Montana Watercourse Ground Water Brochure, available on the publications page.
Groundwater is water found underground in the cracks and spaces in soil, sand and rock. These underground areas that are filled with water are called aquifers. Groundwater is stored in and slowly moves through aquifers.
More than half of Montanans depend on groundwater for their primary water supply. According to the Natural Resource Information Service, groundwater provides 94 percent of Montana’s rural domestic-water supply and 39 percent of the public-water supply. Montana uses over 188 million gallons of groundwater per day for domestic use, public water supplies, irrigation, livestock and industry (US Geological Survey,Estimated Use of Water in the United States in 2000).
What’s the connection between groundwater and surface water? As water from irrigation and precipitation soaks through the surface, or when it seeps out of wetlands, rivers, streams or lakes, it recharges the groundwater. When the water table (the line that separates the saturated and unsaturated zones) is above the level of a stream, the groundwater flows into the stream (gaining stream). Conversely, when the water table is below the level of the stream flow, some of the water in the stream soaks into the ground (losing stream).
Due to the connection between ground and surface water, it is important to protect both water sources from human–induced impairments. The most commonly reported types of human-induced groundwater contamination come from: leaking underground storage tanks, septic tanks, municipal landfills, agricultural activities and abandoned hazardous waste sites. It is important to note that naturally occurring constituents are found in groundwater, and can negatively affect water quality. Most notable in Montana are: salts, and minerals such as iron and manganese, which generally pose no health concerns, and potentially toxic arsenic (associated with geothermal activities) and uranium (found within the Boulder Batholith in southwest Montana). Arsenic and uranium can also be found in other aquifers depending on geology.
The following links provide more information about groundwater and what you can do to protect this valuable resource.