San Francisco Bay Atmospheric Deposition Pilot Study Part 1: Mercury

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ABSTRACT

This report is the first in a three-part series presenting a Pilot Study that was conducted from August 1999 through November 2000. The objective was to estimate the inputs of selected trace metals and trace organic pollutants from the atmosphere to the San Francisco Estuary in California. Ambient air samples and precipitation samples were collected at three sites strategically located around the Bay Area. This report covers the mercury component of the Pilot Study.

The Pilot Study evaluated deposition of atmospheric mercury to the San Francisco Estuary through dry and wet deposition. Analysis of total mercury in the ambient air and precipitation provided results for estimating mercury loading from dry and wet deposition, respectively. Direct atmospheric deposition of mercury to the Estuary includes loading from dry deposition and wet deposition. Indirect deposition of mercury from atmosphere to the Estuary is derived from the chemical being initially deposited to the watershed and then transported to the Estuary through runoff and tributaries, and constitutes the loading from this pathway that is atmospheric in origin. This report provides an estimate of the indirect atmospheric deposition through runoff and tributaries from the local watersheds in the San Francisco Bay Region, but excludes coastal areas that drain water directly to the Pacific Ocean. Estimate of indirect atmospheric loading through more remote watersheds such as the Sacramento River-San Joaquin River drainage areas is not included in the assessment.

Total mercury was detected in ambient air samples at concentrations of 1.5 to 4.2 ng/m³, with an average concentration of 2.1 ng/m³. Although this concentration is within the range detected in other areas of the U.S. and similar to those detected around the Chesapeake Bay area, it is about twice the concentration considered to be the global background. Flux of mercury to the Estuary from dry deposition was rather uniform with a mean of approximately 19 Œµg/m²/yr and a range of 18 to 21 Œµg/m²/yr estimated for different segments of the Estuary. Volume-weighted average mercury concentration in precipitation was 8.0 ng/L, ranging from 6.6 to 9.7 ng/L, within the range detected in other areas around the U.S. The average concentration detected in the San Francisco Bay Area is lower than the concentration of 11 to 15 ng/L found around the Chesapeake Bay area, but twice as high as those measured at some background locations along the U.S. west coast. Flux from wet deposition to the entire Estuary was estimated at 4.2 Œµg/m²/yr, ranging from 3.5 Œµg/m²/yr at the South Bay site to 4.5 Œµg/m²/yr at the Central Bay site, attributable to the differences in the amount of rainfall observed at different segments of the Estuary.

On an annual basis, it was estimated that direct atmospheric deposition, via both dry deposition and wet deposition, contributed approximately 27 kg of total mercury to the Estuary. Direct wet deposition constitutes about 18% of the total atmospheric deposition. Indirect inputs via deposition and runoff from the watersheds in the San Francisco Bay Region that drain water directly to the Estuary were estimated to contribute another 55 kg/yr of mercury loading to the Estuary. To put air-deposition loading into perspective, we compared mass loadings of mercury among the various major conveyances or pathways. Any comparison of loading estimates presented in this report only serves as the first-level screening purposes, because these loading estimates are derived from different reports with various degree of uncertainty.

The uncertainty level in the estimates derived from this study is low (with an error of within 50%) for wet deposition, moderate-high (with an error of two to five folds) for dry deposition, and moderate-high for indirect deposition to surrounding land surfaces and subsequent tributary inputs attributable to atmospheric deposition. This results in a moderate-high uncertainty for overall estimates. It is very difficult to assess the uncertainty associated with mercury loadings estimated by other authors because the level of documentation differs, and different data sources, calculation approaches, and assumptions may have been used. It is believed that the mass-loading estimate of mercury from point-source discharges is fairly accurate due to recent implementation of clean-sampling, handling, and processing techniques, as well as the advanced technology in detecting mercury at ultra-low levels. The uncertainty associated with estimates of mercury inputs from watersheds and sediment remobilization is unknown and is likely very high in comparison.

Combining loading from atmospheric deposition of mercury directly to the Estuary and indirectly through runoff was estimated to contribute almost seven (7) times of the loading from wastewater discharges, an external point source. Results and evaluation presented in this report indicate that loading of mercury from wastewater discharges to the San Francisco Estuary likely constituted less than 2% of the total load from all sources and pathways, and less than 15% of the load from atmospheric deposition. Atmospheric deposition contributes a sufficient enough load of mercury to the Estuary to warrant further evaluation. Similar to the loading from watersheds, atmospheric deposition is an external loading conveyor of pollutants to the Estuary that is more feasible to manage than those from internal pollutant redistributing mechanisms like remobilization of buried sediment. In addition, pollutant loading from watershed runoff includes a component that is atmospheric in origin. Exploring a suitable strategy for minimizing mercury loading to the San Francisco Estuary should include an investigation of measures that can mitigate the sources and pathways that contribute to the releases of mercury to the atmosphere.