Applied numerical modeling of saturated / unsaturated flow and ...

norm. degradation rate constant [-]
100
10
1
0.1
W S = 4 m W S = 16 m
single realization result
ensemble mean
A1 1 A2 2 A3 3 4B
A1 1 A2 2 A3 3 4B
method
method
Figure 7: Comparison of normalized degradation rate constants estimated with methods A1 – A3 and B for two single
plume realizations with different source widths (Ws = 4 m: left diagram; Ws = 16 m: right diagram). Both plume
realizations were repeatedly and independently investigated by 13 individual persons each.
Summary and Conclusions
Table 5: Normalized degradation rate constants estimated with methods
A1 – A3 and B for two individual plume realizations with different source
widths (Ws = 4 m and 16 m). Both plume realizations were independently
investigated by 13 individual persons each.
WS = 4 m
method A1 method A2 method A3 method B
mean 5.26 6.25 4.56 4.26
median 5.02 5.88 2.94 3.47
stdv. 1.59 2.11 3.10 3.33
cv 0.30 0.34 0.68 0.78
N 13 13 13 13
WS = 16 m
method A1 method A2 method A3 method B
mean 1.01 1.28 1.00 1.21
median 0.97 1.15 0.87 1.20
stdv. 0.31 0.51 0.48 0.51
cv 0.31 0.40 0.48 0.42
N 13 13 13 13
In this study, the frequently used center line approach for estimation of degradation rate constants is compared to a
methodology suggested by Stenback et al. (2004), which uses all concentration measurements downstream of the
contaminant source zone to estimate the degradation rate. In a numerical experiment, both strategies are applied to a set
of 85 synthetic contaminant plumes, subject to first order degradation, and investigated using extensive monitoring
networks. Rate constants are estimated using concentrations, hydraulic heads and conductivities locally measured at the
observation wells of the monitoring network.
Results show that rate constants tend to be overestimated in general. Using the center line approach (strategy A) and a
simplified observation well set up causes a general overestimation due to three factors:
• biased estimates of the mean flow velocity,
• inadequate parameterizations of longitudinal and transverse dispersivities and
• off center line measurements.
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