Role of macropores in solute transport under ponded water condition produced by laboratory simulated intense storms.

Soil macropores are widely studied because they are known to serve as avenues for rapid contaminant transport. Currently, it is understood that a macroporous medium develops regions of mobile and immobile water during flow. Consequently, solutes in the mobile phase bypass a considerable portion of the medium, thus limiting their natural attenuation. It is, however, not adequately documented in the literature what role the macropores play during intense rain events that result in variable ponding depths. Macropore flow under ponded water conditions was studied on two undisturbed soil columns taken from the loam soils of northeastern Iowa, U.S. The columns were placed on a four-segment outflow collector system. A slug of water labeled with Br was applied to the columns followed by simulation of 58 mm of rain on column 1 and 53.6 mm on column 2, in one hour, 30 minute, and 15 minute events. During the highest intensity rain event (15 minutes), the total volume of water that exited through the bottom of segments 1,2, 3, and 4 were 21.6%, 34.5%, 28.5%, and 15.4% in column 1, and 1.3%, 15.9%, 72.2%, and 10.5% in column 2, respectively. As the intensity of rain was increased from one hour to the 15 minute event, the total bromide mass recovery increased from 45% (29.2 mg) to 57% (36.9 mg) of the total application (65 mg) for column 1, and from 51 % (33.2 mg) to 59% (38.2 mg) for column 2, respectively. Finally, an examination of dye-stained high flow regions in column 1 demonstrated that macro-porosities made approximately 20% of the total cross-sectional area of the column. These results suggest that solute transport behavior in macroporous soils can be impacted by ponding depths, and intense rain events can severely restrict contaminant degradation in soils.