Monitored Natural Attenuation Calculator

Estimate MNA cleanup timeline from contaminant half-life and monitoring network costs.

Initial concentration (µg/L)

Cleanup target (µg/L)

Contaminant half-life (years)

Plume length (m)

Plume width (m)

Monitoring events/year

Cost per well per event ($)

Groundwater velocity (m/yr)

Tab to move between fields

Estimated cleanup time (years)

19.9

Monitoring wells needed

Total monitoring cost ($)

$1,275,620.00

≈ 3 average U.S. homes

Decay constant (per year)0.35

Annual concentration reduction (%)29.3

Concentration at 5 years (µg/L)883.9

Concentration at 10 years (µg/L)156.3

Concentration at 20 years (µg/L)4.9

Annual monitoring cost ($)$64,000.00

Steady-state plume length (m)86.6

Plume Migration M597.95%

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 ($)
10	99.7	8	$6,378,102.00
35	348.8	8	$22,323,357.00
65	647.8	8	$41,457,663.00
90	896.9	8	$57,402,917.00

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.

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.

Identify Input Parameters
4 parameters
Initial 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 time
Estimated cleanup time = abs(ln(targetConc / initialConc)) / decayConstant
19.9 = 19.9
Calculate Monitoring wells needed
Monitoring wells needed = centerlineWells + flankingWells + 1
8 = 8
Calculate Total monitoring cost
Total monitoring cost = annualMonitoringCost * cleanupTime
1275620 = $1,275,620
Calculate Decay constant
0.3466 = 0.3466
Calculate Annual concentration reduction
29.3 = 29.3