Concentrations and loads of suspended sediment and trace element pollutants in a small semi-arid urban tributary, San Francisco Bay, California

Abstract

Water-quality policy documents throughout the world often identify urban stormwater as a large and controllable impact to sensitive ecosystems, yet there is often limited data to characterize concentrations and loads especially for rare and more difficult to quantify pollutants. In response, concentrations of suspended sediments and silver, mercury and selenium including speciation, and other trace elements were measured in dry and wet weather stormwater flow from a 100% urban watershed near San Francisco. Suspended sediment concentrations ranged between 1.4 and 2700 mg/L and varied with storm intensity. Turbidity was shown to correlate strongly with suspended sediments and most trace elements and was used as a surrogate with regression to estimate concentrations during unsampled periods and to compute loads. Mean suspended sediment yield was 31.5 t/km(2)/year. Total mercury ranged between 1.4 and 150 ng/L and was, on average, 92% particulate, 0.9% methylated, and 1.2% acid labile. Total mercury yield averaged 5.7 μg/m(2)/year. Total selenium ranged between non-detect and 2.9 μg/L and, on average, the total load (0.027 μg/m(2)/year) was 61% transported in dissolved phase. Selenate (Se(VI)) was the dominant species. Silver concentrations ranged between non-detect and 0.11 μg/L. Concentrations and loads of other trace elements were also highly variable and were generally similar to other urban systems with the exceptions of Ag and As (seldom reported) and Cr and Zn which exhibited concentrations and loads in the upper range of those reported elsewhere. Consistent with the semi-arid climatic setting, >95% of suspended sediment, 94% of total Hg, and 85-95 % of all other trace element loads were transported during storm flows with the exception of selenium which showed an inverse relationship between concentration and flow. Treatment of loads is made more challenging in arid climate settings due to low proportions of annual loads and greater dissolved phase during low flow conditions. This dataset fills an important local data gap for highly urban watersheds of San Francisco Bay. The field and interpretative methods, the uniqueness of the analyte list, and resulting information have general applicability for managing pollutant concentrations and loads in urban watersheds in other parts of the world and may have particularly useful application in more arid climates.

Assessment of the contribution of utility vault water to surface water pollution

“…For example, median Cu and Zn concentrations in utility water in this study were 9.66 and 81.6 μg/L, substantially lower than the median concentrations in Paris stormwater of 55 and 270 μg/L, respectively (Zgheib et al 2012 ). Median Cu and Zn concentrations in utility vault water were comparable to the range of stormwater concentrations measured in a local study of urban runoff from Hayward, CA (McKee and Gilbreath 2015 ). That study, which focused on the smaller urban San Francisco Bay Area outfalls over 4 years (from water years 2007–2010) reported that Cu concentrations ranged from 2.3 to 50 μg/L and Zn concentrations ranged from 2.4 to 280 μg/L (McKee and Gilbreath 2015 ), which is similar to low and averaged utility vault water data.…”

Spatial variation of metals and phosphorus in sediments of a river influenced by urbanization

“…According to literature, the occurrence of metals such as Cd, Cr, Ni and Zn is not a surprise. The literature suggests a strong correlation between these metals and urban areas (GOMES et al, 2009;MINELLA, 2009;RIVARO et al, 2011;MCKEE;GILBREATH, 2015).…”

“…Some authors found no significant variation in metals concentrations (ZGHEIB; MOILLERON; CHEBBO, 2011;GALLO et al, 2013), but others found significant variation (VALTANEN; SILLANPAA; SETALA, 2014;MCKEE;GILBREATH, 2015). The main potential sources of metals in sediments, especially in urbanized areas, are paved and unpaved streets, residential and industrial areas, also sewage discharges (SILVA; SIQUEIRA; LEANDRO, 2015;FRANZ et al, 2014;MIGUEL et al, 2005).…”

“…Chemical elements (metals and semi-metals) such as As, Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn have been considered as a fingerprint to trace pollution in urban areas (GOMES et al, 2009;MINELLA, 2009;RIVARO et al, 2011;MCKEE;GILBREATH, 2015). Despite the discussion about iron, some authors also include this metal as tracer of pollution (POLETO; MERTEN; MINELLA, 2009;MCKEE;GILBREATH, 2015;SILVA;SIQUEIRA;LEANDRO, 2015), even been one of the most abundant on Earth as well Al, Ca, K, Mg, Si and Ti.…”

“…Despite the discussion about iron, some authors also include this metal as tracer of pollution (POLETO; MERTEN; MINELLA, 2009;MCKEE;GILBREATH, 2015;SILVA;SIQUEIRA;LEANDRO, 2015), even been one of the most abundant on Earth as well Al, Ca, K, Mg, Si and Ti.…”