Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/17/2001
Publication Date: 9/6/2001
Citation: Jaynes, D.B., Ahmed, S.I., Kanwar, R.S., Kung, K.J. 2001. Temporal dynamics of preferential flow to a subsurface drain. Soil Science Society of America Journal. 65:1368-1376. Interpretive Summary: There is great concern for the role agrochemical plays in the contamination of our Nation's surface and ground water resources. One way agrochemical can move rapidly from fields to the underlying ground water is through preferential flow paths in soil. Preferential flow paths allow for rapid movement of chemicals with relatively little infiltration of rain or irrigation water and effectively bypass much of the soil profile within which agricultural chemicals are normally bound and/or degraded. Scientists have only a poor understanding of how preferential pathways are formed and how water and chemicals move along them and are thus unable to develop management options for reducing preferential flow. This research confirmed that preferential paths can play a significant role in moving surface applied pesticides rapidly through the soil. In addition, this is the first study to measure the highly nonuniform behavior of chemical movement through preferential pathways over short time intervals (minutes to hours). This information will be used by scientists to better understand preferential flow, to develop models able to predict the contribution of preferential flow to groundwater contamination, and to develop new management schemes for farmers that will reduce this route of contamination of ground waters.
Technical Abstract: We conducted a sequential tracer leaching study on a 24.4- by 42.7-m field plot to investigate the temporal behavior of chemical movement to a 1.2-m deep field drain during an irrigation and subsequent rainfall events over a 14 day period. The herbicides atrazine and alachlor along with the conservative tracer Br were applied to a 1-m wide strip, offset 1.5-m laterally from a subsurface drain pipe, immediately before a 11.3 hour, 4.2-mm one hour irrigation. Three additional conservative benzoate anion tracers, PF, TF, and DF were applied to the strip during the irrigation at two hour intervals. Breakthrough of the conservative tracer Br occurred within the first two hours of irrigation, indicating that a fraction of the solute transport was along preferential flow paths. The herbicides applied with Br also arrived at the drain within the first two hours of the irrigation indicating preferential transport. Retardation and attenuation of the herbicides indicated that there was interaction between the chemicals and the soil lining the preferential pathways. The conservative tracers applied during the later stages of irrigation arrived at the subsurface drain much faster than tracers applied earlier. The final tracer, applied six hours after the start of irrigation (DF), took only 15 minutes and 1 mm of irrigation water to travel to the subsurface drain. Model simulations using a two-dimensional, convective/dispersive numerical model without an explicit preferential flow component failed to reproduce Br tracer concentrations in the drain effluent, confirming the importance of preferential flow. This study showed that preferential flow in this soil is not a uniform process during a leaching event.