Department of Geology
Eastern Washington University

Recent Masters theses in Geology

SOIL-GAS RADON MAPPING AND SEASONAL VARIABILITY,
SPOKANE AREA, WASHINGTON

Eric A. Johnson

Abstract

Soil-gas radon sampling conducted in the Spokane area, Washington, found
elevated levels of radon concentrations in areas underlain by
Tertiary-Cretaceous and Cretaceous granites, and pegmatite dike-like
bodies.  The Tertiary-Cretaceous unit Tkiaa that consists of pegmatite, and
aplite sill- and dike-like bodies produced a maximum value of 6842 pCi/L
with an average of 1230 pCi/L from 11 samples.  The Cretaceous unit Kiats
(Mount Spokane granite) produced a maximum level of 891 pCi/L and an
average value of 578 pCi/L.  Surprisingly, Quaternary flood gravel deposits
(Qfg) did not show elevated soil-gas radon levels as expected.  From 48
samples, a maximum of 510 pCi/L and an average of 220 pCi/L were recorded
from Qfg deposits.  Indoor radon levels over this unit commonly produce
high readings but those observations are not explained by soil-gas radon
values.

Areas with geologic potential to produce elevated soil-gas radon levels
are found to the north and south of the Spokane River in the Selkirk
Mountains in regions where granites, pegmatite dikes, and metamorphic rocks
are present.  These areas produced repeatedly higher soil-gas radon levels.
Areas such as the Columbia Basin and the Spokane valley, where flood
gravels or basalts are present, did not show higher soil-gas radon levels.
Two basalt units, Priest Rapids Member of the Wanapum basalt (Mvwp) and
Grande Ronde basalt-magnetostratigraphic unit N2 (MvgN2), produced maximum
levels of 275 pCi/L and 380 pCi/L with average levels of 170 pCi/L and 214
pCi/L respectively.

Concentrations of soil-gas radon were measured weekly at a permanent
monitor in north-central Spokane for a 1 year period from Feb. 4, 1994, to
Feb. 25, 1995.  Seasonal temperature fluctuations are strongly correlated
with soil-gas radon.  A general rise in radon levels was observed during
the spring and early summer until levels peaked in mid summer.  A general
decline in radon levels in the autumn months and low levels were observed
during the winter.  However, when temperatures dropped below freezing for
an extended period of time in late December and early January, a frozen
ground capped the soil and radon levels increased until temperatures rose
above the freezing point.