The occurrence of metals in ground water often is an unwelcome surprise -- both at sites of known contamination and in drinking water wells located far away from any known source of contamination, explains Thomas Kwader.
Clays are Mother Nature's "metal magnet." In the sedimentary record, after the sands and limestones are sorted out, the clay layers collect the leftover impurities that often include metals and trace elements. As you already know, some formations have relatively no clay or clay layers, or clays may appear ubiquitously (evenly) distributed throughout the formation.