IMPACT OF FUNNEL FLOW ON CONTAMINANT TRANSPORT IN SANDY SOILS - NUMERICAL-SIMULATION

Agrichemicals have played an essential role in maintaining the product ivity of arable lands in modern agriculture. Recently, it was found th at preferential flow could greatly enhance the leaching of agrichemica ls 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 b y 6-m-deep two-dimensional hypothetical soil profiles with embedded co arse sand layers. The soil layering structures in these profiles were generated according to what was observed in the vadose zone of a Plain field sand (sandy, mixed, mesic Typic Udipsamment) in central Wisconsi n. Corresponding simulations were conducted in one-dimensional profile s as controls. Results showed that contaminant breakthrough time in th e two-dimensional profiles was only 25% of that in one-dimensional hom ogeneous profiles. The ratio of the total mass leached from the two-di mensional profile to that from the one-dimensional profile increased e xponentially as the water application rate decreased. This implies tha t: (i) the impact of funnel flow on contaminant transport is most dras tic when the net infiltration is low; and (ii) minimizing excess leach ing by carefully controlling the water budget alone might not prevent groundwater contamination. For a pesticide with a degradation coeffici ent of 3 x 10(-7) s(-1) and an adsorption coefficient of 0.5 cm(3) g(- 1) in the top 30 cm and 0.05 cm(3) g(-1) from 30 to 600 cm, about 4% o f the total mass would reach the groundwater through two-dimensional f unnel Bow paths with an averaged daily net infiltration rate of 1.15 m m d(-1).