Tracing flocculent industrial and domestic sewage transport on San Pedro Shelf,Southern California,by nitrogen and sulphur isotope ratios

Domestic and industrial sewage is discharged at a depth of 60 m near Whites Point on the San Pedro Shelf, Southern California borderland. A density-stratified thermocline, above the outfall at a depth of 20 to 30 m, is thought to prevent most effluent particles from reaching shallower depths and recreational facilities. In this investigation, measurement of the natural abundance of nitrogen isotopes (${}^{15}\text{N}{}^{14}\text{N}$) is used to determine the level of sewage contribution to flocculent suspended material persisting at water depths of 7, 13 and 20 m. Organic nitrogen at 20 m depth was shown to be predominantly of sewage-origin and at 7 m, predominantly of marine origin. Organic nitrogen at 13 m depth and within 3 km of the outfall pipes is predominantly sewage in origin.Stable isotope composition of sulphur (${}^{32}\text{S}{}^{34}\text{S}$) in the flocculent material indicates that the effluent particles contain metabolic sulphur, incorporated from dissolved seawater, as well as bacterially produced mineral sulphide.