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Aquatic Geochemistry (v.9, #3)
Biogeochemical Processes in Coastal Aquifers and Permeable Sediments
by Timothy J. Shaw (pp. 165-169).
Permeability of Sands in the Coastal Areas of the Southern Baltic Sea: Mapping a Grain-size Related Sediment Property by S. Forster; B. Bobertz; B. Bohling (pp. 171-190).
We compared measurements of vertical permeability, grain size distribution, and porosity from sandy surface sediments in the southern Baltic Sea. Using this information we constructed maps reflecting the permeability within the study area. We found that the formula suggested by Krumbein and Monk (1942) overestimates measured permeabilites on average by a factor 2.6 for the area investigated. The results of a formula including porosity, as in the Carman-Kozeny relation (Carman, 1937), deviated even more from our observations. Vertical heterogeneity of the sediments between 0–10 cm depth and the content of fine particulate material were likely responsible for this phenomenon.Our findings suggest that sediments, classified by averagegrain size based nomenclature as sands, cannot generally be assumed to be highly permeable. Nevertheless, maps constructed mirror the distribution of the average grain size in two data sets we used. For the shallow southern Baltic and an assumed threshold for permeability-effects of2.5 × 10-12 m2, wecalculate >41% of the sea floor to be permeable. An additional 27% may be permeable, but were excluded from calculations due to poor sorting.
Keywords: permeability; Carman–Kozeny; Krumbein and Monk; mapping; shallow water sediment; grain size; biogeochemistry; Baltic Sea
Nutrient and Radium Fluxes from Submarine Groundwater Discharge to Port Royal Sound, South Carolina by Andrew M. Crotwell; Willard S. Moore (pp. 191-208).
Water exchange between the coastal ocean and underlying aquifers provides a newly-recognized source of materials to the ocean. The flux of materials into the ocean from this process is termed submarine groundwater discharge (SGD). Both surficial and semi-confined aquifers contribute to SGD. Here we use 226Ra and 228Ra to quantify fluxes of SGD to Port Royal Sound, South Carolina, and to separate fluxes from the Upper Floridan (UFA) and surficial aquifers. Higher activity ratios of 228/226Ra in the surficial aquifer make this separation possible. We estimate total SGD fluxes of about 100 m3 s-1 with about 80% being derived from the surficial aquifer. The SGD flux provides about1.8 × 106 mol d-1 of NH4 with almost 90% from the surficial aquifer. Because of strong differences in the concentration of PO4 within the UFA, PO4 fluxes areless certain. Using the UFA wells with low PO4 concentrations yields a flux of 1.2 × 105 mol d-1; using wells with high concentrations yields a flux of 2.0 × 105 mol d-1. In the first case virtually all of the PO4 flux is from the surficial aquifer; in the second case, 40% is from the UFA.The UFA in this region has experienced dramatic changes as a result of withdrawals for human use. Prior to these withdrawals, total nutrient fluxes from the UFA may have been even larger. These changes in the UFA and similar coastal aquifers worldwide have the potential to significantly alter a major nutrient source for the coastal ocean.
Keywords: submarine groundwater discharge; radium; nutrients; Port Royal Sound; Upper Floridan Aquifer
Seasonal Dynamics in Dissolved Organic Carbon Concentrations in a Coastal Water-Table Aquifer at the Forest-Marsh Interface by Miguel A. Goñi; I. Robert Gardner (pp. 209-232).
The seasonal dynamics of dissolved organic carbon (DOC) in a subterranean estuary were examined in a coastal water-table aquifer extending across a forest-marsh interface into an adjacent tidal creek that leads to North Inlet (SC). The aquifer is characterized by groundwater flow from the forest recharge area towards the creek. DOC concentrations range from 50 to 140 mg L-1 in the shallow portions of the aquifer below the forest and undergo seasonal changes that are inversely related to temperature and precipitation conditions. Markedly lower DOC concentrations (<10 mg L-1) in the deep portion of the aquifer are consistent with the loss of a large fraction of the original DOC along the groundwater flow paths. Mass balance estimates indicate that over 60% of the DOC losses are due to sorption reactions whereas the rest appear to be caused by heterotrophic decay. Groundwater DOC discharge from the forest, which occurs in a restricted zone of the high marsh, is 5.5 mg carbon m-2 d-1 and accounts for a minor component of the annual carbon export from North Inlet. In contrast, moderately saline (2–12 ppt) ground waters below the marsh display elevated DOC concentrations (∼20 mg L-1) that appear to be the result of mixing of fresh ground waters and surface seawater during tidal seepage and concentration during evapotranspiration. The flux of DOC associated with the discharge of these saline ground waters is 600 mg carbon m-2 d-1, which represents a significant fraction of the annual DOC budget for North Inlet.
Keywords: dissolved organic carbon; coastal aquifer; subterranean estuary; ground water
The Mobility of Rare Earth Elements and Redox Sensitive Elements in the Groundwater/Seawater Mixing Zone of a Shallow Coastal Aquifer by Thomas Duncan; Timothy J. Shaw (pp. 233-255).
The concentrations of Rare Earth Elements (REE) and Redox Sensitive Elements (RSE) were measured in groundwaters along a transect of the forest-marsh interface of a surficial aquifer system in North Inlet, SC. The well transect extended from a forest recharge area across the marsh and tidal creek to a tidal recharge area of beach ridge. The concentrations of the RSE (Fe, Mn, and U) were consistent with reducing conditions through the transect. Fe was present at concentrations ranging from a few micromolar to greater than one hundred micromolar in most wells. U was depleted with respect to salinity predicted concentrations, indicating removal with respect to the seawater endmember. Dissolved Mn concentrations were generally low in all wells, indicating no significant solid source of Mn (as MnOx) in this system. When extrapolated to a global scale, estimates of U removal during seawater exchange with the aquifer solids equaled 10–20% of the total riverine dissolved U input flux. REE concentrations in the forest recharge area were high in shallow wells, and showed a light enriched fractionation pattern, characteristic of soil leaching by Natural Organic Matter (NOM) rich waters. A decrease in REE concentration with depth in the forest wells coupled with a trend towards Heavy REE (HREE) enriched fractionation pattern indicated removal of the REE coincident with NOM and Dissolved Organic Carbon (DOC) removal. The saline waters of the beach ridge wells show a Light REE (LREE) enriched fractionation pattern and have the highest overall concentrations of the REE, indicating a significant REE source to the seawater endmember waters. The concentration gradients along the beach ridge flow path indicate a large source in the deep wells, and net export of dissolved REE to the tidal creek system and the coastal ocean. Ultrafiltration experiments indicate a transition from a colloidal dominated reservoir for the REE in the forest wells to a colloidal and dissolved reservoir in the beach ridge wells. The ultrafiltration data coupled with a correlation with Dissolved Inorganic Carbon (DIC) release suggest that there is diagenetic mobilization of an REE rich organic carbon phase in the saline endmember wells. We suggest here that degradation of this relic terrestrial organic carbon and REE rich phase results in the export of dissolved REE equal to or exceeding river inputs in this region.
Redox Chemistry in the Root Zone of a Salt Marsh Sediment in the Tagus Estuary, Portugal by Bjørn Sundby; Carlos Vale; Miguel Caetano; George W. Luther III (pp. 257-271).
Measurements of O2, Fe(II), Mn(II)and HS5 in salt marshsediments in the Tagus Estuary, Portugal, made with a voltammetric microelectrode, reveal strong seasonal differences in pore water composition within the 20~cm deep root zone. In spring, oxygen was below detection limit except close to the sediment surface. Fe(II) was present below 5 cm in concentrations ranging from detection limit to 1700 μM. In summer, oxygen was present in the pore water almost to the bottom of the root zone in concentrations ranging from detection limit to more than 100 μM. The spatial variability was intense: O2 concentrations as high as 78 μM and as low as 25 μM existed within 2~mm of each other. Fe(II) was below detection limit except towards the bottom of the root zone. In late fall, oxygen was found to 8 cm depth, but in concentrations lower than in summer, and Fe(II) was present below 9 cm. Mn(II) was found at levels declining from typical values of 200 μM in spring to less than 20 μM in late fall. With one exception, sulfide was below the detection limit in all measurements. During periods when dissolved Fe(II) is available in the pore water at the same time as 2 is delivered by roots, iron-rich concretions can form on roots. These conditions, which lead to precipitation of iron oxide in the sediment adjacent to roots, exist in spring, when new roots infiltrate anoxic Fe(II) containing sediment. They do not exist in summer, when dissolved Fe(II) is unavailable, or in winter, when oxygen is unavailable. The seasonal redox pattern revealed by the pore water chemistry is driven by the annual cycle of growth and decay of roots.