Selenium distribution and fluxes in intertidal wetlands, San Francisco Bay, California.

Selenium (Se) concentrations exceeding ecological guidelines for sediments and suspended particulate matter (SPM) have been observed in the northern reach of the San Francisco Bay estuary. Longterm availability of elevated Se in wetland sediments depends in part on the fluxes controlling Se distribution. The relative contribution of sedimentary vs. post-depositional Se fluxes in two San Francisco Bay intertidal wetlands was estimated. Selenium concentrations on surface wetland sediments were compared with levels on SPM, and with previously established background levels in San Francisco Bay sediments. Sediment Se fluxes to the wetlands were measured directly using sediment traps. Although dissolved Se concentrations are higher than particulate Se concentrations in San Francisco Bay water, sediment input into the system provides the major flux of Se. Strong correlation between Se and C on SPM (r2 = 0.81) indicates the importance of organic particulate deposition. Dependence on sediment texture was qualitatively established by measuring Se on particle-size separates. Normalization to Al showed that 65% of Se spatial variability is related to sediment texture. Selenium is further enriched in the marsh via post-depositional inputs, probably due to in situ adsorption from overlying water and chemical reduction. According to sediment flux measurements, enrichment in the marsh is equivalent to 20 to 25% of the particulate Se flux, thereby defining the marsh as a Se sink. These findings highlight the need for more intensive monitoring of SPM as the major source of Se to intertidal wetlands.

In vitro permeation of nickel salts through human stratum corneum.

Allergic contact dermatitis due to nickel salts is common. It is therefore important to measure the permeation of these salts through the stratum corneum (SC), the primary rate-limiting domain in skin. An advanced diffusion system and analytical techniques now enable better measurement of the flux than was possible in earlier experiments. Human SC was prepared by trypsinization of dermatomed cadaver leg skin. The diffusion system included diffusion cells with a spiral line. Aqueous solutions of nickel salts (Ni(NO3)2, NiSO4, NiCl2 and Ni(-OOCCH3)2 at 1% Ni2+ concentration) were used as the donor solution (400 microL/cell). The receptor fluid, pure water, was collected up to 96 h after application of the donor solutions. Nickel concentrations in the donor and receptor fluid, as well as in the SC, were analysed using inductively coupled plasma mass spectrometry (ICP-MS) with a confidence limit of 0.5 ppb. Based on the total recovery of nickel from the experiments, about 98% of the dose remained in the donor solution, whereas 1% or less was retained in SC and less than 1% was found in the receptor fluid. Following an early surge, nickel permeates slowly across SC. The steady-state permeability coefficients of nickel were calculated from the flux data (approximately 5.2-8.5 x 10(-7) cm/h) with no significant difference among the salts. The results concur in principle with earlier studies conducted using the full-thickness human skin in vitro, and suggest that in vivo nickel ions may permeate simultaneously by routes of diffusion such as the shunt pathway, apart from slow transcellular/intercellular diffusion alone.