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On the safety of mining salt under Lake Cayuga

… an evaluation of the safety of salt mining under Lake Cayuga

Three Letters to Governor Andrew Cuomo on the Safety of Continued Salt Mining under Lake Cayuga

On July 13th I sent this open letter to the New York State Department of Conservation and Governor Andrew Cuomo addressing arguments by Cayuga Lake Environmental Action Now (CLEAN) that the NYSDEC should rescind its permission to emplacement of Shaft #4 so that mining to the north is stopped.  The letter argues that the construction of Shaft #4 should proceed because it poses no substantial risk to the mine, and that mining should proceed because there is no reason to believe that the NYSDEC’s current yearly monitoring of the Cargill Mine will fail to assure continued safe salt mining under Lake Cayuga.

On July 21, CLEAN responded to my letter with their own letter to Governor Cuomo reiterating their concerns.

On August 7th I responded to CLEANs comments with a second open letter to Governor Cuomo.

Salient points in this discussion include:

  1. Shaft #4 poses no risk of mine flooding. The strata into which it will be emplaced are of low permeability, and there are no salt excavations into which the shaft the shaft could trigger a collapse.  The shaft will be reamed upward from a ~1/2 mile tunnel connecting the mine.  The Retsof analogy is not valid.
  2. Pillar design may need to change to match conditions to the north, but with proper design and monitoring by the NYSDEC there is no reason that mining cannot continue as safely to the north as in the past 95 years of mining operation to the south.
  3. Mining under Lake Cayuga is safer than mining under land adjacent to Lake Cayuga because the stress on supporting Pillars is less, any confined aquifers are deep, probably saline, and either not or only distally and weakly connected to water wells, and much less human infrastructure can be impacted by mine subsidence. The increase in horizontal stress in valleys is an advantage because it presses vertical fractures together and reduces their permeability.
  4. Natural flow through Lake Cayuga and the slow nature of mine collapse means that flooding of the Cayuga mine does not pose a significant risk of lake salinification.

In subsequent discussion with Cargill several points arose which substantially further reduce the concerns raised by CLEAN:

  1. The 100 ft lower-than-lake-level head in the test well for the Cargill #4 Shaft is due to its 130,000 ppm salinity.  I should have realized this.  It means that the inflow is not anomalously over- or under-pressured, but equilibrated in pressure with the lake, indicating a low permeability normally-pressured hydrologic environment.   There is no need for any connection to the mine to explain the lower head.  The lower head is due to the higher density of the saline water in the test well.
  2. The 4 gallons per minute (gpm) “inflow” to the mine is from treated water. Cargill is allowed to desalinate some of the water entering the mine from the existing shafts and discharge the desalinated portion into the lake. The nearly-saturated brine residual (4 gpm) goes into the mine. This means that there is no leakage into the mine (the mine is bone dry), other than the water inflow through the shafts.
  3. A flooded but sealed mine will collapse much more slowly than I have calculated because the roof will be supported by pressurized water. This is apparently verified by observations at Retsof and the collapse could be slowed by a factor of 6 to 8.  This dramatic slowing would greatly reduce the 12 ppm potential (worst case) Cayuga salinification calculated in my second letter to Governor Cuomo.
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