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Surface Water

Reporting the BOD results

As required by its Surface Water Discharge permit, our example town: Townsville, sampled three times the first week of its discharge and once per week after that until the discharge was terminated.   Lab results for Townsville.

Townsville is required to report its seven-day average and 30-day average for these parameters. Let's start with the seven-day average.


Seven-Day Average BOD values

Remember, the seven-day average is based on a calendar week, Sunday - Saturday. Townsville discharged for 3 weeks, and sampled at least once in each week. Therefore, they will have 3 weekly averages to calculate. 

Week 1 (May 27 - June 2)

The city sampled three times the first week (View the Townsville May Calendar). To calculate the seven-day average for this week, we must use all 3 samples collected during that week. These results are summarized below:

May 28        26 mg/L

May 30        25 mg/L

May 31        28 mg/L

26 + 25 + 28 = 79

79 divided by 3 = 26.33333333

Therefore, the seven-day BOD average for the first week is 26.3 mg/L (Note: It is not necessary to include all of the numbers after the decimal point).

Week 2 (June 3 - June 9)

There was only one sample collected during this week. Therefore, we do not need to calculate an average. The seven-day average would be the same as the one sample taken that week - 30 mg/L

Week 3 (June 10 - June 16)

There was also only one sample collected during this week, and the discharge was stopped on June 15. Therefore, we do not need to calculate an average. The seven-day average would be the same as the one sample taken that week - 35 mg/L.

Reporting the Seven-Day Averages

Let's summarize our seven-day BOD averages for this discharge:

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