Drinking Water Monitoring - The Drinking Water - page 221

the correlation between different bacterial indicators and the occurrence of
digestive system illness at swimming beaches suggest that the best indicators
of health risk from recreational water contact in freshwater are Escherichia
coli and Enterococcus spp. For saltwater, enterococci are the best. Interestingly,
fecal coliforms as a group were determined to be a poor indicator of the
risk of digestive system illness; however, many states continue to use fecal
coliforms as their primary health risk indicator. If your state is still using the
total of fecal coliforms measurement as the indicator bacteria and you want
to know whether the water meets state water quality standards, you should
monitor fecal coliforms. If, however, you want to determine the health risk
from recreational water contact, the results of the USEPA studies suggest
that you should consider switching to the E. coli or enterococci method for
testing freshwater. In any case, consulting with the water quality division of
your state's environmental agency is best, especially if you expect them to
use your data.
Sampling and equipment Considerations
Bacteria can be difficult to sample and analyze, for many reasons. Natural
bacteria levels in streams can vary significantly; bacteria conditions are
strongly correlated with rainfall; thus, comparing wet and dry weather bac-
teria data can be a problem. Many analytical methods have a low level of
precision yet can be quite complex to accomplish; and absolutely sterile con-
ditions are essential to maintain while collecting and handling samples. The
primary equipment decision to make when sampling for bacteria is what
type and size of sample container you will use. Once you have made that
decision, the same straightforward collection procedure is used, regardless
of the type of bacteria being monitored.
When monitoring bacteria, all containers and surfaces with which the
sample will come into contact must be sterile. Containers made of either
some form of plastic or Pyrex ® glass are acceptable to the USEPA; however, if
the containers are to be reused, they must be sturdy enough to survive steril-
ization using heat and pressure. The containers can be sterilized by using an
autoclave, a machine that sterilizes with pressurized steam. When an auto-
clave is used, the container material must be able to withstand high tempera-
tures and pressure. Plastic containers—either high-density polyethylene or
polypropylene—might be preferable to glass from a practical standpoint
because they will better withstand breakage. In any case, be sure to check
the manufacturer's specifications to see whether the container can with-
stand 15 minutes in an autoclave at a temperature of 121°C without melting.
(Extreme caution is advised when working with an autoclave.) Disposable,
sterile, plastic Whirl-Pak ® bags are used by a number of programs. The size
of the container depends on the sample amount needed for the bacteria anal-
ysis method you choose and the amount needed for other analyses.