Mr. James L. Falter and Dr. Francis J. Sansons of the Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa., 20000914, Pore water studies reef flat sediments, Kaneohe Bay, Oahu, HI (NODC Accession 0000271): University of Hawaii at Manoa, Honolulu, HI.
Mr. James L. Falter and Dr. Francis J. Sansone
The purpose of this study was to examine the behavior of the water composition within these permeable sediments and their relationship to sediment redox structure under varying physical condtions.
Well points were fabricated from 1/8 in. diameter schedule 80 PVC pipe and used to sample pore water at depths of 7,15,25,35,50, and 70 cm in the reef. The well points were driven into the reef framework with the aid of a stainless steel installion tool. A set of wells for sampling each of these depths defined an array. The arrangement of wells within a single array was designed to minimize the degree of lateral separation of any two wells within an array while preventing the pore water volume extracted from one well point from overlapping or interfering with the volume extracted from an adjacent well. The sampling volume was estimated to be a sphere ~6 cm in radius, based upond a total extraction volume of ~250 ml per sampling and an estimated sediment porosity of ~0.3. The porosity of modern aragonitic sands is typically ~0.45; however, a more conservative estimate of porosity is used for Checker Reef sediments due to the presence of the larger pieces of broken coral rubble.
Pore water samples were extracted from the sediment using a battery-powered perstaltic pump. An amount in excess of the dead volume in the tubing that connected the well point to the pump (~270ml) was collected and discarded before taking each 250 ml pore water sample to ensure a pure pore water sample. The mixing of pore water during its passage through the tubing acted to homogenize the samples; however, samples collected from individual well points on and after 1 March 1997 were homogenized in a glass flask free of any head space to ensure uniformity between subsamples intended for different chemical analyses. There did not appear to be any significant change in the relative distribution of pore water species once this deliberate homogenization step was employed. An additional experiment was performed to determine how much pore water composition would change with the volume of water extracted. Four 60 ml pore water samples were sequentially taken from 7-, 25-, and 50-cm well points at each station using 60 ml syringes attached directly to the well points (without using the pump and tubing). These samples showed that nitrate and ammonium concentrations did not change significantly with the amount of pore water extracted, even for the 7-cm well points, suggesting that there was no enhanced movement of pore water along the sides of the well points.
Two arrays ~1.5 m apart were installed at varying distances from the forereef (Stations S and C) and sampled over a 9-month period. At no point was there a freshwater flow into the bay large enouch to effect surface salinities over the reef. Samples for the determination of dissolved nitrate, nitrite, and ammonium were filtered through glass fiber filters and analysed using standard colorometric techniques. Dissolved oxygen was measured in the field using an Orion model 820 dissolved oxygen meter equipped with a Clark-type electrode. Pore water nitrous oxide concentrations were measured following the method of Pierotti and Rasmussen (1980) with only minor modifications.
Sediment hydraulic conductivity was estimated for each of the well points using a falling-head permeameter. These estimates assumed that the sediments were homogeneous and extended infinitely around a given well point. These assumptions were made to simplify the required calculations; however, the sensitivity of the calculations to these assumptions at distances greater than 7 cm away from the well point was very small ( less than 5%).
#INSTRUMENT TYPES: Wells: 1/8 in. diameter schedule 80 PVC pipe
Water extraction: battery-powered perstaltic pump and 60 ml syringes
Dissolved oxygen was measured in the field using an Orion model 820 dissolved oxygen meter equipped with a Clark-type electrode.
See originator data files and methodolgy description.
See originator data files and methodolgy description.
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|Data format:||Original file received by NODC via FTP in file, appx.xls (Microsoft EXCEL). Redundant ASCII text files were made of each individual sheet within the spreadsheet file at the NODC for archival purposes. Original files are: sheetx.prn, where x varies from 1 to 8. The format is straight-forward with well-defined columns. Rows are clearly marked by date and depth. Dataset size: 91 Kbytes. Number of data units: 2 stations, 4 arrays. in format Microsoft XLS Not applicable|
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