Groundwater Transport Calculator

Calculate contaminant travel time and concentration in groundwater using advection-dispersion.

Groundwater velocity v (m/day) (m/day)

Dispersivity α (m) (m)

Longitudinal dispersivity ~0.1×plume length

Distance to receptor x (m) (m)

Retardation factor R

R=1: nonreactive, R>1: sorbs to soil

Initial concentration C₀ (mg/L) (mg/L)

Please check your inputs and try again.

Why Contaminants Travel Slower Than Water

Groundwater moves at a velocity set by hydraulic gradient and permeability. Dissolved contaminants move more slowly, because they repeatedly sorb onto aquifer solids and desorb again. The retardation factor R quantifies that lag.

R = 1 + (ρ_bK_d)/n v_contaminant = v_water/R

R value	Meaning	Typical compound
1	No sorption — travels at water velocity	Chloride, nitrate
2–5	Moderately retarded	Benzene, atrazine
10–100	Strongly retarded	PAHs, many pesticides
> 100	Essentially immobile	PCBs, dioxins

R depends on the compound's sorption coefficient and, critically, on the aquifer's organic carbon content. The same contaminant is far more mobile in clean sand than in an organic-rich silt, which is why site characterisation matters as much as chemical identity.

Advection, Dispersion and Why Plumes Spread

Process	What it does	Dominant when
Advection	Bulk movement with the water	Always the main transport
Dispersion	Spreading from varied flow paths	Grows with travel distance
Sorption	Retards movement	Organic-rich media
Degradation	Reduces mass	Biodegradable compounds

Dispersion means the leading edge of a plume arrives before the calculated advective travel time. This is why monitoring wells detect contamination earlier than simple velocity calculations predict, and why plume modelling uses arrival-time distributions rather than a single number.

Worked Examples

Example 1: Benzene in sandy aquifer: v=1m/day, R=1, x=100m

t=100×1/1=100 days

Result: Arrives in 100 days — mobile contaminant

Benzene is relatively nonretarded in groundwater

Example 2: Atrazine: v=0.1m/day, R=4, x=50m

t=50×4/0.1=2000 days

Result: 5.5 years — retardation slows transport

Sorption to soil organic matter retards transport

Example 3: Effect of organic carbon

Same compound, f_oc = 0.001 versus 0.02

Result: R differs roughly twentyfold

Sorption scales with organic carbon content. A contaminant mobile in clean sand can be almost immobile in organic-rich sediment.

Example 4: Why early detection happens

Advective travel time 100 days, with dispersion

Result: Leading edge detected around 70–80 days

Dispersion spreads arrival over a range. Monitoring wells see the front well before the calculated centre-of-mass arrival.

Common Mistakes

⚠️

Assuming contaminants travel at groundwater velocity

Only non-sorbing tracers such as chloride do. Most organics are retarded by a factor of 2 to 100 or more.

⚠️

Using generic retardation factors

R depends on the aquifer's organic carbon fraction and bulk density. The same compound behaves very differently in sand versus organic silt.

⚠️

Ignoring dispersion in arrival-time estimates

Dispersion spreads the plume so its leading edge arrives before the advective travel time. Monitoring must anticipate early detection.

⚠️

Forgetting degradation

Biodegradable compounds lose mass in transit. A conservative transport calculation that ignores this overestimates the arriving concentration.

Frequently Asked Questions

What is retardation factor R?

R = 1 + ρ_b×Kd/θ where ρ_b=bulk density, Kd=soil-water partition, θ=porosity. R=1: travels at groundwater velocity. R=4: travels 4× slower than groundwater. R for conservative tracers (Cl-) =1.

Dispersion vs advection?

Advection: bulk transport at mean flow velocity. Dispersion: spreading due to velocity variations at pore scale. Peclet number Pe=vx/D: Pe>>1 advection-dominated, Pe~1 dispersion significant. Real plumes: spreading in all directions.

What is the retardation factor?

The ratio of groundwater velocity to contaminant velocity. R = 1 means no sorption; R = 10 means the contaminant moves ten times slower than the water.

Why does organic carbon content matter?

Most organic contaminants sorb to organic matter rather than mineral surfaces. Higher organic carbon means stronger sorption and greater retardation.

What is the difference between advection and dispersion?

Advection is bulk transport with the flowing water. Dispersion is spreading caused by varied flow paths and molecular diffusion, which broadens the plume.

Why do monitoring wells detect contamination early?

Because dispersion spreads the plume ahead of its centre of mass. The leading edge arrives before the calculated advective travel time.

Formula Explorer connections

Interpretation: This formula connects molecular properties, material structure or environmental transport to a macroscopic behavior or exposure estimate. Assumption: Use parameters measured for the same material, solvent, temperature and environment. Empirical correlations may not transfer outside their calibration range.

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