Wednesday, December 05, 2007 Osmeña: Risks of groundwater contamination By Antonio V. Osmeña Estatements
ONE of the major water pollution problems in urban areas is the growing contamination of underground aquifers, which provide drinking water to 95 percent of Cebu’s inhabitants.
There are many sources of groundwater contamination. It may be caused by sewer leaks, accidental chemical or oil spills, leaking underground gasoline storage tanks, active and abandoned landfills and toxic waste dumps that are unlined and located above or near aquifers.
Groundwater can also be contaminated by leaking septic tanks, runoff of animal waste from feedlots (bacteria and nitrates), seawater intrusion, fertilizers (nitrates) and pesticides, and leaks from industrial, mining, municipal, and agricultural surface water impoundments (ponds, pits and lagoons).
Groundwater pollution differs from surface water pollution in several ways:
l For one, some bacteria and most suspended solid pollutants are removed as contaminated surface water percolates through the soil into aquifers. But the effectiveness of this process varies according to the type of soil in the area.
l Another difference is caused by the fact that bacterial degradation of oxygen-demanding wastes reaching aquifers does not occur readily because of the lack of dissolved oxygen in groundwater.
l Soil is not effective in filtering out viruses and most synthetic organic chemicals.
l The rate of movement of most groundwater is so slow (typically about 30 centimeters or one foot a day) that there is relatively little dispersion and dilution of wastes.
l It is difficult to monitor pollutants or to predict what happens after they enter the groundwater table because they tend to spread in a plume whose shape and rate of movement depend on the permeability and slope of the aquifer as well as the rate at which water is pumped from the aquifer by wells. Groundwater may be heavily contaminated in one place and safe for drinking only a few hundred feet away. A well that is pollution-free can suddenly become contaminated when it is reached by a plume that may have been spreading for decades.
Groundwater pollution is essentially irreversible because most aquifers are recharged very slowly.
Preventing contamination in the first place appears to be the only effective solution.
