When Does Aquifer Heterogeneity Matter? Predicting the Influence of Alternative Conceptual Models on Contaminant Plume Migration A classroom exercise for Teaching Hydrogeology in the 21 st Century

Milford, Michigan Approximate limit of Pleistocene continental glaciation

Remedial Extraction Wells -- August 2003 Municipal Water Supply Wells

Conceptual Model 870’ 830’ VOM PW-1,2 520 gpm N S 900’ 180 ft 100 ft Downward migration driven by gravity (recharge) and pumping. DLZ-1,2,3 300 gpm Remedial pumping wells intended to intercept contaminant plume(s)

Conceptual Model Uncertainty Aquitard geometry poorly constrained by eight wells – only two of which penetrate > 5’. SN DLZ PW-1, 3, 2 VOM PW- 1,2 Aquifer Aquitard (a) Regional Aquitard Model

Alternative Conceptual Models SN DLZ PW- 1, 3, 2 VOM PW-1,2 9 Aquifer Aquitard A: Regional Aquitard Model

Alternative Conceptual Models SN DLZ PW- 1, 3, 2 VOM PW-1,2 9 Aquifer Aquitard B: Extended Aquifer Model

Question: 870’ VOM PW-1,2 N S 900’ DLZ PW- 1, 2, 3 830’ 870’ 830’ VOM PW-1,2 N S 900’ DLZ PW- 1, 2, 3 If the two conceptual models suggest different flow paths for the contaminant plumes, will these differences be detectable in the extraction wells?

To Answer the question… Construct two flow and transport models Regional Aquitard Regional Aquitard Extended Aquifer Extended Aquifer Calculate contaminant breakthrough curves MTBE MTBE cDCE cDCE Note: extraction well concentrations were measured in the combined effluent stream entering the air stripper at the site.

MTBE Layer 1 – Shallow Day 1 MTBE Layer 1 – Shallow Day 365 MTBE Layer 1 – Shallow Day 730 MTBE Layer 1 – Shallow Day 1095 MTBE Layer 1 – Shallow Day 1460 MTBE Layer 1 – Shallow Day 1825 VOM-PW-1/2 DLZ-PW1/2/3 Note how the plume evolves from the initial steady state condition after the remedial wells begin pumping. What does this imply about the transport path for the plume – is it becoming shallower or deeper after pumping begins? Contaminant Transport Model (MT3DMS) output for MTBE in a shallow model layer.

Extraction Wells

870’ VOM PW-1,2 N S 900’ DLZ PW- 1, 2, 3 830’ 870’ 830’ VOM PW-1,2 N S 900’ DLZ PW- 1, 2, 3 Implication: Small differences in flow paths may not matter when the contaminant signal is integrated across long screen intervals. Observation: Both conceptual models agree well with observed data.

Implications for Monitoring Well Network Remedial Extraction Wells -- August 2003 Municipal Water Supply Wells RL-1 What about monitoring wells with smaller screened intervals?

Question: Could a monitoring well detect migration of the plume in response to remedial pumping? RL-1 cDCE Answer: Yes, if it is in the right place.

Question: Does one end-member conceptual model better fit the data than the other? Answer: You decide! Extended Aquifer Regional Aquitard

Discussion Questions Can end-member models be used to evaluate the consequences of conceptual model uncertainty? Can uncertainty be more important for some modeling purposes than others? Integrated breakthrough curves Integrated breakthrough curves Performance monitoring networks Performance monitoring networks