Table 7. HMX concentrations from on-site colorimetric method for duplicate subgrid subsamples

Figure 11. Dependence of RSDs of duplicate subgrid samples on mean HMX concentration

Comparison of HMX concentration estimates from on-site colorimetric and laboratory methods

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17

Table 7. HMX concentrations from on-site colorimetric method for duplicate

subgrid subsamples.

HMX concentration (mg/kg)

Subgrid

Mean

Mean replicate variability (all values)

5.15

Mean replicate variability (values >50 mg/kg)

1.24

Mean RSD (all values) = 27.9%

Mean RSD (conc. > 50 mg/kg) = 16.3%

* Replicate variability is the higher value/lower value for replicates.

error, and the sampling error due to spatial het-

combination of spatial heterogeneity of analyte

erogeneity in analyte distribution within the

distribution, subsampling error, and error due to

subgrid. To assess the magnitude of these various

analysis. Likewise the mean RSD in Table 7 for

contributions to the total uncertainty, a series of

duplicate subsamples for samples with concen-

replicate subsamples were collected. For the nine

trations above 50 mg/kg was 16.3% and is due to

subgrids that were resampled, duplicate pile

the contribution of subsampling error and analy-

subsamples were collected and analyzed by the

sis error. From a series of measurements on a

on-site colorimetric method (Table 7). For eight of

single extract, the RSD for analysis was only 3.2%,

the nine subgrids, the ratios (higher duplicate

so the major portion of this 16.3% estimate is as-

value divided by the lower value) ranged between

sociated with subsampling and extraction. Based

1.02 and 1.60, with a mean of 1.24 indicating that

on a comparison of the variances associated with

we were able to reproducibly obtain subgrid

RSDs of 30.1% and 16.3%, only about 30% of the

subsamples. The replicates for subgrid D9C dif-

total variance is associated with subsampling and

fered by a factor of 36.4, but the concentrations

analysis while 70% is due to spatial heterogeneity

for both replicate samples were low. This anoma-

within the subgrid. It is interesting to compare

lous result appears atypical. With the contribu-

this result with those from the seven core samples

tions of spatial heterogeneity excluded as a source

in a wheel pattern, both here and in the earlier

of uncertainty, the results were in very good agree-

studies (Jenkins et al. 1992). The mid-range spa-

ment. An ANOVA was not conducted for this

tial heterogeneity error for area integrated samples

data set since variances were not homogeneous;

was only a little more than twice as large as the

however, RSD values were much more normally

subsampling plus analytical error, whereas the

distributed and were once again inversely related

comparable relationship for the core samples

to concentration.

yielded a difference of about 10 times. Clearly the

Taking the results from Tables 6 and 7 together,

area integrated samples do a better job of mini-

we can compare the relative magnitudes of un-

mizing error due to spatial heterogeneity. We

certainty due to spatial heterogeneity and

should note that area integrated samples can be

subsampling error. The mean RSD estimate when

obtained by a variety of protocols which might be

sampling a subgrid with concentrations above 50

equal to or better than the circular path used in

mg/kg was 30.1% (Table 6). This can be consid-

this study.

ered an estimate of total uncertainty due to the

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