Transport of bacteria on sloping soil surfaces by runoff

Pathogenic bacteria exist at soil surfaces as a result of practices as spre ading of liquid manure on agricultural lands or use of treated wastewater f or irrigation. Rainfall is a major factor affecting vertical and horizontal movement of bacteria in soil. Surface runoff carries bacteria significant distances downstream causing serious threats to ground and surface waters. This study uses a nalidixic acid-resistant Escherichia coil strain as a bio tracer monitoring extent of bacterial migration on sloping soil surfaces by runoff action. Two 10 x 10-m plots in two sites having different slopes we re sprayed with water containing biotracer. Soil texture at sites was clay loam. Sixteen days after spraying, two heavy rainfalls that caused runoffs were recorded. First rainfall occurred 2 days after spraying plots. Samples were collected from soil and runoff at different distances downstream of t he plots. Biotracer was found in soil and runoff samples some 20 m downstre am from center point of plot having the milder slope. Biotracer was found i n soil and runoff samples further downstream of the second plot with the st eeper slope reaching a 35- and 30-m distance respectively. Most soil and ru noff samples collected after the second rainfall, occurring 15 days after i noculation, contained no biotracer except small numbers found in soil sampl es taken from center point of each plot 5 m downstream. Results confirm the important role of runoff in bacterial transport on soil surfaces. They sho w E. coli survives in semiarid areas for a long time and increases potentia l of contamination. (C) 2000 by John Wiley & Sons, Inc.