Vol. 18, No. 1, 1997
HOW DO BACTERIA MOVE THROUGH SOIL?
M.S. COYNE, J.M. HOWELL, and R.E. PHILLIPS
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.
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.
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
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.