I should mention that at the outset I am not intentionally selecting topics that will lead to the rather pessimistic columns I often write.  I just pick a topic, follow the data, and see where it leads.  I started this series because I find water, the molecule itself as well as its uses, interesting.  It’s a five-part series, and here in Part IV, the story is about to become darker.  To start at the beginning, check out “Water Part I: Is God a Mathematician?”, “Water Part II: Water, Water, Everywhere, Nor Any Drop to Drink”, and “Water Part III: Get the Salt Out”.  Part IV is about aquifers, underground geologic structures which contain water.  It turns out our aquifers are in big trouble.
The United States uses approximately 400 billion gallons of fresh water per day, 320 billion from surface water sources, rivers and lakes, and 80 billion from underground aquifers.  The slow intrusion of water into the earth from rainfall has created aquifers under almost all land around the world.  When the geologic conditions in the aquifer allow for water to flow, such as porous rock, gravel, or sand, you can dig a well into the aquifer and pump the water out.  Other formations which hold water too tightly, like clay, are not useable for wells. 
Subsurface aquifers are replenished by rain at varying rates which depend on both depth and geologic structure.  If you extract water from the aquifer at a rate which is faster than it is replenished by rain, the water level in the aquifer drops.  If this continues for too long, one of two bad outcomes is inevitable; either the well will run dry, or it will be infiltrated by salt water. 
Salt water infiltration typically occurs near the coast, where fresh water and salt water aquifers exist in close proximity but remain unmixed due to carefully balanced hydrologic forces, the details of which I will omit for the sake of brevity.  This hydrologic balance is disturbed if the level in the fresh water aquifer drops too low.  This allows the neighboring salt water to infiltrate the fresh water system, rendering it useless for either drinking or irrigation.  See Part II for more details on why.
The Ogallala aquifer, which spans the eight states which make up the Great Plains from South Dakota to Texas, is a compelling example of the unsustainability of overpumping an aquifer.  It was formed about 12,000 years ago during the end of the last ice age and contains water from melted glaciers.  The extraction of water from formations like these is often referred to as the mining of “fossil water”.   The Great Plains were largely desolate and unpopulated before the exploitation of the Ogallala for irrigation.  Once we started exploiting the Ogallala, the Great Plains were converted into an agricultural belt of wheat, corn, soybeans, and cattle.    It’s difficult to overstate the importance of irrigation to agricultural production.  The 20% of the farmland around the world which is irrigated farmland produces approximately 80% of our food.  In addition to driving agricultural production in the Great Plains, the Ogallala provides 82% of all of the drinking water in that part of the country. 
The replenishment of the Ogallala from surface water is quite slow, with only 10% of current extraction rates being replaced by rain intrusion each year.  At this pace the Ogallala will be substantially exhausted by 2030.  The implications of this impending crisis for the Great Plains, the United States, and the rest of the world are staggering and should be a key focus of our national political discussion.  Sadly there are not. 
At some point in the next 10 years, as the exhaustion of the Ogallala becomes too obvious to continue to ignore, I suspect the first response from Washington, D.C. will be to try to wish the problem away.  Be on the lookout for statements like this one, “we don’t have a water supply problem, we have a water distribution problem”, leading to the suggestion that we start diverting water from the Mississippi river for irrigation in the Great Plains.  This type of thinking leads to over-exploitation of river systems.  Consider than until new restrictions were enacted recently, the Colorado River, a river so strong that it carved the Grand Canyon out of the rock, dried up before it hit the Pacific due to the volume of water removed for irrigation.  Draining the Mississippi to irrigate the Great Plains is not a rational or long term solution to our issues of trying to bring population and resources into a sustainable balance.
In addition to the Ogallala, most major world aquifers are under similar strains.  A significant number of aquifers, particularly in less-developed countries, have been spoiled by salt infiltration.  The water table in north-west India, the largest food producing area in this country of over one billion people, is currently dropping by nearly two inches per year. It’s estimated that this region will be running out of water in the next 15 to 20 years, with dramatic implications for the Indian People and South-East Asia in general.
Next week, in the final installment, I will address the parallel resource constraint issues of both water and oil, a parallel which I hope is creeping up on you as you have been reading this series on water.  As we began the Industrial Revolution in 1850, we were sitting on tremendous endowments of both subsurface water and petroleum which had been accumulating over the ages.  The overexploitation of both of these key resources has created a mirage of plenty and never-ending progress, a mirage which is about to evaporate.
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