The effects of scale and spatial heterogeneities on diffusion in volcanic breccias and basalts: Amchitka Island, Alaska

Amchitka Island, Alaska: moving towards long term stewardship

“…Diffusion coefficients and porosities of samples of rock from pre-shot exploratory cores, most in the vicinity of the Cannikin test shot, have been measured in laboratory experiments by the authors (Benning 2008). The results of the experiments indicate that on a finer scale than indicated by MT investigations, porosities vary significantly with geologic layers from approximately 9% for andesites, to 12–14% for basalts, to 13–29% for breccias, with low porosities layers interspersed with high porosity layers throughout the depth of a core.…”

“…At Amchitka Island, in particular, it is noted that the CRESP model that included consideration of preferential flow through the highly fractured andesite sill located at the depth of the Long Shot detonation resulted in significantly shorter travel times, 400 to 1,400 years (Powers and others 2005), as opposed to travel times greater than a 2,200 year model time in other conceptualisations (Hassan and Chapman 2006). In addition to the potential impacts of increased flow through this sill layer, estimates on the diffusion into matrix rock in andesite indicate the probability of little retardation in this layer (Benning 2008). Furthermore, uncertainties in the actual fracture porosities still exist, which is of concern since modelling has shown that the simulated travel time is very sensitive to this parameter.…”

“…Thus, the estimated travel times are significantly increased or decreased when the porosity changes by a few percent, making the prediction of the exact radionuclide arrival times difficult. Data on the porosity and diffusion characteristics of the subsurface materials at the Cannikin location indicate a highly variable and structurally layered subsurface (Benning 2008), which has not been included in groundwater modelling approaches to date. In the LTMSP, while it acknowledged the existence of the much shorter travel time indicated in the CRESP groundwater model, preference is given to the later modelling by Hassan and Chapman (2006) that indicated a much greater travel time.…”

Stochastic orders of log-epsilon-skew-normal distributions

“…A key advantage of the LESN family is that it includes as a subfamily the log-normal distributions, which are well known to be in use in a wide spectrum of disciplines. Examples include astrophysics, Gandhi (2009), environmental sciences, Benning and Barnes (2009), computer science, Doerr et al (2013), economics, Cobb et al (2013), biomedical, Feng et al (2013), and radiology, Neti and Howell (2008). Limpert et al (2001) compared the use of the log-normal distribution across several different science disciplines.…”