Water... The Universal Solvent
The Hydrological Cycle
As water continuously cycles & comes in contact with other matter, it picks up various impurities such as mold, dust, smog, bacteria, smoke, sulfur from industrial smokestacks (acid rain) and carbon dioxide.
Surface water (lakes, rivers, reservoirs) can become contaminated with impurities such as nitrates or animal waste from livestock or fungicides, pesticides and herbicides. From industry it can pick up things like arsenic and detergents.
Ground water is water that has percolated down through different layers of earth like clay, limestone and shale picking up impurities such as radioactivity, sodium, iron, calcium, magnesium and hydrogen sulfide.