Legacy brewery phosphorus as a management constraint in the Mashapaug Watershed: unresolved reservoirs and pathways in an urban pond cascade

Suzannah Rutherford 

Spectacle and Mashapaug Ponds have been listed as impaired waters in Rhode Island since 2002, with 20 public-health advisories issued since 2011. Phosphorus is treated as the primary limiting nutrient for harmful algal blooms in the watershed, and Spectacle Pond is the largest direct phosphorus source to Mashapaug Pond and the lower pond cascade. In February 2026, the Rhode Island Department of Environmental Management issued a permit requiring 65–68 percent stormwater phosphorus reductions from regulated parcels but did not address legacy phosphorus from industrial sources. Tongue Pond, the uppermost kettle pond, received Narragansett/Falstaff brewery effluent from 1890–1981 and once discharged to Spectacle Pond through a constructed outlet. To evaluate whether brewery-derived phosphorus remains a plausible management concern, I estimate phosphorus delivered to, exported from, and retained in or near Tongue Pond during brewery operations. Using 1974 EPA water-quality data and a 1976 federal discharge permit application, I developed a literature-based input model, outlet-constrained export model, and mass-balance retention estimate to partition phosphorus loads. An estimated 281,000–1,405,000 kg P was delivered to Tongue Pond, with a best estimate of 54,000 kg P reaching Spectacle Pond via Tongue Pond outflow. This model yields inferred legacy residuals of 227,000–1,351,000 kg P in or near Tongue Pond, with a central estimate of 643,000 kg P. Sediment cores, porewater profiles, groundwater transects, storm-event flow monitoring, and benthic-flux measurements are needed before managers can determine whether stormwater controls alone are sufficient or whether legacy reservoirs require targeted intervention.