Storm Water Sampling Requirements and Techniques

Specific industries and operations are subject to stormwater sampling requirements under the Iowa Department of Natural Resources (DNR) National Pollution Discharge Elimination System (NPDES) General Permit No. 1 for stormwater discharge associated with industrial activity. Part V of the General Permit identifies these industries based on operations, materials stored, and/or the Standard Industrial Classification (SIC) Code of the industry.

Each facility covered under General Permit No. 1 is required to produce a Stormwater Pollution Prevention Plan (SWPPP) which lists the monitoring requirements for the facility. The specific monitoring requirements are detailed under the individual sections in Part V for each industry and/or operation requiring sampling. In some cases, more than one section may apply to a facility because multiple regulated operations are occurring.

Sampling locations will be dictated by the flow of stormwater at the site, the location of the industrial activity, and the ability to produce a representative sample. As described in the General Permit, annual stormwater samples should be collected from a storm event that has greater than 0.1” rainfall and occurs at least 72 hours after the previous storm event with greater than 0.1” rainfall. Which type of sample you collect is described as follows:

  • If the discharge comes from a holding pond with a retention period greater than 24 hours, a grab sample should be taken during the first hour of discharge. The retention period of the holding pond can be determined by dividing the volume of the holding pond by the discharge rate.
  • If the outfall does not flow through a sufficiently sized holding pond, then a grab sample and a composite sample will be needed. The grab sample should be taken within the first hour of discharge. The composite sample should be flow-weighted or time weighted. A continuous sampler may be used, or sample aliquots (fractions) can be collected by hand and mixed in a clean container. The first sample aliquot should be collected within the first hour of discharge. A minimum of three sample aliquots should be taken each hour at least 15 minutes apart. The collection should last for the first 3 hours of discharge or until discharge stops, whichever occurs first.

The parameters to be analyzed are dictated by the industry type and each applicable industrial activity. Collected samples will need to be delivered to an Iowa-certified laboratory for analysis within the applicable hold times. The laboratory will be able to identify samples that have a short hold time and may require overnight or courier delivery to meet that requirement.

Many samples also have acidic or alkaline preservation requirements. Sample bottles provided by the laboratory usually come prefilled with preservative so be careful not to overfill and dilute the preservative, otherwise it may not count as a valid sample. Generally speaking, filling bottles up to the shoulder is sufficient for laboratory analysis. It is also important to ensure the samples are not contaminated by touching inside the bottle or lid or allowing in excess sediments, grass, bugs, etc. These will all count towards the constituent levels in the sample.

For each sampling event, be sure to record relevant information about the storm event including date and duration (in hours) of the storm event sampled, the rainfall measurement, the duration between the storm event sampled and the end of the previous storm event sampled (greater than 0.1”), and any other relevant notes about the discharge event. These sampling records and the laboratory analysis are required to be kept with the SWPPP for a period of at least three years and presented to DNR upon request or during periodic inspections.

If you need any assistance in completing or updating a SWPPP or have any further questions about stormwater sample collection, feel free to contact our team at 515-256-8814.