Copper - a very practical building piping material, will degrade when it comes in contact with low pH water.  The blue-green stains of leaching copper oxide indicate copper plumping is being compromised.  There are other, more obscure conditions that can leach copper into water also.  (usually, electrical in nature)  Copper is rarely found in local groundwater.  1.3 ppm is the action level.  Water standing in pipes can have much higher results.