A row of fast-transpiring trees (poplar/willow are the standard field choice) planted across a shallow contaminant plume acts as a distributed line of pumping wells. Each tree's roots continuously draw groundwater upward through the xylem to replace what is lost by leaf transpiration, and that draw creates a hydraulic capture zone — a strip of the plume that gets pulled toward the roots instead of continuing downgradient.
Capture width: W = (N · q) / (v · b)
N = number of trees in the row
q = transpiration draw per tree (m³/day)
v = regional Darcy flux / groundwater velocity (m/day)
b = aquifer thickness engaged by roots (here 4 m)
This is the same mass-balance logic used to size a line of groundwater extraction wells for plume containment, applied to biological "pumps" instead of mechanical ones — it is the design basis behind real tree-plantation hydraulic barriers (e.g. poplar rows used at US DOE and industrial sites to contain TCE and nitrate plumes).
Contaminant that enters the transpiration stream doesn't just vanish — its fate depends on the compound, described by the Transpiration Stream Concentration Factor (TSCF) and what happens once it's inside the plant:
- TCE (trichloroethylene) — moderately volatile; a large share passes largely unchanged up the xylem and is released through leaf stomata (phytovolatilization), the rest is oxidised by plant/associated microbial enzymes.
- MTBE — highly volatile and poorly metabolised, so an even larger fraction volatilizes straight through the leaves.
- Nitrate — not volatile at all; it is a plant nutrient, so it is almost entirely assimilated into root and leaf biomass (uptake, not release) rather than emitted to the atmosphere.
Sliders change N, q and v and instantly resize the translucent green capture strip at the root line. Contaminant buttons change the leaf-volatilization vs. tissue-assimilation split applied to every particle the roots intercept. Particles that drift past the capture zone reach the downgradient monitoring point uncontained — exactly the "breakthrough" that real hydraulic-barrier designs are sized to avoid.