|Malone, Robert - Rob|
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2012
Publication Date: 11/1/2012
Citation: Fox, G., Marvin, M., Guzman, J., Hoang, C., Malone, R.W., Kanwar, R., Shipitalo, M.J. 2012. E. coli transport through surface-connected biopores identified from smoke injection tests. Transactions of the ASABE. 55:2185-2194. Interpretive Summary: Analysis of bacteria concentrations and transport to tile drainage systems through cracks and worm holes (preferential or macropore flow) following irrigation or rainfall events are important when assessing the risk of contamination. Macropores are the primary mechanism by which bacteria from surface-applied manure are transported into subsurface drains or shallow groundwater, which results in the flow bypassing much of the filtering capacity of the soil matrix. Limited research has been performed investigating fecal bacteria transport through specific macropores identified in the field. Also, research at the USDA-ARS-National Laboratory for Agriculture and the Environment recently determined that several rational polynomial equations (RPE; a ratio of two polynomials) have the potential to be very useful tools to investigate bacterial tranport through macropore flow but they were only tested under one set of environmental conditions. Our objective was to investigate fecal bacteria (E. coli) transport through naturally occurring macropores that were directly connected to a subsurface drain. The best performing RPE accounted for 97% or more of the variation in E. coli load rates over time in the subsurface drain, which is similar to the more commonly used "lognormal" equation. Even though this study directly injected E. coli solution into macropores directly connected to a subsurface drain, the soil filtered approximately 90% of the E. coli load that entered the macropore. However, even with this substantial reduction in E. coli transport, high concentrations and loads are still possible in subsurface drainage due to the high E. coli concentrations resulting from manure application. This research will help agricultural and environmental scientists more fully understand microbial transport through soil, which will facilitate the design of more effective systems that protect the environment.
Technical Abstract: Macropores are the primary mechanism by which fecal bacteria from surface-applied manure can be transported into subsurface drains or shallow groundwater bypassing the soil matrix. Limited research has been performed investigating fecal bacteria transport through specific macropores identified in the field. The objective of this research was to better understand how fecal bacteria, using Escherichia coli (E. coli) as an indicator organism, are transported through naturally occurring macropores and potential interactions between the macropore and soil matrix domains in the field under controlled experimental conditions. Direct injection/infiltration tests were conducted in two naturally-occurring, surface-connected macropores (biopores) that penetrated to the subsurface drain depth, as suggested by smoke tests. Data included total drain flow rate (baseflow rate and biopore flow rate), biopore inflow rate and Rhodamine WT and E. coli concentrations in the drains. Analysis techniques included determining increases in subsurface drain flow rates due to infiltration tests and percentage of the injected concentration reaching the subsurface drains after dilution with the drain baseflow. In the absence of data for mechanistic models, empirically-based rational polynomial models were compared to the more commonly utilized lognormal distribution for modeling the load-rate breakthrough curves. Load estimates were derived from integrated forms of these empirical functions and percent reductions were calculated for Rhodamine WT and E. coli. Peak total drain flow rates increased nearly two-fold due to direct injection into biopores. Less than 25% of the initial concentrations injected into the biopore reached the drain after dilution with the baseflow in the drain. Lognormal distributions best fit the Rhodamine WT load-rate breakthrough curves (R2 = 0.99 for both biopores) and E. coli for one of the biopores (R2 = 0.98); a rational fractional polynomial model that tailed off more slowly best fit the E. coli load-rate data for the other biopores (R2 = 0.98). Approximately one log reduction was estimated for E. coli loads due to interaction with the soil profile as water flowed through the tortuous path of the biopores; in other words, the soil surrounding a biopore filtered approximately 90% of the E. coli load that entered the biopore. Considering that applied animal manure can contain millions of bacteria per mL, high concentrations and loads are still possible in the subsurface drain flow if macropores are present.