Water Education - Water Quality

General Differences in The Natural Quality of Groundwater and Surface Water

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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. Each source of water has a unique set of contaminants; groundwater stores pesticide chemicals and nitrate while surface water contains most bacteria and other microorganisms. Because of the interconnectedness of groundwater and surface water, these contaminants may be shared between the two sources. Neither water source can ever be entirely free from water contaminants.


Groundwater supplies in aquifers below the surface of the Earth are among the nation's most important natural resources. Groundwater 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 groundwater for all their needs. Between 30 and 40 % of the water used for agriculture comes from groundwater. Withdrawals of groundwater 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 groundwater is that groundwater moves much slower than surface water. This is because groundwater 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 groundwater. Filtration through the soil helps clean groundwater.

The hydrologic cycle interconnects ground and surface water which means they can contaminate one another. A better understanding of how they are interconnected will be explained. As rain or snow falls to the earth’s surface, some water runs off the land to rivers, lakes, streams, and oceans (surface water). Some water is also evaporated and absorbed by plants or continues to move down to become groundwater. Groundwater very slowly moves toward low areas such as streams and lakes which once again end up in surface water. This cycle is continuous and shows how the two are interconnected.

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