The Li+ ion is used frequently as an environmentally acceptable surrogate for sorbing radionuclides in field tracer tests, and experiments using Li are an important part of assessing the potential transport of radionuclides in saturated alluvium south of Yucca Mountain, Nevada, the site of a proposed nuclear waste repository. Equilibrium partition constants (Li+ Kds) were measured using batch studies incorporating a wide range of Li+ concentrations and two different grain-size fractions of alluvium samples from multiple depth intervals in two wells. Cation exchange capacity, surface area, bulk mineralogy from quantitative X-ray powder diffraction, and trace Mn- and Fe-oxyhydroxide mineralogy from extractive studies were evaluated as predictors for linearized Li+ Kd values (K1in) in the alluvium. Many of the predictor variables are correlated with each other and this was considered in the analysis. Linearized Kd values were consistently higher for fine particle-size fractions than for coarse fractions. Single and multivariate linear regression analyses indicated that the clinoptilolite + smectite content, taken together as a combined variable, was the best predictor for Li+ sorption in the alluvium, although clinoptilolite content was clearly a better predictor when the two variables were considered separately in simple linear regressions. Even so, Li+ Klin predictions based on clinoptilolite and smectite abundance were accurate only to within about ±100%. This uncertainty suggests that there is either a high inherent variability in Li+ Klin values or that additional alluvium characteristics not measured or evaluated here may play an important role in simple Li+ cation exchange in the alluvium.

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 KD for soil organic carbon content and computing the organic carbon partition coefficient (Koc). Koc can be estimated from the octanol-water partition coefficient (Kow) of organic compounds, but the correlation weakens as compound polarity increases. Koc also 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 KD or Koc have 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.

Hydrolysis and adsorption of the sulfonylurea herbicides rimsulfuron and primisulfuron were investigated under laboratory conditions. Degradation in aqueous solutions was determined between pH 3 and 8 and between 5 and 35 C. Rimsulfuron degraded faster than primisulfuron. Half-lives at different temperatures at pH 4 ranged from 0.16 to 5.8 d for rimsulfuron and from 0.47 to 16.1 d for primisulfuron. Primisulfuron hydrolysis at 30 C decreased with increasing pH, but rimsulfuron decreased up to pH 7, then increased, suggesting rapid degradation in alkaline conditions. Adsorption, measured in six Colorado soils, was complete within 2 h in all soils. Adsorption isotherms were determined by batch equilibration of 75 to 600 μg L−1. Rimsulfuron was adsorbed in amounts 3.7 to 7 times higher than primisulfuron, with adsorption distribution coefficients (Kd) ranging from 0.1 to to 1.18 for primisulfuron and from 0.71 to 5.1 for rimsulfuron. Adsorption of both herbicides was slightly correlated with soil clay content, and primisulfuron adsorption was also correlated with organic matter content.

Metsulfuron sorption, dissipation, and leaching were studied in six Colorado soils. Sorption was studied in the laboratory by batch equilibration of soil horizons from the surface to 1 m deep found in each study area. Kd was correlated to several soil parameters with pH (-0.773) and percent organic matter (OM) (0.666) the strongest, although low. Kd generally decreased with depth at each site and ranged from 0.10 to 0.83 among surface soils. The lowest Kd was in the soil with the highest pH. Leaching and dissipation were studied by high-performance liquid chromatography/ultraviolet analysis of field samples collected 1 m deep. Bromide ion as conservative tracer was applied concurrently with metsulfuron at the four dryland and two irrigated sites. In most cases, metsulfuron was found only in upper horizons, and the half-life ranged from 11.8 to 27.7 d, the shortest being in the soil with highest percent OM and lowest pH. Very little leaching below the surface horizon was detected in any soil.

The adsorption of atrazine and alachlor was studied on samples of three horizons from soils with different textures and organic carbon contents. Soils were equilibrated with five concentrations of atrazine and alachlor using batch techniques. Adsorption affinity for atrazine and alachlor was approximated by the Freundlich constant (Kf), distribution coefficient (Kd), and the normalized Kd based on organic carbon (Koc). Adsorption was not significantly correlated with soil depth, clay content, or organic carbon. Atrazine adsorption was a linear function of equilibrium concentration for nearly all soil horizons but was nonlinear in most horizons for alachlor. The extent of atrazine adsorption was greater in all horizons of the fine-textured soils (Kd = 1.5 to 5.5) compared to coarse-textured soils (Kd = 0.40 to 0.87). The same general trends with texture were not apparent for alachlor. Conversion of Kd to Koc failed to reduce the variability in the linear adsorption coefficient for atrazine and alachlor in the different soils of this study.

The effect of soil composition on adsorption of chloridazon by 18 samples of natural soils, with different chemical and textural characteristics, obtained from west-central Spain, was investigated. The effect of removal of organic matter on adsorption and adsorption of the herbicide by the active components of the soils (montmorillonite and humic acid) was also studied. A highly significant correlation was found between the distribution coefficient (Kd) and organic matter content when all the soils or the soils with organic matter content >2% were considered. According to the determination coefficients (r2), organic matter content accounted for 72% of the variance in adsorption in the former case and 92% in the latter. In soils with an organic matter content <2%, there was no correlation between Kd and organic matter content. A significant correlation was observed between Kd and smectite content for soils containing smectite in their clay fraction. Both simple and multiple correlations showed that clay content, excluding the smectite fraction, had a relatively small effect on adsorption of the herbicide. Adsorption of chloridazon by oxidized soils, montmorillonite, and humic acid confirms the effect of organic matter and smectite on adsorption in natural soils.

Laboratory studies were conducted to compare the soil adsorption of aminopyralid and clopyralid with the use of batch-slurry and centrifugation assays. The calculated soil binding constants for both herbicides varied between the two techniques, but the centrifugation assay had a lower coefficient of variation compared to the batch-slurry assay. These results indicate that a centrifugation assay is a more accurate procedure for measuring the interaction of aminopyralid and clopyralid with soils. Aminopyralid adsorbed more tightly than clopyralid to six of the eight soils tested. Adsorption Kd values ranged from 0.083 to 0.364 for clopyralid and 0.106 to 0.697 for aminopyralid. Pearson correlation analysis indicated that binding of both herbicides was highly correlated to soil organic matter and texture but not to soil pH. On average, soil thin-layer chromatography indicated that aminopyralid was less mobile (Rf = 0.82) than clopyralid (Rf = 0.91), although both were mobile. These results suggest that aminopyralid will have a lower leaching potential than clopyralid. Lower potential aminopyralid soil leaching, coupled with low use rates, suggests it may be the herbicide of choice in areas where potential for leaching could be a concern.

Sulfentrazone adsorption and mobility in six soils with varying soil properties were evaluated under laboratory conditions. Adsorption was evaluated using a modified slurry technique. Mobility was evaluated using packed-soil columns under saturated flow conditions. The order of adsorption to soil was Sequatchie loam > Dothan loamy sand = Bosket fine sandy loam > Malden loamy sand > Commerce silty clay loam > Harkey clay loam. Greater sulfentrazone adsorption occurred in soils with lower pH. Sulfentrazone movement under saturated flow conditions in 27-cm soil-packed columns was greater in soils with low adsorption, high pH, and coarse texture. Sulfentrazone movement was limited in the Sequatchie loam but was greater in the other soils examined. No clear relationship was evident between sulfentrazone mobility and adsorption in these soils.

Clomazone has been successfully used for weed control in rice, but crop injury is a potential problem on light-textured soils. Experiments were conducted to determine the effect of soil characteristics and water potential on plant-available clomazone and rice injury. A centrifugal double-tube technique was used to determine plant-available concentration in soil solution (ACSS), total amount available in soil solution (TASS), and Kd values for clomazone on four soils at four water potentials. A rice bioassay was conducted parallel to the plant-available study to correlate biological availability to ACSS, TASS, and Kd. TASS was significantly different in all soils. The order of increasing TASS for the soils studied was Morey < Edna < Nada < Crowley, which correlated well with soil characteristics. The order of increasing TASS after equilibrium was − 90 < − 75 < − 33 < 0 kPa. TASS values at 0 kPa were greater than two times the TASS values at − 90 kPa. It appears that severe rice injury from clomazone on these soils could occur if TASS > 110 ng g−1 and Kd < 1.1 ml g−1. We propose that the double-tube technique provides a more accurate estimate of available herbicide because the solution–soil ratios are < 0.33:1 and would be more representative of a plant root–herbicide relationship. This technique or some variation possibly could be further developed such that clomazone rates could be more clearly defined particularly on lighter-textured soils. TASS may be a better predictor of plant-available herbicide than ACSS when evaluating moderately to highly water-soluble herbicides in a nonsaturated soil environment.

Herbicide soil/solution distribution coefficients (Kd) are used in mathematical models to predict the movement of herbicides in soil and groundwater. Herbicides bind to various soil constituents to differing degrees. The universal soil colloid that binds most herbicides is organic matter (OM), however clay minerals (CM) and metallic hydrous oxides are more retentive for cationic, phosphoric, and arsenic acid compounds. Weakly basic herbicides bind to both organic and inorganic soil colloids. The soil organic carbon (OC) affinity coefficient (Koc) has become a common parameter for comparing herbicide binding in soil; however, because OM and OC determinations vary greatly between methods and laboratories, Koc values may vary greatly. This proposal discusses this issue and offers suggestions for obtaining the most accurate Kd, Freundlich constant (Kf), and Koc values for herbicides listed in the WSSA Herbicide Handbook and Supplement.

It has been previously reported that POST-applied isoxaben can effectively control established hairy bittercress. Experiments were conducted to determine the relative importance of root vs. foliar entry of POST-applied isoxaben. At a common isoxaben rate of 0.56 kg/ha, foliar-only and foliar plus soil applications provided 10.5 and 23.3% control, respectively, as determined by fresh weight reduction. In contrast, soil-only application provided 47.0% control. Hairy bittercress foliar absorption of 14C–isoxaben did not exceed 15% of the amount applied after 72 h. Therefore, the comparatively less effectiveness of foliar-only applications may be attributed primarily to limited absorption. Minimal isoxaben concentration required to inhibit root growth of hydroponically grown hairy bittercress was 0.0025 mg/L. Higher concentrations were required to produce a response in the foliage. Sorption of isoxaben by pine bark rooting substrate, typical of what is used in container nursery production, exceeded 99% of amount applied after 36 h. Even with 99% sorption, the probable concentration within the aqueous phase remains sufficient to inhibit hairy bittercress root growth. Additional studies with 14C–isoxaben established that approximately 35% of the root-absorbed isoxaben was translocated into the foliage. Translocation from the roots into the foliage was reduced to 16% when the experiment was repeated during environmental conditions less favorable for vegetative growth (i.e., longer day length and higher temperature). Results indicate that the control of hairy bittercress with POST-applied isoxaben is likely the result of root absorption and root-growth inhibition. Expression of phytotoxicity within the foliage is also a component, but is dependent upon the root-absorbed isoxaben being translocated into the foliage. Extent of this translocation is dependent upon plant maturity and prevalent environmental conditions.

Le paramètre le plus simple et le plus commun pour modéliser la mobilité d’un radionucléide dans un sol est le coefficient de distribution, Kd. Réciproquement, la valeur empirique de Kd est supposée représenter différents mécanismes biochimiques et physiques eux-mêmes influencés par de nombreuses variables. Les valeurs de Kd varient avec les sols et les radionucléides. En conséquence, l'utilisation de valeurs de Kd « par défaut » dans un modèle d'évaluation du comportement d’un radionucléide dans des sols peut aboutir à une incertitude non négligeable. L’étude systématique de cinq radionucléides (Cs, I, Sr, Se, Tc) est faite en termes de comportement dans les sols, mécanismes principaux, cinétiques, réversibilité/irréversibilité de la sorption... Pour chaque radionucléide, une base de données de Kd est constituée, comprenant les données primaires utilisées pour établir les valeurs par défaut les plus communes et de nouvelles données issues des publications récentes. Une fiche synthétique récapitule les paramètres les plus importants influençant significativement leur mobilité, propose une valeur par défaut pour le Kd et suggère les expériences minimales qui pourraient être utiles pour acquérir des valeurs de Kd plus appropriées au comportement du radionucléide.