A Regional Model Comparison between MODPATH and MT3D of Groundwater Travel Time Distributions

Area of Science:

• Hydrogeology
• Environmental Modeling
• Contaminant Transport

Background:

• Groundwater quality changes lag land use alterations due to travel times.
• Accurate simulation of groundwater travel time distributions (TTDs) is crucial for assessing contaminant transport.
• Understanding the limitations of different numerical methods is essential for reliable groundwater modeling.

Purpose of the Study:

• Compare simulated groundwater travel time distributions (TTDs) using MODPATH and MT3D.
• Assess the strengths and weaknesses of each method for simulating flow to wells and streams.
• Identify methods to improve the accuracy of contaminant transport simulations.

Main Methods:

• Compared MODPATH (particle tracking) and MT3D (age-mass) simulations for TTDs.
• Evaluated 9 MODPATH and 13 MT3D implementations in the Wisconsin Central Sands.
• Focused on advective groundwater flow and age-mass transport without dispersion for direct comparison.

Main Results:

• MODPATH accurately simulates TTDs to surface discharge but struggles with weak-sink well cells.
• MT3D better represents weak-sink well cells but neglects travel time within them and has less accurate surface water boundary treatment.
• Simulations showed median travel time differences up to 18 years between methods.

Conclusions:

• Both MODPATH and MT3D have distinct strengths and weaknesses for simulating TTDs.
• MT3D is superior for weak-sink well representation, while MODPATH excels in surficial recharge and discharge.
• Methodological choices significantly impact simulated TTDs, necessitating careful consideration for contaminant transport modeling.