Coliform bacteria from diffuse sources as a factor in estuarine pollution

The contribution of 849 ha of rural watershed to the fecal coliform (FC) pollution of the Rhode River, a subestuary of Chesapeake Bay with a surface area of 485 ha. was estimated. The watershed with an animal population of 0.6 animal unit ha⁻¹ discharged between 7.5 × 10⁶ and 669 × 10⁶ FC ha-day⁻¹. The FC discharge rate was seasonal and largely dependent upon the water flow. Total coliform (TC) discharge was influenced by the same factors as the FC discharge. It was calculated that on the average less than 1% of FC produced by the animals on the land was washed down by water runoff. Occasionally the FC discharge rate in the runoff reached 4–6% of the FC produced by the animals. Fecal coliforms persisted in the water. The FC numbers were high in the Rhode River close to the discharge points and further away were diluted by the river volume. It was estimated, using the maximum number of FC in the runoff, that 2600 m³ of well mixed receiving water was needed for every ha of watershed area not to exceed the safe water standards (14 FC MPN 100 ml⁻¹ for shellfish harvesting. Three factors having a role in FC pollution of an estuarine ecosystem were emphasized as a result of this study: (1) the rural watershed contributed substantial quantities of FC to the estuary; (2) the season of the year may determine the level of pollution entering from rural sources into the estuary; (3) the persistence of bacteria in the estuary may increase the pollution level contributed by the watershed especially at low water temperatures.