Trigger Values for Storm Water Discharges to Water

The Water Framework Directive (WFD) requires no deterioration in the status of a water body.  The status of and possible impacts on receiving waters from storm water discharges can affect this status and must be considered.  Trigger levels on the discharge can be utilised as a tool to ensure the maintenance of the status of the stream.

Trigger level is defined in the licence as a parameter value, the achievement or exceedance of which requires certain actions to be taken by the licensee.

The trigger level condition has the function of providing for early detection of likely contamination problems such that intervention can be put in place (e.g. shut off discharge, stop leak etc.).  It is not an ELV or an Environmental Quality Standard (EQS) as prescribed in the European Communities Environmental Objectives Page 3 of 7[Surface Waters] Regulations 2009 (SI No. 272 of 2009), and should not be relied on as such.

• Trigger levels should be set on a site specific basis and should be set after a sufficient period of monitoring (12 months ideally, to account for seasonal variation) of the storm water discharge to establish  the range of  ‘normal’ background levels.
• If contamination is already on-going at the site, then  other options can be considered,such as sampling from clean yard areas or roof water; however if this is not an option it may be the case that trigger levels have to be set on a generic and fit for purpose basis at contaminated sites, with reference to the appropriate standards e.g. European Communities Environmental Objectives [Surface Waters] Regulations 2009 (SI No. 272 of 2009).
• The number of samples that are taken to establish the background baseline variability is extremely important.  The more sample data available, the better the estimate of background levels.
• A simple check on the adequacy of the sampling number is to look at the values for average and standard deviation.  A large standard deviation (SD) may indicate insufficient sample numbers.
• It is desirable that standard deviation is calculated on at least 20 data points to reflect  the range of natural variability over most sampling conditions.

Once the data is available, the next step is deciding how the trigger values should be set.

• The average plus 2 standard deviations for  warning limit and  the average plus 3 standard deviations for the action limit.
• The 90%ile and 95%ile values could be used as the warning and action limits respectively.

There is little point in a site putting warning and action levels in place for storm water monitoring if this is not linked to a documented response programme  for the achievement or exceedence of trigger level values.