Introduction
Bacteria levels (specifically E. coli ) in Lake Wingra are closely monitored
in the summer by the City of Madison Public Health Department to assess swimming
safety.
Vilas Beach, Madisons most heavily used beach, is often closed to swimming
because of elevated bacterial levels.
Geese feces contain high levels of bacteria, and it is believed that these
feces contribute to the high levels in Lake Wingra.
Our hypothesis is that the sediments are acting as a reservoir for bacteria
and should be taken into consideration when testing the water for levels of
bacteria when determining if the beach should be closed.
We sampled both water and lake sediment from five locations in Lake Wingra
to determine the potential importance of sediment as a reservoir for E. coli.
We diluted the sediment samples with de-chlorinated tap water (100:1) so that
results were in a readable range with our total coliform and E. coli test.
Steps of sediment sample processing is shown in Figure 1.
Step Description
1 Original Sediment Sample was shaken up
2 10ml of shaken sample was poured into small beaker (6times)
3 Took left over sediment and dried it and weighed it
4 Took sample sediment and dried and weight it
5 Took 10 ml sub sample, added it to 990 ml of water to create 1000ml diluted
sample
6 Took 100 ml of the diluted sample and used it for colilert test
All 5 locations had high levels of E. coli in the water (Figure 2).
The levels of both total coliform bacteria and E. coli in lake sediments were
much higher than those of the overlying water, supporting the idea that the
sediment is serving as a reservoir for bacteria, perhaps arising from goose
feces.
Lagoon 1 contained higher levels of bacteria per mg dry weight sediment while
the bridge contained higher levels of bacteria per 100mL shaken sediment.
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| Fig.2 E. coli and bacteria concentration of water sample |
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| Fig. 3 Average E. coli and bacteria concentration of sediment. |
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Fig. 4 Average E. coli and bacteria concentration of dry weight sediment. |
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| Fig. 5 Amount of sediment collected and the amount of sediment in a 10 mL sample |
We recommend that in future studies a standard procedure be developed in
order to collect the same amount of sediment at each location.
Future research could focus on how this bacteria gets into the lake, grows,
and multiplies in the sediment.
Because geese feces contribute to the high levels of bacteria in the water and
sediments, future studies could attempt to correlate sediment bacterial levels
with the number of geese near the testing areas.
As geese management efforts continue around Lake Wingra, this type of research
can be used to evaluate the effects of this management on the levels of bacteria
found in the water and sediment.
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Conclusion
When beginning this experiment, we felt we would find that the sediment
contained high levels of bacteria.
In the summer, the beach is sometimes closed due to high bacteria levels in
the water. Therefore, we thought that bacteria would be held in the sediment
too.
Our water testing supports our hypothesis. The bacteria levels in the sediment
have tested higher than the bacteria levels in the water.
Therefore, we think that the bacteria settle to the sediment at bottom of the
lake.
The results of our experiment are important to Lake Wingra users. We hope that
the Madison Public Health Department will consider testing the sediments in the
summer months for levels of coliform bacteria when deciding to close the beach.
If there are high levels of bacteria in the sediments during the winter months
there should also be high levels in the summer months.
The high levels of bacteria in the sediment should be a concern to the lake
users because this is not a factor in determining if the beaches of Madison
should be closed.