MODELING MACROPORE TRANSPORT OF AGRICULTURAL CHEMICALS ON A RIVER FLOODPLAIN: ATRAZINE TRANSPORT SIMULATION

Abstract

The United States Geological Survey (USGS) model Variably Saturated Two Dimensional Transport (VS2DT)was used to simulate macropore transport of atrazine through a highly structured clay soil to a sand and gravel floodplainaquifer at a field research site in Burleson County, Texas. A simulation of preferential flow through the surface clay wassuccessfully coupled with a simulation of groundwater flow in a floodplain aquifer. Simulated groundwater flow and atrazinetransport compared favorably with field measured values. The water levels in the floodplain aquifer were primarilyinfluenced by fluctuations in river stage. Simulated groundwater and surface water flows into and out of the aquifer werecalculated. Groundwater discharge from the aquifer to the river averaged 0.023 m3/s/km during low river stages. Surfacewater recharge to the aquifer averaged 0.022 m3/s/km during high river stages. Simulated atrazine transport through theclay soil domain resulted in atrazine losses (percent of total applied) of 7% in 1994 and 15 % in 1995. A pumping test at theresearch site and more rainfall in 1995 than in 1994 resulted in simulated atrazine transport to the bottom of the sand andgravel aquifer (19 m). Atrazine was not transported to the river in groundwater flow during model simulations. Simulatedatrazine concentrations were validated using measured concentrations throughout the study period.