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# Initial/boundary conditions for discharge from spring (ECO2N)

Hello,

I am trying to use ECO2N to model a system where salty, CO2-rich water is upwelling into a shallow freshwater aquifer that is horizontal for some distance, then dips upward to a discharge point at the ground surface (a spring). The model seems to work fine under single-phase conditions, but fails to converge when gas phase evolves. Below is a summary of what I did and a request for suggestions on how to solve my problem.

I set up a 2D cross-sectional model that is rectangular except with the bottom-right corner removed to represent the dipping section. I then set no-flow boundaries along the bottom and right sides, hydrostatic pressures in boundary nodes down the left side, steadily decreasing pressures in boundary nodes along the horizontal top, and an upwelling source of water, salt, and CO2 at the bottom-middle. I then ran simulations for long periods to try and achieve steady state conditions. This works and the results make sense when the CO2 concentration is low and the gas remains dissolved, but when I increase the CO2 concentration of the source, gas phase evolves near the spring and convergence failures occur. I suspect that setting up the "spring" boundary condition differently could solve the problem, so I'm looking for suggestions on how to do that.

I've attached the input and output files from my most recent (failed) attempt here for reference. In the input file, you can see that I assigned a rock type "Atmos" to the top-right node, and "Sprng" to the node just below that. To allow fluids to move freely from the spring into the atmosphere, I set IRP=5 (all phases perfectly mobile) for the "Sprng" rock type, and set MOP(11)=0 for upstream weighting. To prevent infiltration of water from the atmosphere, I set the gas saturation in the "Atmos" node to 0.95, which is less than the residual liquid saturation for that rock type (0.1).

Can anyone suggest a better way to represent the spring discharge boundary, such that convergence failures aren't encountered when gas phase evolves near it?