Your NPDES permit specifies whether each parameter must be collected as a grab sample or a composite sample. Using the wrong sample type is a permit violation, and mixing sample types incorrectly in DMR calculations produces inaccurate compliance data. Understanding the distinction — and tracking it correctly in water quality software — is fundamental to a defensible monitoring program.
Grab Samples
What Is a Grab Sample?
A grab sample is a single, discrete sample collected at one specific point in time and location. It represents water quality conditions at that exact moment — a snapshot. Grab samples are typically collected by hand by dipping a sample container into the discharge or using a sampling pole, though automatic samplers can also collect discrete grabs at programmed times.
When Grab Samples Are Required
Certain parameters must be collected as grab samples because they change rapidly, are volatile, or degrade during the extended collection period of a composite:
- pH — Changes with temperature, aeration, and biological activity
- Temperature — Must reflect conditions at the moment of measurement
- Dissolved oxygen — Changes rapidly with temperature and biological activity
- Residual chlorine — Dissipates quickly after collection
- Oil and grease — Adheres to container walls during extended compositing
- Bacteria — Total coliform, E. coli, fecal coliform (short holding times)
- Volatile organic compounds — Lost to headspace during compositing
- Sulfide — Oxidizes rapidly after collection
Composite Samples
What Is a Composite Sample?
A composite sample is a mixture of multiple individual aliquots collected at regular intervals over a defined period, typically 24 hours. Each aliquot is combined into a single container, and the resulting composite represents the average conditions over the entire collection period. Composites provide a more representative picture of discharge quality than any single grab sample.
Time-Weighted Composite
A time-weighted composite collects equal volumes of effluent at equal time intervals. For example, a 24-hour composite might collect 200 mL every 30 minutes (48 aliquots). The resulting composite represents the time-averaged concentration regardless of how discharge flow varied during the period. This method is simpler but less representative when flow rates fluctuate significantly.
Flow-Weighted Composite
A flow-weighted composite adjusts the volume of each aliquot proportionally to the discharge flow rate at the time of collection. During high-flow periods, larger volumes are collected; during low-flow periods, smaller volumes. This ensures the composite accurately reflects the mass of pollutant discharged, not just the average concentration. Flow-weighted composites require a flow signal from the discharge monitoring system.
Side-by-Side Comparison
| Characteristic | Grab Sample | Composite Sample |
|---|---|---|
| Collection | Single discrete sample at one moment | Multiple aliquots combined over 24 hours |
| Representativeness | Snapshot of conditions at one point | Average conditions over the collection period |
| Equipment | Sample container, sampling pole | Automatic sampler (ISCO, Hach), refrigerated |
| Staff time | Minimal — single collection event | Setup + retrieval; sampler runs unattended |
| Best for | Unstable/volatile parameters, instantaneous limits | Stable parameters, average limits, mass loading |
| DMR application | Daily maximum, instantaneous maximum | Monthly average, weekly average, loading calculations |
| Mass loading | Instantaneous load = conc × flow at that moment | Representative of total load over collection period |
How Sample Type Affects DMR Calculations
Separate Limit Types
Permits often assign different limits to grab and composite results. For example, a permit might set a BOD monthly average limit of 30 mg/L (based on composites) and a BOD daily maximum of 45 mg/L (which could be based on either sample type). Mixing grab results into a composite average calculation or vice versa produces incorrect DMR values.
Mass Loading Calculations
For mass loading calculations, composite samples paired with average daily flow provide the most accurate load estimate. Grab samples paired with instantaneous flow give a snapshot load that may overstate or understate the actual daily load depending on when the grab was collected relative to flow variations.
Software Tracking
Software records the sample type (grab, time-weighted composite, flow-weighted composite) for every sample event and uses it to apply the correct calculation method on the DMR. This prevents the common error of accidentally averaging grab results with composite results or applying the wrong statistical method for the sample type.
Related Ecesis Solutions
Water Quality Software
Lab imports, data validation, permit tracking and DMR reporting.
Environmental Data
Sensor integration, statistical analysis and trend visualization.
Inspections & Audits
Mobile field inspections with corrective action tracking.
Compliance Obligations
Track all regulatory obligations and recurring deadlines.
Document Control
Manage SOPs, permits, and compliance documents.
Task Tracking
Assign corrective actions with due dates and accountability.
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