A 2-REGION MODEL OF PREFERENTIAL FLOW OF CHEMICALS USING A TRANSFER-FUNCTION APPROACH

Preferential flow Processes are often used to explain rapid leaching Of chemicals through intact soils. In this study we postulate a physical model within the framework of transfer function modeling, to describe the multiple chemical mass peaks in leachate from multiple rainstorms after a one-time chemical application. Two flow regions are assumed, the fast flow zone (preferential flow region) and a slow flow zone. The fast flow zone is completely leached of chemicals during each rainstorm event and subsequently replenished by the slow zone during periods between storms. Using the model equations, the chemical masses are calculated and fitted to the observed values of bromide, atrazine (2-chloro-4-ethylamino-6-isopropylamino-S-triazine) and alachlor [2-chloro-2'-6'-diethyl-N-(methoxymethyl)-acetanilide] and several parameters characterizing the flow are found. These parameters are: cross-sectional areas occupied by the fast and slow flow zones, mean chemical travel time, coefficient of variation (CV) characterizing the spread of the travel times, mixing time describing the rate of the chemical mixing with the pure water, and the coefficient of the effective mass transport between the slow and fast zones. The fitted parameters are examined to see whether they are attributable to any physical or chemical explanation, and if they indicate that the model assumptions are reasonable. The mean chemical travel times and the CV of travel times are similar for all three chemicals, suggesting that sorption is negligible during flow through the fast flow zone. The rate of chemical mixing with the new rainwater, as well as the rate of mass transport from the slow zone to the fast zone between storms, is related to the sorption properties of the chemicals. The model shows promise in describing preferential flow of pesticides under multiple rainstorms from data on bromide under the same water flow conditions.