THE INFLUENCE OF ARTIFICIAL MACROPORES ON WATER AND SOLUTE TRANSPORT IN LABORATORY SOIL COLUMNS

The role of macropores in infiltration through a sandy loam was studie d using laboratory columns pretreated with water possessing a delta(18 )O signature of -8.7 parts per thousand and 15 mg l(-1) Cl-. A simulat ed snowmelt pulse of delta(18)O-depleted water containing 1100 mg l(-1 ) Cl- was added to a control column and two columns containing a singl e vertical macropore, one continuous and the other discontinuous, Macr opores were formed in situ by disintegration of a biodegradable foam t hread inserted during column packing. Macropores were 2 mm in diameter , which has been suggested to be the threshold for significant macropo re flow given the soil's mean textural pore diameter of 0.41 mm. Meltw ater was flushed from the columns by adding two pore volumes of isotop ically enriched water containing 15 mg l(-1) Cl- at a rate of 17.2 mm day(-1). Tensiometers, time domain reflectometry probes and suction sa mplers were used to monitor matric potential, soil water content and s oil water chemistry at 0.1 m Intervals down the columns, Column efflue nt was sampled daily for delta(18)O and Cl-. Mobile soil water content s (theta(m)) and dispersivities (epsilon) were estimated by fitting a one-dimensional analytical solution of the convection-dispersion equat ion to Cl- breakthrough curves (BTCs). theta(m) increased with depth i n all columns, whereas only the discontinuous macropore column showed an increase in epsilon with flow length. Cl- and delta(18)O breakthrou gh occurred earlier at all depths in the macropore columns relative to the control, resulting in larger epsilon values for macroporous soil. epsilon for a given Raw length tended to be greatest in the discontin uous macropore column, reflecting the role of internal catchment proce sses, Macropore presence was associated with decreased theta(m) during infiltration and bimodal BTCs in column effluent. The threshold ratio of macropore-to-micropore diameters at which macropores exert a detec table influence on water transport must be less than that examined her e. (C) 1997 Elsevier Science B.V.