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Browsing by Author "Rocha, C"

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  • High-resolution permeability determination and two-dimensional porewater flow in sandy sediment
    Publication . Rocha, C; Forster, S; Koning, E; Epping, E
    A new, inexpensive method is proposed to measure permeability in natural sandy sediment with high spatial resolution. This methodology allows for a reconstruction of the vertical permeability anisotropy in natural sediments, with a depth resolution of a few millimeters. Thus, the possible intrusion depth of advective flow over the water-sediment interface of sandy sediments can be deduced. Shipboard measurements on five natural sandy sediment cores taken from North Sea sediments are used to demonstrate that both the direction and magnitude of the second-order permeability tensor can be calculated from direct measurements using this method. This presents a major improvement over previous methods particularly in the context of quantifying flow and reaction in permeable sediments.
    2005-01Journal article Open access Show more
  • Rhythmic ammonium regeneration and flushing in intertidal sediments of the Sado estuary
    Publication . Rocha, C
    The effects of exposure to the atmosphere on ammonium cycling in intertidal sediments were examined at a single site in the Sado estuary (Portugal) during a tidal cycle in November 1994. During a 9-h period, covering pre-ebb to postflood, 11 high-resolution vertical profiles of dissolved and sorbed ammonia and water content were collected. In addition, the vertical distribution of temperature was measured, primarily at the beginning and the end of the exposure period. The most dramatic changes occurred at the end of the exposed period when water flooded the sediment; similar to 75% of the dissolved and sorbed NH4+ pool (44.2 mmol m(-2)) was flushed into the water column by buoyancy-driven porewater exchange. Some 64% of the flushed inventory (28.2 mmol m(-2)) was produced during the exposure period, at an average rate of 4.9 mmol NH4+ m(-2) h(-1). The build-up of ammonium in the sediment was faster in the sorbed pool than in the dissolved pool. An average first-order carbon mineralization rate of 85.2 mmol C m(-2) h(-1) during exposure was estimated from the ammonification rate. Residence times for the organic carbon and nitrogen pools were also calculated (37.5 and 43.4 d, respectively), giving indication of the rapidity of the turnover of organic matter possible in estuary intertidal sediments.
    1998-07Journal article Open access Show more