Organic Carbon Partition Coefficient Research Articles

Natural Attenuation of Trichloroethylene in Rhizosphere Soils at the Savannah River Site

Extensive trichloroethylene (TCE) groundwater contamination has resulted from discharges to a former seepage basin in the A/M Area at the Department of Energy's Savannah River Site. The direction of groundwater flow has been determined and a seep line where the contaminated groundwater is estimated to emerge as surface water has been identified in a region of the Southern Sector of the A/M Area. This study was undertaken to estimate the potential of four rhizosphere soils along the seep line to naturally attenuate TCE. Microcosms were setup to evaluate both biotic and abiotic attenuation of TCE. Results demonstrated that sorption to soil was the dominant mechanism during the first week of incubation, with as much as 90% of the TCE removed from the aqueous phase. Linear partitioning coefficients (Kd) ranged from 0.83 to 7.4 mL/g, while organic carbon partition coefficients (Koc) ranged from 72 to 180 mL/gC. Diffu-sional losses from the microcosms appeared to be a dominant fate mechanism during the remainder of the experiment, as indicated by results from the water controls. A limited amount of TCE biodegradation was observed, and attempts to stimulate TCE biodegradation by either methanotrophic or methanogenic activity through amendments with methane, oxygen, and methanol were unsuccessful. The appearance of cis-1,2-dichloroethylene (c-DCE), and trans-1,2-dichloroethylene (t-DCE) confirmed the potential for anaerobic reductive dechlorination. However, these daughter products represented less than 5% of the initial TCE added. The sorption results indicate that natural attenuation may represent a viable remediation option for the TCE plume as it passes through the rhizosphere.

Chemical Dynamics of Persistent Organic Pollutants: A Sensitivity Analysis Relating Soil Concentration Levels to Atmospheric Emissions

A dynamic modeling framework is presented that provides a method to estimate deposition of air emissions in soil, based on either single or multiple time-point soil measurements. These dynamic estimates, made in the context of the uncertainty and variability associated with environmental systems, differ significantly from those based on steady-state single-valued assumptions. A case study on the global mass balance for polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) illustrates that, by explicitly representing the uncertainty of the parameters used to assess mass balance, there exists a discrepancy between deposition and source emissions of between 6- and 20-fold. The uncertainty in these soil deposition estimates was most strongly dependent on two landscape properties (the deposition velocity of air particles and the thickness of the atmosphere) and three chemical properties (the organic-carbon partition coef ficient, chemical degradation within the atmosphere, and vapor pressure). Usin...

Development and evaluation of triggers for sediment toxicity testing of pesticides with benthic macroinvertebrates

AbstractConcerns regarding the potential of pesticides that partition into sediments to affect benthic organisms have led to the development of sediment toxicity test methods. However, not all pesticides have the potential to accumulate in sediments or affect benthic organisms. Thus, a scientific rationale for identifying appropriate compounds (triggering) for regulatory sediment toxicity testing is required. Here we propose that testing should only be required for compounds that are relatively adsorptive and persistent in sediment, and also have the potential for toxicity to invertebrates. Trigger endpoints and values are proposed and include soil aerobic degradation rate (time for 50% degradation to occur ≥ 30 d), soil adsorption coefficients (organic carbon partition coefficient ≥ 1000), and Daphnia acute and chronic toxicity (48‐h median effective concentration < 1 mg/L or 21‐d no‐observed‐effect concentration < 0.1 mg/L). These data are available internationally for the majority of pesticides. Where appropriate, these preliminary triggers can be augmented by additional data (e.g., water‐sediment degradation studies, toxicity data on benthic species) to make more sophisticated triggering cases or risk assessments. An evaluation of the proposed triggers was performed by reviewing regulatory data from standardized studies on 140 pesticides. A total of 22% of the compounds triggered sediment toxicity testing, of which the majority were insecticides and fungicides. For insecticides, fungicides, and herbicides 44, 27, and 6% of compounds within the group met the triggers, respectively. Those compounds that triggered testing are probably representative of the classes of pesticides that might be expected to raise concerns for sediment toxicity.

Influence of sludge amendment on transport and sorption ideality ofs-triazines in soil columns

The aim of this work was to examine the effect of organic soil amendments (municipal sewage sludge and composted municipal sewage sludge) on transport ideality, sorption equilibrium, and s-triazine (atrazine, simazine, ametryn, and terbuthylazine) retardation compared with nonamended soil. Organic-matter amendment resulted in increased nonequilibrium sorption effects, including early breakthrough, increased breakthrough, and elution front-tailing. Organic amendments likewise caused greater solute retardation compared with transport in nonamended soil columns. As the organic matter content increased, retardation increased and the desorption rate constant decreased. The fraction of fast sorption sites also decreased, resulting in greater sorption non-equilibrium. In a given column, the fraction of fast sorption sites was essentially equal for the different s-triazine compounds. The desorption rate constant decreased as the organic carbon partition coefficient (Koc) increased in the order atrazine > simazine ≥ ametryn > terbuthylazine. Soil amendment with sludge and compost resulted in some nonideal physical transport, which was negligible compared with sorption nonideality effects. The linear sorption isotherm nonequilibrium (LNE) model adequately simulated the measured breakthrough curves. The breakthrough curve for ametryn in an amended, nonrinsed column was identical to that in an amended, rinsed column, indicating that sludge-/soil-derived dissolved organic carbon did not affect ametryn sorption and transport in this system.

Adsorption and Desorption of Atrazine, Deethylatrazine, Deisopropylatrazine, and Hydroxyatrazine on Levy Wetland Soil

The adsorption and desorption of atrazine, and its three metabolites, deethylatrazine (DEA), deisopropylatrazine (DIA), and hydroxyatrazine (HA), on Levy wetland soil were measured using the batch equilibration technique. Solution concentrations for each chemical were 1.3, 3.8, 6.6, and 13.2 μmol L-1. Four 24-h period desorptions for each concentration were conducted immediately after adsorption. The adsorption of all the chemicals conformed to linear isotherms. Adsorption coefficients decreased in the order HA (109.7) > atrazine (38.6) > DIA (26.3) > DEA (22.1). The organic carbon partition coefficients (Koc) for HA, atrazine, DIA, and DEA were 1500, 440, 290, and 240 L kg-1, respectively. Percent of chemical desorbed was about 29% for DEA, 24% for atrazine, 23% for DIA, and 16% for HA. The slopes of all desorption isotherms were less than their respective adsorption slopes, indicating hysteresis. Atrazine and its metabolites were relatively more strongly adsorbed and the binding was less reversible in t...

The use of regression equations for quality control in a pesticide physical property database

Quality control is a crucial aspect of database management, particularly for physicochemical parameters that are widely used in modeling environmental fate processes. Complete rechecking of original studies to verify environmental fate parameters is time consuming and difficult. This paper evaluates an alternative, more efficient approach to identifying database errors. The approach focuses verification efforts on a targeted subset of entries by making use of the relationship between water solubility (S) and soil organic carbon partition coefficient (K oc ). Two regression equations, one selected from the literature and one calculated from entries in the database, were used to evaluate the reasonableness of (S, K oc ) pairs among control compared to the targeted outlier group from a total of 59 pesticides. Our hypothesis was that (S, K oc ) pairs that lay far from the regression line were more likely to be in error than those that fit the regression. Database values were checked against original studies. Identified errors in the database included coding mistakes, miscalculations, and incorrect chemical identification codes. The error rate in outlier (S, K oc ) pairs was about twice that of pairs that conformed to the regression equation; however, the error rate differential was probably not large enough to justify the use of this quality control method. Through our close scrutiny of database entries we were able to identify administrative practices that led to mistakes in the data base. Resolution of these problems will significantly decrease the number of future mistakes.

Selection of Herbicide Alternatives Based on Probable Leaching to Groundwater

Extension workers are sensing pressure to use soils information and chemical characteristics data to guide farmers in selecting pesticides least prone to leach into groundwater. Our objective was to estimate differences in herbicide migration to groundwater under conditions typical for the Southeast Coastal Plain, and to consider how a farmer might be advised to use such knowledge in selecting herbicides. We used a simple computer code for microcomputers to predict persistence and migration of 17 herbicides through a hypothetical, coarse-textured soil typical of the Southeast Coastal Plain. Appropriate herbicides were selected for several common crop-weed problems, such as sicklepod in soybean and Palmer amaranth in corn. Groundwater was assumed to be 3.15 m below the soil surface. Herbicides selected covered a broad range of half-lives and organic carbon partition coefficients. Only after the first-order degradation rate constant was reduced by a factor of five did predicted soil water concentrations of several herbicides at the groundwater interface reach normal detection limits. Still, predicted concentrations were below the level established for health effects advisory purposes. Due to the large number of uncertainties and the inability to estimate practical benefits, we conclude that data relating to soil and herbicide characteristics cannot be used at this time to override cost effectiveness, efficacy, and other factors normally considered by farmers and Extension professionals in herbicides for weed control.

Sorption Thermodynamics of Cypermethrin at High Concentrations on Some Indian Soils

Abstract Sorption thermodynamics of cypermethrin on four different type of Indian soils (alluvial, forest, hill and humid) from water/acetone mixtures (fs = 0.666) at 30°C and 50°C has been investigated through sorption isotherms, Freundlich equation, distribution coefficient and thermodynamic parameters. The measured equilibrium sorption isotherm for all soil-cypermethrin combinations were of non-linear Freundlich type and yielded ‘S’ shaped curves for soils at both temperatures. The sorption of cypermethrin was higher on hill silt loam soil followed by forest loam, alluvial sandy loam and humid loam soils and lower at higher temperature. The order of sorption was anticipated with K and Kd values. Comparison of the Freundlich coefficient K-values in relation to soil properties suggests that organic carbon, CaCO3, clay content and surface area are the most important factors affecting cypermethrin sorption. From Freundlich sorption constant K values, the organic carbon partition coefficient Koc and clay co...

Role of Humified Organic Matter in Herbicide Adsorption

Organic matter is the soil constituent most often associated with herbicide adsorption. Structural diversity makes humified organic material an ideal substrate for the adsorption of many pesticides, but variability in composition and distribution in situ complicates interpretation of its quantitative effect on adsorption. Variability in the adsorption distribution coefficient (KD) of a herbicide among soils often is due to differences in organic matter content and can be reduced by adjusting KDfor soil organic carbon content and computing the organic carbon partition coefficient (Koc). Koccan be estimated from the octanol-water partition coefficient (Kow) of organic compounds, but the correlation weakens as compound polarity increases. Kocalso can be correlated with aqueous solubility if a correction is made for the melting point of compounds that are solids at 25 C. Relative adsorption can be estimated from parachor and molecular connectivity indices; but corrections are needed for polar compounds, and correlations with KDor Kochave been variable. Such predictive methods may be useful for broad classification purposes, but accurate extrapolation generally requires site-specific adsorption measurements. Empirical models which accommodate the multiple regression of organic matter content and other soil properties such as clay content, pH, and cation exchange capacity on herbicide adsorption can increase accuracy, but interpretation may be restricted to a small number of sites.

Validation of CHEMEST, an on-line system for the estimation of chemical properties

CHEMEST is an on-line system for estimating physical/chemical properties important in the assessment of environmental fate of chemicals. Measured values of boiling point, melting point, vapor pressure, water solubility, and soil organic carbon partition coefficient ( K oc) were compared to values estimated by CHEMEST, for more than 170 chemicals. The chemicals were composed of seven classes: alcohols, aldehydes and ketones, amines, carbamates, esters, ethers, and phenols. Mean errors for boiling and melting point were 22.6 and 36.2°C, respectively, with relatively little variability among classes. Carbamates were an exception, accuracy being much lower for both properties. Vapor pressures were quite accurately estimated, measured and estimated values generally differing by less than a factor of two. Mean error factors for water solubility and K oc were somewhat higher, but nevertheless better than the order of magnitude accuracy commonly assumed. Solubility was very inaccurately estimated for the phenols, however. The results indicate that, for the chemicals and properties studied, estimated values were sufficiently accurate for screening-level fate assessment. TSTCHEM, a revised version of CHEMEST, out-performed the latter with respect to the accuracy of solubility and vapor pressure estimates.

Chloroform sorption to New Jersey coastal plain ground water aquifer solids

AbstractStudies examining the adsorption/desorption characteristics of chloroform to New Jersey coastal plain aquifer solids were performed. Adsorption to the Cohansey aquifer solids, a coarse‐to‐fine‐grade sand with a 4.4% organic matter content (2.6% organic carbon content), and to the Potomac‐Raritan‐Magothy aquifer solids, a sandy loam with a 2.2% organic matter content (1.3% organic carbon content), was found to be dependent on adsorber mass. Equilibrium adsorption could be characterized by either linear or Freundlich isotherms. Greater adsorption was achieved to the Cohansey material, which was attributed to its greater organic matter (carbon) content. Calculated normalized organic carbon partition coefficients were compared with values obtained from literature correlations. Consecutive desorption experiments displayed an apparent hysteresis.

Diffusion and partitioning of hexachlorobiphenyl in sediments

The diffusion coefficient and partition coefficient of hexachlorobiphenyl (HCBP) in a sediment are determined by using a dual radio-tag experiment that extended over 2 years. Essentially constant coefficients were observed. An independent measurement of the diffusion coefficient for a nonsorbing chemical (tritiated water) allows an estimate to be made of the interstitial water-sediment partition coefficient. The result is in close agreement with that predicted from hydrophobic sorption correlations based upon sediment organic carbon and HCBP octanol-water partition coefficient. It also corresponds to the low particle concentration limit of the partition coefficients found when batch equilibrations of dilute suspensions of the same sediment were used.

Partitioning of naphthalene, methylnaphthalenes and biphenyl between wastewater treatment sludges and water

AbstractPartition coefficients (Kp) describing the partitioning of naphthalene, methylnaphthalenes and biphenyl between organic‐rich wastes and water were determined using 14C‐tracer techniques as well as high performance liquid chromatographic analysis of the wastes and their aqueous extracts. Results of the two procedures were in good agreement. The concentrations of the specific organics in the wastes were not good predictors of concentrations in aqueous extracts, since Kp varied among the materials tested. Predictions of Kp based on organic carbon partition coefficient (Koc) and organic carbon content of the sludges were well below observed values. Oil content of the wastes and oil‐water partition coefficients appeared to be important factors in determining Kp.

Behavior Assessment Model for Trace Organics in Soil: I. Model Description

AbstractA mathematical model is introduced for describing transport and loss of soil‐applied organic chemicals. The model assumes linear, equilibrium partitioning between vapor, liquid, and adsorbed chemical phases, net first order degradation, and chemical movement to the atmosphere by volatilization loss through a stagnant air boundary layer at the soil surface. From these assumptions and the assumption of steady state upward or downward water flow, an analytic solution is derived for chemical concentration and volatilization flux.This model, which is intended to classify and screen organic chemicals for their relative susceptibility to different loss pathways (volatilization, leaching, degradation) in the soil and air, requires knowledge of the organic carbon partition coefficient (Koc), Henry's constant (KH), and net, first‐order degradation rate coefficient or chemical half‐life to use on a given chemical.Illustration of the outputs available with the model is shown for two pesticides, lindane (γ‐1,2,3,4,5,6‐hexachlorocyclohexane) and 2,4‐D [(2,4‐dichlorophenoxy)acetic acid], which have widely differing chemical properties. Lindane, with a large Koc, large KH, and small degradation rate coefficient, is shown to be relatively immobile, persistent, and susceptible to volatilization. 2,4‐D, with a small Koc, small KH, and large degradation rate coefficient, is mobile and degrades rapidly, but is only slightly susceptible to losses by volatilization.