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Because of the proximity of the PGDP to the New Madrid Seismic Zone, some citizens have raised concerns with regard to the potential safety or integrity of the existing buildings, burial grounds, or landfills at the PGDP. These concerns have also been raised with regard to the safety and integrity of any potential new structures such as a CERCLA landfill or future facilities that might be built at the site after the PGDP is shutdown and demolished. In addressing these concerns, the following facts need to be considered:
1) Various faults have been identified in the geology of western Kentucky, including the PGDP site. Faults, by themselves however, do not necessarily pose a significant seismic risk. The actual seismic risk at a particular site will be dependent upon several additional factors such as the expected magnitude of an earthquake at or near that site and the probability of such a magnitude occurring.
2) Kentucky State regulations prohibit the construction of any landfill “within 200 feet of a fault that displays Holocene displacement (i.e. the displacement has occurred in the last 10,000 years). After several geologic studies (both regional and site specific fault and paleoliquefaction studies) no evidence of Holocene displacement has been detected at the PGDP or the surrounding area. Thus, there does not appear to be any seismic regulatory barrier to the construction of a hazardous waste landfill at the PGDP site.
3) Prior to the construction of any future landfill, several additional analyses will have to be performed in order to determine what level of seismic risk to expect at the design location. These include:
a. Determining the magnitude of the expected level of seismic activity at the bedrock – this depends on an analysis of regional historic seismic activity and a specified earthquake probability. 40 CFR Chapter 1 (258.14) requires that any landfill containment system be designed to withstand the peak ground acceleration (PGA) at bedrock “depicted on a seismic hazard (sic) map, with a 90 percent or greater probability that the acceleration will not be exceed in 250 years, or the maximum expected horizontal acceleration based on a site-specific seismic risk assessment.”
The phrase “90 percent or greater probability that the acceleration will not be exceed in 250 years” effectively means that the landfill containment system must be designed to withstands the bedrock Peak Ground Acceleration (PGA) of the strongest earthquake ground motion that will affect the landfill in 2500 years. (This can be expressed in a variety of ways, and is roughly equivalent to a 2% Probability of Exceedence in 50 years).
Note: several different seismic analyses have been done for the PGDP site, based on both probabilistic and deterministic methods. Depending upon the assumptions used in the analyses, results ranging from 0.5 g to 0.7 have been determined and published.
b. Determining the magnitude of the expected level of seismic activity at the surface – this depends on an analysis of the soils between the bedrock and the surface. In theory, this could either lead to and increase (amplification) or decrease (de-amplification) of the impact of the PGA at the surface – where the particular structure is to be built. A preliminary analysis of the conditions at the southern end of the site have indicated that the expected motion at the surface will actually be de-amplified by 33%. Thus, if one were to use a conservative estimate of the PGA to be 0.7, the associate ground motion at the surface would be expected to be 0.67*0.7 = 0.48 g.
c. Determining the frequency of the expected level of seismic activity at the surface – once this is determined, engineers then check to make sure that the frequency does not match the design frequency of the structure (which could cause the structure to resonate leading to an increase in the shaking) or the frequency of the foundation soils (which could lead to liquefaction of the soils) – if either of these conditions are found to exist then either the soils associated with the foundation are modified, or the structure is designed in such a way to avoid this critical frequency.
4) Before any final decision is made with regard to the placement and design of any future landfill, the CERCLA design criteria require that the public be involved in the decision making process. To date, the DOE has held two public informational meetings about current proposals to construct a hazardous waste landfill at the PGDP. The first set of meetings was held on November 18 and 20, 2008 and the second meeting was held on March 24, 2009. DOE is currently planning to hold a third meeting about this structure this fall.
For additional information about seismic issues at the PGDP or the potential CERCLA cell please consult:
Figure 1. Techtonic Map of Midwest Region
Seismicity between 1974 and 2005 in the central United States (Wang and Woolery, 2008)
"Historic Earthquakes: Near Charleston, Mississippi County, Missouri". United States Geological Survey. http://earthquake.usgs.gov/regional/states/events/1895_10_31.php. Retrieved 2009-09-23