"Phytoscreening": the use of trees for discovering subsurface contamination by VOCs.

We tested the possibility of using tree cores to detect unknown subsurface contamination by chlorinated volatile organic compounds (Cl-VOCs) and petroleum hydrocarbons, a method we term "phytoscreening". The scope and limitations of the method include the following: (i) a number of widespread Cl-VOC contaminants are readily found in tree cores, although those with very high vapor pressures or low boiling points may be absent; (ii) volatile petroleum hydrocarbons were notwell-expressed in tree cores; (iii) trees should be sampled during active evapotranspiration and from directions that are well exposed to sunlight; (iv) there is not necessarily a direct correlation between concentrations measured in tree cores and those in the subsurface; (v) detection of a contaminant in a tree core indicates that the subsurface is contaminated with the pollutant; (vi) many possible causes of false negatives may be predicted and avoided. We sampled trees at 13 random locations in the Tel Aviv metropolitan area and identified Cl-VOCs in tree coresfromthree locations. Subsequently, subsurface contamination at all three sites was confirmed. Phytoscreening is a simple, fast, noninvasive, and inexpensive screening method for detecting subsurface contamination, and is particularly useful in urban settings where conventional methods are difficult and expensive to employ.

Suitability of dye-clay complexes for removal of non-ionic organic compounds from aqueous solutions.

Aqueous sorption of phenol, atrazine and naphthalene was measured on complexes formed from Na-montmorillonite (Fischer bentonite) and the organic cationic dyes crystal violet and rhodamine-B. Sorption isotherms were found to be non-linear. This agrees well with the rigid nature of the dye-clay organic coverage, which provides a finite surface for adsorption. High values of organic carbon-normalized distribution coefficients reached 20,000-25,000 for atrazine on rhodamine-B-montmorillonite, 7000 for atrazine on crystal violet-montmorillonite, and 1500 for phenol on crystal violet-montmorillonite. As such, dye-clays may significantly extend the variety of organoclay sorbents that effectively reduce aqueous concentrations of non-ionic organic compounds.

Temporal changes in kerosene content and composition in field soil as a result of leaching.

A field experiment was designed to determine the combined effect of leaching and natural attenuation on the redistribution dynamics of kerosene--a volatile petroleum hydrocarbon mixture (VPHM)--and of its selected individual components in the soil subsurface. Variables included the composition of contaminant spilled, the soil water content before contamination and the leaching pattern. Temporal changes in the residual kerosene concentration and composition in the soil subsurface of the experimental field during 39 days and leaching by 500 mm of irrigation water were determined to a depth of 100 cm. The main processes controlling contaminant attenuation were volatilization and redistribution with depth. Soil hydration status was found to affect the attenuation, redistribution and composition of VPHM in the porous media. An initial relative increase of n-alkanes in the subsurface compared with the total VPHM in the first leaching period was a result of the volatilization of low vapor pressure compounds. The redistribution of individual components in the soil profile during leaching was in accordance with their physico-chemical properties.

Efficacy of Fluazinam in Suppression of Monosporascus cannonballus, the Causal Agent of Sudden Wilt of Melons.

Sudden wilt (vine decline) of melon, caused by Monosporascus cannonballus, is a worldwide problem in arid and semi-arid regions. Soil disinfestation by fumigation with methyl bromide before planting is a common treatment for disease management but, because methyl bromide is expected to be banned from use within the next 10 years, alternative measures for disease suppression are needed. The efficacy of 29 fungicides against M. cannonballus was evaluated in vitro. Among the fungicides tested, fluazinam and kresoxim methyl were the most effective and both totally inhibited the growth of M. cannonballus in culture at concentrations of 10 µg a.i /ml. Because fluazinam also was effective in inhibition of Pythium aphnidermatum, which also may be involved in sudden wilt syndrome, and kresoxim methyl was not, fluazinam was chosen for further tests. The effective dose of fluazinam for M. cannonballus that reduced mycelial growth by 50% was 0.09 µg a.i./ml. Fluazinam efficacy was evaluated in three field experiments conducted in the spring and in the late summer cropping seasons. In two of the experiments, applications of fluazinam resulted in approximately 87% wilt reduction, whereas in the third experiment it was only 32%. The mobility of fluazinam in soil was determined in samples taken from the field. Fungicide mobility in soil was relatively limited; most of the compound was adsorbed to soil particles, resulting in a zone of high concentration that decreased with depth and distance from the application site. Nevertheless, rates measured even at a depth of 25 cm were sufficient to control M. cannonballus. This study shows that fluazinam may be used as one component in an integrated approach for suppression of sudden wilt of melons.

Prediction of Sorption by Soils of Volatile Hydrocarbon Mixtures.

Mixtures of hydrocarbon vapors evaporating from petroleum spills are often transported through the unsaturated zone. To predict the course of evaporation the sorption isotherm of each component, and how this is affected by the other components, must be known. The sorption of m-xylene and n-dodecane vapors sorbed on air- and ovendry soil was studied. Mixtures of m-xylene and n-dodecane containing 0, 20, 40, 60, 80, and 100% n-dodecane by volume were prepared. The pure compounds and each mixture were diluted with nonvolatile paraffin to contain 5, 10, 20, 50, 80, and 100% of the pure compound or mixture of m-xylene and n-dodecane, thus providing a wide range of vapor composition. Samples of Evesham clay (Orthic Grumaquert) were suspended over each of the vapors, and the amounts sorbed by the soil were determined by gas chromatography after extraction of the soil with CCl4 . On oven-dry soil at low relative vapor pressures the two gases competed for sorption sites. At higher relative vapor pressures the increased sorption due to formation of multilayers more than compensated for competition by the other gas, so that addition of the second component increased sorption of the first. On air-dry soil the addition of a second component always increased sorption of the first at all relative vapor pressures. Empirical isotherms of the pure components give no information about their influences on each other's sorption. Hill's theory of multilayer multicomponent sorption allows one to predict such influences from parameters derived solely from the isotherms of the pure components. The Hill equation is shown to be more successful in predicting the sorption from mixtures of m-xylene and n-dodecane vapors than an empirical equation based on the isotherms of the pure components.