predominantly physical type of adsorption. Temperature change was found to have little or no effect upon the adsorption of diquat over the range of observed temperatures, 5° and 25°C. Rates of adsorption for both bromacil and diquat were very rapid, especially on sediments with small organic matter fractions. In general, diquat took slightly longer than bromacil to attain equilibrium. The slower adsorption rate of diquat on high-organic-matter sediments confirms previous findings indicating a possible redistribution of diquat from adsorption sites on the organic fraction to adsorption sites on the clay surface. Varying the initial solution concentration of either bromacil or diquat did not significantly affect the reaction rates. Desorption studies for bromacil and diquat showed that for each sediment, a unique linear relationship existed between the adsorbate concentration at which desorption began and the slope of the desorption isotherm.