Impact of Funnel Flow on Contaminant Transport in Sandy Soils: Numerical Simulation

Abstract

AbstractAgrichemicals have played an essential role in maintaining the productivity of arable lands in modern agriculture. Recently, it was found that preferential flow could greatly enhance the leaching of agrichemicals into groundwater. This study investigated how contaminant transport would be influenced by funnel‐type preferential flow paths in a sandy vadose zone. Numerical simulations were conducted in four 12‐m‐wide by 6‐m‐deep two‐dimensional hypothetical soil profiles with embedded coarse sand layers. The soil layering structures in these profiles were generated according to what was observed in the vadose zone of a Plainfield sand (sandy, mixed, mesic Typic Udipsamment) in central Wisconsin. Corresponding simulations were conducted in one‐dimensional profiles as controls. Results showed that contaminant breakthrough time in the two‐dimensional profiles was only 25% of that in one‐dimensional homogeneous profiles. The ratio of the total mass leached from the two‐dimensional profile to that from the one‐dimensional profile increased exponentially as the water application rate decreased. This implies that: (i) the impact of funnel flow on contaminant transport is most drastic when the net infiltration is low; and (ii) minimizing excess leaching by carefully controlling the water budget alone might not prevent groundwater contamination. For a pesticide with a degradation coefficient of 3 × 10−7 s−1 and an adsorption coefficient of 0.5 cm3 g−1 in the top 30 cm and 0.05 cm3 g−1 from 30 to 600 cm, about 4% of the total mass would reach the groundwater through two‐dimensional funnel flow paths with an averaged daily net infiltration rate of 1.15 mm d−1.