Principles of Sampling: Why, When, and Where to Sample

Why Sampling Matters: The Regulatory Foundation

Sampling is not optional in water and wastewater operations — it is a legal requirement. Drinking water systems in the United States operate under the Safe Drinking Water Act (SDWA), administered by the U.S. Environmental Protection Agency (EPA) and delegated to state drinking water programs. Each public water system must follow a monitoring plan that specifies which contaminants to test, how often, and where. Wastewater treatment facilities discharge under a National Pollutant Discharge Elimination System (NPDES) permit, issued under the Clean Water Act (CWA). That permit lists every parameter the facility must measure at its discharge point, the required sampling frequency, and the sample type required.

Violating monitoring requirements — even if the water quality is actually fine — is itself a permit violation. Regulators cannot verify compliance without data, and data are only valid if samples are collected correctly. As a licensed operator, you are responsible for ensuring that samples are representative, properly collected, preserved, and documented.

The Monitoring Plan and the NPDES Permit

A drinking water monitoring plan is a written document that each water system develops to meet state and EPA requirements. It identifies the regulated contaminants, sampling locations (called entry points and distribution system sites), collection frequencies (e.g., daily, monthly, quarterly, annual), and the approved laboratory that will analyze the samples.

An NPDES permit is the wastewater equivalent. The permit specifies effluent limitations — the maximum concentrations or loads a facility may discharge. Alongside those limits, it defines monitoring requirements: which parameters to measure (e.g., biochemical oxygen demand, total suspended solids, pH, fecal coliform), how often, and the sample type. Most permits also require influent monitoring so that operators can track treatment efficiency.

Both documents are primary references you must read and understand before collecting a single sample. Always work from the actual permit or monitoring plan — not from memory.

Grab Sampling vs. Composite Sampling

Choosing the right sample type is critical because regulations specify which is required for each analyte.

Grab samples are collected at a single point in time from a single location. They represent water quality at that exact moment. Grab samples are required when:

• The analyte changes rapidly and a single moment in time is what matters (e.g., chlorine residual, dissolved oxygen, pH, total coliform, turbidity).
• The regulation requires a specific worst-case or instantaneous reading.
• Field analysis is needed immediately because the analyte is unstable (e.g., dissolved oxygen, chlorine residual).

Composite samples are a mixture of multiple grab samples collected over a defined time period or at defined flow intervals. They are used to represent average conditions rather than a snapshot. Composite samples are required when:

• Flow or concentration varies significantly during the day (e.g., biochemical oxygen demand, total suspended solids, ammonia).
• The permit requires a flow-proportional composite, meaning each sub-sample volume is proportional to flow at the time of collection.
• Time-proportional composites collect equal volumes at equal time intervals.

Never substitute a composite for a required grab or vice versa — doing so invalidates the sample result for regulatory purposes.

Where to Sample: Location Matters

Sampling location is defined by the analyte and the regulatory requirement — not by what is convenient.

In a drinking water distribution system, total coliform and lead/copper samples have specific location rules. Total coliform sites are defined in the monitoring plan and must be spread throughout the distribution system to represent all parts of the system. Lead and copper samples for the Lead and Copper Rule must be collected from first-draw taps at high-risk sites (older homes with lead service lines or lead solder), following a specific stagnation protocol.

At a wastewater treatment plant, the NPDES permit defines monitoring locations precisely — usually the final effluent at the permitted outfall, plus influent and sometimes intermediate points such as after primary clarification. Sampling at the wrong location (e.g., before disinfection when the permit requires post-disinfection sampling) produces data that does not demonstrate compliance.

Use a process and instrumentation diagram (P&ID) to identify where each process stream flows and where instrumentation and sampling ports are located. Cross-reference the P&ID against permit language to confirm exact sampling points.

Safety During Sampling

Sampling exposes you to biological, chemical, and physical hazards. Follow these precautions every time:

• Personal protective equipment (PPE): Wear nitrile or chemical-resistant gloves whenever handling wastewater or chemical samples. Safety glasses or a face shield are required when splash is possible. Wear slip-resistant boots near wet surfaces.
• Biological hazard: Raw wastewater contains pathogens. Treat all wastewater samples as infectious. Wash hands thoroughly after sampling, even if gloves were worn.
• Confined spaces: Some sampling ports are in or adjacent to confined spaces (manholes, wet wells). Never enter a confined space without atmospheric testing, a written entry permit, and a trained attendant outside. Follow OSHA confined space entry procedures.
• Lockout/tagout (LOTO): If you must open a panel, adjust a valve, or work near moving equipment to access a sampling point, follow your facility's LOTO procedure to prevent unexpected energization.
• Sample container handling: Glass containers can break; inspect for cracks. Containers with preservatives (acid, sodium hydroxide) are labeled hazardous — handle carefully and never mouth-pipette.
• Slips and falls: Sampling areas near tanks and channels are wet. Walk, do not run. Use handrails.