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Setting Stormwater Action Limits

In 2012, the EPA published guidance on the setting of trigger levels for stormwater discharges at IE, IPC and Waste facilities. Where a specific stormwater (SW) Emission Limit Value (ELV) has not been assigned in a Licence, the setting of trigger levels is usually required by the Agency. ELVs are usually not assigned in these cases as SW discharges should only consist of stormwater, roof-water or other uncontaminated, natural waters. The definition of a trigger level is ‘a parameter value, the achievement or exceedance of which requires certain actions to be taken by the licensee’. SW discharges should not negatively impact any water body.

The guidance outlines several parameters which are routinely utilised in licences for the monitoring of SW discharges as follows:

  • Chemical Oxygen Demand (COD);
  • pH;
  • Total Organic Carbon (TOC);
  • Suspended Solids (SS);
  • Metals;
  • Ammonia;
  • Temperature;
  • Conductivity.

The guidance recommends that the setting of trigger levels at a site should be carried out with regard to the following:

  • The status and possible impacts on the receiving water;
  • The parameter should be of such significant interest to warrant the setting of a trigger level;
  • An appropriate parameter(s) is selected relevant to the site and its activities.

Trigger levels should be set on a site-specific basis and should be indicative of on-site contamination. To establish ‘normal’ background levels there should be a sufficient period of monitoring (12 months ideally to factor in seasonal variation). In sites that have ongoing contamination, other options must be considered such as sampling of specific areas. If this is not an option, generic levels should be set that are fit for purpose and have regard to relevant standards. A method of checking the adequacy of sampling numbers is to look at the values for average and standard deviation. A large standard deviation is indicative of an insufficient number of samples.

The guidance outlines two methods for the setting of trigger levels, when sufficient data has been collected, as follows:

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

Where possible, the trigger levels should be the same as an established Environmental Quality Standard (EQS) parameter or one where a relationship can be established with a substance for which an ELV has been set for other discharges on-site. If any samples have been identified as likely to have been contaminated by onsite processes, they should not be included when calculating the mean and SD values of the SW discharge, as this would not give a more accurate result.

Monitoring of SW may be either continuous or non-continuous. The potential risk posed to the receiving environment should be assessed when considering the delay in results for different forms of testing. Continuous monitoring is the most effective option when it includes an alarm to provide early warning. Non-continuous monitoring can be less effective as action cannot be taken as soon as the problem occurs. However, not all parameters can be continuously monitored. In these instances, the period allowed between samples should reflect the contamination risk and the risk to the receiving environment. While some parameters may have a slow turnaround time on analysis, there may be an agreement in place between the licensee and the laboratory to have an immediate notification if the trigger level/ELV is exceeded and the necessary actions may be taken.

An investigation is required if:

  • Continuous monitoring shows readings at or in exceedance of a warning limit;
  • If three consecutive grab samples show values equal to or above the warning limit.

An investigation and notification of potential contamination to the Agency is required if:

  • Continuous monitoring shows readings at or in exceedance of the action limit. This also requires the activation of the shut-off valve/diversion to a holding pond;
  • A grab sample value is equal to or above the action limit is obtained.

During an investigation period, the frequency of monitoring should be increased, in agreement with the inspector, until such time that the source of the contamination has been identified and eliminated.

Some sites may find the setting of trigger levels more difficult if activities have been ongoing for several years. ‘Clean’ SW areas may not be available at these sites. To address SW contamination in older sites, a blow out/clean down of the SW system may be an option. This cleaning cannot be directly discharged to the SW system. Some common causes of contamination are as follows:

  • Spillages/overflows, often from sumps nearby;
  • Deliberate actions;
  • Leakages;
  • Misdirection/incorrect connection of pipes, foul sewer pipes, pipes from hand-basins etc. into storm drains;
  • Seasonal factors such as autumnal leaves, salt/grit used in frosty conditions, etc.

When the baseline values have been set up and agreed with the Agency, they should be reviewed periodically to ensure their suitability. Care should also be taken to not set the values too tight so that resources are not wasted responding to non-contamination events.

Licensees should ensure that there is a documented response programme in place in the event that a trigger level value is reached and/or exceeded. Should there be a value in excess of an action limit, the discharge should be diverted to a holding pond or a shut-off valve should be activated. However, this diversion of stormwater should be the last resort with on-site control measures being preferable such as bunding and spill kits, etc.

To see the guidance in full click here.

Environet Solutions can assist with the setting of trigger levels should your company require it. For any assistance or to get more information please contact us.

May 2020