Groundwater Treatment Calculator
Size a pump-and-treat system from plume dimensions, aquifer properties, and cleanup targets.
About this calculator
This calculator sizes a pump-and-treat groundwater remediation system using Darcy's law and pore-volume flushing theory. It first computes the plume's pore volume — the actual water-filled volume within the contaminated zone — as length × width × saturated thickness × effective porosity. Darcy (bulk) velocity is hydraulic conductivity × gradient, and dividing that by porosity gives the true seepage velocity groundwater is actually moving at. The required capture flow rate is set to 1.5 times the natural flow rate crossing the plume's cross-section (width × saturated thickness) — that 50% safety margin ensures the extraction wells pull in contaminated water from the full plume width rather than letting some slip past uncaptured.
Cleanup duration is estimated from first-order pore-volume exchange theory: the number of pore volumes needed to flush equals the retardation factor times the natural log of the ratio between target and initial concentration, since sorbing contaminants (higher retardation factor) take proportionally longer to flush than a conservative tracer would. Multiplying that pore-volume count by the actual pore volume gives total water to extract and treat, and dividing by the pump rate gives the cleanup timeline in years. This is a simplified, homogeneous-aquifer estimate — real sites see tailing and rebound effects (concentrations plateauing well above the theoretical curve as sorbed and low-permeability-zone contaminant mass diffuses back out), so actual cleanup often takes considerably longer than this idealized calculation suggests, particularly for retardation factors above 5-10 typical of chlorinated solvents or metals.
Inputs
Results
Required pump rate (GPM)
5.94
Total water to treat (M gal)
83.98
Estimated cleanup time (years)
26.9
How to Use This Calculator
- Enter plume dimensions (length, width, saturated thickness in meters) from site investigation data.
- Set effective porosity and hydraulic conductivity (m/s) from aquifer testing or boring logs.
- Enter hydraulic gradient and initial and target contaminant concentrations.
- Input the retardation factor (1.0 for non-sorbing contaminants, higher for organics).
- Review required pump rate (GPM), pore volumes to flush, and estimated treatment duration.
How the result changes with Plume width (m)
| Plume width (m) | Required pump rate (GPM) | Total water to treat (M gal) | Estimated cleanup time (years) |
|---|---|---|---|
| 25 | 2.97 | 41.99 | 26.9 |
| 38 | 4.52 | 63.82 | 26.9 |
| 75 | 8.92 | 125.97 | 26.9 |
| 125 | 14.86 | 209.95 | 26.9 |
What each input means
- Plume length (m)
- Downgradient length of the contaminant plume.
- Plume width (m)
- Cross-gradient width of the plume.
- Saturated thickness (m)
- Vertical thickness of the contaminated saturated zone.
- Effective porosity
- Sand ~0.25-0.35, gravel ~0.25, silt ~0.35, clay ~0.06.
- Hydraulic conductivity (m/s)
- Sand ~1e-4, gravel ~1e-2, silt ~1e-6, clay ~1e-9 m/s.
- Hydraulic gradient
- Head drop per unit distance. Typical 0.001 to 0.01.
- Initial concentration (µg/L)
- Average contaminant concentration in the plume.
- Target concentration (µg/L)
- Regulatory cleanup target (MCL). Benzene MCL = 5 µg/L.
- Retardation factor
- Accounts for sorption. 1 = no sorption (Cl-), 2-5 for BTEX, 10+ for metals.
What each result means
- Pore volume (m³)
- Volume of water within the plume zone.
- Pore volume (gallons)
- Pore volume in US gallons.
- Groundwater velocity (m/day)
- Actual groundwater seepage velocity.
- Required pump rate (GPM)
- Extraction rate for full plume capture with 50% safety factor.
- Pump rate (m³/day)
- Extraction rate in cubic meters per day.
- Pore volumes to flush
- Number of pore volumes needed accounting for retardation.
- Total water to treat (M gal)
- Total volume of groundwater to extract and treat.
- Estimated cleanup time (years)
- Approximate time to reach target concentration at given pump rate.
- Plume travel time (days)
- Time for groundwater to traverse the plume length.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersPlume length (m) = 200, Plume width (m) = 50, Saturated thickness (m) = 10, Effective porosity = 0.3 = 9 input(s) provided
- Calculate Required pump rateRequired pump rate = captureFlowRate * 15850.35.94 = 5.94
- Calculate Total water to treatTotal water to treat = totalWaterM3 * 264.172 / 1e683.98 = 83.98
- Calculate Estimated cleanup timeEstimated cleanup time = cleanupTimeDays / 365.2526.9 = 26.9
- Calculate Pore volumePore volume = plumeVolume * porosity30000 = 30000
- Calculate Pore volumePore volume = poreVolume * 264.1727925160 = 7925160
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator size the pump rate at 1.5× the natural flow rate instead of just matching it?
The 50% safety margin exists because pumping at exactly the natural flow rate risks letting some contaminated groundwater slip past the extraction wells uncaptured, especially near the edges of the plume's cross-section. Pumping faster than the natural flow ensures the capture zone extends across the full plume width and saturated thickness, not just its centerline.
What does the retardation factor actually do to the cleanup timeline?
It multiplies directly into the pore-volumes-needed formula (PV = R × |ln(C_target/C_initial)|), so a retardation factor of 10 requires roughly five times as many pore volumes to flush as a factor of 2 for the same concentration reduction. This is because sorbing contaminants (like chlorinated solvents or metals) continually desorb back into the water as it flushes through, so proportionally more clean water has to pass through the aquifer to strip them out.
Why might real cleanup take longer than the estimated years this calculator shows?
The formula assumes a homogeneous aquifer with clean first-order flushing, but real sites experience tailing and rebound — concentrations plateauing well above the theoretical curve as contaminant mass trapped in low-permeability zones or sorbed onto soil slowly diffuses back into the flowing groundwater. This effect gets worse at higher retardation factors, so sites with retardation above roughly 5-10 (typical of chlorinated solvents or metals) often take considerably longer than this idealized estimate.
What's the difference between Darcy velocity and seepage velocity in the results?
Darcy (bulk) velocity is hydraulic conductivity times gradient, treating the aquifer as if the entire cross-section were open to flow. Seepage velocity divides that by porosity to get the actual speed water molecules travel through the interconnected pore spaces, which is always faster than Darcy velocity since only a fraction of the aquifer's volume is open pore space — this is the number used to estimate how quickly groundwater actually crosses the plume.
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