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Stream Assimilative Capacity Calculator

Work out stream assimilative capacity instantly with clear inputs, formula shown and shareable results.

About Stream Assimilative Capacity Calculator

Assimilative capacity is the load a river can accept while still meeting its environmental quality standard: headroom between standard and background, multiplied by the low flow. Rearranging the mixing equation gives the maximum effluent concentration a discharge may have, which is how consent limits are derived.

Formulas

Assimilative load

Capacity (kg/day) = (EQS - background) x low flow (m3/s) x 86.4

Permissible concentration

C_effluent = (EQS x (Q_river + Q_effluent) - background x Q_river) / Q_effluent

Worked example

With the default values below, Stream Assimilative Capacity Calculator returns Permissible Effluent Concentration of 1,041.800. Change any input above and every figure here is recalculated in your browser.

Inputs used

River Low Flowminimum 0 m3/s
20 m3/s
Environmental Quality Standardminimum 0 mg/L
5 mg/L
Background Concentrationminimum 0 mg/L
2 mg/L
Proposed Effluent Flowminimum 0 m3/day
5,000 m3/day

Results

Permissible Effluent Concentration
1,041.800
Assimilative Load Capacity
5,184.000
Concentration Headroom
3.000
Capacity Utilisation
100.48%

Step by step

  1. 1Values used
  2. 2Assimilative load
  3. 3Permissible concentration
  4. 4Permissible Effluent Concentration= 1,041.800
  5. 5Assimilative Load Capacity= 5,184.000
  6. 6Concentration Headroom= 3.000
  7. 7Capacity Utilisation= 100.48

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Assimilative capacity assessments are regulatory determinations requiring statutory low-flow data and approved environmental quality standards. This is an indicative calculation only.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Assimilative capacity assessments are regulatory determinations requiring statutory low-flow data and approved environmental quality standards. This is an indicative calculation only.

Frequently Asked Questions

What if background already exceeds the standard?

There is no assimilative capacity and no new discharge of that substance can be permitted. Catchment-wide measures are needed before any additional load can be accepted.

Does assimilative capacity apply to all substances?

Not to bioaccumulative or persistent substances, where a no-deterioration or zero-discharge approach applies regardless of calculated dilution.

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