Free Online Darcy Flow Calculator: Seepage Velocity and Travel Time

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Specific discharge is what Darcy law gives directly; real water moves faster because it only uses the connected pores, and a sorbing contaminant moves slower again by the retardation factor. Set R to 1 for a conservative tracer like chloride.

Darcy law, and the three speeds of groundwater

Groundwater has one law and three speeds, and confusing them is the most common error in the field. The tool above keeps them straight: specific discharge is conductivity times gradient, the volumetric flux Darcy measured; seepage velocity is that flux divided by effective porosity, the speed actual water molecules travel; and a sorbing contaminant crawls slower still by its retardation factor. Enter six numbers and the tool reports all three, plus the flow per unit width of aquifer and the arrival times at a downgradient receptor.

Cross section of an unconfined aquifer showing the sloping water table driving Darcy flow, with the water seepage velocity arrow longer than the retarded contaminant arrow
Gradient drives the flow, porosity sets the speed, and retardation is why the contaminant always arrives later than the water.

This tool is part of the Hydrogeology and Water Suite in the bar above, and it is the arithmetic inside every contaminant plume timeline and every time-of-travel protection zone.

Why the distinctions matter

  • Using specific discharge as a travel speed understates plume velocity by a factor of one over porosity, typically 3 to 5: the mistake flatters every timeline it touches.
  • Retardation runs the other way: sorbing solutes like many metals and hydrophobic organics arrive years after the water that carries them, and R is measurable from soil partitioning data.
  • Flow per unit width, times the plume width, is the flux a pump-and-treat system must actually intercept.
  • The linear Darcy law holds for essentially all natural groundwater; only extreme gradients in coarse gravel bend it.

What this tool is for

Plume timelines, receptor risk screening, permeable reactive barrier sizing, and the homework of every hydrogeology course. Heterogeneity spreads real plumes ahead of and behind these mean values, which is what dispersion modeling adds; the center of mass, though, obeys exactly this arithmetic.

Related Calculators and Reference

See also the Permeability Converter for getting a conductivity into the units this calculator expects.