This is the 2D plan-view counterpart to the 3D particle-tracking version, and it deliberately solves the problem with a different numerical method rather than just flattening the same camera view. Explosive residues (TNT, RDX, HMX) leaching from a legacy munitions site obey the 2D advection–dispersion–reaction equation:
∂C/∂t + v_eff ∂C/∂x = D_L ∂²C/∂x² + D_T ∂²C/∂y² − λC
v_eff = v / R (retarded seepage velocity)
D_L = α_L · v_eff (longitudinal dispersion coefficient)
D_T = 0.1 · α_L · v_eff (transverse dispersion coefficient)
λ = ln(2) / t½ (biodegradation decay constant)
Instead of tracking thousands of Lagrangian particles like the 3D version, this simulator solves the equation directly on an Eulerian finite-difference grid (first-order upwind advection + central-difference diffusion), the same family of method used by real MODFLOW/MT3D-style groundwater codes when they solve the concentration field rather than track particles. Every frame the whole grid is stepped forward, sub-cycled with an adaptive time step that always satisfies the advective and diffusive stability limits (Courant number ≤ 1, diffusion number ≤ 0.5) — otherwise the field would blow up numerically at high flow or dispersion settings.
The plume is rendered as a live concentration heat-map (colour = local concentration relative to the source), the monitoring-well readout is the transect-averaged concentration 120 m downstream, and the "Attenuated" stat is a genuine mass-balance measurement: it tracks how much contaminant mass has been removed by biodegradation versus how much is still present in the aquifer or has already flushed past the far boundary, so it is always a real percentage of tracked mass, never a fitted number.
- Retardation factor R — sorption of the explosive to soil organic carbon slows it relative to the water itself; RDX is weakly sorbed (R ≈ 1.5–2), TNT sorbs and reacts more (R ≈ 3–5).
- Dispersivity αL — controls how much the plume spreads and smears out along the flow path as it travels; transverse spreading is scaled to 10% of it, matching typical field-measured anisotropy ratios.
- Half-life — natural attenuation from microbial biodegradation and abiotic reduction; shorter half-lives mean the plume self-destructs before reaching a receptor.
- The well status badge turns from green → amber → red as the relative concentration at the monitoring transect rises, mirroring how a real long-term monitoring network flags a plume approaching a compliance boundary.