How Does the Ground Filter Ground Water?

Unknown

Hello Members,



Ground water occurs in the layers of soil, rock and organic material underground. As water moves underground, these materials filter impurities from the water. How Does the Ground Filter Ground Water?





Thanks and Regards,

Alex Johns

Jamie Hall

It varies...

and sometimes the earth filters water astonishingly well -- and sometimes it doesn't do well at all.



Water moving in fractured rock is in the latter class -- the fractures are just too big to do any filtering.  There may, in some rocks, be some adjustement in the water chemistry, however.  This is most notable in limestone and marble, and less so in dolomite.  In these rocks, the pH of the water generally rises as it moves, and at the same time the hardness increases.  Both are the result of the minerals of the rock -- calcite and dolomite -- dissolving in the water.  Another example of changing water chemistry occurs in shales, where it is not at all uncommon for hydrogen sulfide generated from organic material in the shale to become dissolved in the water -- that "rotten egg" smell. 



In soils, however, it is possible to get excellent filtration and, particularly in shallow zones, chemical treatment as well.  The filtration is partly from what might be thought of as a seive effect -- the particulate material simply being bigger than the spaces in the soils.  This effect is fine, but doesn't account for most bacteria and virus.  They, however, if the area in question is shallow enough to have some oxygen available, are removed -- quite effectively -- in medium to fine sands and coarse silts by a combination of biological action (a thin slime forms on the soil particles, which traps and kills the bad guys) and oxidation.  This process is used in water treatment and some waste water treatment and is called "slow sand filtration" -- quite descriptive.  It does take a lot of area, so isn't used that much today.  Two of the most common dissolved contaminants are nitrogen in various forms and phosphorous as phosphate.  Nitrogen can be removed in soils, provided the water is moving in the root zone of plants and there isn't too much of it.  Deeper, or overloaded, and the nitrogen just goes on its merry way.  Phosphate is removed by chemically adhering to soil particles, particularly clays.  There is a very definite limit to how much can be removed in a cubic foot of soil, depending on the soil mineralogy (sands are lousy).  Other dissolved nasties -- including most heavy metals and many dubious organics -- can be removed not so much by the soil, but again by plants and microorganisms in the soil.  Some plants are much better than others at this, and it does require that the groundwater be close to the surface where plants grow -- one of the really beneficial results of wetlands is that they are very very good at this sort of thing.



Some things just aren't removed at all.  Dissolved salt, for instance.  Once salt is in the water, it stays there.