Abstract
Abstract. Preferential flow is a widespread phenomenon that is known to strongly affect solute transport in soil, but our understanding and knowledge is still poor of the site factors and soil properties that promote it. To investigate these relationships, we assembled a database from the peer-reviewed literature containing information on 733 breakthrough curve experiments under steady-state flow conditions. Most of the collected experiments (585 of the 733 datasets) had been conducted on undisturbed soil columns, although some experiments on repacked soil, clean sands, and glass beads were also included. In addition to the apparent dispersivity, we focused our attention on three indicators of preferential solute transport: namely the 5%-arrival time, the holdback factor, and the ratio of piston-flow and average transport velocities. Our results suggest that, in contrast to the 5%-arrival time and the holdback factor, the piston-flow to transport velocity ratio is not related to preferential macropore transport but rather to the exclusion or retardation of the applied tracer. Confirming that the apparent longitudinal dispersivity is positively correlated with the travel distance of the tracer, our results also illustrate that this relationship is refined if the normalized 5%-tracer arrival time is also taken into account. In particular, we found that the degree of preferential solute transport increases with apparent dispersivity and decreases with travel distance. A similar but weaker relationship was observed between apparent dispersivity, 5%-tracer arrival time, and lateral observation scale, such that the degree of preferential transport increases with lateral observation scale. However, we also found that the travel distance and the lateral observation scale in the investigated dataset are correlated, which makes it difficult to distinguish their influence on these transport characteristics. We also found that the strength of preferential transport increased at larger flow rates and water saturations, which suggests that macropore flow was a more important flow mechanism than heterogeneous flow in the soil matrix. Nevertheless, our data show that heterogeneous flow in the soil matrix also occasionally leads to strong preferential transport. Furthermore, we show that preferential solute transport under steady-state flow depends on soil texture in a threshold-like manner: moderate to strong preferential transport was found to occur only for undisturbed soils that contain more than 8% clay. Preferential flow characteristics were also absent for columns filled with glass beads, clean sands, or sieved soil. No clear effect of land use on the pattern of solute transport could be discerned, probably because the available dataset was too small and too strongly affected by cross-correlations with experimental conditions. Our results suggest that, in developing pedotransfer functions for solute transport properties of soils, it is critically important to account for travel distance, lateral observation scale, and water flow rate and saturation.
Highlights
During recent decades the number and quantity of man-made substances that are released onto the soil has been increasing exponentially
We investigated the controls on inert solute transport based on 733 breakthrough curve experiments collected from the peer-reviewed literature, mostly conducted on undisturbed soil columns
We focused especially on four breakthrough curve shape-measures: namely the normalized 5 %-arrival time, the holdback factor, the apparent longitudinal dispersivity and the ratio of piston-flow and average transport velocities
Summary
During recent decades the number and quantity of man-made substances that are released onto the soil has been increasing exponentially. Rose, 1977; Beven et al, 1993; Griffioen et al, 1998; Oliver and Smettem, 2003) In these studies, the authors investigated correlations among CDE and MIM model parameters of between 50 and 359 BTC experiments, but links to soil properties and experimental conditions were hardly discussed. Another large database of BTC data was published by Vanderborght and Vereecken (2007) It contains 635 datasets of flux and resident concentration BTC experiments with conservative tracers on undisturbed soil and covers all scales between the small column-scale and the fieldscale. The same observation had been previously reported for tracer experiments in groundwater (Gelhar et al, 1992; Neuman, 1990)
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