About cows and coliform
Cows are a important source of biological contamination of fresh water by enteric (intestinal)
pathogens, such as anthrax, salmonella, tetanus, giardia and E. coli O157:H7, present in the cow's manure. It is
expensive and time consuming to test for each of the approximately 100 waterborne enteric pathogens, so an indicator
specie, fecal coliform, is tested instead. Cows produce about 1011 (100 billion) colony forming units
of fecal coliform per cow per day, an order of magnitude greater than sheep and hogs.
The Granger Drain is a system of above-ground drainage ditches and subsurface tile drains
and pipes, covering about 18000 acres, that eventually discharges into the Yakima River in east-central Washington
State. This semi-arid region receives about 8 inches of rainfall per year, and is supplemented with about 36 inches
of irrigation water per year, mostly delivered by the traditional method of water running across the ground in
furrows (rills). In addition to orchards and various crops, this area also hosts a number of dairies and animal
feed operations (feedlots). The Yakima Valley is one of the most productive agricultural areas on Planet Earth.
Waterborne pathogens are quickly washed into the Granger Drain system, and hence into the
Yakima River, during the irrigation season. In order to quantify the relationship between cows and contaminated
runoff, the South Yakima Conservation District measured fecal coliform concentrations in water from drains at the
outlets of 8 different sub-basins within the Granger Drain system from 1990 to 1992. Repeated measurements of fecal
coliform are given as mean values in the table. The Washington State Department of Ecology reviewed the data and
compared the FC counts with a number of other variables. The acreage covered by dairies and feedlots was the only
parameter, out of several studied, which correlated well with the fecal coliform data.
As is apparent from the graph and the data that there is a strong positive correlation between
the acreage of cows and the amount of FC in the water. DOE presented a linear regression between acreage and FC
in their report, however an exponential model fits the data with a higher correlation coefficient. The data and
model raise some interesting questions. Why is there 3000 cfu/100 ml of FC in the drainage water when there are
no cows? Why is there a positive correlation between acreage of dairies/feedlots and FC, but little to no correlation
between the number of dairy cows and FC? What percentage decrease in cow acreage must there be to decrease the
FC counts by 90%, which is DOE's goal? What would be the effect on FC counts of recycling the drainage water back
as irrigation water ("gray water"), to conserve water in this arid region?
Reference: Washington Department of Ecology, "Granger Drain Fecal Coliform Bacteria Total Maximum Daily
Load Assessment and Evaluation"; WA DOE Report, 2001, draft.