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Alaskan Earthquake Mysteriously Thieves Water from Pennsylvania Municipal Supply Well?
Presented at the 2003 MDE Groundwater Symposium
by Mark W. Eisner, P.G. and James M. Wilburn, IV
Advanced Land and Water, Inc.
In late 2000 and early 2001, a municipal supply well was drilled and tested at a subdivision being developed in Littlestown, Adams County, Pennsylvania. The underlying geology is the Conestoga Limestone of Ordovician Age. The well is completed to 300 feet, with 70 feet of casing and a single fracture at 127 feet; it had an original blown yield of over 100 gallons per minute (gpm). A step-drawdown and 72-hour pumping test were performed at a constant 90 gpm withdrawal rate. The water level remained well above the fracture, and recovery was both rapid and complete. All surrounding municipal and domestic wells were monitored during testing; none displayed test-correlative drawdown. A permit for 68 gpm was issued, and the well stood by while engineering and construction work to achieve its connection to the Littlestown system could be completed.
Nearly two years later, on November 20, 2002, personnel mobilized for a pre-operations test and found the static level in the well unchanged. Surprisingly, at 68 gpm, the water level in the well drew all the way down to the fracture in just 10 minutes. Subsequent retesting of the well confirmed this change in well performance. Its safe yield had fallen to an estimated 2 gpm; possibly less.
Drilling a replacement well was impossible, as wellhead protection constraints limited options as the subdivision was built out. Rehabilitation and reconstruction options were risky, uncertain and expensive. The unchanged static conditions and still-rapid recovery response ruled out well interference and drought effects. The detailed manner in which the initial test was performed eliminated test water recirculation as a possibility. A downhole camera survey was performed, and eliminated well bore integrity problems as a contributing factor. Two theories appeared the most likely reason(s) for the cutoff of flow to this well: (1) grouting of contributing fractures during the abandonment work of nearby wells, and/or (2) the remarkable and continent-wide effects of the early November 2002 Alaskan earthquake. Newspaper accounts (including a front page article in the Baltimore Sun late in 2002) detailed continent-wide hydrologic effects caused by the earthquake.
According to the USGS, the magnitude 7.9 quake that hit Central Alaska on November 3 (the “Denali” Earthquake) was the world's biggest earthquake in 2002, and the largest to hit the United States since 1996. The USGS also reported that Lake Pontchartrain in Louisiana sloshed about, and wells in several states including Pennsylvania produced muddy water. Water levels fluctuated in several USGS monitoring wells in Pennsylvania, and some have not returned to their previous levels. The USGS now believes that, in some cases in Pennsylvania, the earthquake subtly changed the connection between certain wells and the aquifer.
After due consideration of other rehabilitative techniques such as acid injection and conventional redevelopment, equipment was mobilized to hydro-fracture the well. Field observations suggested success in restoring (at least some) well-aquifer connection, and the earlier step-drawdown and 72-hour constant-rate tests then were repeated.
The retest data indicated that the well hydraulics had improved substantially because of hydro-fracturing. Some benefit clearly had been realized. The well no longer was capable of sustaining the permitted 68 gpm permitted pumping rate. The well probably cannot exceed 40 gpm on a sustained basis, but it may prove capable of providing at least 30 gpm. In conclusion, it appears that the 2002 Denali Earthquake may have robbed more than half of the sustainable yield from this southern Pennsylvania supply well.
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