Preferential movement of solutes through soils
Bruggeman, Adriana C. Jr.
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Detection of unexpectedly high concentrations of agricultural pollutants in ground water have inspired investigations of the role of preferential movement of chemicals through agricultural soils. This research focuses on preferential flow and solute transport processes and the effects of agricultural management practices on these processes. Experimental methods for monitoring preferential flow and solute transport in the field as well as a stochastic, physically-based model for predicting water flow and transport of non-reactive chemicals in heterogeneous soils with naturally occurring macropores were developed and evaluated. Field experiments, aimed at monitoring the occurrence of preferential flow and solute transport, were conducted in a conventionally-tilled and a no-till soybean field in the Coastal Plain of Virginia. A rainfall simulator was used to apply a one-hour storm at rates of 5.0, 6.5 and 7.5 cm/hr to six 1.83 by 1.83 m plots. Chloride was added to the water to serve as a non-reactive tracer. Electrical conductivity equipment provided a useful method for monitoring solute transport. The moisture and solute conditions, observed during a 28-hour period after the start of the rainfall event, clearly indicated the occurrence of preferential flow and solute movement in the field plots. The variability of the solute concentrations in the field plots was generally higher in the no-till plots than in the conventionally-tilled plots. The plots that received rain at 6.5 and 7.5 cm/hr showed more variability than the plots that received rain at 5 cm/hr. The observed solute concentrations indicated that if the solute transport would have taken place by advection only, 61% of the solute transport in the conventionally-tilled plots and 50% of the solute transport in the no-till plots could be attributed to preferential flow. A physically-based, finite element model for simulating flow and solute transport in variably-saturated soils with macropores (MICMAC) was developed. Flow and solute transport are described by the Richards' equation and the convection-dispersion equation. Flow in the macropores is described by the Hagen-Poiseuille equation. An axisymmetric coordinate system is used to simulate the flow and solute transport from the macropore into the surrounding soil matrix, assuming a vertically oriented, surface-vented, cylindrical macropore. Flow and solute transport between the macropore and the soil matrix are driven by the pressure head at the macropore-matrix boundary. To assess the natural heterogeneity of the soil properties a stochastic component was added to the model. Flow and solute transport at the field scale were simulated by regarding the field as a collection of statistically independent, non-interacting vertical soil columns, using Monte Carlo simulation. The sensitivity analysis of the model indicated that, for a soil with macropores, the model is most sensitive to the saturated water content of the soil matrix, the initial moisture content, and the rainfall rate. The model is not very sensitive to the macropore dimensions. Examination of the stochastic approach indicated that the representation of a heterogeneous field as a collection of non-interacting stream columns may substantially underestimate water and solute leaching. A change of 5% in the soil properties of the neighboring soil columns may underpredict the solute leaching, 24 hours after a rainstorm, by 157% for a soil column with a macropore, and by 58% for a soil column without a macropore. These differences decreased to 47% and 8%, respectively, 168 hours after the rainfall. Field application of the model suggested that the model underestimates the leaching of water and solutes from the root zone. However, the computed results were substantially better than the results obtained when no preferential flow component was included in the model. The model performed best under conditions that favored preferential flow, i.e., a high rainfall rate and high initial moisture conditions. The simulated and observed solute concentrations in the root zone agreed reasonably well, although the maxima of the observed data were generally higher than those of the simulated data.
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