ABSTRACT: Investigating macropore flow and transport in the vadose zone using a tension infiltrometer.

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Abstract

Macropore flow has been identified as a significant pathway for preferential migration of water and contaminants through the vadose zone, particularly in low permeability structured soils. In this study, controlled infiltration experiments were conducted on a macroporous soil in southern Ontario. The objective was to evaluate the influence of macropores on the vertical migration of water, solutes, and colloids. Dye and fluorescent latex microsphere tracers were applied in two separate tests using a tension infiltrometer. Test 1 was conducted under larger tensions to limit the influence of macropore flow, and Test 2 was completed under near-saturated conditions where macropore flow was expected to be more prevalent. Infiltration rates and soil moisture content were monitored during the tests to obtain information on bulk flow response and evaluate the shape of the near-saturated hydraulic conductivity function. Following infiltration, the test sites were excavated, the distribution of dye was mapped and photographed, and the soil was sampled for the presence of microspheres. Image analysis was used to quantify the observed dye tracer patterns. Our results show that surface infiltration conditions have a major influence on the initiation of macropore flow. Linear macropores (earthworm burrows and root holes) were a major flow and transport pathway under near-saturated conditions, resulting in dye migration to considerable depth. Dye tracer patterns correlated reasonably well with the presence of microspheres in the soil samples. The study provides valuable insights into macropore flow behaviour with the results having significant implications for groundwater recharge and aquifer vulnerability in macroporous soil environments.

Bibliography of Canadian Geomorphology