Discharge reports: rebuild pollutant loads from monitoring data
A concentration value is not a pollutant load. The mass discharged also depends on the water volume and the period represented. A low concentration during high flow can produce a substantial load, while a permit limit is not evidence that the maximum allowed amount was actually discharged. Research must keep measurements, calculated loads and compliance questions separate.
The deliverable is a US NPDES outfall-and-parameter worksheet reconciled to a specific monitoring period. The examples use fictional constant values within explicitly stated intervals. They do not describe a real facility, estimate an issuer’s liability or establish whether a discharge violated a permit.
Locate the calculation fields and assumptions
EPA ECHO’s Monitoring Period Loads Report Help, under Table Columns, identifies monitoring period, discharge information, NODI, selected daily value, measurement type, wastewater flow and number of days. Expanding a report row exposes its load equation. These fields allow a researcher to inspect which reported value and estimation status produced a displayed load.
The help page also describes special treatment of non-detects and missing periods. The displayed result can therefore be a calculation using selected values or an estimate; it is not automatically a continuously measured mass total. Save the report version and calculation row before comparing the output with the facility’s own figures.
Rebuild the constant-flow main case
Assume a thirty-day period, discharge of 1,000 cubic metres per day and concentration of two milligrams per litre throughout every discharging day. One cubic metre is 1,000 litres and one kilogram is one million milligrams. Daily mass is 1,000 × 1,000 × 2 ÷ 1,000,000 = two kilograms.
Over thirty days, mass is sixty kilograms. Equivalently, with flow in cubic metres per day, load in kilograms is flow × concentration in mg/L × discharging days ÷ 1,000. The conversion factor belongs to these specified units; applying it unchanged to MGD or pounds would be an error.
| Input or step | Fictional value | Check |
|---|---|---|
| Flow | 1,000 m³/day | Actual selected route and interval |
| Concentration | 2 mg/L | Assumed representative throughout |
| Daily water volume | 1,000,000 L | Cubic metres converted to litres |
| Daily mass | 2 kg | Milligrams converted to kilograms |
| Discharging days | 30 | No non-discharge days in this case |
| Period mass | 60 kg | Daily mass times matched days |
If ten days are instead documented as no discharge and the remaining twenty retain those same values, mass is forty kilograms. Extending the operating-day two kilograms across all thirty calendar days would overstate this hypothetical by twenty. No-discharge evidence changes the activity interval; it is not simply another missing concentration reading.
Test why monthly averages can mislead
For a different case, split the thirty days into equal fifteen-day intervals. The first has flow of five hundred cubic metres per day and concentration four mg/L. The second has flow of 1,500 and concentration one. Assume both are constant within their own intervals and that these are matched flow and concentration observations.
First-interval mass is 500 × 4 × 15 ÷ 1,000 = thirty kilograms. Second-interval mass is 1,500 × 1 × 15 ÷ 1,000 = 22.5 kilograms. Total mass is 52.5 kilograms. The simple time-average flow is 1,000, and the simple time-average concentration 2.5, but multiplying those averages by thirty days produces seventy-five kilograms.
The 22.5-kilogram difference results from breaking the pairing between flow and concentration. A volume-weighted concentration for these supplied intervals is 1.75 mg/L, which reproduces 52.5 kilograms when applied to the total thirty thousand cubic metres. Do not assume a reported monthly mean concentration is volume-weighted unless the source establishes that basis.
Separate missing, no discharge and non-detect
Retain the exact NODI code and description. A sampling issue does not supply a zero concentration or prove zero discharge. A no-discharge code addresses a different factual condition. If the discharge occurred but monitoring information is missing, identify what the tool estimated and what the underlying DMR actually reported.
Non-detect means the relevant analytical result is qualified by a detection boundary, rather than a measured zero. The help page describes a hybrid calculation using half the detection limit in certain periods and zero annual load when every period is non-detect. Treat that as the tool’s documented calculation convention, not proof that no pollutant molecules were discharged.
For an explicit sensitivity illustration, suppose the selected non-detect threshold is two mg/L and the applicable calculation uses one mg/L. With the main case’s flow and days, the computed load is thirty kilograms. This is an imputed result under the assumed rule. It is not a new laboratory measurement and should not be described as independently observed thirty-kilogram discharge.
Reconcile direct quantities and selected values
A monitoring record may offer quantity or concentration information, and multiple value fields can exist. Record the field selected in the actual EPA row, its measurement type, units and qualifiers. Do not replace the selected monthly-average value with a daily maximum simply because both appear in the same download.
If the facility reports a direct mass quantity, compare its interval and basis with the concentration-flow reconstruction. A measured mass rate, a calculated period total and a permit maximum are different objects. An apparent mismatch may come from units or aggregation before it says anything about the facility’s environmental performance.
Preserve both raw values and the transformation. For example, record the original flow unit before conversion rather than only storing the converted figure. Another reviewer should be able to reproduce the calculation and identify whether a difference arose at input selection, unit conversion, time weighting or annual extrapolation.
Keep limits and annual estimates distinct
The permit limit has its own units, averaging period and applicable parameter. A sixty-kilogram monthly load cannot be compared directly with a concentration limit or an unrelated daily maximum. Obtain the actual permit conditions and monitoring basis before attempting a compliance interpretation; the load worksheet alone does not decide it.
For a separate annual boundary, assume ten measured periods each contribute forty kilograms and two periods lack data. The sum of supplied measured-period calculations is four hundred kilograms. An illustrative annual extrapolation of forty per missing period would produce 480, of which eighty is estimated. This is an explicitly chosen teaching assumption, not a claim that EPA necessarily uses that extrapolation for every missing case.
Show measured-period sum, estimated contribution and combined annual output separately. A revised year with fewer missing months can change the annual estimate even without a corresponding physical change. Comparing two totals without their estimation coverage can turn a data-completeness change into an unsupported operating trend.
Use the pollutant-load verification sheet
| Field | Obtain | Check |
|---|---|---|
| Identity | NPDES permit, outfall and parameter code | Same route and pollutant |
| Period | Start/end and discharge days | Calendar days distinct from operating days |
| Values | Raw flow, concentration or quantity | Matched interval, units and averaging |
| Qualifiers | NODI and detection information | Missing and non-detect not forced to zero |
| Equation | Expanded row and selected value | Reproduce conversion and estimation |
| Limits | Applicable permit condition | Separate load result from legal conclusion |
Start with a saved report row, retrieve its underlying DMR fields and reconstruct the equation. If the record cannot be expanded or necessary inputs are missing, report the displayed value as a tool output with the known limitations. Do not call the result independently verified merely because the arithmetic on incomplete inputs is easy.
When sources conflict, compare outfall identifiers, parameter definitions, revised DMRs, data dates and estimation flags. Keep unexplained differences visible. New corrected reports, laboratory qualifiers, discharge-day evidence or changed estimation conventions trigger updates to the affected rows, while the previous version remains available for comparison.
FAQ
Does below detection mean a measured zero load?
No. Preserve the qualifier and the calculation convention used for the reported result.
Can monthly mean flow always be multiplied by monthly mean concentration?
No. The pairing and weighting must support that calculation, as the two-interval example demonstrates.