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.

Groundwater contaminated with 500−1200 μg/L trichloroethylene (TCE) was treated in situ over a 410-day period by cometabolic biodegradation through injection of 7−13.4 mg/L toluene, oxygen, and hydrogen peroxide in groundwater circulated between two contaminated aquifers through two treatment wells located 10 m apart. One well pumped contaminated groundwater from the 8 m thick upper aquifer to the 5 m thick lower aquifer, while the other pumped contaminated water from the lower to the upper aquifers using flow rates of 25−38 L/min, effecting groundwater circulation between them. Following 18 days of periodic toluene injection to develop an active biological population, continuous pulses of toluene were added. Over 312 days, an average 87 ± 8% TCE removal was obtained in the upper aquifer with each pass through the treatment well. In the lower aquifer, removals were 83 ± 16% over the last 79 days when peroxide addition was reduced. Treatment reduced TCE in the regional groundwater plumes from about 1000 μg/L in new water entering the 480 m2 monitored treatment zone to an average of 18−24 μg/L in groundwater leaving the treatment zone, indicating total TCE removal of 97−98%. Pumping heads for groundwater recirculation were less than 6 m. Toluene was removed by 99.98% through biodegradation to an average of 1.1 ± 1.6 μg/L at the 22 m × 22 m boundaries of the study zone, well below the goal of 20 μg/L maximum.

This paper focuses on the analysis and application of an analytical model that incorporates solute diffusion within immobile regions into the three‐dimensional advection/dispersion solute transport equation. The diffusion term of the model is formulated using either a first‐order rate expression or an expression assuming Fickian diffusion into spherical, cylindrical, or rectangular immobile regions. In order to assist in the analysis of solute transport behavior by means of the models, a modified form of Aris's method of moments is developed, which permits the calculation of the spatial and temporal moments of solute distributions simulated using the three‐dimensional diffusion models, without having to invert the Laplace‐ or Fourier‐transformed solutions. By using this method, the moments of the diffusion models are compared with one another, with the moments of a model that assumes equilibrium advective/dispersive transport, and with the moments of a model that assumes that a first‐order rate law governs mass transfer between the mobile and immobile regions. The method of moments also is used to assess the differences in the spatial and temporal moment behavior of each transport model under discussion.

The environmental and biodiversity benefits of organic farming are widely recognized, but there is still controversy about the effects of organic production methods on the nutritional composition of food and human health. In the first part of this article therefore, we critically review the evidence that organic farming methods improve the nutritional quality of food crops. Moreover, we summarize our current understanding of how quality gains are linked to the implementation of the “innovations” introduced into conventional crop production during the intensification or “green revolution” of agriculture over the last 100 years. In the second part of the article, we critically review the evidence for the range of health benefits related to organic food consumption. Specifically, we describe and discuss the results from: (i) dietary intervention studies which have found that organic food consumption substantially reduces pesticide exposure in humans and affects feed intake, growth, hormone balances and immune system responsiveness in animal models; (ii) human cohort/epidemiological studies which have reported significant positive associations between organic food consumption and the lower incidence of a range of diseases including obesity, metabolic syndrome, cancer, hypospadias, pre-eclampsia, eczema and middle ear infections in infants; (iii) interactions and trade-offs between diet (e.g., whole-grain, fruit and vegetables and reduced red-meat consumption) and food types (organic versus conventional) concerning public health and future food security. The article also identifies knowledge gaps and highlights the need for (i) long-term, factorial field experiments to understand the relative effects of agronomic and pedoclimatic drivers on crop quality and safety, and (ii) clinical trials and additional human cohort studies to confirm the positive health outcomes linked to organic food consumption. The main conclusions from our review are that there is growing evidence that (i) agricultural intensification has resulted in a reduction in the nutritional quality of food and the sustainability of food production, and (ii) organic farming practices not only improve food quality and human health, but also food security. This is particularly true where current nutritional guidelines (increasing whole-grain, fruit and vegetable products, while reducing red-meat consumption) are implemented.

Tailing of breakthrough responses, which has been experimentally observed during flow through porous media, can be modeled by dividing the porous medium into regions of mobile and immobile water, and coupling the advective-dispersive solute transport equation with expressions to describe diffusional transfer between the two regions. Three-dimensional solutions to this coupled set of partial differential equations with infinite boundary conditions are derived by applying the Laplace transform to the equations with respect to time, and the Fourier transform with respect to space. The solutions presented herein may be useful in applying the two-region models to field settings. Wakao, N.