Selecting a Storm Event for Sampling

A successful sampling is to collect the runoff at the right storm event and at the right time. The general permit recommends that the storm event to be sampled must meet the following two conditions:

• Be preceded by at least 24 hours of no greater than trace precipitation

• Have an intensity of at least 0.1 inches of rainfall (depth) of rain in a 24-hour period

The general permit also recommends that the grab sample be collected within the first 30 minutes after discharge from the facility to a point off site, not from when rainfall begins. It is important that the staff are at the ready status for the sampling event by doing the following:

• Evaluate weather forecasts before deciding whether or not to sample a particular rain event. The National Weather Service is an excellent source of information on upcoming storms or some commercial websites such as http://www.weather.com/, Netscape, and Yahoo also provide weather information and forecasts

• Spend time observing rain events at the facility site with attention (attention) to how rain intensity relates to stormwater discharges from the site, before conducting a real field sampling

Copyright @ January,2005 Prepared by Transportation Environmental Resource Center (TERC)