Basics ..Glossary .. Overview .. Tutorial .. FAQs .. References .. USGS/WRD SPMD Activities .. Other SPMD Links 

6th International SPMD Workshop and Symposium
USGS Columbia Environmental Research Center

SPMD Fact Sheet

The Virtual Fish:  SPMD Basics

By Duane Chapman

Our rivers, streams, and lakes are in peril.  Contaminants destroy aquatic habitats and endanger our own lives. To solve this problem, we must know how much pollution threatens our waterways. 

Scientists at the Columbia Environmental Research Center (CERC) have an effective method for determining the amount of contamination in water.  This article explains our basic approach and also provides a Glossary of related technical terms.

What's the Problem?

Oil and water don't mix, right? We've all heard that before, but consider this: Many harmful pollutants are oils or oil-like and they do dissolve in water but at very low concentrations. These chemicals dissolve in other oils much easier than in water. They are "lipophilic" (fat loving) and "hydrophobic" (water fearing) contaminants.

Because these contaminants are usually present in low concentrations in water, they rarely cause quick death in fish or other aquatic animals. But here's the catch:   Animals have fats (lipids) in their bodies.  Hydrophobic contaminants tend to move from water to the fats in animals where harmful concentration can build up. This is called "bioconcentration." In a sense, animals inadvertently gather the contaminants from the water.

What Is CERC's Solution?

We at CERC have invented an artificial device called a semipermeable membrane device (SPMD) that is designed to mimic the parts of animals that cause bioconcentration. It is a long, flat, plastic tube containing oil. We call them "fatbags." The special plastic of the SPMD allows contaminants to pass through, like membranes of animal cells. The oil inside is similar to a highly purified fish fat. The contaminants dissolve in this oil just as they do in the fats of a fish.

When the SPMDs are put in the water and left for a month, they gather hydrophobic contaminants just as a fish would.  We then collect the SPMDs and chemically analyze them for the contaminants.

How Do SPMDs Work? 

Why don't we just collect some water and analyze it for contaminants? There are several reasons:

bulletOften it is hard to detect the contaminants in the water through normal chemical procedures because they are present at such low concentrations.
bulletBecause of bioconcentration factors, very low concentrations of contaminants may still be important in the environment. The fat-loving chemicals in a large volume of water slowly move across animal membranes (for example, fish gills) and collect in the animal’s fat.
bulletThe concentrations of these chemicals in rivers can change daily or even hourly. To get a true picture of the amount of contaminants present, we would have to take many samples and analyze them all. The SPMD allows us to calculate an average of the concentration of each contaminant while the SPMD was in the water.
bulletThe final reason concerns "bioavailability," the fraction of a chemical that an organism can absorb and incorporate.  Sometimes hydrophobic contaminants are present in a water sample, for example, attached to pieces of dirt or other particles in the water. These contaminants are (1) less available to aquatic animals than contaminants that are free in the water and would therefore be (2) less available to the SPMDs. So by using SPMDs we get a clearer picture of whether the contaminants are present in a form that might cause problems in animals.

The following diagram illustrates how an SPMD works:

The SPMD itself consists of a flat plastic tube with lipid or fat sealed inside.  As you can see from the diagram above, this membrane allows free (bioavailable) contaminants to pass through to the lipid while excluding water.  The SPMD is placed on a rack, which is inserted within a protective "shroud," and is then ready for use in the water.

How Are SPMDs Deployed in the Environment?

An SPMD can also be used vertically and horizontally as illustrated below:

What Are the Advantages of Using SPMDs?

So why don't we just use a fish instead of building a fake fish?

bulletSPMDs don't need to be fed.
bulletThey don't eat each other or die from disease, predators, lack of oxygen in the water, or from the contaminants that we are trying to measure.
bulletAn SPMD doesn't move around, won't escape, reproduce, and create an introduced species problem.
bulletAn SPMD can be used in almost any environment; you don't have to use one kind of SPMD in cold mountain water and another in warm desert water,  seawater, and groundwater. An SPMD can even be used to concentrate contaminants efficiently from the air, soil, and sediment.
bulletFish can actively depurate (get rid of) some kinds of hydrophobic contaminants; SPMDs don't. If a spill happened in the first week of an SPMD deployment, we will still find hydrophobic contaminants from the spill in the device when we come back in a month. With fish, we might not.
bulletAlso, we can use SPMDs to back-calculate a waterborne contaminant concentration, and we usually can't do that with fish.

An SPMD is not a perfect model of a fish. Fish can depurate some contaminants and pick up other pollutants through their diet. Some of these contaminants reach their highest concentrations in animals that are high on the food chain. This is called "biomagnification." An SPMD cannot model biomagnification, but it can alert us to the presence of contaminants that can be biomagnified.

An SPMD is a useful tool that assesses environmental contamination by hydrophobic chemicals. As a "virtual fish," the semipermeable membrane device provides CERC scientists with reliable information that helps determine how much contamination is present in aquatic habitats. Everyone benefits from a cleaner environment!

For More Information

For information on CERC's SPMD technology, contact Dr. Dave Alvarez: (573) 441-2970; e-mail:

You can discover more about SPMD technology by reading answers to frequently asked questions (FAQs).  In addition, the Overview discusses SPMD technology in greater detail.


ambient – surrounding; existing or present on all sides
bioavailability - the fraction of a chemical that is available for uptake by organisms
bioconcentration - concentration of contaminants in animals from their ambient environment
biofouling - contamination in water caused by waterborne organisms
biomagnification - concentration of contaminants through the food chain
biomonitoring - the use of certain organisms to detect and in some cases to determine the toxicity of environmental contaminants (pollutants)
biota - plant and animal life in a given region
deploy - to use, spread out, arrange
depurate - to eliminate a chemical by excretion or by breaking it down (metabolizing it)
food chain – arrangement of organisms within an ecological community: each organisms uses the next lower member of the community as a food source
food web – all of the interacting food chains in an ecological community
fugacity – tendency to escape
hydrophobic - "water hating"; having a low solubility in water
in situ – in the natural or original position
lipophilic - "fat loving"; having a high solubility in oils
partition - divide into parts
sequestration medium - a thin film of neutral lipid (triolein) that retains contaminants
solubility – the amount of a substance that will dissolve in a given amount of another substance
SPMD - semipermeable membrane device; also called "fatbag." It's a device that mimics parts of aquatic animals that cause bioconcentration.  SPMDs are used to assess contamination in water and air
triolein - a neutral lipid (fat) found in aquatic organisms and used in SPMDs

Basics..Glossary.. Overview.. Tutorial.. FAQs..References.. USGS/WRD SPMD Activities.. Other SPMD Links