Experimental Investigation of Direct Connectivity between Macropores and Subsurface Drains during Infiltration

Abstract

Recent research indicates immediate breakthrough of surface‐applied contaminants in subsurface drainage by transport through macropores directly connected to the surface. This “direct connectivity” phenomenon was verified and investigated by conducting infiltration experiments (1‐cm ponded water at the soil surface) in a laboratory soil column (sandy loam soil with bulk density of 1.6 g cm−3) with a vertical artificial macropore placed directly above or shifted away from a lateral subsurface drain. The experimental setup allowed surface‐connected and buried macropore lengths to be varied from the surface to the subsurface drain depth without unpacking or disturbing the soil column between experiments. It was observed that the longer the buried macropore length (i.e., as the macropore approached the soil surface), the more rapid the response at the drain outlet in addition to an increased percentage of total drain flow through the macropore (35–40%). Breakthrough with surface‐connected macropores was significantly faster than with buried macropores, suggesting that breaking surface connectivity of macropores by tillage may be an important management strategy. For surface‐connected macropore experiments, the average ratio of steady‐state total (macropore and matrix) to matrix flow rates decreased as the distance from the drain increased: 2.4, 2.1, and 1.6 for distances of 0, 6.25, and 12.5 cm, respectively. Extrapolating this data to distances beyond 12.5 cm suggested that macropores located within 20 to 25 cm of the drain act as though directly connected in this sandy loam soil. This research verifies the “contributing area” concept hypothesized in previous field and numerical modeling studies.

Heterogeneity of Infiltration Rates in Alluvial Floodplains as Measured with a Berm Infiltration Technique

“…For water movement through soil, macropores have been shown to have a large impact on flow and solute transport (Thomas and Phillips, 1979;Fox et al, 2004;Djodjic et al, 2004;Akay and Fox, 2007;Gotovac et al, 2009). Gold and Kellogg (1997) specifically called for the development of unique sampling schemes and simulation models for situations where substantial infiltration occurs through preferential flow pathways, but limited work in monitoring, theoretical model development, and application has been achieved to date.…”

Evaluation of a Geonics EM31-3RT probe to delineate hydrologic regimes in a tile-drained field

“…Measured and simulated increases in EC were found for shallow (4 m) soil after the rainfall event as expected; however, what was unexpected was that the increase in EC was smaller above the tile drains compared to several meters away from them. This phenomenon could be related to the disturbed soil profile conditions after installation of the drains or perhaps, more likely, due to rapid drainage through macropores concentrated immediately above the tile drains as discussed in Shipitalo et al (2004), and Akay and Fox (2007). This study suggests that the Geonics EM31-3RT magnetic induction probe could be used to detect zones of relatively rapid drainage; such information can be important for refining site-specific management practices to minimize unintended losses of fertilizers and pesticides to the environment.…”

“…Although subsurface drainage removes excess water and improves plant growth, it also affects the amount and quality of water leaving agricultural land and entering surface water systems (Kladivko et al 2001). Moreover, the surface-connected cracks and macropores have been shown to provide a direct route for contaminant transport from the soil surface to subsurface drains (Shipitalo et al 2004;Akay and Fox 2007). In fact, tile drainage systems have been found to play an important role in connecting surfaceapplied contaminants to surface water bodies (Sims et al 1998;Jamieson et al 2002).…”

Modeling Parent and Metabolite Fate and Transport in Subsurface Drained Fields With Directly Connected Macropores1

“…As verified in the field by Shipitalo and Gibbs (2000) and laboratory by Akay and Fox (2006), macropores located within 20-50 cm of the drain tiles will transmit flow and pesticide directly into the drain line. This is currently being accounted for in the EF modification in that an EF of the water ⁄ pesticide reaching the water table is diverted directly into the tile line.…”

“…The model was capable of simulating these early peaks when using EF of 2%, similar to the value used at Allen County (Figure 3). The 2% EF is justified based on the field and laboratory observations that macropores located within 20-50 cm of drain lines are ''hydrologically'' directly connected (Shipitalo and Gibbs, 2000;Akay and Fox, 2006). This 20-50 cm area represents approximately 2-5% of the field.…”