NUMERICAL-ANALYSIS OF FIELD-SCALE TRANSPORT OF BROMACIL

Field-scale transport of bromacil (5-bromo-3-sec-butyl-6-methyluracil) was analyzed using two different model processes for local descriptio n of the transport. The first was the classical, one-region convection dispersion equation (CDE) model while the second was the two-region, mobile-immobile (MIM) model. The analyses were performed by means of d etailed three-dimensional, numerical simulations of the how and the tr ansport [Russo, D., Zaidel, J. and Laufer, A., Numerical analysis of f low and transport in a three-dimensional partially saturated heterogen eous soil. Water Resour. Res., 1998, in press](1), employing local soi l hydraulic properties parameters from field measurements and local ad sorption/desorption coefficients and the first-order degradation rate coefficient from laboratory measurements. Results of the analyses sugg est that for a given flow regime, mass exchange between the mobile and the immobile regions retards the bromacil degradation, considerably a ffects the distribution of the bromacil resident concentration, c, at relatively large travel times, slightly affects the spatial moments of the distribution of c, and increases the skewing of the bromacil brea kthrough and the uncertainty in its prediction, compared with the case in which the soil contained only a single (mobile) region. Mean and s tandard deviation of the simulated concentration profiles at various e lapsed times were compared with measurements from a field-scale transp ort experiment [Tauber-Yasur, I., Hadas, A., Russo, D. and Yaron, B., Leaching of terbuthylazine and bromacil through field soils. Water, Ai r Soil Poln., 1998, in press](2) conducted at the Bet Dagan site. Give n the limitations of the present study (e.g. the lack of detailed fiel d data on the spatial variability of the soil chemical properties) the main conclusion of the present study is that the field-scale transpor t of bromacil at the Bet Dagan site is better quantified with the MIM model than the CDE model. (C) 1998 Elsevier Science Limited. All right s reserved.