Northern Prairie Wildlife Research Center
Sediment interstitial water samples are often collected with methods that destroy "in situ" properties relevant to ecological investigations. Such methods are of limited biological value because the quantity and quality of nutrients that are actually available to biota cannot be determined. Three basic methods have been used to collect samples of interstitial water (Höpner 1981). The first involves extraction of interstitial water from sediments by pressure filtration or centrifugation. Besides disturbing the sediments, those procedures bias analyses because temperature (Mangelsdorf el al. 1969, Bischoffe; et al. 1970, Fanning and Pilson 1971) and pressure other than those "in situ" (Emery and Hoggan 1958, Barnes 1973) alter the concentrations of dissolved chemical species. Further, the technical effort to process such samples is considerable because samples must be processed in an inert atmosphere within a glove box (Höpner 1981).
The second set of methods involves the use of "in situ" samplers (Sayles et al. 1973, Montgomery et al. 1979, Rutgers van der Loeff 1980) in which interstitial water is collected directly from specific sediment profiles via permanently mounted capillary tubes. These "in situ" samplers have the desirable feature of being able to draw repeated samples over time from identical locations, and they minimize mechanical disturbance of sediments because they do not have to be removed to collect samples. The sediment profiles examined are necessarily coarse (i.e., profiles of large depth increments), however, because a syringe is used to extract samples through capillary tubing and vacuum from the syringe draws interstitial water from coarse profiles.
A third set of samplers is based on diffusion of ions and other chemical species across a semi-permeable membrane. A solution is added to internal chambers of the sampler and the device is inserted into the sediment. Chemical species present in interstitial water slowly diffuse through the membrane into the sampler and eventually equilibrate so that the concentrations inside the sampler match those in the interstitial water profiles.
Diffusion-controlled samplers modeled after the device described by Hesslein (1976) have received much use in investigations of sediment dynamics (Gaillard et al. 1989, Simon et al. 1985, Simon 1988) and dissolved heavy metal gradients (Schwedhelm et al. 1988). Those units have provided useful information on nutrient cycles and other dissolved chemical species in benthic sediments and water-column profiles and they are mobile and easy to operate. The units must be inserted and then removed from sediments, however, in order to recover samples, resulting in an unknown degree of sediment disturbance. Such units use polycarbonate membranes that have to be replaced each time samples are collected and sample removal must occur in an inert atmosphere within a glove box. Moreover, up to 10 days may be required for equilibration of divalent ions (Simon et al. 1985).
An additional device for collecting samples of interstitial water to facilitate measurements of plant nutrients in wetland sediments was described by Neeley and Davis (1985). The device is constructed of PVC pipe and functions like a shallow well. It has the desirable characteristic of collecting interstitial water directly and can be used to draw repeated samples over time from identical locations. This device collects samples from single profiles, however, and uses atmospheric gases for venting during sample removal. The introduction of atmospheric gases into the collection chamber during venting may alter certain "in situ" characteristics of subsequent samples and limit the application of this device.