COVARIANCE OF GRADIENTS IN CARBON AND NITROGEN ISOTOPIC RATIOS IN SURFACE SEDIMENTS AND TOTAL DISSOLVED METAL CONCENTRATIONS IN THE SAN FRANCISCO BAY ESTAURINE SYSTEM

Mara Ranville* (Earth Sciences, University of California at Santa Cruz, Santa Cruz, CA 95064, ranville@es.ucsc.edu), James Zachos (Earth Sciences, University of California at Santa Cruz, Santa Cruz, CA 95064), A. R. Flegal (Environmental Toxicology, University of California at Santa Cruz, Santa Cruz, CA 95064).

Abstract

Preliminary analyses of spatial gradients in carbon and nitrogen isotopic ratios (d¹³C and d ¹⁵N) of surface (< 2 cm) sediments in San Francisco Bay indicate they are similar to the spatial gradients of some trace elements in that estuarine system. Trace metals measured include (1) some total dissolved (<0.45 mm) trace metal (Ag, Cu, Ni) concentrations in the water column, (2) some particulate trace metal (Ag) concentrations in benthic sediments, and (3) stable lead isotopic compositions (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb) in both surface waters and sediments. That apparent covariance is now being determined with more detailed carbon and nitrogen isotopic composition analyses of those sediments, which are being concurrently analyzed for trace elemental concentrations. These data are being incorporated in calculations to quantify the range and variation of natural and anthropogenic organic inputs to the estuarine system, which are complicated by pronounced seasonal variations in freshwater water influx, primary productivity, and associated biogeochemical fractionation processes.

Introduction

San Francisco Bay is an extremely important hydrologic system, because it drains 40% of the area of California and is the second largest estuary on the west coast of North America (Conomos, 1979). The Sacramento and San Joaquin rivers account for 90% of the freshwater flow to the bay, but this influx has been greatly reduced by freshwater diversions to support population growth and agriculture in California (Nichols, 1986). The remaining freshwater inputs to the bay come from small local streams that have little flow during the summer and wastewater discharges that are relatively constant (Conomos, 1985).

The temporal and spatial variation in these freshwater inputs strongly influence biogeochemical cycles within the estuary (Flegal, 1996). The rate and magnitude of freshwater flow directly impacts sediment accumulation within the bay. More importantly, it controls the estuary’s ability to “dilute, transform, or flush out contaminants” (Nichols, 1986).

Those impacts on trace metals are being investigated with complementary analyses of C and N isotope ratios to identify the primary sources and mechanisms of transport of organics into San Francisco Bay. Based on preliminary N isotope systematics, relatively low d¹⁵N values are expected at the north end of the bay, where rivers draining the heavily agricultural Central Valley enter the estuary. In contrast, higher d¹⁵N values are expected in the South Bay, where there is a high concentration of wastewater treatment plants that discharge directly into the bay, and freshwater runoff is minor. Carbon isotope ratios also should range between two isotopic endmembers, with marine carbon sources being characterized by less negative d¹³C ratios and terrestrial carbon sources by more negative d¹³C ratios.

Methods

Sediment samples are collected biannually (winter and summer) from 26 sites in San Francisco Bay (Figure 1). Aliquots of homogenized sediments are analyzed for a suite of parameters (e.g. grain size, total organic carbon, total nitrogen) and trace element concentrations (Ag, Al, Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn). Additional aliquots are now being analyzed for C and N isotopic compositions.

For stable isotope analyses, homogenized sediment is acidified with 1N HCl in order to remove inorganic carbon. After drying overnight, sediment is loaded into Fisons tin foil cups and analyzed in a Carlo Erba 2500 elemental analyzer coupled to a Optima gas source mass spectrometer in order to simultaneously obtain d¹⁵N, d ¹³C, and C/N measurements. All values are reported in the delta notation, which for d¹⁵N (in per mil) is defined as:

[¹⁵N/¹⁴N (sample) / ¹⁵N/¹⁴N (standard) –1] x 10³.

Carbon delta notation is defined as:

[(¹³C/¹²C (sample) - ¹³C/¹²C (standard) –1] x 10³

Nitrogen is referenced to atmospheric nitrogen, while carbon is referenced to VPDB.

Isotopic Tracers

The initial phase of this research consists of determining carbon and nitrogen ratios in surface sediment organic matter (OM) at sites throughout San Francisco Bay. Nitrogen (d¹⁵N) ratios can be used to distinguish anthropogenic from natural sources of nitrate to a system. In San Francisco Bay there appear to be three sources of nitrogen to the sediments.

The first nitrogen source reflects a marine contribution and averages 8‰. The second source is from riverine flux. Typically this endmember has d¹⁵N ratios of –4‰ to 4‰, which is tentatively attributed to runoff from agricultural areas that are characterized by heavy use of fertilizers (Heaton, 1986). The third source is derived from anthropogenic sewage and sewage treatment.

Waste derived nitrogen can enter the coastal sedimentary organic pool via two pathways. The most direct path is as a component of untreated solid organic wastes that are discharged directly into marine waters. The d¹⁵N of the N in solid wastes is variable ranging from between 2-4‰ (Sweeney et al, 1980; Jordan, 1997). The second path is as a dissolved component of treated effluent in the form of either NH⁴⁺ or NO₃^-. These species, which enter the nutrient sub-cycle and fuel algal production, tend to have relatively high d¹⁵N in the range of 10-20‰. The enrichment in N¹⁵ results from bacterially mediated fractionation during the treatment process. Despite attempts to lower the levels of dissolved N is discharged wastewater, the amount of N0₃^- in San Francisco Bay is allowed to reach concentrations of 45mg/L.

A second tracer, carbon isotope ratios (d¹³C), can also be used to distinguish marine versus freshwater sources of organic carbon. Previous studies have documented variations in organic carbon d¹³C from –27‰ where the rivers enter the bay to –19‰ near the Golden Gate bridge (Spiker, 1979). This isotopic gradient is attributed to an estuarine mixture of freshwater and marine dominated regions.

Trace Metals

The covariance of surface water trace metal concentrations with sediment light stable isotope ratios is currently being examined. Preliminary limited covariance (r = 0.4, simple linear regression) between silver and d¹⁵N ratios in sediments is seen, but no covariance between lead in either suface waters or sediments and stable isotope ratios is seen. More detailed statistical analysis of the data is currently underway, with the hopes of elucidating the true relationship between trace metals and stable isotopes in San Francisco Bay.

References

Conomos, T.J., 1979, Properties and circulation of San Francisco Bay waters, in Conomos, T.J., ed., San Francisco Bay: The Urbanized Estuary: San Francisco, Pacific Division of the American Association for the Advancement of Science, p. 47-84.

Conomos, T.J., Smith, R.E., Gartner, J.W., 1985, Environmental setting of San Francisco Bay: Hydrobiologia, v. 129, p. 1-12.

Flegal, A.R., Rivera-Duarte, I., Ritson, P.I., Scelfo, G.M., Smith, G.J., Gordon, M.R., Sanudo-Wilhelmy, S.A., 1996, Metal contamination in San Francisco Bay waters: Historic perturbations, contemporary concentrations, and future considerations, in San Francisco Bay: The Ecosystem: San Francisco, Pacific Division of the American Association for the Advancement of Science, p. 173-188.

Heaton, T.H.E., 1986, Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere: A review: Chemical Geology, v. 59, p. 87-102.

Jordan, Marilyn J., Nadelhoffer, Knute J., Fry, Brian, 1997, Nitrogen cycling in forest and grass ecosystems irrigated with 15N-enriched wastewater: Ecological Applications, v. 7(3), p. 864-881.

Nichols, F.H., Cloern, James E., Luoma, Samuel N., Peterson, David H., 1986, The modification of an estuary: Science, v. 231, p. 567-573.

Spiker, E.C., and Schemel, Laurence E., 1979, Distribution and stable-isotope composition of carbon in San Francisco Bay

Sweeney, R.E., Kalil E.K., Kaplan, I.R., 1980, Characterisation of domestic and industrial sewage in Southern California coastal sediments using nitrogen, carbon, sulphur and uranium tracers: Marine Environmental Research, v. 3, p. 225-243.

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