Geoscience Reference
In-Depth Information
Long-term exposure to arsenic in drinking water is causally related to increased
risks of cancer in the skin, lungs, bladder, and kidney, as well as other skin changes
such as hyperkeratosis and pigmentation changes. Conclusions on the causality of
the relationship between arsenic exposure and other health effects are less clear.
The evidence is strongest for hypertension and cardiovascular disease, suggestive
for diabetes and reproductive effects, and weak for cerebrovascular disease, long-
term neurological effects, and cancer at sites other than lung, bladder, kidney, and
skin. Studies on laboratory animals and in vitro systems generally suggest that
inorganic arsenicals are considered more toxic than organic arsenicals, and within
these two classes, the trivalent forms are more toxic than the pentavalent forms, at
least at high doses.
3.5.2 Cadmium
Cadmium is found naturally deep in the subsurface in zinc, lead, and copper ores,
in coal, shales, and other fossil fuels; it also is released during volcanic activity.
These deposits can serve as sources to ground and surface waters, especially when
in contact with soft, acidic waters. Chloride, nitrate, and sulfate salts of cadmium
are soluble, and sorption to soils is pH dependent (increasing with alkalinity).
Cadmium found in association with carbonate minerals, precipitated as stable solid
compounds, or coprecipitated with hydrous iron oxides is less likely to be mobi-
lized by resuspension of sediments or biological activity. Cadmium absorbed to
mineral surfaces (e.g., clay) or organic materials is more easily bioaccumulated or
released in a dissolved state when sediments are disturbed, such as during flooding.
Roughly 15,000 tons of cadmium is produced worldwide each year (McMurray
and Tainer
2003
). It is produced as an inevitable by-product of zinc, lead, and
copper refining and smelting, because these combined metals occur naturally
within the raw ore. The most significant uses of cadmium are metal plating and
coating for corrosion protection in alloys and electronic compounds and in nickel/
cadmium batteries. Additional uses for cadmium are in pigments and stabilizers
for PVC. Cadmium also is present as an impurity in several products, including
phosphate fertilizers, detergents, and refined petroleum products (EPA
2005
; Jarup
2003
). Industrial contamination of topsoil is likely the major source of human
exposure, via uptake into food plants and tobacco (Hayes
1997
).
Cadmium is biopersistent and, once absorbed by an organism, remains resident
for many years (with a biological half-life of 10-30 yr) although it is eventually
excreted (McMurray and Tainer
2003
). Acute exposures may cause several health
effects in humans, including nausea, vomiting, diarrhea, muscle cramps, salivation,
sensory disturbances, liver injury, convulsions, shock, and renal failure. Chronic
effects may include a potential to cause kidney, liver, bone, and blood damage
from long-term exposure at levels above the MCL (EPA
2005
). Cadmium com-
pounds are classified as human carcinogens by several regulatory agencies. The
most convincing data that cadmium is carcinogenic in humans come from studies
