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A large-scale field experiment on natural gradient transport of solutes in groundwater has been conducted at a site in Borden, Ontario. Well-defined initial conditions were achieved by the pulse injection of 12 m s of a uniform solution containing known masses of two inorganic tracers (chloride and bromide) and five halogenated organic chemicals (bromoform, carbon tetrachloride, tetrachloroethylene, 1,2-dichlorobenzene, and hexachloroethane). A dense, three-dimensional array of over 5000 sampling points was installed throughout the zone traversed by the solutes. Over 19,900 samples have been collected over a 3-year period. The tracers followed a linear horizontal trajectory at an approximately constant velocity, both of which compare well with expectations based on water table contours and estimates of hydraulic head gradient, porosity, and hydraulic conductivity. The vertical displacement over the duration of the experiment was small. Spreading was much more pronounced in the horizontal longitudinal than in the horizontal transverse direction; vertical spreading was very small. The organic solutes were retarded in mobility, as expected. ford University advised on the selection of organic compounds; Gary Hopkins was instrumental in the design and implementation of the experiment. Kent Keller, Stephanie O'Hannesin, Ernie Kaleny, and Bill Blackport (University of Waterloo) contributed greatly during the instrumentation of the site and the collection of the samples. Other collaborators from the University of Waterloo included

The sorption of tetrachloroethene (PCE) and 1,2,4,5tetrachlorobenzene (TeCB) was studied on sandy aquifer material from Borden, ON, by using a batch methodology designed to accurately measure sorption over long equilibration periods. Autoclaving was effective in inhibiting biotransformation, and use of fire-sealed glass ampules precluded volatilization losses. Data analysis techniques were developed to accurately account for partitioning to sample headspace and other losses. Sorption isotherms for PCE and TeCB with Borden solids deviated from linearity when a 4-5 order of magnitude range in aqueous concentration was considered. However, in the dilute range (<50 /¿g/L), the deviations from linearity were inconsequential. The sorption of TeCB was approximately 40 times stronger than for PCE, in qualitative accordance with TeCB's approximately 100-fold greater octanol-water partitioning coefficient. For a given solute, the distribution coefficients differed by a factor of 30 among the various size fractions, being greatest for the largest grains. For most Borden solids, the long-term sorption of PCE and TeCB exceeded by more than 1 order of magnitude the predictions of generalized correlations based on hydrophobic partitioning into organic matter. This difference is believed to be partially the result of mineral contributions to sorption, but may also reflect unattainment of equilibrium in previously regressed results-in this study, contact times on the order of tens to hundreds of days were required. For Borden solids, pulverization of solid samples was shown to be a viable expedient to obviate the need for excessively long equilibrations.

Henry's Law constants for fission product elements and important activated elements are estimated for the system: dilute element, 1 atm oxygen pressure, and liquid silicate solvent. A lower oxygen pressure limit for use of these values is also presented. These constants may be used to calculate solubility of fission products in fallout as a function of temperature. In conjunction with diffusivities, half-lives, yields, and detonation parameters, these constants can be used to calculate fractionation effects in fallout. A simple estimating scheme is given for describing fission-product distribution and fractionation in fallout. Q > 1. The system, condensed dtato-near atmospheric pressure oxygen-very dilute gameous fission product (oxide), was chosen. The fission product was assumed to be so dilute that only one fission-product atom per gaseous molecule (or per dissolved ion) was considered possible. This assumption might be in error for Tc , As, andSb, which form polymeric gaseous specie. very readily, but otherwise should be adequate. 2. Where appropriate, the dissolution process was considered to be pure liquid fission-product silicate in the molten silicate fallout particles. This solutior, process was considered to be ideal. In cases where liquid fission-product silicate was believed to be unstable, pure liquid fission. product oxide or liquid fission product was assumed to form an ideal solution with the silicate.

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The long-term behavior of five organic solutes during transport over a period of 2 years in groundwater under natural gradient conditions was characterized quantitatively by means of moment estimates. Total mass was conserved for two of the organic compounds, carbon tetrachloride and tetrachloroethylene, while the total mass declined for three other compounds, bromoform, 1,2dichlorobenzene, and hexachloroethane. The declines in mass for the latter three compounds are interpreted as evidence of transformation of the compounds. Retardation factors for the organic solutes, relative to chloride, ranged from 1.5 to 9.0, being generally greater for the more strongly hydrophobic compounds. The retardation is attributed to sorption. The apparent retardation factor increased markedly for all compounds over the duration of the experiment, by as much as 150%. Results from temporal and spatial sampling were in good agreement when compared at the same scale of time and distance. APPROACH The experimental design and implementation have been described elsewhere [Mackay et al., this issue]. To recapitulate briefly, a uniform solution of five organic solutes and two inorganic tracers was introduced into the saturated zone of a sand aquifer in the form of an approximately rectangular prism [Mackay et al., this issue, Figure 6]. Thereafter the solutes were monitored as they migrated under the influence of the natural hydraulic gradient, which at the Borden site results in an average linear ground water velocity of approximately 0.09 m/day. Retardation factors were estimated in two ways: (1) by comparing average travel times estimated from breakthrough responses for the organic solutes with the travel time of chloride, based on time-series sampling at discrete points, and (2) by comparing the velocities of the organic solutes with that of the chloride tracer, based on analyses of samples taken from a three-dimensional sampling array at a particular time. The former is termed breakthrough sampling, and the latter is referred to as spatial snapshot or synoptic sampling. In interpreting the spatial data, the method of moment calculations described by Freyber•t [this issue] was employed. The zeroth moment of the spatial concentration data was used to estimate the mass in solution, and the first moment was used to estimate the position of the center of mass. The errors implicit in the sampling, analysis, and data interpretation are discussed by Mackay et al. [this issue] and Freyber•t [this issue].In the present paper, organic solute behavior is compared to that of chloride, assuming the latter to be a nonreactive tracer. A previous paper [Freyber•t, this issue] has demonstrated that chloride and bromide behaved similarly in all important respects, and thus the bromide data are omitted from this paper for simplicity.

Equations describing the transport of ion‐exchanging solutes governed by local chemical equilibrium through a saturated porous medium are well established in the literature. Concentration profiles resulting from the numerical solution of the general multispecies equations typically exhibit unusual and complicated features such as multiple fronts and plateau zones. This paper presents an analytical framework, based upon the theory of chromatography, which permits a priori characterization of certain key concentration profile features. The cases studied include both homovalent and heterovalent exchange in binary and ternary systems. In order to test its validity, the chromatographic analysis is applied to a field project involving direct injection of advanced treated municipal effluent into an aquifer. All of the major features observed in the available field data are accurately predicted by the chromatographic theory.

Results are presented from a field study that document the in‐situ biotransformation of trichloroethylene (TCE), cis‐dichLoroethylene (cis‐DCE), trans‐dichloroethylene (trans‐DCE), and vinyl chloride (VC) in a saturated, semiconfined aquifer. The enhanced biotransformation was accomplished by stimulating the growth of indigenous methane‐oxidizing bacteria (methanotrophs), which transform chlorinated aliphatic compounds by a cometabolic process to stable, nontoxic end products. Experiments were performed in the presence and absence of biostimulation by means of controlled chemical addition, frequent sampling, and quantitative analysis. The degree of biotransformation was assessed using mass balances and comparisons with bromide as a conservative tracer. Biostimulation of the test zone was successfully achieved by injecting methane‐ and oxygen‐containing ground water in alternating pulses under induced gradient conditions. After a few weeks of stimulation, methane concentrations gradually decreased below the detection limit within two meters of travel. Under active biostimulation conditions, 20 to 30% of the TCE was biotransformed during the first season of testing. Direct evidence for biotransformation of VC, trans‐DCE, cis‐DCE, and TCE was obtained in the second and third seasons of field testing. In the absence of biostimulation, the organic compounds concentrations at observation wells reached 95% of the injection concentration, demonstrating negligible losses due to abiotic processes. Biostimulation of the test zone resulted in a concurrent decrease in concentration of methane and the halogenated aliphatic compounds. The organic compounds were transformed within two meters of travel as follows: TCE, 20–30%; cis‐DCE, 45–55%; trans‐DCE, 80–90%; and VC, 90–95%. These results are in qualitative agreement with methane‐utilizing, mixed‐culture laboratory studies which indicate that the rate of biotransformation is more rapid when the molecules are less halogenated. A biotransformation intermediate was observed which was identified by GC‐MS analysis as trans‐dichloroethylene oxide (trans‐DCE epoxide), an expected intermediate based on laboratory studies. When methane addition was stopped, the concentration of the intermediate rapidly decreased, while halogenated compound concentrations slowly increased, indicating that active methane utilization was required for biotransformation to occur.