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Fluorescence techniques have been used to identify humic substances, (e.g. aquatic fulvic acids from different origins). Synchronous scans have proven adequate for distinguishing detail and differentiating samples. As well, fluorescence has often been used to probe humic interactions with metals and xenobiotic organics. In this study, the fluorescence of a well-characterized material, Laurentian fulvic acid (LFA) was compared with that of a simple hydroquinone/quinone (H 2 Q/Q) model system. Synchronous fluorescence (room T) and laser-excited fluorescence experiments at 10°K were carried out to characterize the fluorophore. The synchronous fluorescence behavior of LFA displayed similarities to that of an equilibrated, aerated, H 2 Q/Q system, but only at higher pH. (Higher pH favours a shift of the redox equilibrium towards quinone.) In contrast, the spectra of LFA suggest the role of "quinone" groups, even at lower pH. A signif- Aquatic Sciencesicant feature of these spectra is a lowest energy band. At low temperature this band was more selectively excited at 470 nm, but vibrationally resolved line narrowed spectra were not observed. Fluorescence lifetimes were very short compared to the laser pulse width of ca. 10 ns. Under the low T condition the spectra of LFA and the model are essentially identical. We suggest that the principal luminophore in LFA is a quinone-hydroquinone type, perhaps a charge transfer complex consistent with the absence of the fluorescence line-narrowing phenomenon. The spectra imply a lowest energy component of LFA emission from a fluorophore very similar to that of the simplest H 2 Q/Q. The significant similarities of the model system to LFA are underscored by striking parallels in the radicals in the two systems seen in well-resolved electron paramagnetic resonance (EPR) spectra that could be obtained at pH = 6.

The distribution of chlorothalonil among the dissolved, labile sorbed, and bound residue states was monitored during an 18 day period in an aqueous slurry of an analyzed quartz sand soil from Simcoe, ON, Canada. The Simcoe soil is 90.−95.% quartz sand. The online HPLC microextraction method was used for this purpose, because it is the only available technique that can resolve the total amount of a pesticide in a soil into its dissolved, labile sorbed, and bound residue components. The processes for which the molecular level kinetics were determined included labile surface sorption and desorption and bound residue formation. At a reaction time of 14 days, the solution concentration of 0.75 × 10-6 M was 43.3% of the total chlorothalonil, 26.2% was in the labile sorbed state, and 30.5% was a bound residue. There were no chemical reactions and no biodegradation during the 18 day period. The kinetics of mass transfer among the three states were determined and are consistent with intraparticle diffusion. Although the amounts are small, it is suspected that the 5.−10.% nonquartz materials in the Simcoe soil contribute most of the sorption and bound residue effects.

Chlorothalonil has been found to persist but not leach in some quartz sandy soils. For this effect to be understood, it is necessary for the numbers of occupied and empty surface sorption sites to be known. This requires that the labile surface sorption capacity θC, which is the total number of sites, be measured. Wetting effects might however, affect θC. The measurement of θC has been done only recently by very few laboratories, and the wetting effects have seldom been investigated. New analytical chemical methods are therefore required. On − line HPLC micro extraction has been used in the present work to develop a method for the titration of chlorothalonil onto the labile surface sorption sites. Two conclusions have been drawn from the measurements. The first is that θC is one to 2 orders of magnitude greater than it is for the previously reported cases. Second, the effect of wetting on bound residue formation reported by Belliveau and Langford () has been confirmed. These effects help to explain the behavior of chlorothalonil in the quartz sandy soils.

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Two categories of pesticide soil models now exist. Government regulatory agencies use pesticide fate and transport hydrology models, including versions of PRZM.gw. They have good descriptions of pesticide transport by water flow. Their descriptions of chemical mechanisms are unrealistic, having been postulated using the universally accepted but incorrect pesticide soil science. The objective of this work is to report experimental tests of a pesticide soil model in use by regulatory agencies and to suggest possible improvements. Tests with experimentally based data explain why PRZM.gw predictions can be wrong by orders of magnitude. Predictive spreadsheet models are the other category. They are experimentally based, with chemical stoichiometry applied to integral kinetic rate laws for sorption, desorption, intra-particle diffusion, and chemical reactions. They do not account for pesticide transport through soils. Each category of models therefore lacks what the other could provide. They need to be either harmonized or replaced. Some preliminary tests indicate that an experimental mismatch between the categories of models will have to be resolved. Reports of pesticides in the environment and the medical problems that overlap geographically indicate that government regulatory practice needs to account for chemical kinetics and mechanisms. Questions about possible cause and effect links could then be investigated.