Tuesday, November 11, 2008

Stormwater Monitoring - Selecting Appropriate Technique and Equipment

Many States now require dischargers to collect stormwater samples to demonstrate that their stormwater pollution prevention measures are effective. Samples are either visually examined on-site, or sent to a laboratory for chemical analysis. Either way – samples need to be collected.

Because in most States samples need to be collected within the first 30 minutes of discharge, stormwater monitoring presents some special challenges. To further complicate this requirement, rain events may also need meet specific requirements (i.e., 0.25 inch total rain fall, dry for previous 3 days, etc.). There are three basic options available to dischargers:

  1. Train Facility Staff to Collect Samples. In this case, a few staff are trained on how to collect and handle samples.
  2. Contract Third-party Sampler. Here, you would need to contract with a vendor who is located close enough to be at the site and ready to sample within 30 minutes of the start of a rain fall event.
  3. Buy or Lease Automated Sampling Equipment. In this case, equipment is installed at each outfall and samples are collected automatically when water begins to flow. In most cases, equipment would need to be installed by trained personnel.

Each of these options has its unique benefits and drawbacks. Using a third-party sampler has some logistically problems, as most facilities find it difficult to have someone on-site within 30 minutes, especially considering that the vendor may also be providing this service to others. Using automatic equipment eliminates this logistical problem; however, automated equipment tends to be more costly, and often needs to be reset after small rain events or if any other water gets inadvertently discharged to the storm sewer. Therefore many dischargers opt to train their own staff to collect samples, as it is the least expensive and most reliable method.

Selecting the appropriate option for a given discharger requires consideration of all the outfalls that need to be sampled. For example, some outfalls may not be conducive for installing automated sampling equipment, and might require a different technique. Therefore, if a facility has four outfalls to sample and one can not be effectively sampled with automated equipment, staff may need to be trained to collect samples at that outfall anyway, and therefore it may not make much sense to install automated equipment at the other outfalls, unless other factors prevail.

The bottom line is that selecting a reliable and cost effective sampling method is dependant on a number of factors, including:

  • Frequency of monitoring
  • Types of parameters to be tested for
  • Physical layout and constraints of the outfalls
  • Safety considerations
  • Availability of on-site staff
  • Availability of contract sampling vendors
  • Availability of equipment

For further information contact Caltha LLP at
Caltha LLP Website

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