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This Information Prepared by University of Minnesota
Department of Environmental Health and Safety and Facilities Management

What does the term "100-year flood level" mean?

A 100-year flood is the level to which the Mississippi River would rise an average of

once every 100 years. Listed below are the significant flood levels of the Mississippi

River at the Andersen Library as determined by the Federal Emergency Management


10 year 734.5 feet

50year 737.5 feet

100 year 738.5 feet

500 year 741.2 feet

What storm/flood level was used in the design of the caverns?

The roadway in front of the cavern is approximately 17 feet higher than the 500-year flood level. The portal entry is two feet above this level. Therefore the caverns will not flood from overland flow during a 500-year flood. To prevent flooding from a groundwater flow, the caverns have a groundwater drain and pump system installed around the perimeter that is similar to typical building basements. Groundwater modeling shows that the pumps can handle flow corresponding to 500-year flood plus five feet.

How are the caverns protected from this storm/flood level?

The high ground in front of the portal protects the caverns from overland flow. The drain and pump system protects the cavern from groundwater flow. A backflow valve and an automatic butterfly valve prevent flow backup through the groundwater drain system. If the backflow valve fails, the butterfly valve will automatically close. The butterfly valve can also be closed manually. Backup generators would power the pumps in the event of a main power failure. If the groundwater drains and pumps cannot keep up with the groundwater flow additional pumps can be added to the drain system through manholes in the service road area.

Will the sump pumps continue to work if storm sewers are filled to capacity during a significant flood?

Yes. Water could still be pumped out because the pumps can produce a hydraulic head higher than the flood level.

Will the caverns flood in a 500-year flood?

The safeguards discussed above will prevent the caverns from flooding in a 500-year flood condition.

So what's really going on with the air in the Andersen Library caverns?

From 1893 to 1959, Minnegasco produced gas from coal, and stored it in large tanks, including one located on what is now the West Bank ball field. Coal tar from the manufactured gas collected in the bottom of the tanks, and has polluted the ground and shallow groundwater.

The groundwater in the area generally flows toward the river from the polluted area past the Andersen Library caverns.

A system was developed to capture the groundwater in pans so it wouldn't leak into the caverns. The polluted water in those pans provides a food source for a certain type of mold that feeds on petroleum products. The mold itself isn't a problem, but when the mold dies and begins to decompose, it emits hydrogen sulfide.

Some people have reported an oily odor in the cavern driveway that's similar to the odor of a garage or automobile mechanic's shop. That's the coal tar or perhaps vehicle exhaust. Others have reported a rotten egg smell. That's the smell of the mold decomposing. It may be annoying, but it's not dangerous, because the levels of these materials are kept very low.

The only place in the library complex where there is the potential for a significant concentration of hydrogen sulfide is in a mechanical room where a sump pump is located. Facilities Management knows of the problem and has added monitoring devices and has trained their personnel on the precautions necessary to work in the room. In addition, the ventilation system for the room exhausts air directly to the outside of the building rather than through the building's regular return air system.

Access to the room is restricted to certain Facilities Management personnel only. There is no chance that library employees or patrons can inadvertently enter the room.

Does the air in the above-ground levels of Andersen Library contain harmful levels of hydrogen sulfide?

No. Monitoring done in Andersen Library shows that the air quality is no different than the air in Wilson Library or other buildings on campus.

Peer Environmental has also done testing in the library caverns since before the library opened.

But I know I can smell something.

The nose is a highly sensitive detector of odors. While the danger level for hydrogen sulfide is 10 parts per million, the instruments used to test for hydrogen sulfide are sensitive enough to record one part per million. The human nose can detect even less than that. If you notice an odor of rotten eggs, it's likely you smell hydrogen sulfide. It's also likely that any smells are the result of a concentration of less than one part per million.

Will the ventilation system in Andersen Library prevent hydrogen sulfide gas from entering the storage areas where the collections are kept?

Yes. The ventilation system creates positive pressure inside the storage buildings, preventing air transmission from outside areas. The walls, floors and roofs of the buildings are also designed to retard vapor transmission.

What prevents hydrogen sulfide gas from entering the storage areas?

Hydrogen sulfide is heavier than air. If present, it will remain at the bottom of the sealed sumps below the floor of the mechanical room at the lowest cavern level. The room has elaborate sensing, control and exhaust systems. Air quality monitoring has shown no detectable levels of hydrogen sulfide anywhere in the storage buildings.

Is the ventilation system for the underground storage area the system that serves the above ground office area?

No. The systems are totally separate.

What safeguards are in place to handle contaminated groundwater?

Sump pits are sealed and closely monitored. Exhaust fans remove any hazardous gases.

What is the University doing about the contaminated ground water?

A Horizontal Well was installed in 2001 to protect the building systems, materials, and occupants by diverting contaminated water. This Well proved to meet Minnesota Pollution Control Agency (MPCA) performance criteria.

The University continues to investigate a variety of solutions to the contaminated ground water problem and will continue to work with Minnegasco and the MPCA in its efforts to protect its facilities and staff.

Why not just clean up the old gas holder?

The University is anxious to have the gas holder cleaned up. This will be an expensive project and Minnegasco's contribution to the cost needs to be assured before the project can begin.

What if I have an air quality concern?

If you have concerns about air quality in your work area or concerns that your workplace is in any way unsafe, you can contact the Zone office at 625-2001, the Department of Environmental Health and Safety at 626-6002 or your supervisor. The university is committed to workplace safety and will never compromise the safety of employees or students.