Plume tracking and dilution of effluent from the Boston sewage outfall

Field surveys were conducted on the Boston sewage outfall plume to test and certify the outfall's initial dilution in the near field and to investigate its dispersion in the far field. Rhodamine WT dye was added to the effluent at the treatment plant at a constant concentration over a 6-h period and tracked offshore over three days. During the near-field surveys, the current was flowing closely parallel to the diffuser, resulting in the wastefield spreading laterally as a dynamic density current at a rate that was closely predicted by theoretical equations. The plume was submerged by the oceanic density stratification, with a minimum initial dilution of about 102 within a few tens of meters from the diffuser. The initial dilution and the other near-field characteristics were in good agreement with predictions of mathematical models and with the physical model study on which the diffuser design was based. After a travel time of 24h, the dye patch was still intact and oceanographic mixing and dispersion had increased dilution by a factor of about two to more than 200:1. After 48h, the plume had broken into large patches, and most dilutions considerably exceeded 400 with an average dilution of order 1000. For the approximately 52h that the dye patch was followed in the far field, mixing was due to lateral diffusion; vertical mixing was negligible. This slow vertical mixing is due to the stable density stratification in the water column. The outfall is performing as designed. The field surveys provided a strong confirmation of the ability of small-scale laboratory model studies to replicate and predict the near-field characteristics of ocean wastewater outfalls. They also increase the confidence that mathematical models can be used to reliably estimate initial dilution under other effluent flows, oceanographic conditions, and stratification regimes.