Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2006
Publication Date: 8/14/2006
Citation: Kung, K., Kladivko, E., Helling, C.S., Gish, T.J., Steenhuis, T.S., Jaynes, D.B. 2006. Quantifying pore size spectrum of macropore-type preferential pathways under transient flow. Vadose Zone Journal. 5:978-989. Interpretive Summary: Deep movement into soil of chemicals or pathogens can occur preferentially, through soil macropores, thereby contaminating groundwater. Increasing the knowledge about such leaching and other factors affecting environmental fate, especially of pesticides, helps in predicting amounts of agricultural chemicals in soil, water, or plants, and, indirectly, their risk to nontarget species. This research expands on a series of field studies (first with continuous water input, and now with intermittent water additions that better simulate natural rainfall) designed to learn how preferential transport occurs. The outcome of this work has been development of a field method and mathematical models which describe in a fundamentally new way how such complex transport processes actually occur through a range of soil pores, responding to different soil moisture and rainfall conditions. By its major departure from classical theory, the research will provide scientists and regulators with more accurate tools to predict environmental impact from pesticide or other agricultural chemical use, and help guide safer management practices as needed to avoid groundwater contamination.
Technical Abstract: It is well known that there is a spectrum of pores in a soil profile. The conventional use of a single lumped value of soil hydraulic conductivity to describe a spectrum of hydraulically-active pores may have unintentionally impeded development of field-scale chemical transport theory and perhaps indirectly hindered the development of management protocols of chemical application and waste disposal. In this study, three sets of four tracer mass flux breakthrough patterns were measured in field experiments conducted under transient unsaturated flow conditions. The data were used to evaluate the validity of an indirect method to quantify equivalent pore spectra of macropore-type preferential flow pathways. Results indicated that there were distinct trends in how pore spectra of macropore-type preferential flow pathways changed when a soil profile became wetter during a precipitation event. This suggests that the indirect method has predictive value and is perhaps a better alternative to the lumped soil hydraulic conductivity approach in accurately determining the impact of macropore-type preferential flow pathways on water movement and solute transport under transient unsaturated flow conditions.