Some conclusions that may be drawn from this work are
listed below:
• The fraction of organic carbon is highly variable in
the soil profile, ranging from 0.5 near the surface to about 0.08 at about 45 cm.
Sorption appears to be a long-term kinetic process in
most of the soils investigated and the equilibrium distributions are slightly nonlinear.
For the 24 soil samples investigated in this research,
there is a definite correlation between K^ and TOC.
Diffusion rate modeling was only mildly successful in modeling kinetic sorption data due to data scatter and discrepancy between isotherm equilibria and rate data equilibria.
PRZM was capable of predicting soil column flow fairly well, but its ability to track contaminant levels was unsatisfactory.
Soil heterogeneities complicate the predictive ability of general purpose models and must be accounted for to
obtain predictive accuracy.
6.2 Recommendations
The large amount of data that has resulted from this
comprehensive project could well be used to validate
unsaturated zone flow models. Given the desirability of
predicting solute movement in the unsaturated zone, the following recommendations can be made for future studies.
It is important to gain a better identification and quantification of metolachlor's degradation by-products in the field data to allow for comparison to model
output.
• Sorption rate effects should be included in predictive
models to investigate the contaminant retardation
during nonmonotonic flow.
Predictive models should also include the effect of
soil moisture hysteresis and mobile/immobile diffusion limitations. Their impact on solute transport in the
VII SEFEREHCE8
Addiscott, T. M., (1977). A simple computer model for
leaching in structured soils. Journal of Soil Science,. 28.
554-563
Addiscott, T. M., & Wagenet, R. J. (1985). Concepts of
solute leaching in soils: A review of modelling approaches.
Journal of Soil Science. 15. 411-424.
Armstrong, D. E., & Chesters, G. (1967). Adsorption
catalyzed chemical hydrolysis of atrazine. Environmental
Science and Technology. 2, 683-688.
Armstrong, D. E., Chesters, G., & Harris, R. F. (1967). Atrazine hydrolysis in the soil. Soil Science Society of America Proceedings. 31. 61-66.
Ball, W. P., Goltz, M. N., & Roberts, P. V., (1991).
Comment on "Modeling the transport of solutes influenced by
multiprocess nonequilibrium" by M. L. Brusseau, R. E. Jessup, and P. S. C. Rao. Water Resources Research. 27,
653-656.
Bouchard, D. C, Lavy, T. L., & Marx, D. B., (1982). Fate of metribuzin, metolachlor and fluometuron in soil. Weed Science, 30, 629-632.
Bowman, B. T., (1988). Mobility and persistence of the herbicides atrazine, metolachlor, and terbuthylazine in
plainfield sand determined using field lysimeters.
Environmental Toxicology Chemistry, 485-491.
Brady, N. C. (1990). The Nature and Properties of Soils.
New York: Macmillan Pxiblishing Company.
Braverman, M. P., Lavy, T. L., & Barnes, C. J. (1986). The
degradation and bioactivity of metolachlor in the soil.
Weed Science. 34. 479-484.
Briggs, G. G., Bromilow, R. H., & Evans, A. A. (1982). Relationships between lipophilicity and root uptake and
translocation of non-ionised chemicals by barley. Pesticide
Science. 13. 495-504.
Brusseau, M. L., Jessup, R. E., & Rao, P. S. C. (1989).
Modeling the transport of solutes influenced by multiprocess
Brusseau, M. L. & Rao, P. S. C, (1989a). The influence of sorbate-organic matter interactions on sorption
nonequilibrium. Chemosphere. 18, 1691-1706.
Brusseau, M. L. & Rao, P. S. C., (1989b). Sorption
nonideality during organic contaminant transport in porous
media. Critical Reviews in Environmental Control. 19 (1), 33-99.
Carsel, R. F., Smith, L. A., Dean, J. D., & Jowise, P. P.
(1984). User's Manual for the Pesticide Root Zone Model
rPRZM); Release 1. USEPA EPA-600/3-84-109. U. S. Gov.
Print. Office, Washington DC.
Chiou, C. T., Peters, L. J., & Freed, V. H. (1979). A
physical concept of soil-water equilibria for nonionic
organic compounds. Science. 206. 831-832.
Chiou, C. T., Porter, P. E. & Schmedding D. W. (1983). Partition equilibria of nonionic organic compounds between soil organic matter and water. Environmental Science and
Technology. 17. 227-231.
Crittenden, J. C., Hutzler, N. J., Geyer, D. G., Oravitz, J. L., & Friedman, G. (1986). Transport of organic compounds
with saturated groundwater flow: Model development and
parameter sensitivity. Water Resources Research. 22. 271- 284.
Freeze, R. A. (1971). Three-dimensional, transient,
saturated-unsaturated flow in a groundwater basin. Water
Resources Research. 7, 347-366.
Gerstl, Z. (1990). Estimation of organic chemical sorption
by soils. Journal of Contaminant Hydrology. 6, 357-375. Goltz, M. N. fit Roberts, P. V. (1986). Interpreting organic solute transport data from a field experiment using physical nonequilibrium models. Journal of Contaminant Hydrology. 1, 77-93.
Hillel, D. (1971). Soil and Water; Physical Principles and
Processes. New York: Academic Press.
Jaynes, D. B., (1984). Comparison of soil-water hysteresis
models. Journal of Hydrology^ 75. 287-299.
Jury, W. A., Spencer, W. F., & Farmer, W. J. (1983).
Behavior assessment model for trace organics in soil I:
Model description. Journal of Environmental Quality. 12.
Karickhoff, S. W. (1981). Semiempirlcal estimation of
sorption of hydrophobic pollutants on natural sediments and
soils. Chemosphere. 10. 833-846.
Karickhoff, S. W. (1984). Organic pollutant sorption in aquatic systems. Journal of Hydraulic Engineering. 110.
707-735.
Karickhoff, S. W., Brown, D. S., & Scott, T. A., (1979).
Sorption of hydrophobic pollutants on natural sediments.
Water Research. 11, 241-248.
Keller, K. E., (1991). Ph.D. Dissertation, Crop Science Department, North Carolina State University, Raleigh, NC.
Kool, J. B., & Parker, J. C. (1987). Development and
evaluation of closed-form expressions for hysteretic soil
hydraulic properties. Water Resources Research. 23. 105-
114.
League, K., & Green, R. E. (1991). Statistical and
graphical methods for evaluating solute transport models: Overview and application. Journal of Contaminant Hydrology^
7, 51-73.
Luckner, L., van Genuchten, M. Th., & Nielsen, D. R. (1989). A consistent set of parametric models for two-phase flow of
immiscible fluids in the subsurface. Water Resources
Research. 25. 2187-2193.
Miller, C. T. & Weber, W. J., Jr. (1986). Sorption of hydrophobic organic pollutants in saturated soil systems. Journal of Contaminant Hydrology^ 1, 243-261.
Mualem, Y. (1974). A conceptual model of hysteresis. Water
Resources Research, 10, 513-522.
Mualem, Y. (1976). A new model for predicting the hydraulic conductivity of unsaturated porous media. Water Resources
Research. 12, 425-435.
Neuman, S. P. (1973). Saturated-unsaturated seepage by finite elements. Journal of the Hydraulics Division.
Proceedings of the American Society of Civil Engineers. 99.
2233-2251.
Neuman, S. P. (1975). Galerkin's approach to saturated- unsaturated flow in pourous media. In C. Taylor, R. H. Gallagher, J. T. Oden, 0. C. Zienkiewicz, (Eds) Finite
Pedit, J. A. (1988). IAS-6 [Computer Program]. Chapel
Hill, NC: University of North Carolina, Environmental Sciences and Engineering Department.
Pedit, J. A., & Miller, C. T. (1988). The advantage of
higher order basis functions for modeling multicomponent
sorption kinetics. In M. A. Celia, L. A. Ferrand, C. A.
Breb, W. G. Gray, & G. F. Pinder (Eds.), Computational
Methods in Water Resources. Vol. 2. (pp. 293-298). Cambridge, MA: Computational Mechanics Publications.
Peter, C. J., & Weber, J. B. (1985). Adsorption, mobility, and efficacy of alachlor and metolachlor as influenced by
soil properties. Weed Science. 33. 874-881.
Pignatello, J. J., & Huang, L. Q. (1991). Sorptive
reversibility of atrazine and metolachlor residues in field soil samples. Journal of Environmental Quality. 20. 222- 228.
Poulovassilis, A. (1970). Hysteresis of pore water in granular porous bodies. Soil Science. 109, 5-12.
Ritter, W. F. (1990). Pesticcide contamination of
groundwater in the United States - A review. Journal of
Environmental Science. 1, 1-29.
Russo, D. & Bresler, E. (1981). Soil hydraulic properties as stochastic processes: I. and analysis of field spatial
variability. Soil Science Society of America Journal. 45,
682-687.
Russo, D., Jury, W. A. & Butters, G. L., (1989a). Numerical analysis of solute transport during transient irrigation 1. The effect of hysteresis and profile heterogeneity. Water
Resources Research, 25, 2109-2118.
Russo, D., Jury, W. A. & Butters, G. L., (1989b). Numerical
analysis of solute transport during transient irrigation 2.
The effect of immobile water. Water Resources Research. 25.
2119-2127.
Schwarzenbach, R. P. & Westall, J. (1981). Transport of
nonpolar organic compounds from surface water to
groundwater. Laboratory sorption studies. Environmental Science and Technology. 15. 1360-1375.
Topp, G. C. (1971). Soil-water hysteresis: the domain theory extended to pore interaction conditions. Soil Science Society of America Proceedings. 35, 219-225.
van Genuchten, M. Th. (1980). A closed-form equation for
predicting the hydraulic conductivity of unsaturated soils.
Soil Science Society of America Journal. 45. 892-898.
van Genuchten, M. Th. & Wieranga, P. J. (1976). Mass transfer studies in sorbing porous media. Soil Science
Society of j^erica Journal. 40. 473-480.
Voice, T. C, & Weber, W. J., Jr. (1985). Sorbent concentration effects in liquid/solid partitioning.
Environmental Science and Technology. 19. 789-796.
Wagenet, R. J. & Hutson, J. L. (1986). Predicting the fate
of nonvolatile pesticides in the unsaturated zone. Journal
of Environmental Quality. 15. 315-322.
Wagenet, R. J. & Rao, P. S. C. (1990). Modeling pesticide fate in soils. In H. H. Cheng, (Ed.) Pesticides in the Soil Environment (pp. 351-399). Madison, WI: SSSA, Inc.
Weber, J. B., & Miller, C. T. (1989). Movement of organic chemicals over and through soil. In B. L. Sawhney (Ed.) Reactions and Movement of Organic Chemicals in Soils. (pp 305-334). Madison, WI: SSSA, Inc.
Weber, W. J., Jr., & Miller, C. T., (1988). Modeling the sorption of hydrophobic contaminants by aquifer materials 1. Rates and equilibria. Water Research. 22. 457-464.
Wu, S., & Gschwend, P. M. (1986). Sorption kinetics of
hydrophobic organic compounds to natural sediments and soils. Environmental Science and Technology. 20, 717-725. Wu, S., & Gschwend, P. M. (1988). Numerical modeling of sorption kinetics of organic compounds to soil and sediment particles. Water Resources Research. 24, 1373-1383.