Wednesday, November 24, 2010

Aquifers not Aquitards

From the recent post on watershed boundaries, a reader mentioned the concept of underground aquifers and their relation to geographical boundaries and .  My title is in jest (sort of) referring to 'Aquitards' which according to Wikipedia is "a zone within the earth that restricts the flow of groundwater from one aquifer to another", but I thought an apt metaphor for our overuse and depletion of these amazing resources.  So in a crude analysis, the map of US aquifers is pretty amazing (here's a comparison of 'watersheds' and 'aquifers' in two maps with some context of states and cities (images from National Atlas mapping tool)



While many aquifers develop in tandem with surface waterways, others are disconnected from these sources giving them different patterns.  Ancient sources are often tapped, with draw-down causing these to be depleted much faster than they are recharged.  One of the most familiar, the 10 million+ year old Ogallala Aquifer (synonymous with 'High Plains Aquifer') that supplies water to the agricultural bread-basket of the world - centered in Nebraska and spreading from the southern tip of South Dakota into the northern panhandle of Texas.  

:: image via Wikipedia

I hadn't considered the number of aquifers and their distribution (another great tool is an online mapping application from National Atlas, found here), but it's interesting to see the difference between more broadly based, central aquifers (not specifically linked to a river) like the Ogallala, or in Oregon the Pacific Northwest Basaltic rock aquifers (unlike the Columbia River based systems to the north.  These more agriculturally oriented aquifers can be compared to small scale aquifers like the Biscayne which supplies drinking water to much of Central Florida.

:: image via USGS

The interactive mapper allows you to zoom in on state & county boundaries, as well as locations of significant cities, to see the relationship of urban agglomeration to aquifers, for instance a closer look at the area centered on Chicago (mapped from the National Atlas).

The cause and effect of cities and aquifers is probably more significant in the impacts of urbanization on water supplies (both through depletion and pollution) and the delicate interaction between surface and subsurface conditions.

:: image via Wikipedia

While subsurface conditions do exist separate from visible surface conditions, there are impacts as many rivers as charged with these underground sources, and depletion (and diversion) has caused some rivers to no longer reach the oceans - such as the Rio Grande and the Colorado (anyone guess the reasons) or the filling of traditionally large reservoirs like Lake Mead and Powell - creating significant water scarcity issues in certain metropolitan regions.  Another great lens to look at cities, so more on this to come... seems the hydrological cycle is tied to everything we do.

:: image via EDRO

1 comment:

  1. Beautiful post. I don't know how you do it.

    Very interesting - first time I've seen a map of aquifers in the US.

    I understand there's more emphasis in the US now on the importance of maintaining infiltration in land developments, not just controlling peak flows, volumes, and water quality. Still got to wonder if such new requirements are just a insignificant drop in the bucket compared to the huge amounts of groundwater that are consumed by the energy and agricultural sectors.

    Thanks again. Looking forward to your ideas on neighborsheds.


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