Abstract:
A pore-water profiler and method for sampling pore water. The pore-water profiler includes a sample intake probe that receives the fluid to be sampled. A clog-resistant first filter filters the fluid as it enters the sample intake probe. A second filter, which has a pore size less than the pore size of the first filter, filters the fluid a second time before the fluid enters a sample container. A sample triggering system connected to the sample container initiates sampling by causing the fluid to be drawn into the sample intake probe. The profiler provides high-resolution (centimeter-scale) vertical pore-water profiles. The sequential filtration of the pore water avoids the problem of sample-circuit clogging, even in sediments dominated by fine or organic-rich particles. The profiler has all non-metallic, acid-washable components that contact the fluid sample, making the profiler suitable for trace-inorganic studies.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for government purposes without the payment of any royalties therefore. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention relates in general to a fluid sampler and, more particularly, to an in situ, clog-resistant pore-water sampler for use in collecting water samples to determine depth profiles of chemical constituents across a sediment-water interface. 
     2. Description of the Related Art 
     Fluid sampling may be used to monitor environmental changes in water, air, or other desired fluids, and to monitor water quality in ground water and surface water. For example, it is desirable to estimate the potential importance of solute flux from the benthos in aquatic systems where long-term (decadal) sediment accumulation of nutrients or toxic substances are of concern. More information regarding the determination of benthic flux in aquatic systems is found in J. S. Kuwabara et al., “Quantifying the Benthic Source of Nutrients to the Water Column of Upper Klamath Lake, Oreg.,”  U.S. Geological Survey Open File Report  2007-1276, 39 pp., 2007, incorporated herein by reference. 
     Conventional techniques used to sample pore water to quantify the benthic flux of biologically reactive solutes across the sediment-water interface are labor, equipment, and resources demanding. Also, where sediment is dominated by fines (less than 63-micron particles), particularly detrital fines with high-organic content (e.g., in eutrophic, lentic environments), conventional samplers can quickly clog to yield inadequate sample volumes. 
     Therefore, there is a need for a simple, inexpensive, reliable, remote sampling device for use in obtaining test samples of a fluid medium for major and trace solutes from remote sites that will not clog when obtaining samples from fine, organic-rich sediments. 
     BRIEF SUMMARY OF THE INVENTION 
     Water-quality managers and modelers often require measurements of benthic flux to comprehensively and accurately represent the transport of toxic and nutritive substances in surface waters. The pore-water profiler described below is a cost-effective field device to respond to such requirements for a wide range of environmentally significant solutes that are transported across the sediment-water interface. 
     The non-metallic pore-water profiler described herein provides pore-water samples near the sediment-water interface to produce high-resolution (centimeter-scale) vertical concentration profiles for trace solutes, even when the bed material is dominated by fine or organic-rich particles that tend to clog filters and screens. The pore-water profiler is suitable for investigations involving a wide variety of biologically reactive solutes (e.g., micronutrients, macronutrients, and toxic trace metals), some requiring ultra-clean sampling protocols. Concentration profiles can then be used to determine a diffusive flux of solute across the sediment-water interface. Particularly in lentic systems, this benthic flux has been demonstrated to be a significant if not dominant source of biologically reactive solute to the water column. In addition, samples collected by the pore-water profiler have been found to be suitable for analysis of other solutes of environmental interest (e.g., dissolved organic carbon). 
     The pore-water profiler is deployed and triggered to collect filtered pore water from variable depths above and below the sediment-water interface. The profiler collects the filtered pore water through a series of sintered porous polyethylene probes and in-line filters to avoid the problem of sample syringes that become plugged with fine sediments. Unlike conventional samplers, the pore-water profiler is suitable for trace-solute analyses because all wetted surfaces are acid-washable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which are not necessarily drawn to scale. 
         FIG. 1  is a perspective view illustrating a pore-water profiler according to an embodiment of the invention; 
         FIG. 2  is an elevational view of a sampling circuit according to the embodiment of the invention; 
         FIG. 3  is an enlarged view of portion A of  FIG. 2 ; 
         FIG. 4  is an exploded view of portion A of  FIG. 2 ; 
         FIG. 5  is a top view of a lower plate; 
         FIG. 6  is an enlarged view of portion B of  FIG. 2 ; 
         FIG. 7A  is a perspective view of a bracket; 
         FIG. 7B  is a top view of the bracket shown in  FIG. 7A ; 
         FIG. 7C  is a front view of the bracket shown in  FIG. 7A ; 
         FIG. 7D  is a side view of the bracket shown in  FIG. 7A ; 
         FIG. 8A  is a top view of a disk; 
         FIG. 8B  is a side view of the disk shown in  FIG. 8A ; 
         FIG. 9A  is a bottom view of an upper plate; 
         FIG. 9B  is a top view of a retaining nut; and 
         FIG. 9C  is a side view of the retaining nut shown in  FIG. 9B . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An exemplary embodiment of a pore-water profiler  10  is illustrated in  FIG. 1 . The pore-water profiler  10  (also referred to as a “fluid sampler”) is a non-metallic field-sampling device with one or more towers  12 , each tower  12  (also referred to as a “sampling circuit housing”) housing a portion of a sampling circuit  14  (see  FIG. 2 ). Each sampling circuit  14  is exposed to the water column and collects pore water from a defined depth relative to the sediment-water interface. All sample-wetted parts are acid-washable and hence compatible with studies involving trace-inorganic solutes. 
     In the embodiment shown in  FIG. 1 , the pore-water profiler  10  includes six independent sampling circuits  14 .  FIG. 2  illustrates only one of the sampling circuits  14  for clarity. The other sampling circuits are similar. Each sampling circuit  14  includes a probe  16  (also referred to as a “sample intake probe”) that is connected to a lower plate  18  through an opening  19 . The lower plate  18  is preferably made of polyvinyl chloride (PVC). An enlarged view of the probe  16  is shown in  FIG. 3 . An exploded view of the probe  16  is shown in  FIG. 4 . The probe  16  includes a probe stem  20 , an inner probe fitting  22 , an outer probe fitting (also referred to as a “tip”)  24 , and a filter  26 . The filter  26  is preferably a sintered porous polyethylene ring  26 . 
     The probe stem  20  has external threads to connect to the lower plate  18  and internal threads to receive the inner probe fitting  22 . The probe stem  20  is preferably made of polypropylene. The tip  24  is preferably made of polyvinylidene difluoride (PVDF) and is conical to facilitate sediment penetration, but may be flattened or rounded. The sintered porous polyethylene ring  26  is slipped over the threaded upper perimeter of the tip  24 , and the tip  24  is attached to the bottom of the inner probe fitting  22 . The sintered porous polyethylene ring  26  is preferably made from a porous, 30-micron polyethylene pipe. Pore water enters the probe  16  through the sintered porous polyethylene ring  26 , which filters the pore water. The length of each probe stem  20  may be varied to place each sintered porous polyethylene ring  26  at a desired sediment depth. 
     The inner probe fitting  22  is preferably made from a 25-mm PVDF rod. The inner probe fitting  22  is both threaded and bored to create a channel  27  to transfer pore water from the sintered porous polyethylene ring  26  to an acid-washed tube  28 . The inner probe fitting  22  also has a groove  29  cut circumferentially to receive the pore water from the sintered porous polyethylene ring  26 . The tube  28  transfers the pore water to an in-line filter  30  (see  FIG. 2 ). The tube  28  is connected to the inner probe fitting  22  and the in-line filter  30  using commercially available barbed fittings  110  and  112 , respectively, which are preferably made of PVDF. The tube  28  may be made of, for example, Viton® or C-Flex® tubing. 
     Referring back to  FIG. 2 , the in-line filter  30  is preferably a 0.2-micron filter and has a membrane composition that is selected based upon the solutes of interest. For example, hydrophilic PVDF (polyvinylidene fluoride) membranes are acid-washable and may be useful for trace-inorganic solutes. Filtrate from the in-line filter  30  then passes through a commercially available plastic valve  32  into a sample container  34 . In the embodiment shown in  FIG. 2 , the sample container  34  is a commercially available all plastic 60-mL syringe  34 . The volume held by each syringe  34  can be selected based on the intended application. For example, a typical commercially available all plastic syringe can hold a volume of up to 60 mL. Alternatively, different-sized syringes may be used with the pore-water profiler  10 . 
     Referring to  FIGS. 1 and 2 , each of the sampling circuits  14  is supported above the lower plate  18  by one of the towers  12 . In the embodiment shown in  FIGS. 1 and 2 , each tower  12  includes an upper tower member  36 , a lower tower member  38 , a tower union  40 , a lower tower member stop  42 , and a tower base  44 . The upper tower member  36 , the lower tower member  38 , and the tower union  40  are preferably made from a transparent PVC pipe to aid in viewing assembly of the pore-water profiler  10  and subsequent sample retrieval. For example, the upper tower member  36  and the lower tower member  38  may be made from 2-inch PVC pipe, and the tower union  40  may be made from a 2-inch PVC pipe coupling. The tower base  44  and the lower tower member stop  42  are preferably made from PVC. After the upper tower member  36 , the lower tower member  38 , and the tower union  40  are joined together (e.g., by adhesively affixing the tower union  40  to the upper tower member  36  and the lower tower member  38 ), a portion is cut out or removed to create a vertical slot  46  that facilitates access to and assembly of the sampling circuits  14  and sample retrieval. The lower tower member  38  rests in the lower tower member stop  42 , and the lower tower member stop  42  is attached to the lower plate  18  through the tower base  44 , which is attached to the lower plate  18 . The tower bases  44  may be attached to the lower plate  18 , for example, by being affixed to grooves  130 , as shown in the embodiment of the top side of the lower plate  18  in  FIG. 5 . The grooves  130  form seats for attaching (e.g., by cementing) the tower bases  44  to the lower plate  18 . 
     An enlarged view of the syringe  34  within the tower  12  is shown in  FIG. 6 . The syringe  34  includes a flange  150 , a plunger  152 , and a syringe tip  154 . Within each tower  12 , slots  156  are provided to slide the syringe flange  150  in place with the syringe tip  154  and the valve  32  facing in a downward direction. A bracket  48  is slid onto the top of the syringe plunger  152 . The bracket  48  is preferably made of PVC and is illustrated in more detail in  FIGS. 7A to 7D . Referring to  FIGS. 2 and 7C , a spacer rod  50  presses against the bracket  48  to hold the syringe plunger  152  in position until the pore-water profiler  10  is triggered. The spacer rod  50  is preferably made of PVC. Elastomeric rings  52  are slipped into a central hole  160  of the bracket  48  and are connected to the top of the tower  12  to generate tension on the syringe plunger  152 . In the embodiment shown in  FIG. 2 , the elastomeric rings are rubber o-rings  52 , which are connected to the top of the tower  12  using a hook  54  that hangs from the top of the upper tower member  36 . The hooks  54  are preferably made of PVC. 
     Referring back to  FIG. 1 , the pore-water profiler  10  is assembled from the lower plate  18  upward before being deployed. To aid assembly, the lower plate  18  is supported at four corners by legs  56  that attach to the lower plate  18 . The legs  56  may optionally be removed to facilitate sampling. The legs  56  are preferably made of PVC and are threaded to attach to the lower plate  18  through openings  58 . The lower tower member stops  42  are attached to the tower bases  44  on the lower plate  18 , and the lower tower member  38 , which has been affixed to the tower union  40  and the upper tower member  36 , is placed in the lower tower member stop  42 . Then, the probes  16  are attached to the under side of the lower plate  18  through the openings  19 . 
     Referring to  FIG. 2 , for each tower  12 , one end of the tube  28  is fed through the probe  16 , and the other end is fed through an opening  62  in the tower base  44 , into the lower tower member  38 , to the in-line filter  30 . The acid-washed syringe  34 , filled with 10 Mohm double-dionized water, is emptied through the sampling circuit  14  for a final rinse of all acid-washed, wetted parts. The syringe  34  is secured in the tower slots  156  as described above and the bracket  48  with the o-rings  52  is slid onto the syringe plunger  152 . The lower end of the spacer rod  50  is placed against the bracket  48  and the upper end abuts the top  64  of the vertical cutout  46  to secure the position of the syringe plunger  152  before the pore-water profiler  10  is triggered. 
     The o-rings  52  are extended from the bracket  48  onto the hooks  54  hanging off the walls of the upper tower members  36 . Tension from the o-rings  52  pulls on the syringe plungers  152  to collect the samples after the pore-water profiler  10  is triggered. The end of each spacer rod  50  touching the bracket  48  is connected to a central hub  66  that slides along (i.e., up and down) a central pipe  68  (also referred to as a “central support”). In the embodiment shown in  FIGS. 1 and 2 , the central hub  66  is a disk  66 , and the spacer rod  50  is tied to the disk using a line  69 , preferably a ⅛-inch nylon cord. The central disk  66  is illustrated in more detail in  FIGS. 8A and 8B . In the embodiment of the central disk  66  shown in  FIGS. 8A and 8B , the ends of the lines  69  are knotted to secure each into one of six vertical holes  170 . The lines  69  are further secured in the central disk  66  with an o-ring  70  placed around the central disk  66 . The central disk  66  is preferably made of PVC. The central pipe  68  is preferably made of PVC and is threaded to attach to the lower plate  18 , as shown in  FIG. 2 . 
     As shown in  FIGS. 1 and 2 , an upper plate  71  is placed on the tops of the upper tower members  36  after all of the sampling circuits  14  have been assembled. In the embodiment of the upper plate  71  shown in  FIG. 9A , the under side of the upper plate  71  has grooves  180  to receive the upper tower members  36 . Referring to  FIGS. 1 and 2 , the upper end of the central pipe  68  is fed through an opening  72  in the upper plate  71 . The upper tower members  36  and the upper plate  71  are secured with a fastener  74 . In the embodiment shown in  FIGS. 1 and 2 , the fastener  74  is a retaining nut  74 . The retaining nut  74  is preferably made of PVC and is shown in more detail in  FIGS. 9B and 9C . Plastic-coated weights  76  are secured to the pore-water profiler  10  (e.g., by being tied onto the tower bases  44 ) to control buoyancy. 
     In addition to the line  69  that connects the spacer rod  50  to the central disk  66 , several additional lines are used to deploy the pore-water profiler  10 , as shown in  FIGS. 1 and 2 . A first line  78  is attached to the central disk  66  through a hole  172  (see  FIG. 8A ), fed through an opening  80  in the upper plate  71 , and connected to a top ring  82 . A trigger line (not shown) is fed through the top ring  82  so that when quickly tugged, all the spacer rods  50  are dislodged and sample collection in all of the sampling circuits  14  begins. Alternatively, the pore-water profiler  10  may be adapted so that other forms of energy may be used to trigger the pore-water profiler  10 . For example, a plastic or plastic-coated spring may be used rather than the o-rings  52 . A buoy line  86  connects to the retaining nut  74  on the upper plate  71  through a hole  182  (see  FIGS. 9B and 9C ) and to a buoy (not shown) to keep the pore-water profiler  10  vertical while deployed. A tether line  88  is fed through the central pipe  68  and secured to the lower plate  18  through an opening  90  in the lower plate  18 . The pore-water profiler  10  is lowered onto the bottom sediment using the tether line  88 , after which the trigger line is tugged and pulled out of the water. The tether line  88  may be secured to another buoy (not shown) to facilitate retrieval of the pore-water profiler  10 . 
     Upon retrieval of the pore-water profiler  10 , each of the valves  32  is shut, and the sample syringes  34  are removed from the towers  12  and doubled bagged in argon-filled, plastic, zipped bags. Argon-filled bags containing the filtered samples are then transported to the laboratory in refrigerated coolers for chemical analyses. 
     In one embodiment of the invention, in addition to sampling the water just above (approximately 1 centimeter) the sediment-water interface, the pore-water profiler  10  was used to collect interstitial water from five depths within the top 10 centimeters of a lakebed, with the length of the probe stems  20  being varied to place the sintered porous polyethylene rings  26  at 1.0, 2.0, 3.3, 5.5, and 10.0 cm, to characterize dissolved solute vertical gradients (that is, six independent sampling circuits  14 ). Each sampling circuit  14  collected filtered (0.2 micron) water into acid-washed 60 mL syringes  34 . After being lowered onto the lakebed, the pore-water profiler  10  was tripped mechanically to begin sample collection and retrieved approximately 24 hours later. Dye experiments indicated that this extended sampling period avoided short circuiting of samples between depths and along pore-water profiler  10  surfaces. After retrieval, the valves  32  for the sample syringes  34  were closed, placed in argon-filled bags, and refrigerated in darkness for subsequent chemical analyses. 
     The pore-water profiler described herein provides numerous benefits. For example, in a single deployment, the profiler collects multiple pore-water samples from different depths to generate a high-resolution (centimeter-scale) vertical concentration profile, minimizing relative errors between depths. This permits the determination of diffusive-flux measurements for solutes. 
     The sequential filtration of pore water avoids the problem of sample-circuit clogging, even in sediments dominated by fine or organic-rich particles that would typically plug sample-collection ports of conventional samplers. 
     The pore-water profiler is non-metallic with wetted parts (i.e., parts exposed to the sample) that are all acid-washable. No electronic or motorized parts are used to avoid sample contamination by exposed metal parts. As all parts exposed to the sample are acid-washable, the pore-water profiler is suitable for trace-inorganic studies requiring ultra-clean (sub-micromolar) sampling techniques. In addition, samples collected by the pore-water profiler have been found to be suitable for analysis of other solutes of environmental interest (e.g., dissolved macronutrients and organic carbon). 
     The pore-water profiler collects the pore-water sample directly (in situ) without the need for an ultra-clean field laboratory to process the samples. The filtered sample retrieved from each sampling circuit can be transported directly from the field for chemical analysis, which minimizes the potential for field contamination of samples and minimizes sample-storage time where particulate matter may alter the speciation or partitioning (i.e., the chemical forms) of the analytes. The direct sampling afforded by the pore-water profiler permits greater spatial and temporal coverage (i.e., less field time and effort is required to collect the sample). No incubation experiments are needed. Facilities are often not available, or too resource-intensive to construct, to perform core extrusion studies. The pore-water profiler offers a convenient and cost-effective alternative. 
     Benthic-flux measurements have been determined by various methods, but are consistently resource intensive. Because all of the parts can be machined from commercially available stock or purchased directly, the pore-water profiler can cost-efficiently provide spatial and temporal coverage of a wide range of aquatic systems where contaminants may accumulate and remobilize in, and transport from, benthic sediments. 
     The pore-water profiler described herein, is easily assembled, disassembled, transported, and deployed. Because the pore-water profiler does not require the use of any heavy equipment (e.g., a high-speed centrifuge, pump, motor, etc.), the pore-water profiler is a stand alone device that can be carried by one person for field deployment to determine pore-water concentration gradients. 
     Although the invention has been described relative to a specific embodiment thereof, it is not so limited and many modifications and variations thereof will be readily apparent to those skilled in the art in light of the above teachings. For example, the lengths of the probe stems  20  can be modified (lengthened) so that the profiler can sample the water column of a lake or estuary. Such water-column profiles have been used to estimate benthic flux by eddy diffusivity. Similar stem-length modifications can be employed to sample the air column, using a suspended profiler. 
     These and other variations and modifications of the illustrated embodiment will become readily apparent to those skilled in the art in light of the above teachings without departing from the spirit and scope of the present invention. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.