Submitted to: International Symposium on Preferential Flow
Publication Type: Proceedings
Publication Acceptance Date: October 1, 2000
Publication Date: January 3, 2001
Interpretive Summary: In the past decade, several studies have shown the impact of runoff water and sediment on the ecology of surface streams. Although the runoff process has been studied extensively, evaluating the impact of subsurface clay lenses, especially at the field and watershed scale, is severely lacking. Yet, with the advent of various new technologies like ground penetrating radar (GPR), global positioning systems (GPS), and geographic information systems (GIS), detailed analysis of the subsurface soil can be obtained which will enhance our knowledge of surface and subsurface water flow interactions. In this study, a 20 ha research site, which contains four small watersheds, was used to evaluate surface-subsurface water and chemical interactions. By combining GPR, GPS, and GIS technologies it was shown that discrete subsurface water channels could be detected. Under certain physical constraints these discrete subsurface channels could re-emerge at the soil surface; called seepage zones. Although the watersheds had similar organic matter content, texture, and chemical spatial distributions, the only watershed with seepage zones had 18 times more nitrate runoff than the other three. As a result, this study shows that the subsurface soil structure can have a profound impact on water quality and that a knowledge of the subsurface will be critical in developing management practices that can conserve soil and water resources.
Technical Abstract: Determining the interaction between surface runoff and subsurface hydrology has been hindered by our inability to characterize the subsurface stratigraphy on a watershed scale. Over 40 km of ground-penetrating radar (GPR)data were collected and evaluated in determining subsurface stratigraphy and flow patterns on a 20 ha research site in Beltsville, Maryland. The research site has four watersheds (about 4 ha each) with a confining clay lens that varies in depth between 0.9 m and 3.5 m from the soil surface; the watersheds have similar textures, organic matter content, and yield distributions. Although the surface slope was greater on one of the watersheds, slope alone could not explain why it also had a nitrate runoff flux that was 18 times greater than the other three watersheds. Combining GPR data with global positioning systems and a geographic information system revealed that this watershed had primary subsurface flow wpatterns that followed surface topography. Some of these discrete channels re-emerged at the surface resulting in the seepage zones. The seepage zones were responsible for the increased nitrate flux. As a result, this study demonstrated the dramatic impact subsurface stratigraphy can have on surface chemical runoff fluxes, even when soil properties, yield distributions, and climate are similar.