Unsaturated Column Experiments Research Articles

Rate-Limited Transport of Hydroxyatrazine in an Unsaturated Soil

Unsaturated column experiments on an intact soil core were conducted under two different soil pressure heads, -8 and -19 cm, to characterize chemical sorption and desorption for hydroxyatrazine. Breakthrough curves for 3 H 2 O were similar under the different experimental conditions. The advection-dispersion transport model described 3 H 2 O transport well with Peclet numbers of 0.17 and 0.21, respectively. The transport of hydroxyatrazine, a common degradation product of atrazine, was slightly enhanced at the -8 cm head relative to the -19 cm head (84% vs 79% mass recovery). A model that describes sorption with a single rate coefficient and desorption as a distribution of rate coefficients fits the hydroxyatrazine breakthrough curve well. The strong similarities in transport properties at different soil-water tensions suggests that water and contaminants can be transported through macroporous soils across a range of moisture conditions in the vadose zone in agricultural fields.

Modeling Mass Transfer Processes in Soil Columns with Pore‐Scale Heterogeneity

AbstractRemoval of dissolved organic compounds from natural porous media is rate limited by multiple, simultaneous mass transfer processes, including slow diffusion from immobile zones of varying sizes and shapes, and rate‐limited sorption. We examined the variability in mass transfer rates and quantified the effects of multiple, simultaneous mass transfer processes on unsaturated column experiments using a diffusion model with a statistical distribution of diffusion rate coefficients. We examined the validity of conventional first‐order mass transfer and diffusion models to represent mass transfer in subsurface systems and compared this with a diffusion model with a lognormal distribution of diffusion rate coefficients. The main conclusions of our work are: (i) even in relatively homogeneous porous media, extreme variability (exceeding four orders of magnitude) in diffusion rate coefficients (Da/a2) must be invoked to represent mass transfer in the experiments we examined; (ii) models using a lognormal distribution of diffusion rate coefficients, while employing only one more mass transfer parameter than conventional models, generally represent mass transfer much better; (iii) single‐rate‐coefficient models (either first‐order or diffusion) very poorly represent mass transfer in all experiments examined, although some of this failure is attributed to our use of a linear isotherm; (iv) models with two diffusion rate coefficients, although containing twice as many estimated parameters, also offer poor representations of mass transfer.

VOC Accumulation and Pore Filling in Unsaturated Porous Media

A series of unsaturated column experiments was conducted to study different grain-scale accumulation mechanisms affecting total uptake of volatile organic compounds (VOCs) onto a model solid and subsequent removal of VOCs from the porous media. Experimental variables included VOC (benzene, methylbenzene, 1,4-dimethylbenzene, and 1,3,5-trimethylbenzene), moisture content (primarily water-unsaturated conditions), and influent VOC concentration. Calculations of the mass distributions of benzene indicated that it was primarily in the aqueous and air phases with a small fraction at the air−water interface. Similar calculations for the other VOCs indicated that greater than 50% of the accumulated mass of these VOCs was located within intraparticle pores and on the substrate surface. Analysis of the sorption data in terms of a pore-filling model support the hypothesis that a capillary phase separation (CPS) process occurred within the pores and produced a neat, separate VOC phase. We suggest that CPS will become more critical in materials with small mesopores or micropores, and that it is partly responsible for the existence of a resistant fraction of VOCs present within water-filled intraparticle pores.

Assessment of the Fate of Two Herbicides in a Wyoming Rangeland Soil: Column Studies.

Extensive use of dicamba (2-methoxy-3,6-dichlorobenzoic acid) and picloram (4-amino-3,5,6-trichloropicolinic acid) in arid Wyoming, along with large volumes of irrigation water used in some areas, has created a concern for the potential contamination of surface and groundwaters by these herbicides. Persistence and mobility of dicamba and picloram were investigated in a Wyoming rangeland soil using batch adsorption and soil column studies. The objectives of this study were to characterize soil chemical and physical properties that affect herbicide transport, examine herbicide sorption, model herbicide movement, and estimate degradation rate constants. Essentially no sorption of dicamba was detected; however, picloram sorption was greatest in the highest organic C content horizon. Both saturated (5.90, 2.96, and 0.82 kg ha-1 dicamba and 1.85, 0.97, and 0.47 kg ha-1 picloram) and unsaturated (2.76 and 1.00 kg ha-1 for dicamba and picloram, respectively) column experiments were conducted. The herbicides and Br tracer (34, 38, 69, and 137 mg L-1 ) were displaced through the soil columns using distilled water that was added in daily increments (60 mL d-1 ). Degradation rate constants were calculated using both a simple recovery fraction technique and by matching LEACHP-generated breakthrough curves to experimental data. For the two columns receiving intermediate application rates, anaerobic picloram dissipation was more rapid (t1/2 = 19 d) than for aerobic conditions (t1/2 = 87 d). The rate of dissipation of dicamha was approximately the same under aerobic and anaerobic conditions (t1/2 = 15 and 17 d in the saturated and unsaturated columns, respectively). Piclorum and dicamba dissipation was more rapid at the lowest application rates, with t1/2 of 13 and 10 d. At the highest application rates, t1/2 of 23 and 17 d were measured for picloram and dicamba, respectively. Both herbicides were found to be highly mobile, with the mobility of picloram increasing at higher pore-water velocities.