HOW DO BACTERIA MOVE THROUGH SOIL?
M.S. COYNE, J.M. HOWELL, and R.E. PHILLIPS

Introduction

The contamination of water supplies by fecal bacteria is an important 
water quality issue in Kentucky. Contamination may come from point 
sources, such as straight pipes depositing raw sewage into streams, or 
nonpoint sources, such as manure runoff from cropland. A direct cost 
of contaminating water supplies is the expense that homesteads or 
water companies incur to chlorinate, filter, and otherwise treat water 
to make it potable. Indirect costs are the time lost to illness from 
drinking inadequately treated water, slower weight gain in livestock 
drinking contaminated water, and the degradation of aquatic habitats. 
Many Kentuckians obtain their potable water from ground water. We 
depend on soil to filter and purify wastes and waste water that are 
land-applied before they affect ground water. However, in central 
Kentucky, we have observed a direct relationship between grazing 
cattle in pasturefields and fecal bacteria in shallow wells and wet 
weather (breakout) springs. To understand how these water supplies 
become contaminated by fecal organisms, we decided to first study 
how fecal bacteria moved through soil. With this understanding, we 
could test various control methods. We specifically looked at fecal 
coliforms in this report, since they are the bacteria used to indicate 
potential fecal contamination in water quality assessment.

Methods 
 
We extracted intact soil blocks (13 x 13x 13 inches) from two soils: a 
Nolin silt loam and a Lowell silt loam. They were placed on a 
collection chamber that partitioned leachate into 100 individual 
sections (10 x 10). This let us map where water and bacteria exited the 
blocks. Although the blocks were too shallow to realistically assess the 
depth of fecal bacteria movement in the field, they were large enough 
so that we could examine the pattern of bacteria movement in a 
representative mass of undisturbed soil. Before each experiment, we 
centered about one half pound of fresh dairy manure (about a tenth of 
a pound dry weight) on top of a soil block and shaped it to resemble a 
voided dung deposit that covered 38% of the soil block surface. Each  
deposit had approximately 5 billion fecal coliforms. We used a 
laboratory rainfall simulator to rain on the soil blocks at a rate of about 
1 inch per hour to cause leaching. Leachate was periodically collected 
from the 100 individual sections beneath the soil block, and analyzed 
for total water flow and fecal coliform concentration (the background 
contamination was not detectable).

Results and Discussion

  The saturated hydraulic conductivity of the Lowell soil was 0.3 
inch/hour. Leachate was collected in most sections beneath the block 
(Figure 1) but 20 sections in it accounted for about 60% of the leached 
water. In contrast, the Nolin soil had a saturated hydraulic 
conductivity 3 times greater (1 inch/hour) and 20 sections accounted 
for nearly 100% of the total leachate; these sections were widely 
distributed (Figure 2).

  In both soil blocks, the 20 most rapidly flowing sections accounted 
for almost 100% of the fecal coliforms that leached. In the Lowell soil, 
these sections were immediately below the manure deposit (Figure 1) 
and one section alone accounted for almost 40% ofthe fecal coliforms. 
In the Nolin soil, fecal bacteria followed the same distribution pattern 
as leachate (Figure 2) and 55% of the coliforms that leached were in 
just one section.

  Far more fecal coliforms were trapped in the soil, usually within the 
first few inches, than were transported through it, however. The fecal 
coliforms collected in leachate only accounted for 0.01% of the total 
fecal coliforms in the Lowell soil and 0.1% in the Nolin soil.

  In both soils, fecal coliform concentrations steadily increased with 
time even though the flow through each soil remained constant. After 
just 1.2 inches of rain was applied, fecal coliform concentrations in the 
Lowell soil ranged between 2 and 34 fecal coliforms/100 ml. In the 
Nolin soil, there were 15 to 680 fecal coliforms/100 ml depending on 
the section we examined. For comparison, the potable water standard 
in Kentucky is <1 fecal coliform/100 ml and the primary water contact 
standard (bathing and swimming water) is 200 fecal coliforms/100 ml. 

Conclusions

  Fecal coliform concentrations in excess of water quality standards 
rapidly leached through soil blocks with freshly applied dung deposits. 
The bacteria moved through the most rapidly flowing pores of these 
soils. The Lowell soil had the lowest saturated hydraulic conductivity, 
the most evenly distributed water flow, and the lowest fecal coliform 
concentrations in leachate. The Nolin soil, which had greater saturated 
hydraulic conductivity and several high-flowing pores, also had the 
highest fecal coliform concentrations in leachate and transmitted more 
of the total fecal coliforms from the manure deposit.

  Given the many fecal coliforms in leachate from a single manure 
deposit, and the number of manure deposits typically found on grazed 
land, it is obvious why wet weather springs and shallow wells 
underlying pasture lands frequently exceed water quality standards. 
While the greatest bacterial filtration occurs at or near the soil surface, 
bacteria moving past this zone can be transported to whatever depth 
that pores are continuous. The potential for bacteria movement 
depends on soil characteristics such as soil structure, and will affect 
groundwater to different extents depending on rainfall intensity and 
duration, and the depth of soil to ground water. The risk it represents 
for water supplies is presently unknown. Our current challenge is to 
assess that risk and develop management recommendations to reduce 
it.

                         FIGURE LEGENDS
                                
Figure 1. The distribution of leachate and fecal coliforms at the bottom 
of a 13 inch block of Lowell silt loam. The height of the bars is 
proportional to the % of total flow or fecal coliforms.

Figure 2. The distribution of leachate and fecal coliforms at the bottom 
of a 13 inch block of Nolin silt loam. The height of the bars is 
proportional to the % of total flow or fecal coliforms.