Numerical Analysis of Transport of Trifluralin From a Subsurface Dripper

gations that leached the material from the emitter zone reduced its effectiveness. Quantitative descriptions of The transport of a pulse of trifluralin (2,6-dinitro-N,N-dipropyl-4the fate and transport of trifluarlin in the soil, therefore, [trifluoromethyl] benzenamime) applied via a subsurfacedripper was analyzed numerically. Results of the analyses suggest that the moveare essential for the management of this potentially ment and spread of trifluralin in the soil is considerably retarded by hazardous chemical in the ecosystem with the aims of its strong adsorption to the solid phase of the soil. This is particularly maximizing its efficiency to prevent clogging of subsurso in soils which contain aconsiderable fraction of organic C and in face drip emitters and minimizing the potential of drainfine-textured (clayey) soils with low hydraulic conductivity and high age water to pollute underlying groundwater supplies. water retentivity. Water uptake by plant roots and the resultant rapid To the best of our knowledge, there has been no attempt decrease of water velocity with increasing distance from the dripper to quantify the fate and transport of trifluarlin in the soil. restricts further the downward movement of trifluralin and its potenThe general objective of our study is to investigate tial to pollute the groundwater. The presence of dissolved organic the fate and transport of trifluralin applied to the soil matter (DOM) in the irrigation water may enhance both the movevia a subsurface dripper. The specific objectives of this ment and the spread of trifluralin in the soil, particularly in coarsetextured soils with a relatively small fraction of organic C. Because study, were: (i) to investigate the effects of the soil hyof the strong adsorption of trifluralin to the soil, its concentration in draulic properties, the fraction of organic C in the soil the aqueous phase of the soil is very low and it decreases further with and the concentration of DOM in the irrigation water on increasing time because of degradation and nonequilibrium sorption. the fate and transport of trifluralin for a given applied This is particularly so in coarse-textured soils with a relatively large mass of trifluralin and a given dripper discharge, and (ii) fraction of organic C. Nevertheless, results of the analyses suggest to assess the effect of the aforementioned characteristics that for soils of quite widely differing textures and organic C contents, of the soil and the irrigation water on the persistence of a trifluralin concentration of ct 10 6 kg m 3 may persist in the vitrifluralin in the vicinity of the subsurface dripper, and on cinity of the dripper for a relatively long period of time (90 d) even its potential to pollute underlying groundwater supplies. with relatively small applied mass (3 10 5 kg). To pursue these objectives, we used the Richards equation to describe the flow, and the convection–dispersion equation (CDE) with a bicontinuum sorption model to T which is a potentially hazardous chemidescribe the transport, modifying an efficient numerical cal, is a soil-active preemergence herbicide that has method to solve the pertinent partial differential equabeen extensively used to control annual weeds. Its pritions (Russo et al., 1998). The present study is a numerimary mode of action is by affecting the physiological cal experiment that provides detailed information on the growth processes associated with root growth and elonconsequences of characteristics of the soil and the irrigagation. Most of the studies conducted to determine the tion water for the transport of trifluralin under realistic effect of trifluralin on root growth used relatively high conditions. At the price of reduced generality, this aptrifluralin concentrations (Talbert, 1965; Hackskaylo proach circumvents most of the stringent assumptions and Amato, 1968; Mallory and Bayer, 1972). Thus, no of theoretical studies and facilitates analysis of simpliminimum concentration has been reported below which fied yet realistic situations, at a fraction of the cost of plant roots grow and divide normally. In a recent study physical experiments. (Ganmore-Neumann and Gerstl, 1996), it was found that concentrations as low as a few 10 6 kg m 3 could THEORY inhibit corn (Zea mays L.) root growth and elongation. In recent years trifluralin has been used with much The Physical Domain and success to prevent clogging of subsurface drip emitters the Simulated Scenario by plant roots. In initial studies, commercially available We considered the movement of trifluralin from a subsurtrifluralin was applied via the irrigation water and was face dripper in a flow domain in the shape of a parallelepiped found to remain in the immediate vicinity of the emitter. consisting of a rigid, homogeneous, and isotropic soil with a However, degradation of the chemical and ensuing irriprescribed initial pressure head, i. A scheme of the flow domain is depicted in Fig. 1. Water is infiltrated into the D. Russo, A. Laufer, Department of Environmental Physics and Irrisoil from a subsurface dripper by an imposed time-dependent gation, Institute of Soils, Water and Environmental Sciences Agriculdischarge, [F(t) Q, t j t t′′ j , F(t) 0, elsewhere], where tural Research Organization The Volcani Center, Bet Dagan 50250, Q is the dripper discharge, t t′′ j t j is the duration of an Israel; J. Zaidel, Department of Environmental Physics and Irrigation, irrigation event, t t j t′′ j 1 is the time interval between AGRA Earth & Environmental Ltd., 160 Traders Blvd. East, Suite successive events, j 1 to NI, NI is the number of events, 110, Mississauga, ON, L4Z 3K7 Canada; Zev Gerstl, Department of and t t is the duration of an irrigation cycle. Soil Physical & Environmental Chemistry, Institute of Soils, Water Neglecting evaporation, water may leave the flow system and Environmental Sciences Agricultural Research Organization The Volcani Center, Bet Dagan 50250, Israel. Received 27 Oct. 2000. Abbreviations: 3-D, three-dimensional; CDE, convection dispersion *Corresponding author ([email protected]). equation; DOM, dissolved organic matter; HOC, hydrophobic organic compounds; PPCG, polynomial preconditioned conjugate gradient. Published in Soil Sci. Soc. Am. J. 65:1648–1658 (2001).