Semipermeable Membrane Devices

Semi-Permeable Membrane Devices (SPMD's) are housed in cages such as shown here, and placed at different depths of the water column. SPMD's allow us to detect the presence and measure the amount of pollutants such as hydrocarbons, in the water.

Semipermeable membrane devices (SPMDs) are a passive sampling device used to monitor trace levels of organic contaminants. When placed in an aquatic environment, SPMDs accumulate hydrophobic (water-“hating,” fat-“loving”) organic compounds, such as polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), and organochlorine pesticides from the surrounding waters.

With increasing development along our nation’s coasts, chemicals from urban runoff, storm sewer overflows, pesticide use, and industrial use have significantly contributed to the pollution of coastal waters. Monitoring programs employing SPMDs and other sampling technologies are necessary to determine the status and trends of contaminants in coastal areas. With the appropriate data in hand, natural resource managers can make informed decisions and formulate effective strategies to minimize water pollution.

Even extremely low levels of some organic contaminants can be harmful to wildlife and people. These contaminants can be acutely toxic (cause a quick death), but more often they accumulate in an organism’s tissues, resulting in chronic problems such as reproductive disorders, muscular and neurological abnormalities, immune deficiencies, and genetic defects.

Imitating Aquatic Organisms to Monitor Chemicals

This canister, used for deploying SPMDs, holds five spiders stacked on top of each other.

This canister, used for deploying SPMDs, holds five "spiders" stacked on top of each other.

Despite today’s advanced technology, laboratory equipment often cannot detect low levels of such contaminants in water samples. To get around this problem, scientists commonly use aquatic organisms such as mussels, oysters, and fish to detect contaminants in water. These animals are useful for analysis because their bodies bioconcentrate chemicals; that is, their tissues absorb a certain amount of the chemicals (contaminants) that pass through them.

SPMDs have been designed to imitate the process that takes place in these aquatic organisms. Thus, they can be used to monitor low levels of waterborne pollutants. They are constructed of a tubular polyethylene membrane that lies flat (see photo, left) and is filled with a thin film of triolein, a lipid (fat or fat-like substance) commonly found in aquatic organisms. The polyethylene membrane acts similar to a biological membrane by allowing selective organic compounds in.

Advantages of SPMDs

SPMD technology is relatively new. Scientists are still working to improve this technology and its associated protocols. Though some fine tuning is still needed, there are several potential advantages of using SPMDs over aquatic organisms. For one thing, SPMDs can be deployed in waters that are not inhabited by bivalves or that are too toxic for organisms to live. They are more easily standardized than aquatic organisms, and, for studies performed at different sites or in different seasons, the data have better compatibility. Migration, mortality, and metabolism issues, which are common when dealing with living organisms, are nonexistent with SPMDs. Additionally, SPMDs can be deployed for long periods of time (days to months) and used to estimate the time-weighted mean concentrations of contaminants, which take into account surges in contaminant concentration levels due to surface runoff events, chemical spills, etc.

When working in the marine environment, scientists deploy SPMDs in protective stainless-steel canisters. These canisters protect the SPMD from damage and also provide a solid structure for anchoring it in the water column. SPMDs may be deployed for periods ranging from one week to several months.

The Web Team gratefully acknowledges this contribution by Paula Souik, a technical analyst in NOAA's Office of National Marine Sanctuaries.