M4L8Selection

Module IV

Lesson Eight: Stormwater Sampling Plan

Sampling Points Selection

The first step in selecting sampling points is to consider the areas draining the facility. The site map in the SWPPP should show the drainage areas. Areas of particular concern are those where raw materials or finished product are exposed to rainfall and/or runoff, and areas where leaking fluids such as petroleum products and hydraulic fluids have the potential to enter stormwater runoff.

Source: Image of Department of Ecology, Washington State

The next step is to determine where the runoff from each drainage area is discharged from your facility. If there are separate drainage areas with separate discharge points, stormwater sampled at one discharge sampling point may not represent the facility’s stormwater quality overall.

Making a determination of whether a discharge is likely to have stormwater quality that differs from other discharges and requires separate sampling requires a review of the site map in the SWPPP with consideration to sources of pollutants in each drainage area. This should be followed up with an on site assessment of activities, sources and quantities of pollutants in each drainage area. This information will help the operator to document the decision as to whether two or more drainage areas can be represented by a single sample site.

Common Sampling Points

Pipes discharging a facility’s stormwater offsite
Ditches carrying a facility’s stormwater offsite
Manhole access to storm sewer carrying a facility’s stormwater, so sampling personnel can lower a sample bottle attached to a pole into the manhole

In general, manhole access on a facility own property may be simpler and safer than access off property and more readily verifiable as carrying only a facility’s stormwater. These three types of sampling points are not too difficult to access and the flow within them tends to be fast enough, with enough turbulence, to allow collection of well mixed, representative samples.

When sampling from a manhole, use apole to safely sample from above ground. Avoid touching the sides of the manhole or piples with the bottle to prevent contamination. Please the opening of the bottle upstream so that the flow enters the bottle directly.

Source: Image of Department of Ecology, Washington State