Key takeaway
A time composite and a flow-proportional composite can describe the same discharge differently. Preserve the sampling weights before turning concentration into a load.
Ask what the bottle represents
A wastewater archive labels a result as a two-hour average. That label leaves an important question unanswered: did the sampler take equal volumes at equal times, or did it collect in proportion to the wastewater passing the point? A laboratory concentration cannot recover aliquot weights that were lost before the sample arrived.
EPA Region 4’s February 2026 procedure distinguishes equal-volume, constant-interval time composites from flow-proportional methods. The latter can vary aliquot volume at constant intervals or vary collection intervals with flow. Its current revision is inside a URL containing 2017; the document cover, rather than that directory name, identifies the edition.
Two hours produce two defensible concentrations
Hypothetical example: an invented discharge has two consecutive one-hour periods. During the first, flow is a constant 10 m³/h and concentration is a constant 100 mg/L. During the second, flow is 30 m³/h and concentration is 20 mg/L. Assume a well-mixed stream, an unchanged constituent and perfect collection and preservation. No real sampling or laboratory test occurred.
Equal-volume aliquots representing the two equal-duration periods produce a time-average concentration of (100 + 20) / 2 = 60 mg/L. Weighting by discharged volumes instead gives (10 × 100 + 30 × 20) / 40 = 40 mg/L. Both results follow from their declared weighting; neither is a transcription error.
| Invented period | Duration | Flow / discharged volume | Concentration | Constituent mass |
|---|---|---|---|---|
| First hour | 1 h | 10 m³/h / 10 m³ | 100 mg/L | 1.0 kg |
| Second hour | 1 h | 30 m³/h / 30 m³ | 20 mg/L | 0.6 kg |
| Total | 2 h | 40 m³ discharged | 40 mg/L volume-weighted | 1.6 kg |
Reproduce the load without changing the question
In these compatible units, 1 mg/L equals 1 g/m³. The first period therefore contributes 10 × 100 = 1,000 g and the second 30 × 20 = 600 g. The total is 1.6 kg. Multiplying the volume-weighted 40 mg/L by 40 m³ reproduces that mass. Multiplying the time-average 60 mg/L by the same volume gives 2.4 kg, overstating this stipulated total by 0.8 kg.
The mistake would be using a correct answer to one question as if it answered another. A time-weighted concentration can be the intended quantity. The internal review should establish the intended statistic before choosing a reported value, rather than automatically replacing every time composite with a flow composite.
Recover the collection evidence
For a historical bottle, request the start and stop boundaries, actual aliquot times and volumes, sampler program, flow record and any missed collections. Record whether the flow series used for compositing belongs to the same point and period. A programmed setting describes intended collection; a fault or an empty intake can change what actually entered the container. Include the missed-aliquot count even when the final bottle volume looks normal.
EPA’s inspection manual explains that the data objective, analytical method and permit affect sample selection, and distinguishes grab observations from composites. Its guidance does not choose a permit or approve a parameter for compositing in this illustration. The responsible sampling reviewer must establish those actual requirements and decide how exceptions affect interpretation.
Keep the unresolved weights in the archive description
Suppose the archive retains the 60 mg/L result but loses the sampler program and aliquot log. The completed description can say that a composite result exists while its weighting basis remains unresolved. It cannot promise a reconstructed mass-loading history merely because a daily flow total survives elsewhere. Recovering that evidence is a specific task for the sampling owner.
Keep original concentration, collection basis and any later derived load in separate fields. A revised interpretation should identify which recovered record changed it. Inventory can describe those evidence categories without transferring facility data. A fictional calculation like this is enough to explain the issue while client terms, facility details and any proposed disclosure receive their own review.