Trends in land use, sewage, and stormwater and their relationship with coastal water quality in St. Thomas, United States Virgin Islands

This research investigated the health of coastal embayments adjacent to urbanized watersheds of a subtropical, Caribbean island by analyzing land use changes, historical water quality monitoring data, and sewage treatment and disposal information, and modeling stormwater runoff quantity and quality. The results of this research found both temporal and spatial trends in degraded water quality in coastal embayments where treated sewage is discharged directly to coastal waters. Water quality improved in embayments shortly after sewage discharges ceased; however, physical and environmental changes (i.e. removing mangroves, opening salt ponds, dredging, bottom scouring by vessels), boat sewage, and stormwater runoff are currently threatening the health of these coastal waters.; One of the major sources of contaminants degrading water quality was sewage (treated, partially treated, and raw) discharged from municipal sewage treatment plants. All the municipal sewage treatment plants examined in this study were working poorly and records show they have been for years. Most of the sewage treatment plants did not meet secondary treatment levels as required by the federal Clean Water Act. Recent efforts to the supply of piped, potable water to new areas served by improperly working sewage treatment plants are further threatening the marine environment. Piped water will likely lead to increased water consumption rather than moderation, because residents will no longer need to rely on limited, stored water supplies in their cisterns. This will lead to further increases in sewage discharged to coastal waters.; Cisterns play an important role in controlling nonpoint source pollution from increasing urbanization. Modeling results suggest there is no increase in stormwater runoff due to development because a significant portion of stormwater is diverted to cisterns. However, pollutant concentrations in stormwater are predicted to have increased with time due to urbanization resulting in pulses of stormwater runoff to coastal waters that are higher in biological oxygen demand, nutrient, and sediment concentrations.; Resorts and condominium complexes, which own and operate unregulated private sewage treatment plants, reuse treated wastewater for irrigation. This may increase human health risks and runoff resulting in nonpoint pollution to coastal waters. This practice needs to be further examined.; Potable water supply and sewage sanitation should be considered together and developed as a single system. A single water supply-sewerage system integrated into a comprehensive land and water use plan that balances growth management with water supply, and adaquate sewage treatment and disposal would be considerably more protective of the marine environment than practices in place today.