Significance of Caution Levels Findings for
Percent Saturation of Dissolved Oxygen
Percent Saturation in Early October Survey
Baseline Monitoring:
In 1994 and 1995, when work began on establishing caution level and warning level thresholds for the Contingency Plan, caution level thresholds were set at 6.5 mg/l and 80% saturation so as to be more conservative than the standards set for Class SA waters in the Massachusetts Water Quality Standards, 6.0 mg/l and 75%, respectively. These water quality standards were used to set the warning levels. At that time, outfall siting and early baseline monitoring results had documented natural excursions below the caution threshold (and also below the warning levels) at both the nearfield and Stellwagen Basin areas, for example in 1994. No information was then available, however, on the frequency of such natural excursions.
Further baseline monitoring established that during the period (generally June through October) when the Bay is stratified, bottom water dissolved oxygen saturation in the nearfield and in Stellwagen Basin regularly falls below the Contingency Plan threshold levels. These below-threshold natural excursions were observed, in fact, in 7 of the 8 baseline years.
The baseline data shows, then, that natural excursions (often taking the measures to levels much lower than the early October instances found by MWRA in 2000) regularly occur in these areas. According to the baseline information, natural events cause the dissolved oxygen to rise again after.
The early October 2000 sampling shows that this natural pattern has occurred again this year in the nearfield and in Stellwagen Basin. Following the cycle of the natural pattern, additional sampling conducted later in October showed an average dissolved oxygen saturation of about 82.7% in the nearfield. The levels of percent saturation of dissolved oxygen observed earlier, therefore, appear to be attributable to naturally-occurring phenomenon.
The levels of dissolved oxygen in Massachusetts Bay during the baseline monitoring and after the outfall went on-line in September 2000 have been well above levels that support a healthy marine ecosystem. This is discussed further in Attachment C.
During the time that the below-caution level excursions were observed, the treatment plant itself was performing in an excellent fashion. The cBOD monthly average for September was 13 mg/l and for October was 9.6 mg/l (preliminary data), well under permit limit of 25 mg/l established in the Deer Island NPDES permit.