Description
In November 2010, the South Bay International Wastewater Treatment Plant (SBIWTP) was upgraded from primary to secondary treatment to conform to the Clean Water Act (CWA). In this study, statistical comparisons were made of the bacterial water quality (total and fecal coliforms and enterococci densities), both before and after discharge of secondary treated sewage to the SBOO began, so that the effect of this treatment upgrade on ocean water quality could be quantitatively assessed. The frequency of exceedance of bacterial indicator thresholds was statistically analyzed for 11 shore, 7 kelp, and 21 offshore stations throughout US and Mexico using the Fischer’s Exact test, for the years before (2001- 2010) compared to after the secondary discharge began (2011-2018). It was determined that the 2 offshore stations that showed significant (p <0.05) improvement in water quality postsecondary treatment and were immediately near the point of discharge (I14, I16). On the other hand, shore stations (S3, S4, S5, S6, S10, S11, S12) evidenced a significant (p <0.05) decrease in water quality during non-chlorination, but a significant increase (S2, S3, S4, S5, S6, S10, S11, S12) in microbial water quality during chlorination. Analysis of kelp stations indicated a significant increase in water quality at two stations (I24, I25) during nonchlorination; however, during the period of chlorination all 7 stations showed a significant improvement. Wilcoxon rank-sum test was conducted to analyze total bacterial density for each of the monitoring stations. Results from Wilcoxon analysis followed similar trends of the Fischer’s exact test results. Additionally, Wilcoxon rank-sum test was used to assess the impact of the treatment upgrade on chlorophyll a and dissolved oxygen at offshore stations (I1-I40). Statistical was conducted to determine whether or not the addition of secondary treatment had any significant effect on fecal indicator bacteria levels and water quality parameters dissolved oxygen and chlorophyll a. Ultimately this study found that the addition of secondary treatment to the SBIWTP lead to minimal improvement of ocean water quality in this region. There are a number of other factors that could be responsible for the continual presence of bacteria in the ocean water in this region.
