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| Titel |
Relations between macropore network characteristics and the degree of preferential solute transport |
| VerfasserIn |
M. Larsbo, J. Koestel, N. Jarvis |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 18, no. 12 ; Nr. 18, no. 12 (2014-12-16), S.5255-5269 |
| Datensatznummer |
250120568
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| Publikation (Nr.) |
 copernicus.org/hess-18-5255-2014.pdf |
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| Zusammenfassung |
| The characteristics of the soil macropore network determine the potential for
fast transport of agrochemicals and contaminants through the soil. The
objective of this study was to examine the relationships between macropore
network characteristics, hydraulic properties and state variables and
measures of preferential transport. Experiments were carried out under
near-saturated conditions on undisturbed columns sampled from four
agricultural topsoils of contrasting texture and structure. Macropore network
characteristics were computed from 3-D X-ray tomography images of
the soil pore system. Non-reactive solute transport experiments were carried
out at five steady-state water flow rates from 2 to 12 mm h−1. The
degree of preferential transport was evaluated by the normalised 5%
solute arrival time and the apparent dispersivity calculated from the
resulting breakthrough curves. Near-saturated hydraulic conductivities were
measured on the same samples using a tension disc infiltrometer placed on top
of the columns. Results showed that many of the macropore network
characteristics were inter-correlated. For example, large macroporosities
were associated with larger specific macropore surface areas and better local
connectivity of the macropore network. Generally, an increased flow rate
resulted in earlier solute breakthrough and a shifting of the arrival of peak
concentration towards smaller drained volumes. Columns with smaller
macroporosities, poorer local connectivity of the macropore network and
smaller near-saturated hydraulic conductivities exhibited a greater degree of
preferential transport. This can be explained by the fact that, with only two
exceptions, global (i.e. sample scale) continuity of the macropore network
was still preserved at low macroporosities. Thus, for any given flow rate,
pores of larger diameter were actively conducting solute in soils of smaller
near-saturated hydraulic conductivity. This was associated with larger local
transport velocities and, hence, less time for equilibration between the
macropores and the surrounding matrix which made the transport more
preferential. Conversely, the large specific macropore surface area and
well-connected macropore networks associated with columns with large
macroporosities limit the degree of preferential transport because they
increase the diffusive flux between macropores and the soil matrix and they
increase the near-saturated hydraulic conductivity. The normalised 5%
arrival times were most strongly correlated with the estimated hydraulic
state variables (e.g. with the degree of saturation in the macropores
R2 = 0.589), since these combine into one measure the effects of
irrigation rate and the near-saturated hydraulic conductivity function, which
in turn implicitly depends on the volume, size distribution, global
continuity, local connectivity and tortuosity of the macropore network. |
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