Source or Cause


Ammonium/Ammonia (+as N)

2 mg/L**

Decomposition of animal and plant proteins, agricultural, urban runoff, and effluent from wastewater treatment plants or industrial contamination.

Can cause unwanted algal blooms. Can indicate sewage pollution and consequent possible presence of pathogenic microorganisms.

Calcium (Ca2+)


Occurs in rocks, bones, shells, etc. Very abundant.

Causes water hardness. Essential for normal growth and health (1 - 2 g daily requirement).

Lead (Pb2+)

MCL 15 μg/L* 50 μg/L**

Leaching from ores and erosion of natural deposits; effluent discharges; corrosion of household plumbing systems.

A cumulative poison particularly in infants and children; Delays physical and mental development; May cause kidney problems and high blood pressure in adults.

Magnesium (Mg2+)

35 mg/L**

Major constituent of geological formations.

A major dietary requirement for humans (0.3 - 0.5 g/day) and the second major constituent of water hardness.

Potassium (K+)


Geological formations.

No health concerns except at gross levels.

Sodium (Na+)

20 mg/L**

Abundant constituent of rocks and soils.

Essential dietary requirement (5 g or more). Causes hypertension if in excess.