Water surfacing downhill from a sewage disposal system or other concentrated source of animal waste may originate from a failing leach field, a cracked septic tank, leaking sewer lines, or sub-surface run-off from feedlots, kennels, etc. The further away from the source of the waste, the more difficult it is to pinpoint whether the surfacing water is from sewage (including concentrated animal wastes) or from natural springs, leaking water lines, irrigation run-off, etc.
One way to check for sewage is to test for fecal coliform bacteria. These bacteria are found primarily in the feces of warm-blooded animals (birds and mammals, including humans). Fresh sewage can contain up to 1 million or more fecal coliform bacteria per 100 milliliters (MPN/100 mL). However, fecal coliform bacteria may die off once they leave the fecal matter from which they came. After enough distance traveled through the soil, sewage may eventually become diluted and cleaned up enough that it more resembles natural springs. A natural spring may have no fecal coliform, and usually not more than ten or so, depending on the concentration of wild animals in the area and the source of the water feeding the spring. Drinking water has no fecal coliform in the pipes, but may pick up a few while traveling through the ground. Surface water may have anywhere from no fecal coliform (Lake Tahoe, mountain streams) to several hundreds or even thousands (farm ponds with water fowl, creeks in populated areas, etc.).
There are no hard and fast rules as to what fecal coliform result indicates sewage. It is more of a continuum – the higher the result, the more likely that the water is sewage-related. Distance from the potential source must always be taken into consideration in evaluating the results. The most relevant State regulation is that which governs water contact sports, such as swimming. Beaches are posted (No Swimming) if the fecal coliform result exceeds 400 MPN/100 mL From a practical level, then, it is a good policy to avoid contact with water that contains more than 400 MPN/100 mL of fecal coliform.