Monitored Natural Attenuation Calculator
Estimate MNA cleanup timeline from contaminant half-life and monitoring network costs.
About this calculator
This calculator projects a Monitored Natural Attenuation (MNA) cleanup timeline using first-order decay kinetics: concentration is assumed to fall exponentially over time as C(t) = C0 × e^(−λt), where the decay constant λ equals ln(2) divided by the contaminant's half-life. Solving that equation for the time needed to reach a regulatory cleanup target gives the headline "estimated cleanup time" output, and the same formula projects concentrations at the 5-, 10-, and 20-year marks so you can see the decay curve's shape rather than just its endpoint. The annual percentage reduction is derived directly from the decay constant as (1 − e^(−λ)) × 100.
Because MNA relies on combined natural processes — biodegradation, dispersion, dilution, sorption, and volatilization — rather than active pumping, the calculator also sizes a long-term groundwater monitoring well network: centerline wells spaced roughly every 75m along the plume's length (plus one upgradient background well), flanking well pairs roughly every 50m of plume width, and one additional source-area well, with total projected monitoring cost scaled by sampling frequency, per-event cost, and the full cleanup duration. A separate "steady-state plume length" output estimates the equilibrium distance a plume can travel before decay balances advection (groundwater velocity divided by the decay constant) — useful for judging whether the plume is likely to stay within a property boundary. The core assumption throughout is a single, site-wide first-order half-life; real sites often show non-uniform degradation rates and rebound, so half-life values should come from site-specific data or conservative literature ranges, not guesswork.
Inputs
Results
Estimated cleanup time (years)
19.9
Monitoring wells needed
8
Total monitoring cost ($)
$1,275,620.00
≈ 3 average U.S. homes
How to Use This Calculator
- Enter initial concentration (µg/L) and cleanup target from your remediation action levels.
- Set the contaminant half-life in years based on site-specific attenuation rates or literature values.
- Enter plume dimensions to estimate total mass and monitoring network size.
- Review the estimated cleanup time and projected concentrations at 5, 10, and 20 years.
- Use annual concentration reduction to confirm attenuation rates meet regulatory requirements.
How the result changes with Contaminant half-life (years)
| Contaminant half-life (years) | Estimated cleanup time (years) | Monitoring wells needed | Total monitoring cost ($) |
|---|---|---|---|
| 1 | 10 | 8 | $637,810.00 |
| 1.5 | 14.9 | 8 | $956,715.00 |
| 3 | 29.9 | 8 | $1,913,431.00 |
| 5 | 49.8 | 8 | $3,189,051.00 |
What each input means
- Initial concentration (µg/L)
- Source area contaminant concentration.
- Cleanup target (µg/L)
- Regulatory cleanup level (MCL). Benzene = 5, TCE = 5 µg/L.
- Contaminant half-life (years)
- First-order biodegradation half-life. BTEX ~0.5-3 yr, chlorinated solvents ~2-20 yr.
- Plume length (m)
- Current downgradient extent of the plume.
- Plume width (m)
- Cross-gradient width of the plume.
- Monitoring events/year
- Quarterly (4) is typical for MNA programs.
- Cost per well per event ($)
- Includes mobilization, sampling labor, and analytical costs per well.
- Groundwater velocity (m/yr)
- Average seepage velocity of the groundwater.
What each result means
- Estimated cleanup time (years)
- Time to reach target concentration via natural attenuation.
- Decay constant (per year)
- First-order rate constant λ = ln(2)/half-life.
- Annual concentration reduction (%)
- Percentage reduction in concentration per year.
- Concentration at 5 years (µg/L)
- Predicted concentration after 5 years of MNA.
- Concentration at 10 years (µg/L)
- Predicted concentration after 10 years of MNA.
- Concentration at 20 years (µg/L)
- Predicted concentration after 20 years of MNA.
- Monitoring wells needed
- Recommended number of monitoring wells for MNA program.
- Annual monitoring cost ($)
- Yearly cost for MNA sampling and analysis program.
- Total monitoring cost ($)
- Total projected monitoring cost over the cleanup period.
- Steady-state plume length (m)
- Maximum plume extent if attenuation balances advection.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersInitial concentration (µg/L) = 5000, Cleanup target (µg/L) = 5, Contaminant half-life (years) = 2, Plume length (m) = 300 = 8 input(s) provided
- Calculate Estimated cleanup timeEstimated cleanup time = abs(ln(targetConc / initialConc)) / decayConstant19.9 = 19.9
- Calculate Monitoring wells neededMonitoring wells needed = centerlineWells + flankingWells + 18 = 8
- Calculate Total monitoring costTotal monitoring cost = annualMonitoringCost * cleanupTime1275620 = $1,275,620
- Calculate Decay constant0.3466 = 0.3466
- Calculate Annual concentration reduction29.3 = 29.3
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
How does the half-life value drive the estimated cleanup time?
The decay constant λ is calculated as ln(2) divided by the half-life you enter, and the cleanup time is then solved from the exponential decay equation as |ln(C_target/C_initial)| divided by λ. A shorter half-life produces a larger λ and therefore a faster projected cleanup, while doubling the half-life roughly doubles the estimated years needed to reach the target concentration.
How does the calculator decide how many monitoring wells I need?
It places centerline wells roughly every 75m along the plume's length (plus one upgradient background well), flanking well pairs roughly every 50m of plume width, and adds one dedicated source-area well, summing all three groups into the total. This is a rule-of-thumb network design meant to bracket the plume for regulatory monitoring, not a substitute for a site-specific hydrogeologic monitoring plan.
What does the "steady-state plume length" output mean?
It's the equilibrium distance the plume can theoretically travel before natural decay balances out advective transport, calculated as groundwater velocity divided by the decay constant. It's useful for judging whether an actively degrading plume is likely to stabilize within a property boundary or a nearby receptor distance rather than continuing to migrate indefinitely.
Why might my actual site take longer to clean up than the projected years shown here?
The whole model rests on a single, uniform, site-wide first-order half-life, but real sites frequently show non-uniform degradation rates across different zones of the plume, along with rebound effects where concentrations plateau instead of following a clean exponential curve. If your half-life estimate comes from generic literature ranges rather than site-specific data, treat the projected cleanup time as optimistic and lean toward the conservative end of published half-life ranges.
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