Naturally occurring contaminants can also be found in drinking water. Some contaminants come from erosion of natural rock formations. Groundwater, which moves slowly through the pores or cracks in underground layers of rock, dissolves minerals as it travels. Dissolved solids are minerals or salts that have been dissolved in the water while in the aquifer. Dissolved solids can be calcium, magnesium, salt, iron or other minerals.
Fluoride can be added to water supplies to promote healthy teeth. It can also be present in water from the erosion of natural deposits or discharge from fertilizer and aluminum factories.
Water can also pick up naturally occurring radium or man-made radionuclides as it flows to the water source. The radioactive gas radon-222 occurs in certain types of rock and can leach into ground water. All rock contains some radium, usually in small amounts. Testing for radionuclides is a relatively recent change in DATCP requirements. The testing process for water samples begins with a screening for "gross alpha particle activity" which measures the total amount of one type of radioactivity given off by the water. If high levels of gross alpha activity are found, further testing for radium is conducted. Radioactivity levels are measured in "picocuries" per liter of water (abbreviated "pCi/L").
In Wisconsin, most of the community water supplies which exceed the radium standard draw water from a deep sandstone aquifer and are located in a narrow band which stretches from Green Bay to the Illinois state line. In addition, a few high radium levels have been found in groundwater from sandstone formations in west central Wisconsin and in granite formations in north central Wisconsin. In all cases, the radium was present in the rock and water long before the first well was drilled.