Zerdly

Contaminant Plume Travel Time Estimator

Estimate how long a dissolved contaminant plume takes to travel a set distance through an aquifer, using Darcy's law and the seepage velocity. Enter the hydraulic conductivity, gradient, effective porosity, distance and an optional retardation factor to get the pore-water velocity, the contaminant velocity and the travel time in days and years.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the hydraulic conductivity K in metres per day and the hydraulic gradient i as a dimensionless slope (drop in head over distance, m/m).
  2. Enter the effective porosity n_e (the connected pore fraction that actually carries flow, typically 0.1–0.35) and the travel distance L to the receptor in metres.
  3. Leave the retardation factor R at 1 for a conservative (non-sorbing) tracer, or enter a larger value for a contaminant that sorbs to the aquifer solids, then read the seepage velocity, contaminant velocity and travel time.

How it works

The pore-water or seepage velocity is v_s = K·i / n_e: darcy flux K·i divided by the effective porosity gives the true speed of water in the pores. A reactive contaminant is slowed by sorption by the retardation factor R, so its velocity is v_c = v_s / R and the time to travel distance L is t = L / v_c = L·R / v_s. Dividing the day figure by 365.25 gives years. The model assumes steady, uniform, one-dimensional advective flow only.

Worked example

Worked example. With K = 5 m/day, i = 0.01, n_e = 0.25, L = 100 m and R = 1, the seepage velocity is 5 × 0.01 / 0.25 = 0.2 m/day, so the travel time is 100 / 0.2 = 500 days (about 1.37 years). Doubling the retardation factor to R = 2 halves the contaminant velocity and doubles the travel time to 1000 days.

Common mistakes

  • Using total porosity instead of effective porosity — only the connected, flow-carrying pores set the seepage velocity, so total porosity underestimates the speed.
  • Entering the hydraulic gradient as a percentage or an angle; it is a dimensionless ratio (head drop divided by flow-path length, m/m).
  • Treating the answer as an exact arrival time — dispersion means the leading edge of the plume arrives earlier than the advective (centre-of-mass) travel time this tool reports.

Frequently asked questions

What is the retardation factor R?

R accounts for a contaminant sticking (sorbing) to the aquifer solids, which slows it relative to the groundwater. R = 1 means no sorption (a conservative tracer moves at the water velocity); R = 2 means the contaminant moves at half the water speed and takes twice as long. It is derived from the soil bulk density, porosity and a distribution coefficient (Kd) for the specific chemical.

Why is my real plume arriving sooner than the calculated time?

This tool gives the advective travel time of the plume's centre of mass. Real plumes also spread by hydrodynamic dispersion and preferential flow through more permeable layers, so the leading edge can arrive noticeably earlier than the average travel time — always treat the result as an order-of-magnitude screening estimate.

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