In Montana, water quality is measured against established beneficial uses. These are the various ways that a water body is suitable for use by people, wildlife, and livestock. Montana’s beneficial uses include recreation, drinking water, aquatic life, and agriculture. Water quality goals are based on the maximum amount of a pollutant a waterbody can have while still maintaining its designated beneficial uses. If a waterbody is determined to be “impaired,” it means that its suitability for those beneficial uses are limited or harmed to some extent.
Suitability for Recreation
Lakes are valued for scenery, swimming, boating, and fishing, but under some circumstances these activities may not be desirable or safe. Algal blooms diminish recreation opportunities and might be toxic and potentially harmful. High fecal coliform bacteria counts at a local beach or in a bay can require temporarily closing an area to swimming. A day or two is usually sufficient to allow nature to lower bacteria counts, unless the source is a poorly maintained septic system or a break in a nearby sewer line.
Suitability for Drinking
Do not drink lake water without disinfecting it. Bacteria, viruses, parasites and toxic algae in the water can cause sickness. Bacteria may naturally occur or come from a malfunctioning septic system, a sewage treatment plant, or stormwater runoff.
Suitability for Aquatic Life
Wildlife including fish and other aquatic organisms rely on suitable water quality conditions for their habitat and survival. Pollutants may cause a decline or disappearance of some species and a dominance of others, shifting the lake food web, and altering the lake dynamics. Another path that is important to consider is the bioaccumulation and biomagnification of pollutants, such as heavy metals, in the food web. This poses a risk to humans consuming fish from contaminated lakes. The pathway of bioaccumulation and biomagnification is not always known and may vary between pollutants.
Suitability for Agriculture
Many of Montana’s lakes are used to irrigate pastures and crops or provide drinking water for livestock. High levels of salinity, a common pollutant, may make water unsuitable for irrigation depending on the salt tolerance of the crop. It is important that livestock have access to clean water, as water with high levels of salinity or other contaminants can cause ailments or inability to drink the water.
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