Rivers, lakes, and coastal areas can become contaminated with fecal pathogens that threaten the health of people who recreate in the polluted water. Fecal contamination can be persistent over time or occur in short-lived events and has a variety of causes, such as combined sewer overflows, street water runoff during storms, wastewater treatment plant discharges, leaking pipes or septic tanks, and even fecal waste from wildlife, swimmers, or boats. Identifying and reducing the sources of fecal contamination to a particular beach is often hindered by the presence of multiple sources, natural variability in bacterial indicator concentrations over space and time, and the dynamic currents, weather patterns, and natural processes that affect these concentrations.