The Influence of Macropores on Flow and Transport to Subsurface Drains in Low Permeability, Salt Affected Soils.

摘要

The use of subsurface tile drains in conjunction with surface irrigation is proposed as a method for remediating salt affected soils, which are a major environmental issue for the oil and gas industry. Understanding the influence macropores have on the effectiveness of the tile drains in removing salts from the soil is required for future development of similar remediation sites. A multi-year study using conservative benzoic acid groundwater tracers was performed at a field site located in central Alberta in order to examine the affect macropores have on fluid flow and solute transport to subsurface tile drains through low permeability salt affected soils. Regular surface irrigations occurred during the 2009 and 2010 field seasons over a 20 × 20 m test plot and were combined with detailed temporal monitoring of tile drain discharge rates and effluent tracer concentrations. The spatial distribution and concentration of the tracers were also monitored using soil and pore water sampling. Results were then used to construct a dual permeability flow and transport model using the software package HYDRUS to aid in the understanding of flow and transport dynamics occurring on site. The tracer breakthrough to the tile drains occurred 46.75 hours after application to the ground surface. After two field seasons, 7% of the initial applied tracer mass was recovered in the tile drains and 32% of the initial mass was accounted for in soil core extractions taken from directly below the plot which indicates solute flushing is occurring under the plot. The tile drains were able to capture 5.9% and 51% of the applied irrigation (including precipitation) water volume in 2009 and 2010 respectively. The large discrepancy between the tile drain capture volumes is thought to be a function of the antecedent conditions in 2009 and 2010. Initial modeling results were successful at simulating the tile drain discharge, but were less successful in matching the effluent tracer concentrations. Field observations and simulation results suggested that although macropores accounted for the majority of the fluid and solute transport, flushing of tracer from the soil to the tile drains was still occurring.