Sunday, May 11, 2008

Sinkhole Risk in Missouri

Sinkholes have become a topic of renewed interest in various parts of the country; specifically, in Missouri where some areas have noticed an increase of phenomenon. However, this type of geologic formation has not been given the same level of attention as more exciting geology, such as volcanoes and earthquakes. It has remained mostly misunderstood and understudied, except by the few that have devoted their life’s work to the study of the type of terrain harboring them, known as karst.

Beneath the state of Missouri is one of the largest, most continuous karst terrains known.  It is primarily created by dissolution of rock, sometimes leaving the landscape looking pockmarked, and a bit “like Swiss cheese.”  (Robinson, 2006) Natural springs, magnificent cave structures, and complex underground river systems characterize this landscape; as well as hidden dangers, such as deep vertical shafts and sinkholes.  Usually sinkholes, sometimes referred to as collapses, form slowly and on such an insignificant scale, that they seldom affect everyday life.  (Kauffman, 2007) However, sinkholes can form suddenly, sometimes on a massive scale, and with devastating consequences.

Natural sinkholes typically form where the underlying bedrock is composed primarily of fractured limestone and dolomite, from as far back as 500 million years ago.  (Elliot, 2008)  As water trickles down through the soil, this underlying bedrock erodes away, leaving an empty cavity.  Combined with sediment particles, the infiltrating water continues to erode the bedrock, creating its own path through the varying layers of sedimentary rock, widening and opening the cavity as it goes.  (Kauffman, 2007)  It is hard to predict what will happen next in this equation.  It may develop a huge cave that continues to expand for millions of years, sustaining a unique and alien ecosystem of its own, a sinkhole could develop, or both.

The United States Geological Survey identifies two different types of sinkholes.  The most benign are solution and cover-subsidence sinkholes, which are usually nothing more than an inconvenience.  The ground above the eroding rock layers slowly sinks into the cavity, forming a small surface depression.  This process might stop or may continue repeatedly over hundreds of thousands of years in the same location, with only incremental changes to the overall surface structure.  

The second type is a cover-collapse sinkhole, which is the biggest concern, because they often develop quickly and unexpectedly.  These sinkholes usually develop where the ground above the bedrock is comprised of a significant amount of clay or simply can no longer withstand the weight above.  As the water and sedimentary particles filter through, and erode the underlying bedrock, the clay remains in place above, forming an arch-like structure, or cavern.  These caverns can be just a few feet wide and deep, or hundreds of feet in depth and width.  As erosion of the overlying materials continues, the roof of the cavern progressively collapses in on itself until it eventually reaches the surface, and the entire structure collapses.  (Kauffman, 2007)  

From 1970 to current date, the Missouri Department of Natural Resources investigated nearly 200 reported collapses, and found that most were less than 10 feet in diameter and depth.  (Kauffman, 2007)  However, there have been some documented that have covered numerous acres, and have caused massive property and ecosystem damage.  Recently in Missouri, there have been a few cities which have been experiencing an increase of the appearance of this nuisance; indeed, just a few years ago, an entire lake recently disappeared because as the result of a sinkhole.

Surrounded by a suburban subdivision, the 23-acre Lake Chesterfield in St. Louis drained in just a few days in 2004, after a sinkhole emerged in the lakebed.  (Currier, 2004) The man-made lake, and accompanying subdivision were built in the mid-1980s.  The only reported review of the structural integrity of the area had been through aerial photos taken by the county.  The photos did not indicate the presence of past sinkhole formation, or that the landscape would be prone to this sort of development.  As Glenn Powers, planning director for St. Louis County indicated, “These things happen.”  (Price, 2004) If there had been a minor, hidden collapse, it may have gone unnoticed by developers.  The water from the lake might have begun to seep into a small crack around an old overgrown sinkhole and only leaked out a thimble full of water every day for years, until just enough erosion occurred.  

In this scenario, once it reaches this point, it is much like someone pulling a plug on drain.  The Missouri Department of Natural Resources (MoDNR, 2004) reviewed this particular case, and found that the rock layers underlying lake were very “prone to solution activity.”  (MoDNR-176, 2008) This indicates that the areas have always been a prime candidate for sinkhole development, and intensive studies should have probably been done, prior to any development.  

Plans are being made to seal the sinkhole and refill the lake, at great cost to the homeowners of the subdivision.  As private property, the city is not obligated to restore the lake, or repair any damages done.  This is not an isolated episode, either.  Property owners are facing growing repair costs, as the incidents increase, especially in the southern part of the state, where karst terrain is most dominant.

Near Springfield, in 2006 a sinkhole measuring approximately 60 feet wide and 75 feet deep, opened and swallowed portions of a couple of homes in a half-block area in Nixa, Missouri.  One homeowner that lost half of house said that he had been reading the paper, when he heard a rumbling noise.  Initially, he thought that it was a thunder or something similar.  He was surprised to look out of his window and see part of his house, garage, and car swallowed by the earth.  

City spokesman, Bryan Newberry stated, “These homes had been there without problems until this point.”  (AP, 2006)  This is typical of sinkhole formation; they do not send up a warning flare prior to their arrival.  Small towns like Nixa and rural areas are ill prepared for situations such like this.  Building codes are either not in place, or because the construction is technically outside of city limits, do not apply.  

In a recent Geographic Information Systems (GIS) analysis, which accompanies this article, it was discovered that there are areas within Missouri, which are at a higher risk than others are, and Nixa is no exception.  Situated just a few miles immediately south of Springfield, the city sits squarely on one of these higher risk areas, and as recent as April 2008, new sinkholes have begun to form.  There are no existing regulations requiring developers to conduct research on the geology before construction.  Nixa city leaders have taken notice though and are beginning to discuss how to address the issue, considering changes to regulations concerning construction around sinkholes.  (KY3, 2008)

Another city, which has also been witnessing a steady increase in sinkhole activity, is Cape Girardeau.  As of April 2008, the city has recorded 20 collapses over the past nine months.  According to Ken Eftnick, Director of Development Services for the city, they “have been seeing one show up every week or so and don’t anticipate that changing anytime soon.”  (McNichol, 2008)  

Workers in the city continue efforts to stabilize and fill the sinkholes, but there really is no good way to fill a sinkhole and be certain it will not reappear.  They are finding it difficult to keep up though, as heavy rains and flooding continue in low-lying areas of the Mississippi River, near the Cape.  Documenting the sinkholes, through site visits and aerial photography, the Geographic Information Systems Coordinator for Cape Girardeau continues to attempt to make sense of it all.  The holes do appear related, and the cause is still a mystery, while property owner concerns mount.  

The biggest concern for the city is a wastewater treatment facility, which lies perilously close to the area that the collapses have been occurring.  Widespread contamination of precious groundwater and ecosystems would then be the biggest concern, should a collapse occur beneath this facility.  (Arnand, 2008) In the meantime, industry is adjusting.  Utility lines, both gas and electric, have had to be moved and Burlington-Northern Sante Fe Railroad has installed ground movement sensors to alert oncoming trains if there has been significant disturbance to the integrity of the rails.  At the local rock quarry, it is business as usual.


Sinkholes can be problematic enough without human intervention.  Research has drawn correlations between some of these sinkholes and human land-use practices, specifically as they relate to ground water pumping.  (Barnett, 2007)  In this case, the water pumped out is acting as buoyant for the ground surface above, helping it to maintain a relative degree of structural integrity.  When ground water is pumped out, the earth above it no longer has any sort of support, and the structure could collapse.  The same effect may be occurring in Cape Girardeau, with the river acting as the pump.  Higher than average precipitation and flooding could be the catalyst, causing the Mississippi River and its tributaries to swell and recede.  This action fills the gaps in the rock underground, and stabilizes it somewhat.  When the water retreats though, it leaves only air to support the ground above, disrupting previous structural integrity.  In addition, earth movement from minor earthquakes, as well as human disturbances, intensify shifting.


Cape LeCroix Creek, around which many of the sinkholes have been developing, indirectly empties into the Buzzi Unicem quarry.  Water from the creek disappears underground and reappears in the walls of the quarry, further eroding the rock layers during its underground journey.  Regular blasting at the quarry site may be compounding the problem.  When asked about the significance of the nearby quarry activity, the GIS coordinator for Cape Girardeau dismissed the idea as not particularly relevant to the current situation, and that the blasting was insignificant.  (Arnand, 2008) However, a visiting geology professor noted that there was regular significant scale blasting occurring at the quarry, and that alone could be enough to create the problem.  He indicated that during his brief visit, he could feel vibrations from the blasting, as much as a mile away.  (Hageman, 2008) Similar to a carefully balanced house of cards, these intense vibrations can cause the loose layers of rock to shift and settle.  Water may intrude, washing away particles that have filled gaps, and when blasting resumes, the underlying rock settles further.  If a significant gap opens in the rock layers under the ground, it could cause those rocks to shift and settle, resulting in a collapse.  

While scientists are known to not agree on just about everything from A to Z, this does lead to an interesting question.  Given the set of circumstances, is it possible that the public is being put at risk, in the interests of preserving a business, which is the “biggest employer and highest taxpayer in Cape Girardeau?”  (McNichol, 2008) There is no question that in order to dismiss the quarry as an antagonist to the problem, an impartial study should be conducted in and around the quarry.  The city recently reached out to the U.S. Army Corps of Engineers, to help identify and propose remediation to the problem.  

The U.S. Army Corps of Engineers will have its hands full, investigating the exact cause of the sudden increase in sinkhole activity.  For years, karst landscapes were ignored, except by mining and industry interests.  A notable few have done extensive studies, but when compared with other types of terrain, research has been lacking.  Perhaps, it has been the level of accessibility, or simply the basic assumption by everyday people that the ground is as solid as it will always be.  In either case, situations such as those in Cape Girardeau, have demanded that scientists turn their attention to the topic.  At the time that this article was initiated, there was very little to be found about sinkholes, excepting news reports, and a few in-depth geologic studies.  Since that time, numerous new studies have appeared each with their own hypothesis on the exact nature of this problem, and with a different approach to solving it.


The initial GIS analysis and mapping done in combination with this article reviewed the rock layers within Missouri, and their relationship to sinkhole incidents Using the latest GIS software, two separate analyses were conducted (refer to analysis maps at end of article).  The study only scratched the surface; excepting geology, no other factors that contribute to sinkhole development were considered in the analysis.  As with any science, confirmation of previous findings is essential to forming a solid base for future research.  The analysis did confirm that the state is indeed “textbook karst.”  (Elliott, 2008) The regions with the highest incident of sinkhole activity are comprised of Paleozoic era, Cambrian, Mississippian, and Ordovician systems.  These are some of the weakest rocks, when pitted against the scouring power of running water.  Future research can use the study as base to continue comparisons with the other factors, in order to narrow down the possibilities until a specific cause is identified.

Identifying the cause of sinkhole development could take years.  Many of the factors that contribute to this problem are not constants, and monitoring of these phenomenon will be essential in understanding their significance.  Until a cause and permanent solution can be agreed upon, there are measures that individuals living in affected areas can take to avoid the problems associated with these formations and avoid the disaster that has befallen some of the previously mentioned communities.  

the state of Missouri should initiate a thorough investigation of its karst regions and make recommendations to the local governments to regulate construction in these areas.  The local governments could then work towards identifying their own specific areas of concern and inform their citizens of potential hazards.  Findings should be communicated quickly, and in comprehensible terms.  As Assistant Professor of Geography, David Fox of Park University indicated during a brief interview, it is important that the message about geographic dangers “be communicated effectively to the general population [and] focus on real people and their personal relationship with the environment, so that they walk away with a better understanding of the impact.”  (2008)  Indeed, with a better understanding, there will be a lot fewer surprises, and might just save a life.  At minimum, property owners can take charge of the situation themselves, and attempt to understand the structural integrity of their property, and insure themselves appropriately.  Only through awareness, will sinkholes be more understood, and their effects on human habitation, be minimized. 

 References

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Location: Missouri, USA

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