| Year | Fecal coliform bacteria | Dissolved oxygen | Total phosphorus | Total cadmium, dissolved | Total lead, dissolved |
| percent of all measurements exceeding national water quality criteria | |||||
| 1975 | 36 | 5 | 5 | * | * |
| 1976 | 32 | 6 | 5 | * | * |
| 1977 | 34 | 11 | 5 | * | * |
| 1978 | 35 | 5 | 5 | * | * |
| 1979 | 34 | 4 | 3 | 4 | 13 |
| 1980 | 31 | 5 | 4 | 1 | 5 |
| 1981 | 30 | 4 | 4 | 1 | 3 |
| 1982 | 33 | 5 | 3 | 1 | 2 |
| 1983 | 34 | 4 | 3 | 1 | 5 |
| 1984 | 30 | 3 | 4 | <1 | <1 |
| 1985 | 28 | 3 | 3 | <1 | <1 |
| 1986 | 24 | 3 | 3 | <1 | <1 |
| 1987 | 23 | 2 | 3 | <1 | <1 |
| 1988 | 22 | 2 | 4 | <1 | <1 |
| 1989 | 30 | 3 | 2 | <1 | <1 |
| 1990 | 26 | 2 | 3 | <1 | <1 |
| 1991 | 15 | 2 | 2 | <1 | <1 |
| 1992 | 28 | 2 | 2 | <1 | <1 |
| 1993 | 31 | <1 | 2 | na | na |
| 1994 | 28 | 2 | 2 | na | na |
| 1995 | 35 | 1 | 4 | na | na |
| 1996 | na | 1 | 1 | <1 | <1 |
| 1997 | na | 1 | 2 | <1 | <1 |
Source: U.S. Geological Survey, national-level data, unpublished, Reston, VA, 1999.
Notes: *Base figure too small to meet statistical standards for reliability of derived figures. na = not available. Violation levels are based on the following U.S. Environmental Protection Agency water quality criteria: fecal coliform bacteria -- above 200 cells per 100 ml; dissolved oxygen -- below 5 milligrams per liter; total phosphorus -- above 1.0 milligrams per liter; cadmium, dissolved -- above 10 micrograms per liter; and total lead, dissolved -- above 50 micrograms per liter.
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