Caye M. Drapcho, James F. Beatty and Eric C. Achberger
A major water quality concern in Louisiana is the concentration of fecal coliform bacteria in our streams and bayous. Currently four health advisories that limit primary contact recreation such as swimming are in place because of elevated fecal coliform counts. These advisories are for the south shore beaches of Lake Pontchartrain, 18 miles of the Tchefuncte River, 12 miles of the Bogue Falaya and 79 miles of the Tangipahoa River. The Louisiana Department of Environmental Quality identified three potential sources of fecal coliform contamination in the Tangipahoa River: leakage from faulty home septic systems, effluent from municipal wastewater treatment plants not operating properly and runoff from confinement areas of dairy farms where cattle are held for feeding. Dairy farms in the watershed were required to install waste treatment lagoons and apply the water from the lagoons to nearby pastures to prevent transport of fecal coliform bacteria to nearby surface waters.
Fecal coliform bacteria are found naturally in the intestinal tract of warm-blooded animals. Average fecal coliform counts per gram of waste vary from 230,000 for cattle to 13 million for humans. Escherichia coli is the predominant fecal coliform in the waste of most warm-blooded animals. The percentage of the fecal coliform count in animal waste attributed to E. coli ranges from 84 percent for pigs, 97 percent for sheep and humans, and 99.9 percent for cows. Although fecal coliform bacteria as a group are not pathogenic, they are used as a test of recent fecal contamination from either animal or human origin; however, the fecal coliform test as an indicator of water contamination in subtropical and tropical climates has been questioned. This is because the presence of some soil bacteria from nonfecal sources may show up as “false positives.”
A research project was initiated at the LSU AgCenter Southeast Research Station in Washington Parish to evaluate the effect of dairy farming on surface water quality. The objectives were to quantify the fecal coliform content in surface water runoff from grazed pastures as compared to drainage from nongrazed pastures and forested areas and to determine if bacteria from nonfecal sources were contributing a significant portion to the fecal coliform count. The research consisted of a watershed study and a field-scale study.
The dairy farm at the Southeast Research Station is a fairly typical size for Louisiana, with a dairy herd of 180 milking cows. As with most Louisiana dairy farms, the cows are fed forage in confinement systems about 25 percent of the time and graze on pastures the rest of the time. Waste from confinement areas is collected and treated in a lagoon system. Dairy cattle are grazed at an average density of one cow per acre.
For the watershed study, water samples were collected at 10 points along the natural drainage at the station after rainfall. These points included drainage from a 300-acre wooded area containing no livestock and drainage from grazed pastures.
For the field-scale study, plots with water collection troughs were installed in bermudagrass pastures. Dairy manure was applied evenly over the surface of four of the plots; no manure was applied to the other four plots. During periods of drought, a device called a rainfall simulator was used to water the plots. Runoff water was collected to determine the fecal coliform count and the percentage of the fecal coliform count caused by E. coli. This information can help determine if the fecal coliform bacteria in a water sample originated from dairy cattle or another animal source. No attempt was made in this study to find pathogenic strains of E. coli, such as O157:H7. These strains would be expected to account for less than 0.5 percent of the total E. coli count.
Results from the watershed study indicate that the fecal coliform levels in surface runoff from timberland with no livestock frequently exceeded the standard set by LDEQ for swimming and other primary contact recreation. Fecal coliform counts in surface runoff from grazed pastures routinely exceeded the primary standard by a factor of 10 or more (Figure 1). The values increased as the water flowed along the drainage through the station. Fecal coliform counts at the last sampling location were similar to levels in the liquid portion of the lagoon.
For the field-scale study, the fecal coliform counts in the surface runoff from the pasture plots that received manure were 10 to 30 times higher than the primary standard when collected one day after manure application. Fecal coliform colonies in these samples included at least 90 percent E. coli. For rainfall applied one and two weeks after manure application, the fecal coliform count in the surface runoff declined to near the standard set for primary contact recreation. This is caused by decay of the bacteria and physical removal of the bacteria from the soil surface. E. coli accounted for only 10 percent to 50 percent of the fecal coliform count.
For the nontreated plots, the fecal coliform count in the surface runoff samples was not zero as might be expected. Rather, the fecal coliform count was near the primary standard for most periods of rainfall and did not decline with time. For rainfall applied one and two weeks after the beginning of the study, fecal coliform counts in the runoff from the manure-applied plots and the nontreated plots were essentially the same. Further, fewer than 5 percent of the fecal coliform colonies in the runoff from the nontreated plots were E. coli.
The information obtained from these studies suggests that bacteria from sources other than dairy cattle may contribute a substantial portion of the fecal coliform count. The fecal coliform counts from the forested area containing no livestock in the watershed study indicate that wildlife contribute fecal coliform bacteria to water. In addition, the data from the control plots in the field-scale study indicate that soil bacteria may be causing “false positive” readings on the fecal coliform test. Although the fecal coliform test uses an elevated incubation temperature to prevent the growth of nonfecal bacteria, researchers have shown that soil coliforms from the Klebsiella and Citrobacter groups are able to grow at the high incubation temperature.
This study also suggests that grazing cattle may represent a significant source of fecal coliform contamination to surface waters. The fecal coliform counts from the grazed pastures in the watershed study and from the manure-applied pasture plots in the field-scale study were above the primary standard, and most of the fecal coliform count was caused by E. coli. Further research on management measures and grazing practices that could reduce fecal coliform transport from grazed pastures is needed to ensure that grazing is maintained as a viable option for Louisiana cattle farmers.
Two problems that limit the development of effective and sensible control measures are: (1) distinguishing soil coliform bacteria from true fecal coliform bacteria so that accurate assessments of the level of fecal contamination are made and (2) identifying the source of fecal coliform contamination by differentiating coliforms from cattle and those from other sources, such as humans, wildlife and pets.
We are studying the feasibility of using DNA fingerprinting techniques to correlate specific strains of E. coli to each common source of fecal contamination. This approach will require developing a database of DNA patterns attributed to E. coli strains from human and animal sources. In practice, E. coli isolated from water samples would be fingerprinted and compared with the database to identify the likely source of contamination. Once the source was identified, appropriate control or treatment methods could be used.