1. Applications
Porewater, also called interstitial water, is the water located between particles in a sediment matrix. Equilibrium-partitioning theory suggests that contaminants in porewater are the most bioavailable and so are most likely to be toxic to sediment-dwelling organisms.
Porewater toxicity testing has several practical advantages over whole-sediment toxicity testing. It removes some of the confounding factors associated with whole-sediment tests (e.g., particle size, sediment avoidance, and presence of indigenous predators or other organisms), usually takes less time, and requires smaller test chamber volumes (although analysts may need more sediment from which to extract the porewater). Porewater tests also incorporate sublethal endpoints more easily and often use organisms at sensitive life stages, which can provide more sensitive results than many whole-sediment tests. Testing a dilution series of porewater is much easier than testing whole sediment, helping to determine a concentration–response relationship.1,2
A few drawbacks should be considered when choosing to conduct porewater testing over whole-sediment testing. The lack of sediment in porewater testing removes the possibility that sediment-ingesting organisms will ingest contaminants adsorbed onto sediment particles, potentially reducing the uptake of contaminants below that which would take place in real-world conditions.1 Porewater samples may be anoxic, which can
• produce environmentally unrealistic results because organisms usually avoid anoxic porewater in the environment, and
• necessitate aeration to increase the dissolved oxygen (DO) concentration to a level that will sustain organisms during testing. Aeration may change the redox status, and ultimately the bioavailability, of porewater contaminants.3
Also, whole-sediment samples for toxicity testing often may be held for multiple weeks (depending on the contaminants of concern), but extracted or recovered porewaters should be tested within 1 to 3 d.
Analysts use porewater tests to rapidly screen for hot spots and prioritize sampling areas.2 Porewater testing can be one of a suite of tests, improving predictions of impacts on the benthic community (e.g., when developing sediment-quality guidelines).4,5 Porewater tests also are useful in toxicity identification evaluations (TIEs) to determine toxic contaminants in sediment tests, partly because porewater samples are easier to manipulate than whole-sediment samples.5–9
Porewater may be collected either ex situ or in situ. During ex situ collection, analysts collect an intact sediment sample in the field and extract its porewater in the laboratory. During in situ collection, analysts directly insert porewater samplers into sediment in the field. Both methods are addressed below.