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ISGP: Sampling

Sampling is a critical component of the ISGP. It allows you to ensure that your sites' stormwater discharges comply with the water quality requirements given to their facility and that the waters your facility discharges to will be clean for others to enjoy for generations to come.

Identifying what sample parameters your site is required to monitor, finding a laboratory that can analyze your samples, and using proper sampling techniques are all critical elements of an effective stormwater management program at a facility.


As mentioned on the Make a Plan page, you are required to develop a sampling plan outlining who is going to sample, what they will be sampling, and how those samples will be processed. Ecology has created a stormwater sampling manual that is a good reference for developing your program

*Note: Some items such as sampling dates and communication procedures have changed since this document was published- use this as an in-depth sampling guide, but please double-check any timelines or communication procedures.

Identifying Sample Parameters

Section S5 of the ISGP identifies the parameters and benchmark or effluent levels that a facility is required to sample for. There are several parameters that all facilities must sample for. These are Turbidity, pH, visual oil sheen, zinc, and copper. Some facilities may have additional sampling requirements based on 2 factors:

What NAICS code(s) Does the Site Operate Under?
Certain industry types have additional sampling requirements based on the pollutants commonly associated with that industrial activity. For a list of additional sampling, requirements see section S5 of the ISGP.

Does the Facility Discharge to an Impaired Waterbody?
If a facility is discharging into an impaired waterbody they may have to sample for pollutants that are causing the waterbody impairment. For example, a site might have to sample for E.coli if they discharge to a creek that is impaired due to high levels of E.coli.

It is important for the permit holder to determine what they are required to sample and stay up to date with their facility’s requirements. Ecology will input sampling requirements into your SAW account in the WQPermitPortal, but will usually not directly contact you with this information. In some instances, a facility may be prescribed additional sampling requirements seemingly without notice. This can occur if a receiving body is classified as an impaired waterbody. It is recommended to check your SAW account before sampling to verify what you are being asked to sample matches what you expected to sample.

Finding a Lab

Finding a lab to process the samples you have taken is another important step in ensuring compliance with permit requirements. The lab you chose plays a critical role in analyzing the samples collected and providing accurate and reliable results that are used to determine compliance with the benchmarks and or effluent limits for your facility. Your lab must also be accredited for each water quality parameter you are required to sample. This provides assurance that the laboratory has the necessary expertise and equipment to perform accurate and reliable results. 

The lab can also be a great resource. Ask a lot of questions. Some labs subcontract parameters that they cannot process internally and it can affect hold time criteria. Your lab is your partner in this effort, and you need to feel confident in their abilities.

Ecology has created a laboratory search resource to help identify labs in the market. We also have a lab questionnaire that outlines important topics to discuss with your lab.

Sample Location(s)

Identifying where samples should be collected is critical and should be identified on the site map and supported by a narrative in the sampling plan. Sampling locations are typically safe for personnel to sample from, at a discharge location representative of the industrial activity, downstream of your BMPs, and not commingled contaminated or diluted with other sources of water.

It is important to identify any special circumstances for each sample location in your narrative description. For example, your site might have 2 catch basins with one requiring sampling from a specific pipe feeding into it, and the other requiring sampling within the catch basin. An “x” on a map over the catch basins wouldn’t capture this nuance, so a description is needed to avoid confusion.    

Locations that include run-on from other properties, places where groundwater might dilute the sample, and places that put the sampler at risk would all be considered poor sampling locations. Not all sampling locations are at the same points identified on the site map as discharge points due to influences such as accessibility, commingling, or safety-related issues. It is important to note that the proper selection of the sampling location that is representative of the facility discharge is critical to permit success.

Sample(s) must be collected from all identified sampling locations from every discharge point every quarter unless they are identified as substantially identical or the sample point has achieved consistent attainment. Substantially identical discharge points means those that share the following characteristics with another discharge point: 

  • The same general industrial activities conducted in the drainage areas of the discharge points 
  • The same BMPs are implemented in the drainage areas of the discharge points
  • The same type of exposed materials and pollutant sources in the drainage areas of the discharge points
  • The same type of surfaces in the drainage areas (e.g., asphalt, crushed rock, grass)

Sample Timing

Samples are required to be collected once per quarter. These samples are required to be collected within 12 hours of discharge from the facility. It is important to note that the timing of “discharge” is likely different than when it started to rain on site, and each discharge point at the facility may start to discharge at a different time.

Multiple samples can be taken per quarter so that the average of all of the monitoring results for each parameter (except pH and visible oil sheen) can be submitted on the DMR. However, if Permittees collect more than one sample during a 24-hour period, they must first calculate the daily average of the individual grab sample results collected during that 24-hour period; then use the daily average to calculate a quarterly average. 

Monitoring your site is critical to ensuring that the site SWPPP is accurate and effective for the site. The monitoring data generated through inspections and sampling needs to be kept on-site and reported on the DMR once per quarter. See the Reporting & Recordkeeping page for more information.

Other Considerations

Source Evaluation vs Reportable Discharge Data

In some instances, there is a need to try and identify where the source of a pollutant is being generated on the property (e.g. roof runoff, truck scales, a certain drain, etc.). Source sampling can be conducted on a property and not reported as long as the sampling occurs inside of the property and not at the discharge locations. Samples collected at the discharge locations generate reportable data.

Response to Exceedances of Standards

When your sampling parameter exceeds the prescribed benchmark, you are required to perform what are known as Corrective Actions which require the implementation of additional BMPs on your site. Exceedance of numeric effluent limits that are sometimes prescribed due to proximity to impaired waters is more serious as it triggers a violation of the permit and requires immediate action. For more information on exceeding these standards, see our Corrective Action page.