Subsurface Water Flow and Its Subsequent Impact On Chemical Behavior.

The impact of the subsurface stratigraphy on crop growth and agrichemical behavior has been studied for several years at the OPE3 research site located at the USDA-ARS Beltsville Agricultural Research Center, in Beltsville Maryland.  This site contains subsurface restricting layers that have been identified with ground-penetrating radar and reside between 1 to 4 m below the soil surface.  Since the subsurface flow pathways are 3-dimensional,ow the depth to the restricting layer varies in depth along the length of the subsurface flow pathway.  Depressions along the subsurface flow pathways are common and these depressions form cascading pools of water when the pathways are actively flowing.  If the pathways are not flowing (i.e no lateral water flow) then water that has accumulated previously within these localized “pools” will behave as a “local” perched water table.  As a result, the subsurface flow pathways have both a lateral flow and perched water table component.  Impact of these subsurface flow pathways on corn grain yields and turbulent pesticide volatilization fluxes will be briefly discussed.  Furthermore, recent chemical transport studies were conducted by applying tracers (chloride in 2008 and pentafluorobenzoic acid 2009) over a 10 m by 10 m area and observing breakthrough curves in 9 observation wells. Chemical transit times varied widely between well locations and suggest a complex combination of matrix and preferential flow processes.  Lateral preferential flow in presence of perched water substantially affects the chemical transport.