Metals and water quality
The metals scan includes calcium, magnesium, and iron which play major roles in water
chemistry. Other metals include aluminum, barium,
cadmium, chromium, lead,
sodium, and zinc, which tend to be present in smaller amounts. The toxicity of metals is
dependent on their solubility, and this in turn, depends heavily on pH and on the presence of
different types of anions and other cations in the water.
Methodology: Atomic Absorption Spectrometer analysis has been used in the past to identify
these metals. However, in addition to atomic absorption, laboratories now use Inductively
Coupled Plasma Emission Spectroscopy (ICP).
High pH in a stream can cause precipitation of metal salts, which makes them temporarily
unavailable to organisms. Because of this relationship Warm Water Aquatic Habitat
Criteria for metals are calculated by a rather complex mathematical formula employing the
natural log of the hardness. As hardness increases, the allowable concentration increases. The
metal criteria in some manuals are calculated based on a hardness of 100 mg/L. If the hardness
values in the test results vary much from 100 mg/L, the criteria can be recalculated. Even though
metal concentrations may be very low (below a toxic level), aquatic organisms can bioaccumulate
(or concentrate) certain metals (for example, mercury, lead, and cadmium). If more is absorbed
than excreted, the levels can then build up over time to a toxic level.
Environmental Impact: Metal ions are dissolved in groundwater and surface water when the
water comes in contact with rock or soil containing the metals, usually in the form of metal salts.
Metals can also enter with discharges from sewage treatment plants, industrial plants, and other
sources. The metals most often found in the highest concentrations in natural waters are calcium
and magnesium. These are usually associated with the carbonate anion (CO32-) and come from
the dissolution of limestone rock. As mentioned under the discussion of hardness, the higher the
concentration of these metal ions, the harder the water; however, in some waters other metals can
contribute to hardness. Calcium and magnesium are non-toxic and normally absorbed by living
organisms more readily than the other metals. Therefore, if the water is hard, the toxicity of a
given concentration of a toxic metal is reduced. Conversely, in soft, acidic water, the same
concentrations of metals may be more toxic.
When looking at the metals individually, the intended use of the water is very important.
Industry requires varying amounts of metals and or hardness for many of its manufacturing
techniques, while agriculture has its own requirements.