In situ Pore Water Sampler: RISS
The chemical composition of marine pore water as well as groundwater on land holds detailed information about the ecological and geochemical status of sediments and soils. Oxygen depletion, formation, fate and release of methane, as well as turnover of nutrients and trace elements can be quantified by pore water analysis. Consequently, pore water and groundwater sampling is an important issue for environmental investigations in the marine and terrestrial environment.
Several, techniques exist to sample pore water in aquatic systems. Onboard ship or in the lab, ex situ pore water extraction methods like squeezing sliced sediment by pressure are applied. Ex situ techniques are very suitable for several research objectives.
In cases, where degassing of methane from sediments, oxygen contamination during pore water extraction, high resolution sampling, or combined benthic chamber and pore water studies are of major relevance, in situ pore water analyses are superior.
Schematic diagram of a Rhizon (length 5 and 10 cm, respectively, outer diameter 2.5 mm, dead volume 0.5 mL, pore size 0.1 um). Rhizon are applied for porewater sampling (B) on board ship and in the lab, (C) for long-term incubation experiments, and (D) for in situ measurements.
A Rhizon In Situ Sampler (RISS) was developed for in situ pore water sampling from coastal as well as deep sea sediments.
Rhizones: Hydrophilic porous polymer, pore diameter of 0.1 µm. The fluid sampled from the sediment flows into the tiny space between the porous tube and the supporting wire.
The key parts of our sampling method are Rhizons (Rhizosphere Research Products, NL-6706 Wageningen). They are made of a hydrophilic porous polymer tube, with a typical pore diameter of 0.1 um, extended with a polyvinyl chloride tube. The outer diameter of a Rhizon is 2.4 mm, and the filter section has a length of 5 or 10 cm.
To support the polymer, a wire is fixed to one end of the porous polymer. The wire is made of either stainless steel, carbon fiber or nylon, the latter in case pore water is used for metal measurements. The fluid sampled from the sediment flows into the tiny space between the porous tube and the supporting wire.
A Rhizon has several advantages compared to other sampling devices: low mechanical disturbance of the sediment due to small diameter (2.4 mm), low dead volume (0.5 mL including standard tubing), minimized sorption processes on the inert polymer, no aging during long-term deployments. The pore size (mean 0.1 um, max. 0.2 um) ensures the extraction of microbial- and colloidal-free, ready-to-analyze solution.
One of the advantages of the RISS system is the sampling of pore water from gassy, methane-rich sediments. We applied this system at the Haakon Mosby Mud Volcano, for example. The device was deployed by the ROV Victor (IFREMER).
Examples for the application of the RISS
Comparison of pore water profiles sampled Rhizons with low pressure squeezing. The sediment samples were gathered in the Southern Ocean by a Multicorer. A good correspondence between both techniques was observed.
Silicic acid profiles from three stations in the Atlantic sector of the Southern Ocean: (A) PS65/600-2, (B) PS65/701-2, and (C) PS65/594-3. At each site, at least two cores from the Multicorer system were processed either by low pressure squeezing or with Rhizons for porewater sampling.
Combined sampling system based on a benthic chamber and a Rhizon pore water sampler. This device allows to determine benthic fluxes by sediment incubation and pore water sampling at the same location.
Schematic diagram of a RISS combined with a benthic flux chamber. The platform made of poly carbonate (thickness 0.6 cm) can be pushed into the sediment with minimum disturbance, and the Rhizons are then moved horizontally into the sediment as well as back into the protective grooves by nylon cords. Porewater profiles can be determined underneath the benthic chamber, with negligible effect for the benthic flux measurement.
Date derived by the combine benthic chamber – RISS sampling system
Change of chloride and silicic acid over time in a benthic chamber deployment on a tidal flat near Sahlenburg/Cuxhaven Germany. (B) Two silica and two chloride profiles taken with a Rhizon in situ sampler beneath the benthic chamber at the (I) beginning of chamber deployment and (II) end of chamber deployment.
Application of the RISS sampling system deployed by the ROV Victor at the Haakon Mosby Mud Volcano.
Literature
Seeberg-Elverfeldt, J., Schlüter, M., Feseker, T., Kölling, M. Rhizon sampling of porewaters near the sediment-water interface of aquatic systems. Limnol. Oceanogr.: Methods 3, 2005, 361–371.