Post-Audit of a Groundwater Model Used in Designing a Remediation System

A groundwater model was developed to select optimum placement and flow rates for a remediation system consisting of a continuously pumping sump and a series of bioreactor infiltration trenches within a contaminant plume.  After two years of system operation, a post-audit was performed to determine the accuracy and viability of the model for full-scale facility remedial design.

A quantitative comparison of simulated and measured heads showed the best overall fit, as indicated by the lowest absolute mean residual and normalized root-mean squared error, corresponded to a mid-stand of water table.  The best head correlation, as indicated by the highest correlation coefficient, corresponded to a high water table.  The worst overall fit to the model corresponded to a low water table.  The less model accuracy during high and low water tables suggested that the model is desensitized by specified head boundaries near the recirculation trenches.     

A comparison of simulated flow lines with those drawn from measured heads showed the best overall fit corresponding to a low water table, despite this condition also corresponding to the worst quantitative fit of heads.  The projected flow lines showed a reasonable match to mid-stand of water table.  The worst match between flowlines corresponded to a high water table, although this condition also corresponded to the best statistical correlation of heads.    

This study illustrates the importance of: 1) post-auditing to confirm model accuracy;  2) avoiding use of head boundaries near flow boundaries (e.g., pumping and infiltration); and 3) evaluating the post-audit in terms most relevant to the purpose of the simulations (e.g., flow lines and targeted treatment areas rather than heads).  Based on an accurate groundwater model, a full-scale remedial design can be prepared with a higher degree of confidence in the actual effects of the selected remedy.