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- The distribution and speciation of mercury in the California Current: Implications for mercury transport via fog to land
- Unfiltered seawater samples from vertical profiles collected at 60 stations within the California Current during four summer cruises spanning a two-year period from 2014 to 2015, were analyzed for elemental mercury (Hg°), monomethyl mercury (MMHg), dimethyl mercury (DMHg) and total mercury (THg). Fog water samples, taken at sea and throughout a network of land based stations were also analyzed for MMHg and THg. Vertical profiles indicate that midwater regions around 300 m are associated with concentration maxima in methylated species. Cyclonic mesoscale eddies were shown to be strong sources of the gaseous mercury species to the lower atmosphere and a likely source of these species to fog. Calculated evasive flux of Hg0 and DMHg were greatest in these regions (34 and 11 pmol m−2 d−1, respectively), whereas anticyclonic eddies support little or no sea-air evasion. Incubation experiments showed that DMHg is stable over short time scales at natural seawater pH (7.8 to 8.2) but degrades rapidly to MMHg at low pH. Demethylation of only a small percent of the evading DMHg, on acidic marine aerosols associated with fog condensation nuclei, can account for over 100% of the MMHg observed in fog. The surface microlayer, enriched in MMHg (by 30×), may also contribute to sea-air flux through aerosol production. Neither shelf sediments nor oxygen minimum zones appear to be a major source of methylated mercury in the California Current.
- Coale, Heim, Negrey, Weiss-Penzias, Fernandez, Olson, Chiswell, Byington, Bonnema, Martenuk, Newman, Beebe, Till
- Long-term variation in concentrations and mass loads in a semi-arid watershed influenced by historic mercury mining and urban pollutant sources
- Urban watersheds are significantly anthropogenically-altered landscapes. Most previous studies cover relatively short periods, without addressing concentrations, loads, and yields in relation to annual climate fluctuations, and datasets on Ag, Se, PBDEs, and PCDD/Fs are rare. Intensive storm-focused sampling and continuous turbidity monitoring were employed to quantify pollution at two locations in the Guadalupe River (California, USA). At a downstream location, we determined loads of suspended sediment (SS) for 14 yrs., mercury (HgT), PCBs, and total organic carbon (TOC) (8 yrs), total methylmercury (MeHgT) (6 yrs), nutrients, and trace elements including Ag and Se (3 yrs), DDTs, chlordanes, dieldrin, and PBDEs (2 yrs), and PCDD/Fs (1 yr). At an upstream location, we determined loads of SS for 4 yrs. and HgT, MeHgT, PCBs and PCDD/Fs for 1 yr. These data were compared to previous studies, climatically adjusted, and used to critically assess the use of small datasets for estimating annual average conditions. Concentrations and yields in the Guadalupe River appear to be atypical for total phosphorus, DDTs, dieldrin, HgT, MeHgT, Cr, Ni, and possibly Se due to local conditions. Other pollutants appear to be similar to other urban systems. On average, wet season flow varied by 6.5-fold and flow-weighted mean (FWM) concentrations varied 4.4-fold, with an average 7.1-fold difference between minimum and maximum annual loads. Loads for an average runoff year for each pollutant were usually less than the best estimate of long-term average. The arithmetic average of multiple years of load data or a FWM concentration combined with mean annual flow was also usually below the best estimate of long-term average load. Mean annual loads using sampled years were also less than the best estimate of long-term average by a mean of 2.2-fold. Climatic adjustment techniques are needed for computing estimates of long-term average annual loads.
- McKee, Bonnema, David, Davis, Franz, Grace, Greenfield, Gilbreath, Grosso, Heim