As Above, So Below: Rainwater=Groundwater – January 6, 2009


Living in the Northwest, we are fortunate to have a seemingly abundant supply of water. Although there is a considerable amount that falls from the sky, there is an equally important amount of water that lies below us.

The rain that falls from the sky eventually finds its way into the aquifers below ground. We rely on this water for everything from drinking water to water for our crops.


Hydrogeology is the study of the distribution and movement of groundwater. Hydrogeology takes into account many factors that comprise ground water systems. Geologic strata, soil composition, and rate of re-charge/discharge are all very key components to understanding groundwater and aquifer dynamics.

In simplistic terms, rainwater permeates the ground and is filtered by many layers of rock and sediment on its way to an underground aquifer. An aquifer is a giant holding tank for water. It is from this aquifer that we get our drinking water.

Depending upon the type of soil that is above the aquifer, this will dictate just how fast the aquifer is filled up. Soils with loose sand and gravel will allow rainwater to permeate the ground quickly, thereby filling the aquifer quickly. Other soils that contain more dense materials such as clay, may take thousands of years for the water to permeate. This results in a very slow filling aquifer.

Surface Area Required

Since aquifers are typically very large, they rely on the very large surface area above them to collect the water necessary to supply them. The cycle begins when rain permeates the ground. It then filters down to the aquifer at differing rates depending upon the soil and ground geology above it. Sometimes water filters very quickly. Other times it takes thousands of years to reach the aquifer. This cycle has been ongoing for many millennia.

Interruption in Replenishment Cycle

Today, we are seeing an interruption in this replenishment cycle. It is due to the ever growing population of mankind. Man is drawing down the levels in aquifers all over the world at a record pace. The water is being consumed not just for drinking, but also for the watering of cropland. While consumption is certainly a major issue, of greater concern is the rate at which the water is NOT being replenished.

As noted above, aquifers need large surface areas above them so that they can be replenished. These surface areas must be permeable to water so that the rain can reach the aquifer. In the urban environment of today, there is considerably less surface area available to collect rainwater. Why? The reason is simple. Concrete and asphalt are man made water barriers. They do not allow rainwater to permeate into the soils they cover. So, not only do we have aquifers being depleted at a record rate, but because of concrete, asphalt and other non-permeable surfaces, rain water is becoming “run-off”. This run-off is creating other problems besides its inability to recharge an aquifer.


If run-off was simply a focused amount of rain water, then it might be reasonable to assume that it could be funneled to an area where it could easily recharge an aquifer. Unfortunately, it is not that simple. Today’s rainwater causes flooding because its normal path into the ground has been re-routed or impeded. The sheer volume of water that is generated by a typical rainstorm will quickly overwhelm any collection basin made to contain it. It now becomes easy to see why a large, natural surface area is so important for the recharging of an aquifer.


Another problem of the run-off is that in a rainstorm, much of the rain is quickly channeled away from any open land and allowed to quickly flow into rivers and streams. While this is a natural occurrence for rainwater, it historically only does so after the ground is saturated and there is no where else for the rain water to flow. At this point, the ground is at its maximum saturation level for the given event. This cycle will then begin again when the surface water has absorbed into the ground sufficiently to allow for the next storm.

This excess run-off causes severe flooding in areas where there may not have been severe flooding prior to the introduction of concrete and asphalt.

Chemical Cocktails

Further complicating matters is that when rain water falls onto concrete and asphalt, it typically flushes away many nasty chemicals that often collect there. Motor oil, anti-freeze, transmission fluid, gasoline and pesticides are but a few of the man-made chemicals that are commonly washed away when rain water flows across paved streets.

These chemicals are then washed into open areas and are allowed to then soak into the ground. Where do they go then? Eventually, they end up in the aquifer. What does not soak into the ground flows into streams and rivers and is eventually carried downstream to the oceans. Once these chemicals find their way to the oceans, we begin to see dead zones appear.

So, how do we help mitigate the problems with run-off and chemical flushing? Here are several ideas:

  • Limit/eliminate chemical usage — find alternatives to pesticides and lawn and garden chemicals. Properly dispose of any chemicals that are old or outdated. Do not pour them into the ground.
  • Maintain your automobile — a properly maintained automobile will have a much lower chance of any spill or leak of automotive chemicals.
  • Minimize the use of Asphalt and Concrete. If you are planning a driveway for your home, consider using gravel or some other porous material. This helps rainwater into the ground upon which it falls and helps to prevent “run-off”.
  • Build a rain barrel — rain barrels will allow the usage of stored rainwater to be used for watering of a garden or lawn. This prevents using water from the aquifer.

Responsibility of Government Agencies

Although there is much individual citizens can do to preserve our groundwater, government must ensure that laws are in place and properly enforced to effectively clean up our waterways, protect groundwater from contamination and restore aquifers.

Washington State has mandated the Stormwater Management Manual for Western Washington (revised 2005), which requires adoption by Washington cities and counties by August 2009. Click here on the Department of Ecology’s website for more information:

The Environmental Protection Agency’s Clean Water Act (CWA) “is the cornerstone of surface water quality protection in the United States.” Visit their website for more information at:

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