In 1990 and 1991, particles from buoyant and neutrally buoyant hydrothermal plumes above hydrothermal vents at the North Cleft segment of the Juan de Fuca Ridge were sampled to study their changing composition and fluxes away from the vent field. In the rising buoyant plume, >75% of the P, V, Cr, and As scavenging from seawater by hydrothermal precipitates occurs in the first 50 m above the vent. Cu and Zn are most enriched in buoyant plume particles collected from the first few meters above the vent. However, the degree of enrichment decreases very rapidly with increased height above the vents due to sedimentation of the more dense Cu‐ and Zn‐rich sulfide phases. Using the plume data, coupled with the results of our analysis of sediment trap samples, we estimated that more than 99% and 99.9%, respectively, of the total hydrothermal Fe and Mn produced at the vent field are transported beyond the vent field and dispersed in the open ocean.
Geochemical tracer data (i.e., 222Rn and four naturally occurring Ra isotopes), electromagnetic (EM) seepage meter results, and high-resolution, stationary electrical resistivity images were used to examine the bi-directional (i.e., submarine groundwater discharge and recharge) exchange of a coastal aquifer with seawater. Our study site for these experiments was Lynch Cove, the terminus of Hood Canal, WA, where fjord-like conditions dramatically limit water column circulation that can lead to recurring summer-time hypoxic events. In such a system a precise nutrient budget may be particularly sensitive to groundwater-derived nutrient loading. Shore-perpendicular time-series subsurface resistivity profiles show clear, decimeter-scale tidal modulation of the coastal aquifer in response to large, regional hydraulic gradients, hydrologically transmissive glacial terrain, and large (4-5 m) tidal amplitudes. A 5-day 222Rn time-series shows a strong inverse covariance between 222Rn activities (0.5-29 dpm L(-1)) and water level fluctuations, and provides compelling evidence for tidally modulated exchange of groundwater across the sediment/water interface. Mean Rn-derived submarine groundwater discharge (SGD) rates of 85 +/- 84 cm d(-1) agree closely in the timing and magnitude with EM seepage meter results that showed discharge during low tide and recharge during high tide events. To evaluate the importance of fresh versus saline SGD, Rn-derived SGD rates (as a proxy of total SGD) were compared to excess 226Ra-derived SGD rates (as a proxy for the saline contribution of SGD). The calculated SGD rates, which include a significant (>80%) component of recycled seawater, are used to estimate associated nutrient (NH4+, Si, PO4(3-), NO3 + NO2, TDN) loads to Lynch Cove. The dissolved inorganic nitrogen (DIN = NH4 + NO2 + NO3) SGD loading estimate of 5.9 x 10(4) mol d(-1) is 1-2 orders of magnitude larger than similar estimates derived from atmospheric deposition and surface water runoff, respectively.
During the extraction of previously acidifled estuarine samples, altered organic material still retains some capacity to inhibit the extraction of trace metals by Cheiex-100. Previous studies have indicated that heating or UV oxidation of samples reduces the capacity of this organic matter to inhlbit the extraction of trace metals by Cheiex-100. The results of this study using recently collected samples indicate that decreasing the flow rate to 0.2 mL min-' is also an effective means of increasing the retention of trace metals by Chelex-100. Additional benefits of the slow-flow column extraction method include improvements in precision and the diminatlon of pretreatment procedures that could cause contamination or reduce the extractability of Fe. Aged acidified samples require heating of the sample prior to extraction. Controlled contamination can be minimized for most metals by preextraction of the buffer solution.Since Riley and Taylor (1) introduced the Chelex-100 ionexchange method for the determination of trace metals in seawater, it has been used in numerous investigations with varying degrees of success. In a batch method, complete extraction was achieved for Mn after 16 h of equilibration (2). In a column method employing a flow rate of 4 mL min-l, extraction of open ocean water a t natural pHs resulted in significantly higher imprecision for Zn and lower values for Cu and Ni relative to an organic solvent extraction method (3). While column extraction of coastal waters at a flow rate of 1 mL min-' or greater resulted in low values or high imprecisions for some metals ( 4 4 , pretreatment of the samples by heating (4,5) or UV oxidation (4) significantly increased the yield. Significant advancements in the column method were achieved when Kingston et al. (7) demonstrated the importance of pH control for the extraction of Fe, Mn, Cu, Cd, Ni, Pb, Zn, and Co from seawater and the need to rinse major cations from the column before elution with nitric acid. Although good recoveries of metals added in microgram per liter quantities were found, significant contamination problems were still encountered (8). Incorporating these modifications (7) into a column procedure with no pretreatment of the sample, Bruland (9) found that extraction of open ocean water by Chelex-100 a t a flow rate of 4 mL min-' produced values comparable to the solvent extraction method for Cu, Ni, Cd, and Zn. Incorporating these same modifications (7) into a column procedure employing a rate of 1-2 mL m i d for the analyses of unpolluted previously acidified offshore water, Sturgeon et al. (10) also found good agreement with solvent extraction methods for Fe, Mn, Cu, Ni, Cd, and Zn but not for Pb.The success of Chelex-100 methods for the analysis of open ocean (9) and offshore samples (10) would suggest that the method of Kingston et al. (7) has eliminated many of the causes of poor performance reported earlier (3). However, the modifications of Kingston (7) have not alleviated all of the problems reported for coastal and estuarine samples (...
Reservoir sediments from Lake Roosevelt (WA, USA) that were contaminated with smelter waste discharged into the Columbia River (BC, Canada) were examined using three measures of elemental release reflecting varying degrees of physical mixing and time scales. Aqueous concentrations of Cd, Cu, Pb, and Zn in the interstitial water of reservoir sediments, in the gently stirred overlying waters of incubated sediment cores, and in supernatants of aggressively tumbled slurries of reservoir sediments generally were higher than the concentrations from a reference site. When compared to chronic water-quality criteria, all three measures of release suggest that slag-contaminated sediments near the U.S.-Canadian border are potentially toxic as a result of Cu release and Pb release in two of the three measures. All three measures of Cd release suggest potential toxicity for one site farther down the reservoir, probably contaminated as a result of transport and adsorption of Cd from smelter liquid waste. Releases of Zn and As did not appear to be potentially toxic. Carbonate geochemistry indirectly affects the potential toxicity by increasing water hardness.
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