Grossmann: Our Collapsing Planet—Aquifers & Sinkholes in Salty Kansas

7 November 2013

“Aquifer” is a sophisticated name for what we used to call groundwater.  You dig down, and you find water at a certain depth.  That’s the water table – the underground “water level.”  That’s what makes water wells possible.   Groundwater is found in formations called aquifers.  And aquifers are a little like lakes.  But, unlike lakes, the water doesn’t rest in a depression on the surface of the land.  Rather, groundwater permeates the land like water permeates a sponge.  You can’t see or feel an aquifer even when you standing on the ground right above it.

Aquifers are also like lakes because they are bodies of water with a bottom and distinct “sides” or boundaries.  So, you can dig in one place and find water near the surface.  However, in a second place, just a few miles away, you might dig and dig, but find no water at all.  In the first place, you were above an aquifer but, in the second, you weren’t. [1]

In Kansas, farming was very difficult until they discovered ground water in 1911. [2] They didn’t just find some water.  They found a lot of it.  Relative privation gave way to agricultural prosperity.  At the time, the best “explanation” for the water was that there was an “underground river.”  And, from that “river,” water was drawn for irrigation.  Kansas farmer Rodger Funk remembers that there was also a conventional wisdom:  They said the river was inexhaustible.  Now, Funk laments, “they were wrong.” [3]

What “they” called an underground river was, in fact, the Ogallala Aquifer, a massive reservoir of groundwater covering a substantial part of eight states from Texas to South Dakota.  The water accumulated in the ground thousands of years ago as the last glaciers from the last ice age melted. [4] Replenished by light yearly rains, the water remained intact until it was discovered, but misidentified as an underground river in 1911.  Pumping water from the aquifer for agricultural and, later, residential use began.  Then, slowly, the groundwater began to disappear.  Slowly, but surely.

The history of groundwater use in Kansas was the focus of a recent study by researchers from Kansas State University in Manhattan, Kansas and published in the Proceedings of the National Academy of Sciences.  In 1960, the Kansas aquifer’s water reserves had declined only 3% as a result of systematic use. However, by 2010, the reserves had declined by 30%.  And, with projected usage, we can expect a 69% decline by 2060. Aquifers can run dry and, with the modest rainfall in the region, it would take hundreds, if not thousands, of years to replenish this massive aquifer once it was “tapped out.” [5]

The study’s lead researcher, David Steward, Professor of Civil Engineering at Kansas State University explained that no one can be sure how long the aquifer’s waters will last.  The study proposed a plan to reduce water usage through more efficient use in the hope of maintaining the aquifer, as a water source, indefinitely.  That would be a positive step, but the challenge is to actually reduce water use.  While not all states touched by the aquifer have such a positive prognosis, there is reason for cautious optimism for the future of the Kansas portion of the Ogallala Aquifer. [6]

However, low water production is only one of the dangers posed by a depleted aquifer.  Pumping aquifers dry can create another problem: Subsidence. Perhaps a more familiar term would be sinkholes.

An aquifer’s waters provide internal support for the soil around the groundwater deposit.  When the water disappears too quickly, air fills the void left in the pores of the rock and the earth.  And air is anything but good supporting material.  So, a dry aquifer may not only mean a sudden lack of water for agriculture and residential use.   It also can “result in land subsidence, cracking house foundations, and changing drainage patterns.” [6.1]

Strictly speaking, sinkholes form in lands with substantial limestone strata beneath the surface.  Water permeates and flows through limestone and, very gradually, erodes it away.  This is how the typical cave is formed.  Over extremely long periods of time, the limestone bed become thin and finally collapses.  When it does, the soil above it also collapses.  Suddenly, a small but significant area of the earth’s surface disappears into a roughly circular hole.  The effects are sudden and sometimes devastating. [7]

Quite specifically, subsidence might be the more technical term to apply to a collapse caused by air in the soil resulting from depletion of an aquifer.  With subsidence, everything just starts to sink. [8] Mexico City, for example, was built on an old lakebed resting above a large underground aquifer.  Because the city has no other cost effective source of water, this aquifer, though not dry, was substantially depleted by heavy use during the last decades of the 20th century.  The resulting land subsidence is so severe that whole areas of city are sinking rapidly.  Not only are foundations undermined, but the sewer system is subject to continual damage.  In spite of constant repair efforts, fresh and contaminated water become mixed spreading contamination to the city’s drinking water. [9]

Last summer, Kansas was in the middle of a draught.  On July 31, 2013, Wyatt Hoss noticed a 90 foot deep roughly round crater in the pasture north of Sharon Springs in Wallace County, Kansas.  The sinkhole was 200-300 feet across.  The discovery turned the area into a tourist attraction or in the words of Wyatt Hoss’s 82 year old mother, Margaret, “a three-ring circus.”  A local commentator, Kate Wilkins-Wells, was amused to see a TV reporter on the scene standing within the concentric fissures near the rim of crater while soberly warning would-be visitors that the place the reporter was standing was too dangerous for curious visitors. [post] The pasture became a local tourist attraction much to the chagrin of property (and sinkhole) owner Margaret Hoss who valued her privacy. [10]

The sinkhole was in an area of the county with little oil, gas, or groundwater extractions.  But there was a draught in progress.  And the sinkhole was quite near the Ogallala aquifer which is noticeably thin or absent in that area.  So, could the Wallace County sinkhole be the result of aquifer depletion? [11]

Probably not, The Director of the Kansas Geological Survey, Rex Buchanan, quickly noted that, in spite of the closeness of a relatively depleted portion of the aquifer, both water extraction and activities related to oil or gas extraction had been scarce in the area.  [12]

But if the cause wasn’t a depleted aquifer or the extraction of oil or gas, what happened?   And, unlike most areas vulnerable to frequent sinkholes, Kansas doesn’t have underground limestone deposits or the caves that go with them.  So, it had to be something else.  To find that “something else,” you need to look at the history of sinkholes in Kansas.

Not only have there been a lot of sinkholes in the past, but Wallace County is famous for its “collapses” including the Smoky Basin Cave-in that developed about 5 miles east of Sharon Springs in 1926 and grew to an estimated 350-by-250-foot irregular oval.  [13] And, in the more distant past, another sinkhole, the circular Old Maid Pool, is located about 6 miles northwest of Sharon Springs. [14]

But what could be causing these sinkholes?

Salt.  Kansas is unusually rich in halite deposits.  Halite is rock salt.  The presence of a 200 foot thick bed of rock salt about 2,000 feet below the surface likely led to the formation of the old sinkholes as well as this newest addition in Wallace County. [15]

Why is salt so important to the development of sinkholes?  Well, you have to see the effect of water on salt to understand.  Exposed to water, salt dissolves almost instantly.  And I mean instantly.  In other words, it was the presence, rather than the absence, of groundwater “penetrating through fractures in overlying layers” of soil that dissolved the salt and “eventually created a substantial void.” “When the rock above could no longer sustain the weight, everything from the surface down suddenly collapsed.”

So, before we look for water depletion or imprudent oil or mineral extraction, subsurface salt deposits are a notoriously “usual suspect” in sinkhole formation.   Indeed salt has been mined around nearby Hutchison, Kansas for decades.  And past sinkholes have been definitely linked to the dissolution of underground salt, gypsum, or other sedimentary mineral deposits. [16]

The Wallace County sinkhole presented a little mystery because the area’s subsurface geology is not so well known.  Among Kansas counties, Wallace County is “short on oil and gas and its salt is too deep to mine economically.”  So less data is available on the subsurface geology of the area, but it is assumed to be no different than the areas all around and, so, is vulnerable to sinkhole formation. [17]

Meanwhile, Margaret Hoss was offended that her pasture turned into to tourist attraction, and the visitors ignored the family’s pleas to stay away.  Barricades didn’t work, and Hoss was afraid the traffic would damage her pasture’s grass, which was needed for their cattle.  Still, she made no reports about the trespassers to County Sheriff Townsend.  Hoss explained that, while she was angry about the trespassing, she still didn’t believe in “petty arrests.”  [18]

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