pH is required to be averaged by the CGP, but averaging pH is cumbersome in general and especially cumbersome for sites with multiple drainage areas.

Determination of Average Daily pH

The Construction General Permit (CGP) has established Numeric Action Levels (NALs) for pH on all Risk Level /Type 2 and 3 projects. A minimum of three samples are to be taken for each designated drainage area once rainfall reaches Qualifying Event (QE) status. A Qualifying Event is defined as a ½” of rainfall with a discharge that occurs during normal business hours (not evenings or weekends). The pH scale runs from 1 to 14, with 7 designated as neutral, below 7 is Acidic and above 7 is Alkaline (base). The Numeric Action Levels (NALs) for pH are 6.5 to 8.5. A pH reading (average) which exceeds the NAL standard, e.g. below 6.5 or above 8.5 requires action to be taken.

It is good to remember that pH is a field test which is designed to trigger immediate action if there is a Numeric Action Level exceedance. For ease of use within the field it is acceptable to perform a basic arithmetic average, as long as a minimum of 3 readings are taken and are all either above 7 or below 7. For example the following three samples would be averaged as follows: 7.6 + 8.1 + 8.0 = 23.7 / 3 = 7.9 pH average. Example 2: 8.5 + 8.4 + 8.9 + 8.6 = 34.4 / 4 = 8.6 pH average, which is an exceedance of the pH NAL. (all example readings were above 7 on the pH scale)

A Numeric Action Level exceedance triggers action which means an attempt must be made to understand and control the source of high alkalinity.

As a field test, the arithmetic average, although not as technically accurate as a true pH reading, provides a clear indicator of the general pH quality, and is sufficient enough to identify a problem which can then trigger immediate action.

The pH calculator below provides a comparative tool for those that wish to analyze the arithmetic pH average with the true pH average. See CGP Technical Bulletin Series – Issue 2013.1 for more information.

pH Calculator

Please enter one pH value in each cell below and click on “Calculate”.