Pesticide transport and significance of macropore flow in the coastal plain soils

Many processes are known to contribute to groundwater contamination. Extensive groundwater monitoring and experimental data, however, show that macropore flow is likely to be one of the principal mechanisms responsible for accelerated movement of chemicals in the soils.;A physically based two-domain model of water flow and solute transport (MACRO) was applied in a sandy loam soil to quantify the significance of macropore flow in Coastal Plain soils of Maryland. The simulations were compared with the measured data in order to evaluate the applicability of such a model in predicting the environmental fate of atrazine in Coastal Plain Soils. The field site is located in the Coastal Plain Physiographic Region of Maryland near Upper Marlboro, Maryland.;First, six most sensitive input parameters for the model were identified by performing a thorough sensitivity analysis. Second, the model was calibrated using the field data from 1992. Finally, the model simulations of atrazine concentrations for 1993, 1994 and 1995 were compared with the means of lysimeter concentration data and the 95% confidence intervals of the means of these measured data.;The results indicated a reasonable agreement between simulated and measured atrazine concentrations in the soil. About 80% (average) of the model predictions of 1993, 1994 and 1995 fell within the 95% confidence intervals of the measured data. This study concluded that preferential flow plays an important role in water movement and solute transport in the Coastal Plain Soils, thus models with macropore capability such as MACRO should be used for pesticide screening instead of Darcian-based models.;To further evaluate the MACRO model's capability in predicting water movement in the Coastal Plain Soils, infiltration tests were conducted at the soil surface and 30 cm below the soil surface using tension infiltrometer. Infiltration tests were run at zero tension (indicative of macropore flow condition) and -5 cm tension (indicative of matrix flow condition). The cumulative infiltration predicted by the MACRO model was compared with the measured tension infiltrometer data. The results indicated a good agreement between the model predictions and the measured data at the soil surface under pressure heads of 0 and -5 cm.