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

Preferential flow Processes are often used to explain rapid leaching O f chemicals through intact soils. In this study we postulate a physica l model within the framework of transfer function modeling, to describ e the multiple chemical mass peaks in leachate from multiple rainstorm s after a one-time chemical application. Two flow regions are assumed, the fast flow zone (preferential flow region) and a slow flow zone. T he fast flow zone is completely leached of chemicals during each rains torm event and subsequently replenished by the slow zone during period s between storms. Using the model equations, the chemical masses are c alculated and fitted to the observed values of bromide, atrazine 2-chl oro-4-ethylamino-6-isopropylamino-S-triazine) and alachlor hloro-2'-6' -diethyl-N-(methoxymethyl)-acetanilide] and several parameters charact erizing the flow are found. These parameters are: cross-sectional area s 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 th e pure water, and the coefficient of the effective mass transport betw een 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 flo w 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 t o the fast zone between storms, is related to the sorption properties of the chemicals. The model shows promise in describing preferential f low of pesticides under multiple rainstorms from data on bromide under the same water flow conditions.