RDX was the explosives-related analyte found at highest concentration in the 27
surface soils, although the concentrations found at the Fort Richardson range
were generally an order of magnitude lower than those found at Fort Lewis. The
reason for this difference may be the practice at Fort Richardson of filling in the
craters with clean gravel after each training session, or a lower usage of the range
in terms of total grenades thrown than at the Fort Lewis range. Several RDX
concentrations exceeded 100 :g kg-1 at the Fort Richardson range, but most of
the values were below 30 :g kg-1, indicating that the distribution of these data
was also nonnormal. Overall, RDX concentrations in the surface soil ranged from
1.7 to 518 :g kg-1 with a median value of 28.9 :g kg-1.
RDX was detected in all of the 21 subsoil samples collected at Fort
Richardson with concentrations ranging from 1.8 to 193 :g kg-1 (Figure 12).
Median values for soil samples collected at 15-, 30- and 45-cm depths were 15.0,
5.2, and 4.3, :g kg-1, respectively. These results suggest that RDX is leaching
downward into the soil profile at the hand grenade range, but the concentrations
must be very low. These residual concentrations in subsurface soils are often
barely detectable, even using the new GC-ECD method with much lower
detection limits than the method that had been used traditionally, RP-HPLC
Method 8330. Had Method 8330 been used to analyze the soils samples at Fort
Richardson, explosives analytes in most samples would have been below
detection.
The distribution of TNT concentrations in surface soil samples at Fort
Richardson was similar to that of RDX. Several values were above 50 :g kg-1,
but most of the values were less than 20 :g kg-1 (Figure 13). As observed pre-
viously, concentrations were an order of magnitude or more lower than found at
Fort Lewis. In surface soil samples TNT concentrations ranged from 0.9 to
134 :g kg-1 with a median value of 9.7 :g kg-1. The two environmental transfor-
mation products of TNT, 4ADNT and 2ADNT, ranged from <1 to 48.8 :g kg-1
and <1 to 28.0 :g kg-1, respectively, with median values of 10.1 and 7.3 :g kg-1.
Concentrations of TNT in 16 of the 21 subsoil samples were <1 :g kg-1. Con-
centrations of 4ADNT and 2ADNT were also generally <1 :g kg-1 indicating that
neither TNT nor its daughter products were leaching at detectable concentrations
below the surface soil. The behavior of TNT relative to RDX is not surprising,
since TNT and its daughter products are known to adsorb to soils to a much
greater extent than RDX (Brannon et al. 1999).
Concentrations of HMX in surface soils at Fort Richardson ranged from <1
to 80.4 :g kg-1 with a median value of 6.9 :g kg-1 (Figure 14). A ratio of the
median values for RDX/HMX in these soils is 4.2 compared with a mean ratio of
7.6 for the Composition B extracted from a low-order hand grenade. Concen-
trations of HMX in the subsoil were <1 :g kg-1 in 14 of the 21 samples. The
lower solubility and rate of dissolution of HMX compared with RDX are prob-
ably responsible for a lower residual concentration ratio of RDX to HMX in
surface soil than present in Composition B, and for the higher ratio of RDX to
HMX found in the subsoil samples.
27
Chapter 2 Characterization of Explosives Contamination at Military Firing Ranges
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