Sampling solute concentrations in the vadose zone provides an early‐warning system for groundwater pollution. Various sampling devices are available. This study evaluates the functionality of fiberglass wicks in a soil solution sampler. Fiberglass wicks act as a hanging water column, drawing water from the undisturbed field soil without external application of suction. Flow characteristics of wicks were observed in the laboratory by miscible‐displacement tests using Br and an organic dye. The matric potential in the wick as a function of flow rate was measured. Fiberglass wicks behaved like a porous medium that effectively could apply a suction to the soil while only minimally retarding the organic dye. The relationship between matric potential, moisture content, and unsaturated conductivity was determined for several wicks. Based on these relationships, a set of curves was produced depicting travel time of nonadsorbed chemicals and the matric potential in the wick as a function of flow rate. These curves can be used for selecting optimum wick length and diameter for a given sampler configuration.
Both physical and biological processes have temporal and Structural pores associated with macropore-type preferential flow spatial patterns of formation and destruction cycles pathways can accelerate chemical transport in unsaturated soils, thereby potentially causing groundwater contamination. To predict chemical (Gupta et al., 2002). Tillage practices and compaction, transport through these pathways, classical deterministic models de-for example, often destroy the continuity of large strucpend on soil hydraulic conductivity, which effectively lumps flow contural pores (Isensee et al., 1990). The coefficient of varitributions from all individual pathways. We contend, however, that quanation of soil hydraulic conductivity, which is often dictifying the pore spectrum of preferential pathways, without lumping the tated by the soil structural pores, ranged from 100 to contributions of individual pores, is the appropriate method for simu-400% (Libardi et al., 1980; Warrick and Nielsen, 1980). lating convective chemical transport through macropore-type prefer-This suggests that the spatial variability of soil structural ential pathways. In this study, we conducted field-scale experiments pores measured by using core-or block-sized samples is by using an improved tile drain monitoring protocol to measure the very large. As a result, the size spectrum of large strucmass flux breakthrough patterns of conservative tracers. The tails of tural pores measured at several random locations by these patterns suggested that the impact of preferential pathways on contaminant transport can be conceptualized as that occurring through small sample sizes in a field may not represent the speccylindrical capillary tubes. We then proposed a distribution function trum of the entire field. Temporal extrapolation of meabracketed by sharp cutoff points to represent the pore spectrum of surements may be similarly invalid. For these reasons, these tubes. Finally, we used the measured tracer breakthrough curves it is difficult to directly measure the field-scale spectrum (BTCs) as data sources to find the parameters of the proposed funcof the large structural pores, yet this property is among tion. Our results, based on the best fitting, showed that the preferential the most important soil properties when dealing with pathways are naturally clustered into domains; preferential pathways issues related to water quality. with a wide range of pore radii could become active simultaneously Soil characteristic curves and hydraulic conductivity when infiltration rate increases. Because the derived pore spectra sicurves were used in past research from the 1950s and multaneously satisfy both water movement and solute transport, pore 1960s [summarized by Hillel (1980, p. 183-185) and Jury spectra can be used to (i) calculate soil hydraulic conductivity of preferential pathways in deterministic approaches, and (ii) construct multi-et al. (1991, p. 89-94)] to quantify the soil pore-size ple probability density functions (PDFs) for the transfer function ap-d...
Abstract. Artificial capillary barriers are being used to divert water away from sensitive underground regions. Conversely, funneled flow over natural capillary barriers may increase the danger of groundwater contamination by decreasing the travel time and contact area. There have been relatively few experimental studies of capillary barrier flow patterns. In this study, water was applied uniformly across the top surface of a backlit tilting chamber, 1 cm thick, 110 cm high, and 180 cm long, in which a coarse sand layer was imbedded in a fine sand. Bedding slope and water application rates were varied between 0 ø and 12 ø and • 1 and $ cm h-, respectively. After attaining steady state, matric potential was measured along the textural interface, and photos of dye traces were taken in order to visualize streamlines. The funneled flow was characterized by three discrete regions: an initial capillary diversion, a breakthrough region, and a toe diversion. The breakthrough region consisted of a significant zone of partial breakthrough where the vertical flux into the coarse layer was less than the water application rate. The lateral distance of the capillary diversion was explained well by previously published relationships when the water entry value at the textural interface was replaced by lower, observed matric potential at which breakthrough occurred at the most upslope point. The length of the capillary diversion was overpredicted using the air entry value. Finally, the toe of the coarse layer had significant, observed effects on funneled flow patterns, which have previously received little, if any, attention. The results of this study imply that the slope of the coarse layer and infiltration rate will largely govern the effectiveness of capillary barriers and that capillary barriers are less effective than previously assumed.
Solute concentration and soluble dye studies inferring that preferential flow accelerates field-scale contaminant transport are common but flux measurements quantifying its impact are essentially nonexistent. A tile-drain facility was used to determine the influence of matrix and preferential flow processes on the flux of mobile tracers subjected to different irrigation regimes (4.4 and 0.89 mm h(-1)) in a silt loam soil. After tile outflow reached steady state either bromide (Br; 280 kg ha(-1)) or pentafluorobenzoic acid (PFBA; 121 kg ha(-1)) was applied through the irrigation system inside a shed (3.5 x 24 m). Bromide fluxes were monitored at an irrigation rate of 4.4 mm h(-1) while PFBA fluxes were monitored at an irrigation rate of 0.89 mm h(-1). At 4.4 mm h(-1) nearly one-third of the surface-applied Br was recovered in the tile line after only 124 mm of irrigation and was poorly fit by the one-dimensional convective-dispersive equation (CDE). On the other hand, the one-dimensional CDE fit the main PFBA breakthrough pattern almost perfectly, suggesting the PFBA transport was dominated by matrix flow. Furthermore, after 225 mm of water had been applied, less than 2% of the applied PFBA had been leached through the soil compared with more than 59% of the applied Br. This study demonstrates that the methodology of applying a narrow strip of chemical to a tile drain facility is appropriate for quantifying chemical fluxes at the small-field scale and also suggests that there may be a critical input flux whereby preferential flow is initiated.
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