Transient groundwater flow may severely hamper the monitoring and evaluation of natural attenuation
processes at contaminated sites, as concentration distributions derived from point measurements at monitoring
wells may neither be temporally nor spatially representative. This problem is studied using numerical modelling
of transient flow and reactive contaminant transport within synthetic aquifers. Uncertainties in site investigation
due to transient flow are assessed by simulating plume sampling: concentration measurements at control planes
in different spatial configurations are used to characterize the plumes and attenuation processes. It is found that
local concentration measurements may be significantly influenced by transient flow conditions. Control planes
with insufficient spatial well density involve the risk of missing the plume centre and temporal concentration
peaks. Transient flow conditions may thus be a strong source of uncertainty for the assessment of contaminant
attenuation based on local measurements in space and time.