Landfill Gas Well 2D: Pressure-Field Capture Model

This is the plan-view counterpart to the 3D landfill gas-well simulator, and it computes its physics independently rather than reusing the 3D scene's assumed velocity profile. A finite-difference relaxation solves the steady 2D Darcy pressure field for the whole site footprint — a Poisson equation with the well's vacuum as one boundary and the atmosphere at the site margin as the other, and continued anaerobic gas generation as a distributed source term everywhere in between. Methane parcels are advected by the actual gradient of that solved field, and the well's radius of influence is read directly off it as the radius where the net radial flow reverses — the same conservation-based definition real landfill gas engineers use when spacing extraction wells, verified here by checking that the net flux through that radius is zero.