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Data Set #047

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About lake bacteria

    Some bacteria, especially intestinal bacteria discharged in feces, can pose a health risk to bathers and swimmers in public waterways such as lakes and rivers. Fecal coliform bacteria are found in the feces of most mammals and birds, whereas Escherichia coli (abbreviated E. coli) are found almost exclusively in humans. These bacteria enter lakes and streams from leaking sewer lines, discharges from sewer overflow systems during periods of heavy rainfall, effluent from improperly functioning septic systems, feces-contaminated runoff in streams, and discharges from "heads" on boats, especially in marinas. The significant role of pets in contaminating surface waters is now being recognized. In a 1993 study of the ribonucleic acid in bacteria in Piper's Creek, Seattle, the main source of bacteria was discovered to be domestic cats.

    King County's Division of Water and Land Resources collected data on bacterial counts and water temperature at a large number of public beaches around Lake Washington and Lake Sammamish, east of downtown Seattle. Juanita Beach on the northeast side of Lake Washington was sampled approximately every week from May through September, 2000, at a depth of 1 meter. Bacterial concentrations are given in colony forming units (CFUs) per 100 milliliters (0.1 liters) of water. Typical bacterial counts in the middle of Lake Washington away from public beaches average around 20 CFU/100 ml. King County uses the "10 state standard" for water quality for swimmers; the mean of all fecal coliform samples cannot exceed 200 CFU/100 ml and no single sample can exceed 1000 CFU/100 ml. As can be seen from the data, this water quality standard was exceeded during the summer of 2000 at Juanita Beach.

    The graph shows fecal coliform abundance plotted against E. coli abundance, to see how these two bacterial types are correlated. There is a strong linear relationship between the two, suggesting a similar source for these bacteria, or the same environmental control on the abundance of these two bacterial types. A 1998 study of the genetic makeup of fecal coliform bacteria at Juanita Beach showed that ducks and geese were the primary source of contamination, with seagulls and dogs as secondary sources. Note that there is a spike in bacterial counts in early July, following a warm water period at the end of June. It would be interesting to look at rainfall and runoff from this period. The 12 July data have been left off the graph for ease of viewing the rest of the data; is this a true outlier?

Reference: King County (Washington) Division of Water and Land Resources
http://dnr.metrokc.gov/wlr/waterres/lakes/

     
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Juanita, Lake Washington, water quality
King County Div of Land and Water resources
day 0 = 17 May 2000
bacteria: colony forming units per 100 milliliters (CFU/100ml)
temperature: ° C

 

date

coliform

E. coli

temp

0

350

350

15.3

7

   

17.2

14

73

90

14.6

21

180

120

16.8

28

150

330

16.8

35

200

200

18

42

130

120

24

49

2200

1600

19.6

56

6300

5800

21.7

63

360

300

20.1

70

1200

980

19.4

77

560

630

22.1

84

1700

1800

24.1

91

19

16

22

98

430

270

23

105

320

270

19

112

87

160

18

119

380

210

18

 

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