Sinkholes: a Hidden, Real New Mexico Geohazard
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winter 2016 SINKHOLES: A HIDDEN, REAL NEW MEXICO GEOHAZARD The majority of New Mexicans probably underground water systems formed by the regard sinkholes as an exotic geohazard dissolution of soluble rock by circulating that occurs far away in other parts of groundwater. What does this mean? For the country. In places such as Florida or groundwater to circulate, it must be able Kentucky, sinkholes have opened beneath to move through the subsurface bedrock, and “swallowed” houses, roads, and primarily through cracks, called “joints.” classic Corvettes in the National Corvette In regions where bedrock is composed Museum (which occurred on February 12, of soluble rock types, such as limestone, 2014). However, New Mexicans living in dolomite, or gypsum, groundwater moving the east-central and southeastern parts of through joints actually dissolves the rock, the state may view sinkhole development creating cavities in the subsurface. as a real and present danger. Indeed, they The word karst is based on a 19th might be aware that Blue Hole, in the Century German modification of the Santa Rosa area, and most of the lakes Slavic word krs and the Italian carso, at Bottomless Lakes State Park east of which essentially both mean “a bleak, Roswell are, in fact, sinkholes (see photo waterless place.” Although this description on this page). Large cavities have been Mirror Lake, one of several sinkholes at Bottomless could apply to many desert landscapes, it Lakes State Park east of Roswell, where upward encountered when drilling water wells in artesian flow of groundwater through gypsum originally referred to a specific region—the this region, and oil and gas drillers have bedrock has produced solution-collapse sinkholes. barren, high plateau area of what was then discovered large voids in the subsurface. Photo by Lewis Land. northwest Yugoslavia (current-day Slovenia Residents in these regions may have also and Croatia). This area is characterized by state and U.S. highways, warning of the observed sudden muddying of regional well dry stream valleys, often ending abruptly, potential for a sinkhole encounter (see waters after major rainfall events, as well as and numerous depressions. Water, instead second photo on page two). standing water collecting in low-lying areas of running freely over the land, seemingly Sinkholes associated with oilfield activities in pastures and fields. The phenomena of sinks into the ground. differ from the numerous others throughout sinkholes, subsurface cavities, intercon- All rocks, given sufficient time, undergo the region in that they are man-made, or nectivity of water-bearing strata directly physical weathering, which results from the anthropogenic. We now know, largely affected by large weather systems, and disintegrative erosional processes caused through geophysical work carried out by the slow, barely noticeable subsidence by flowing water and wind, and chemical the National Cave and Karst Research of depressions in the land surface are all weathering, which results when rocks and Institute (NCKRI) with the New Mexico related; they occur in areas characterized by minerals are dissolved into constituent Bureau of Geology and Mineral Resources soluble bedrock and are all part of so-called molecules that can be washed away from (NMBGMR), that another potentially cata- karst landscapes. More than 25 percent of the rock mass. In the case of carbonate strophic anthropogenic sinkhole could form the world’s population either lives on karst and evaporitic rocks, water and carbonic on the south side of Carlsbad, beneath the terrain or derives its water from karstic acid act as the chemical weathering agents intersection of U.S. Highway 285 with U.S. aquifers. In 2008, two very large sinkholes that break these rocks down. Carbonic Highway 62, an interchange known locally (approximately 111 m [364 ft] and 64 acid forms when meteoric water (rain) as the South Y. This hazard is discussed later m [210 ft]) in diameter, respectively, combines with carbon dioxide, which is in this article. abruptly formed in the oil fields east of produced by organic constituents in the the Pecos Valley and made national news. Karst, hydrology, and solution soil. Similar dissolution can take place on See photo on page two. If southeastern processes your teeth, through exposure to carbonic New Mexicans were previously unaware of acid in soft drinks, and can cause cavities. The term karst describes a landscape this hidden geohazard, they were alerted Most karst features in the eastern United containing caves, sinkholes, and extensive by news reports and hazard signs on both States and other karst areas around the Published by the New Mexico Bureau of Geology and Mineral Resources • A Division of New Mexico Tech JWS Sinkhole: Karst features and landforms 07-28-2008: 10:00 am Limestones and dolomites, wherever 111 m (364 ft) diam exposed at the surface, will, over time, show numerous minor solution effects. These small (one to several centimeters in size) etched pits, grooves or runnels, hollows or flutes, are often separated by small, knife-like ridges and pinnacles. Collectively, these features are known by the German term, karren. Dissolution may 111 m be concentrated along joints in the bed- rock, and widening of these joints forms large conduits, enhancing the infiltration and circulation of water through the rocks. While karren terrain might be painful to bare feet and hard on shoes, it is relatively harmless compared to the hazards wrought by the sudden formation of large-scale karst 108.65 m features. Subsurface karstic cavities enlarge and coalesce, forming underground chan- nels for groundwater flow, ultimately allow- 0 50 m ing collapse of the overlying land surface. Of County Rd-217 the resulting karst landforms, such as closed depressions and sinkholes, are the most The JWS Sinkhole, one of two anthropogenic sinkholes that opened above brine extraction wells common, often occurring by the thousands in 2008 in the Loco Hills area of northern Eddy County. Courtesy of National Cave and Karst Research in major karst areas. Disappearing streams, Institute. large springs, and cave systems also typify world are formed in the carbonate rocks, These evaporitic rocks are much more karst terrain. limestone (calcium carbonate) and/or dolo- soluble than carbonates in the presence of Streams “disappear” when surface water mite (calcium-magnesium carbonate). In fresh water, and do not require carbonic flows or infiltrates into underground cavi- arid and semi-arid areas such as southeast- acid for dissolution to occur. ties. This process creates landforms such ern New Mexico, however, as blind valleys (where a valley terminates karst features more com- against a rock wall and the stream abruptly monly form in evaporitic disappears), pocket valleys (essentially the bedrock. Evaporites are a opposite, where groundwater discharges type of rock produced by from a spring and flows down a valley), salts left behind during the and dry valleys (occupied by no surface evaporation of saline water. stream, but often above a subsurface one). The most common types of During a storm event, surface runoff in dry evaporite rock are gypsum, valleys infiltrates rapidly into subsurface composed of calcium sul- conduits (see photo on page three). fate, and halite, the mineral name for sodium chloride, Disappearing reservoirs or table salt. Thick deposits Somewhat similar to disappearing of gypsum and halite are Highway sign adjacent to NM State Highway 360, east of streams, two New Mexico reservoirs, the present at the surface and in Hondo Reservoir and Lake McMillan, the subsurface throughout Artesia, warning of the possible sinkhole/subsidence hazard. Photo by Lewis Land. “disappeared” during the 20th Century the Permian Basin region of and ultimately had to be abandoned. southeastern New Mexico and west Texas. In some geologic settings, karstic cavities Problems began for the Hondo Project From roughly the vicinity of Carlsbad may also form by dissolution related to even before the reservoir was built to store northward, evaporite rocks were formed hydrogen sulfide. This gas rises up from water for irrigation. In 1904, the director in mudflats of vast, shallow lagoons, deep reservoirs of hydrocarbons. When of the newly created U.S. Reclamation between 270–250 million years ago. As mixed with ground water, sulfuric acid Service was shocked to discover that large the sea retreated, the water evaporated, is formed. This acid attacks carbonates cavities occupied the floor of the proposed resulting in precipitation of the salts. With much more vigorously than meteoric reservoir site, located about 18 km (11 mi) time, these calcium-, magnesium-, and water or carbonic acid. Carlsbad Caverns, southwest of Roswell. Nevertheless, offi- sodium-enriched deposits were lithified Lechuguilla Cave, and other cave systems cials were mollified by proponents’ claims into gypsum and anhydrite (hydrated and in the Guadalupe Mountains of southeast that there were more than sufficient quan- anhydrous calcium sulfate, respectively), New Mexico and west Texas have formed tities of local clay for sealing the floor. The halite, and other more exotic minerals. through this dissolution process. reservoir was completed in 1907 but was new mexico earth matters 2 winter 2016 never able to hold significant quantities of in the world at that time. Nevertheless, water. In several of the post-construction because of the gypsum bedrock beneath the years, attempts were made to repair the lakebed, and adjacent gypsum bluffs of the lakebed, but larger holes kept being dis- McMillan Escarpment east of the valley, covered, and the repairing of the reservoir the reservoir almost immediately began floor was finally abandoned. Around leaking, particularly along the eastern 1917, the Reclamation Service gave up on margin of the lake. Much of the reservoir the project entirely. water, lost through karstic conduits, Lake McMillan Reservoir, which resurfaced in a series of springs in the Pecos stored Pecos River water for the Carlsbad River appproximately 6 km (3.5 mi) down- Irrigation District, also leaked for much stream.