Runoff volume and flow concentration are hydrological factors that limit effectiveness of vegetated filter strips (VFS) in removing pesticides from surface runoff. Empirical equations that predict VFS pesticide effectiveness based solely on physical characteristics are insufficient on the event scale because they do not completely account for hydrological processes. This research investigated the effect of drainage area ratio (i.e., the ratio of field area to VFS area) and flow concentration (i.e., uniform versus concentrated flow) on pesticide removal efficiency of a VFS and used these data to provide further field verification of a recently proposed numerical/empirical modeling procedure for predicting removal efficiency under variable flow conditions. Runoff volumes were used to simulate drainage area ratios of 15:1 and 30:1. Flow concentration was investigated based on size of the VFS by applying artificial runoff to 10% of the plot width (i.e., concentrated flow) or the full plot width (i.e., uniform flow). Artificial runoff was metered into 4.6-m long VFS plots for 90 min after a simulated rainfall of 63 mm applied over 2 h. The artificial runoff contained sediment and was dosed with chlorpyrifos and atrazine. Pesticide removal efficiency of VFS for uniform flow conditions (59% infiltration; 88% sediment removal) was 85% for chlorpyrifos and 62% for atrazine. Flow concentration reduced removal efficiencies regardless of drainage area ratio (i.e., 16% infiltration, 31% sediment removal, 21% chlorpyrifos removal, and 12% atrazine removal). Without calibration, the predictive modeling based on the integrated VFSMOD and empirical hydrologic-based pesticide trapping efficiency equation predicted atrazine and chlorpyrifos removal efficiency under uniform and concentrated flow conditions. Consideration for hydrological processes, as opposed to statistical relationships based on buffer physical characteristics, is required to adequately predict VFS pesticide trapping efficiency.

This study was conducted to reconcile an apparent inconsistency between the simazine laboratory sorption isotherm data and the field lysimeter transport experiment reported by Poletika et al. (this issue). In this investigation, linear and nonlinear one-and two-stage simazine sorption models were fitted to the sorption and desorption isotherm laboratory data to obtain parameter estimates for use in the transport model. Once obtained, the calibrated sorption model was combined with the parameterized outflow concentration record from a mobile Br tracer to represent rate-limited sorption and transport of the simazine added simultaneously with the Br. The calil•rated model did an excellent job of representing the final simazine profile in the soil, particularly with the nonlinear model. This is in contrast to a single-stage adsorption model tested by Poletika et al. (this issue), which reached poor agreement with the field profile when laboratorymeasured sorption parameters were used. The results demonstrate the compatibility of field and laboratory experiments on pesticide movement and also indicate that sorption isotherms may require substantially longer to reach equilibrium than is customarily allowed in current protocols. Pignatello, J. J., Sorption dynamics of organic compounds in soils and sediments, in Reactions and Movement of Organic Chemicals in Soils, edited by B. L. Sawhney and K. Brown, pp. 31-44, Soil Science Society of America, Madison, Wisc., 1989. Poletika, N., and W. A. Jury, Effects of soil surface management on water flow distribution and solute dispersion, Soil Sci. Soc. Am. J., 58, 999-1006, 1994. Poletika, N., W. A. Jury, and M. L. Yates, Transport of bromide, simazine, and MS-2 coliphage in a lysimeter containing undisturbed, unsaturated soil, Water Resour. Res., this issue. Press, W. H., B. P. Flannery, S. A. Teukolsky, and W. T. Vetterling, Numerical Recipes in C, The Art of Scientific Computing, Cambridge University Press, New York, 1988. Rao, P.S. C., and J. M. Davidson, Estimation of pesticide retention and transformation parameters required in nonpoint source pollution models, in Environmental Impact of Nonpoint Source Pollution, edited by M. R. Overcash and J. M. Davidson, pp. 23-67, Butterworth, Stoneham, Mass., 1980. Seber, G. A. F., and C. J. Wild, Nonlinear Regression, John Wiley, New York, 1989. Selim, H. M., J. M. Davidson, and R. S. Mansell, Evaluation of a two-site adsorption-desorption model for describing solute transport in soils, paper presented at Summer Computer Simulation Conference, Simul. Counc., Washington, D.C., 1976. Susarla, S., G. V. Bhaskar, and S. M. R. Bhamidimarri, Adsorptiondesorption characteristics of some phenoxyacetic acids and chlorophenols in a volcanic soil, I, Equilibrium and kinetics, Environ. Technol. Lett., 14, 159-166, 1993. Swanson, R. A., and G. R. Dutt, Chemical and physical processes that affect atrazine and distribution in soil systems, Soil Sci. Soc. Am. Proc., 37, 872-876, 1973. Talbert, R. E., and O. H. Fletchall, The adsorption of some s-t...

The effect of rate-limited adsorption on transport of environmental contaminants is difficult to characterize at the field scale. This study investigated transport, during unsaturated water flow, of pulse inputs of bromide, simazine (2-chloro-4,6bis(ethylamino)-s-triazine), and MS-2 coliphage in a field lysimeter (0.8 rn x 0.8 rn square) containing undisturbed Tujunga loamy sand (mixed, thermic, Typic Xeropsamment). Sixty-four fiberglass wick soil solution samplers collected drainage fractions from the exit surface (30 cm depth) following daily 2-cm water inputs applied at 0.5 cm h -1. After 19.7 cm of cumulative drainage, the soil above 10 of the 64 locations was sampled to determine final depth distributions of simazine and virus. Most of the bromide was leached from the transport volume, while the sorbing pesticide and virus remained in the soil. Variance analysis indicated that local dispersion processes contributed more to the observed bromide spreading than did differences in local water velocities. A linear, first-order, kinetic adsorption submodel was incorporated into a generalized linear transport model relating the bromide flux concentrations to the simazine and virus final resident concentrations. Least squares fitting showed that areaaveraged bromide transport could be described reasonably well by the two-parameter convection-dispersion model (CDM), although the mobile-immobile water model provided a slightly better representation of effluent tailing. The CDM parameters fitted to the bromide data were then held constant while the two parameters of the adsorption submodel were varied to fit the pesticide soil concentrations at the end of the experiment at 10 days. A good fit was obtained for simazine, and the fitted value 0.54 d -1 of the rate coefficient was in the range characterizing nonequilibrium adsorption. A batch adsorption/ desorption experiment produced Freundlich isotherms describing nonlinear adsorption (exponent rn = 0.85) and hysteresis in desorption. There was poor agreement between the retardation factor (R) estimated from a linearized batch distribution coefficient K a and the R fitted to lysimeter data. Virus concentrations fitted to the model yielded coefficients implying strong adsorption (R -254) and rapid inactivation (inactivation rate coefficient of 1.64 d-•), whereas the laboratory sorption study implied that the virus should be very mobile in soil. The difference in field and laboratory sorption may be due to air-water interfacial forces in the unsaturated field experiments. IntroductionLeaching of environmental contaminants such as pesticides and human pathogenic microorganisms through the vadose zone to groundwater represents an important threat to public health [Craun, 1985]. For this reason there is much interest in characterizing the transport and fate processes governing the movement of these contaminants at the field scale. A critical issue in modeling pesticide or pathogen transport and fate in the natural environment is determining the extent to which model parameter...

The fiberglass wick soil solution sampler has shown potential for use in unsaturated soil, but its effect on determination of transport properties is not fully characterized. This study investigated transport behavior in the sampler of Br‐, simazine [2‐chloro‐4,6‐bis(ethylamino)‐s‐triazine], and MS‐2 coliphage. Travel‐time moment analysis for several steady flow experiments involving narrow pulse inputs of these components showed rapid breakthrough with well‐defined peaks and negligible adsorption. Assuming a convection‐dispersion process in a two‐layer system consisting of homogeneous soil overlying a sampler, moment analysis suggests that, for the flow rates tested, both expected travel time and its variance are little influenced by transport through the porous wick for layer thicknesses commonly used in solute transport studies.

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