There are a number of ways that beaches can become contaminated with bacteria. Storm water runoff, combined with sewer overflows, sewage treatment plant bypasses, agricultural runoff, faulty septic systems, and large populations of waterfowl like geese all contribute to water pollution which can result in beach postings.
Beach postings are most likely to occur after rain storms or prolonged hot weather. Rain water washes fecal material from cats, dogs, birds and other wildlife into storm sewers which flow directly into nearby rivers and lakes. Prolonged hot weather conditions promote the growth of the bacteria in the water.
Water temperature also has a significant effect on bacterial levels. Shallow beaches will have warmer water and a higher potential for bacterial growth.