A classroom exercise for Teaching Hydrogeology in the 21 st Century

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

2. Approximate limit of Pleistocene continental glaciation Milford, Michigan

3. Remedial Extraction Wells -- August 2003 Municipal Water Supply Wells

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

5. (a) Regional Aquitard Model S N DLZ PW-1, 3, 2 VOM PW-1,2 1 2 3 4 Aquifer 5 6 7 8 9 Aquitard 10 11 12 13 14 15 Conceptual Model Uncertainty Aquitard geometry poorly constrained by eight wells – only two of which penetrate > 5’.

6. A: Regional Aquitard Model S N 1 DLZ PW-1, 3, 2 VOM PW-1,2 2 3 4 Aquifer 5 6 7 8 9 Aquitard 10 11 12 13 14 15 Alternative Conceptual Models

7. Alternative Conceptual Models B: Extended Aquifer Model S N 1 DLZ PW-1, 3, 2 VOM PW-1,2 2 3 4 Aquifer 5 6 7 8 9 10 11 12 13 Aquitard 14 15

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

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

10. 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 DLZ-PW1/2/3 VOM-PW-1/2 Contaminant Transport Model (MT3DMS) output for MTBE in a shallow model layer. 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?

11. Extraction Wells

12. Extraction Wells

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

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

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

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

17. 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 • Performance monitoring networks