Framework

Seepage Meter Operation

In terms of the actual operation of the seepage meter, the following procedures and practices are suggested:

  1. place the chamber on the bed of the surface water body open-end down and turn slowly (about 1 cm/s) into the sediment until the top is about 2 cm above the sediment surface. The top of the chamber should not stick up too much out of the bed because of upward advection of interstitial water (Bernoulli effect) caused by such positive relief in environments with waves, tides or currents. This process was interpreted to account for anomalous inflows into meters installed in a shallow marine and reef setting (Shinn et al, 2002). Semi-analytical analysis suggests that a seepage chamber set at a depth that is the same as the chamber radius, will collect more than 90% of the ambient flow, assuming efficient design of the bag and tubing (Murdoch and Kelly, 2003). The chamber should be installed as deep as possible to limit the ingress of shallow throughflow or recirculated surface water. However, avoid placing the chamber too deep into the sediment so that the lid is directly on the sediment bed. Also avoid pushing the chamber too rapidly into the sediment as this can cause blowouts that become preferred pathways for water flow (Lee, 1977). In reality the depth of installation is largely predicated on the competence of the sediment, and the need to not excessively disturb the sediment profile. The chamber should be tilted slightly so that the vent hole is relatively elevated, as this allows any entrapped gas to escape freely;
  2. minimise the activity around the meter during installation and operation. By monitoring the pressure within a chamber using a transducer, field studies have shown that walking past or stepping near the meter can effect hydraulic pressure and cause artificial inflows into the chamber (Rosenberry and Morin, 2004). Subsequent measurements of seepage in areas of the sediment bed disturbed by previous installations or by repeated foot traffic can return larger seepage rates. This has been attributed to the disturbance of a thin, lower-permeability sediment veneer (Rosenberry and Morin, 2004);
  3. allow sufficient time between initial installation of the chamber and the commencement of measurements so that hydraulic pressures inside the chamber equilibrate with those of the surface water body. Laboratory tests suggest that 80% of this equilibration occurs in the first 10 minutes (Cherkauer and McBride, 1988; Cable et al, 1997) and investigators have used stabilisation times ranging from 10-15 minutes (Landon et al, 2001) to 2-5 days (Shaw and Prepas, 1989; 1990);
  4. the end of the vent tube can be fixed into position on the bank of the surface water feature using a stake or small star picket. This can be flagged to become a useful marker for the location of the seepage meter during its operation;
  5. pre-fill the collection bag with a known volume of water before attaching the bag to the meter. Plastic bags have an inherent tendency to expand slightly during operation of the meter, inducing a head loss. This causes an anomalous short-term influx of water into the bag after being attached to the chamber (Shaw and Prepas, 1989; Blanchfield and Ridgeway, 1996). This error was effectively eliminated in field trials when the bag was filled with 1000mL of water prior to attachment;
  6. before attaching the tubing (and bag) to the chamber ensure that the water in the bag is in hydraulic equilibrium with the surface water body. This is done by slowly lowering the bag into the water with the valve open and the chamber-end of the tubing above the water surface. This will expel any air within the bag through the tubing, however be careful not to lose any water from the bag. When this is completed, turn the valve closed;
  7. attach the tubing to the chamber via the hose fitting. Before attachment, remove any air within the tubing between the hose fitting and the valve. This is done by submerging the bag and tubing, with the hose fitting directed upwards to allow air bubbles in the tubing to escape. Place the bag inside its protective cover and avoid folding, creasing or kinking the bag as these can result in anomalous and erratic head differences (Murdoch and Kelly, 2003). Weights such as concrete blocks can be placed on both the chamber and the protective cover to prevent any lateral movement due to high stream flow;
  8. with the bag attached and inside its protective cover, the meter is ready for operation. Seepage measurement commences when the valve is opened - make sure that you record the time that this was done;
  9. a control bag can be used to quantify the effects of factors such as waves, wind or currents or the properties of the bag itself (Sebestyen and Schneider, 2004). This is a pre-filled bag and tubing identical to that used in the meter that is submerged and tethered (with valve closed) about 0.15m above the sediment bed at the same time as the meter is installed and operated. The bag is positioned near the meter but not attached to the meter. Any changes in the water volume within the control bag reflects the magnitude of these effects. Field studies in the nearshore coastal environment have also used complete control meters set up in sand-filled plastic swimming pools on the bed, specifically to measure such measurement artefacts (Cable et al, 1997);
  10. after a period of time the seepage measurement is ended by returning to the meter, turning the valve closed and recording the time that this was done. The duration of the test is based on the local seepage regime and can vary from less than an hour to several days, so a trial and error approach is required. The change in water volume in the bag should exceed 50ml. Avoid letting the bag fill close to its maximum capacity due to significantly increased head losses, confirmed by laboratory tests (Murdoch and Kelly, 2003). Likewise, avoid completely draining the contents of the bag in the situation of high negative seepage;
  11. remove the tubing from the chamber via the hose fitting and measure the volume of water in the bag. This can simply be done with a measuring cylinder. An alternative approach is to weigh the pre-test and post-test bag, to define the change in water volume (assuming a density of 1, or alternatively correct for temperature effects). Use Equation A1.1 to derive the seepage rate;
  12. investigators have incorporated a meter correction factor to the calculation of seepage rates, taking account of the measurement artefacts due to frictional resistance and head losses within the meter. Laboratory testing indicated a ratio of measured to actual seepage of 0.77 (Belanger and Montgomery, 1992). For negative fluxes involving movement of surface water into the aquifer, correction factors have ranged between 1.11 and 1.74 (Rosenberry and Morin, 2004). Such correction factors would be unique to a particular seepage meter and would require calibration in a laboratory flume; and
  13. the meter should be routinely inspected and cleaned. Potential problems include leaky fittings, perforations or split seams in the bag, plugging of tubing by algal growth and kinks in the tubing.

References

Belanger TV, Walker RB, 1990. Ground water seepage in the Indian River Lagoon, Florida. In: Tropical Hydrology and Caribbean Water Resources: Proceedings of the International Symposium on Tropical Hydrology and Fourth Caribbean Islands Water Resources Congress. American Water Resources Association. 367-375.

Blanchfield PJ, Ridgway MS, 1996. Use of seepage meters to measure groundwater flow at brook trout redds. Transactions of the American Fisheries Society 125:613-818.

Cable JE, Burnett WC, Chanton JP, Corbett DR, Cable PH, 1997. Field evaluation of seepage meters in the coastal marine environment. Estuarine, Coastal and Shelf Science 45:367-375

Cherkauer DS, McBride JM, 1998. A remotely operated seepage meter for use in large lakes and rivers. Ground Water 26:165-171.

Landon MK, Rus DL, Harvey FE, 2001. Comparison of instream methods for measuring hydraulic conductivity in sandy streambeds. Ground Water 39:870-885.

Lee DR, 1977. A device for measuring seepage flux in lakes and estuaries. Limnology and Oceanography 22(1):140-147.

Murdoch LC, Kelly SE, 2003. Factors affecting the performance of conventional seepage meters. Water Resources Research 39(6), 1163.

Rosenberry DO, Morin RH, 2004. Use of an electromagnetic seepage meter to investigate temporal variability in lake seepage. Ground Water 42(1):68-77.

Sebestyen SD, Schneider RL, 2004. Seepage patterns, pore water and aquatic plants: hydrological and biogeochemical relationships in lakes. Biogeochemistry 68: 383-409.

Shaw RD, Prepas EE, 1989. Anomalous, short term influx of water into seepage meters. Limnology and Oceanography 34:1343-1351.

Shaw RD, Prepas EE, 1990. Groundwater-lake interactions: I. Accuracy of seepage meter estimations of lake seepage. Journal of Hydrology 119:105-120.

Shinn EA, Reich CD, Hickey TD, 2002. Seepage meters and Bernoulli's revenge. Estuaries 25:126-132.