ARE THERE GENERAL DIFFERENCES IN THE NATURAL QUALITY OF GROUNDWATER AND SURFACE WATER?

As you could probably guess, there are some obvious differences in surface water and groundwater quality. But there are more differences than you might have been aware of.

Groundwater supplies in aquifers below the surface of the Earth are among the nation's most important natural resources. Ground water is the source of about 40% of the water used for public supply. It provides drinking water for more than 97% of the rural population who do not have access to public water-supply systems. Even some major cities, such as San Antonio, Texas, rely solely on ground water for all their needs. Between 30 and 40 % of the water used for agriculture comes from ground water. Withdrawals of ground water are expected to rise in the coming century as the population increases and available sites for surface reservoirs become more limited.

Now that you know what groundwater is you're probably wondering what surface water is. And if you guessed it has something to do with above ground ...you're easily correct.

Surface water can be found over the land surface in streams, ponds, marshes, lakes or other fresh (not salty) sources. Other than the location, one of the primary differences between surface and ground water is that ground water moves much slower than surface water. This is because ground water experiences far more friction as it moves through the pores in soil then surface water experiences as it flows over the earth's surface. Surface-water can be affected by numerous physical variables such as topography, land cover, soil conditions, mineralogy, and ground-water conditions, all of which may be affected by geologic conditions. Surface water is also more easily contaminated than ground water. Filtration through the soil helps clean ground water.

The natural quality of groundwater further differs from surface water in that:

• for any given source, its quality, temperature and other parameters are less variable over the course of time; and,


• in nature, the range of groundwater parameters encountered is much larger than for surface water, e.g., total dissolved solids can range from 25 mg/L in some places to 300,000 mg/L in some deep saline waters in other places.

At any given location, groundwater tends to be harder and more saline than surface water, but this is by no means a universal rule. It is also generally the case that groundwater becomes more saline with increasing depth, but again, there are many exceptions. As groundwater flows through an aquifer it is naturally filtered. This filtering, combined with the long residence time underground, means that groundwater is usually free from disease-causing microorganisms. A source of contamination close to a well, however, can defeat these natural safeguards. Natural filtering also means that groundwater usually contains less suspended material and undissolved solids than surface water.

There is little doubt that water coming from your faucet could contain chemicals that are harmful to your health. More and more we are hearing about situations where the quality of our water is not good enough for normal uses. Bacteria and microorganisms routinely penetrate into drinking-water supplies, sometimes causing severe illness in a town; chemical pollutants have been detected in streams, endangering plant and animal life; sewage spills have occurred, forcing people to boil their drinking water; pesticides and other chemicals have seeped into the ground and have harmed the water in aquifers; and, runoff containing pollutants from roads and parking lots have affected the water quality of urban streams.