EXECUTIVE DIRECTOR’S COLUMN
Geologic Hazards, Dramatic and Subtle
Robert A. Stewart, CPG-08332
On January 12, 2015, a sinkhole opened abruptly on a residential street in Lafayette, Colorado, nearly swallowing a car. Happily, the driver was able to escape without injury, and his car was removed with comparatively little damage. The sinkhole was about eight feet deep, and 20 feet in diameter. Its location and the cause of the sinkhole was no surprise. Underneath was the main shaft of the Simpson coal mine, which operated in Lafayette from 1888 to 1927. The Town of Lafayette owes its name to Lafayette Miller, who owned the land with his wife Mary; they were successful farmers. Lafayette died of heatstroke in 1878. Mary continued the farm, and her business interest shifted to coal, which was discovered on her land in 1884. She then platted the original limits for the town, which she named for her late husband. The coal seam was 14 feet thick, and the mine produced over four million tons before it closed. The shaft, about 200 feet deep, was subsequently sealed at the collar with a concrete slab and buried. The town expanded over the original footprint of the mine, but the shaft remained below East Cleveland Street.
The sinkhole opened when the original slab settled, perhaps due partly to routine traffic vibration in the neighborhood built over the mine. Fortunately for the town, the State of Colorado pays for remediation due to this type of mine subsidence. The initial response was to fill the sinkhole with flowable concrete, sub-base and finally bituminous pavement. At a later time, the plan is to grout the entire shaft to prevent further subsid- ence. The coal seam was horizontal, and the big question is to what extent the abandoned workings are still open to the shaft, after nearly 100 years. That could mean a lot of grout.
Lafayette is part of the Boulder-Weld Coal Field, and the legacy of mining is extensive. Moreover, the mining records are quite good, enabling municipal planners and developers to avoid areas at risk from subsidence. The various documents are available through the Colorado Geological Survey, or local municipalities, via website or in person.
Sinkholes are dramatic and get good media coverage. In Lafayette, the cause of the sinkhole was anthropogenic; else- where, sinkholes due to karst open unpredictably. Eliminating the hazard requires avoidance of karst-prone areas, not always easy in urban and suburban areas.
Less appreciated are risks due to naturally-occurring min- erals and elements. Asbestos, radon and lead in paint are perhaps the best examples known to the general population, through routine testing as part of real-estate transactions. More problematic are less-publicized and more subtle geologic hazards due to slow but persistent migration of various metals in vapor and liquid phase through rock and soil. Our ability to understand such hazards reflects the incredible advances
in analytical instrumentation since the mid-20th century, enabling geologists to detect and record natural geochemical processes on local and global scales.
I’m presently reading Four Billion Years and Counting – Canada’s Geological Heritage, published in October 2014 by the Canadian Federation of Earth Sciences. This book takes the reader through geologic time from the beginning (the 4,030-million year Acasta Gneiss of the Hadean Eon) to the present (Anthropocene Epoch). The photographs are beauti- ful, and with colorful maps and explanatory graphics, lead the reader through concepts and places from coast to coast, and the Great Lakes to the Arctic.
Part 3 of Four Billion Years and Counting is Wealth and Health, which contains an interesting chapter, Toxins in the Rocks. One of the central discussion points is how bioconcentration and biomagnification affect the food chain. Cadmium is concentrated by lichens, a staple food of caribou, in which the cadmium collects in certain organs. Mercury vapors naturally emanate from black shales and as a vapor or liquid phase along faults under lakes, biomagnifying in the food chain to apex predators that also happen to be food sources. The lat- ter include humans (“Sick from Sushi” – Consumer Reports, October 2014). Other examples include arsenic and fluoride in well water, and selenium-accumulating plants. The subtle effects of geologic hazards are often difficult to diagnose. Just as the dosage makes the poison, so does knowledge promote avoidance, regardless of the hazard.
Stay tuned for more impressions of Four Billion Years and Counting, a thoroughly enjoyable and engaging read.
The Lafayette sinkhole, mostly filled with grout, January 15, 2015.
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