Meta-analysis of the effects of soil properties, site factors and experimental conditions on solute transport

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, an...

Effects of Past Copper Contamination and Soil Structure on Copper Leaching from Soil

“…A positive relationship between bulk density and preferential transport in undisturbed soil columns has been found in several studies due to the presence of well‐aligned and connected cracks typical of soils with high clay content (Norgaard et al, 2013; Poulsen et al, 2006). After reviewing 580 experiments from the literature, Koestel et al (2012) found a clay content threshold of 8% for preferential flow in undisturbed columns. The clay content in our study field exceeded that threshold but was almost constant and we did not find any relationship with the preferential flow parameters.…”

Pedotransfer Functions in Earth System Science: Challenges and Perspectives

“…The solute spread is sometimes also described as the dispersivity or, in model‐independent studies, as the apparent dispersivity (Vanderborght et al, ). Two recent comprehensive metastudies on dispersivities have been published by Vanderborght and Vereecken (, n = 635) and Koestel et al (, n = 733). These studies have demonstrated that the dispersivity is positively correlated with the lateral and longitudinal scale of the experiment, the flow rate, and the clay content and negatively with the sand content, and the geometric mean particle size (Koestel et al, ).…”

“…Two recent comprehensive metastudies on dispersivities have been published by Vanderborght and Vereecken (, n = 635) and Koestel et al (, n = 733). These studies have demonstrated that the dispersivity is positively correlated with the lateral and longitudinal scale of the experiment, the flow rate, and the clay content and negatively with the sand content, and the geometric mean particle size (Koestel et al, ). It is likewise known from the same study that dispersivities are smaller for repacked than for undisturbed soil by 1 order of magnitude.…”

Solute Transportin Sand Columns as Affected by Effluent Surface Tension

“…Thus, molecular diffusion in the bulk solution and tortuosity of the porous medium () are important determinants of D. Koestel et al [21] reported a positive correlation between water flux rate (q) and D from large number of studies. In addition, the solute dispersion scales with the first power of velocity of water in capillary tubes [22].…”