Lecture No. 16. Metal Toxicity II

Before continuing on metal toxicity, there are a few miscellaneous items to cover.

1. A word that is creeping into the English language, at least in the scientific literature, is diel, usually pronounced "die-eel." It is used to signify phenomena that have recurrence periods that track the day-night cycle, such as the arsenic fluctuations in Whitewood Creek. The word daily does not quite connote the periodicity. Diurnal actually means "by day" as opposed to nocturnal. There is an obscure English word, quotidian, which has the proper meaning, but no one ever uses it. I refuse to use a made-up word that looks like the name of a famous organic chemist (Diels), so I will continue to say daily periodic.
2. I am showing two new E_h-pH diagrams from Dana’s Manual of Mineralogy. One covers iron under diagenetic conditions at 25¡ C and 1 atmosphere. Notice the differences between this diagram and the ones we saw earlier. One difference is that dissolved silica is present at a fairly high concentration, so that greenalite can form under anoxic conditions. There is also a large stability field for magnetite, which can be thought of as Fe₂O₃áFeO. I am not sure exactly what the conditions are that make magnetite formation possible — possibly just a longer time scale. In the banded iron formations, red bands of hematite alternate with greenish bands of quartz and greenalite and generally no magnetite. This suggests that E_h fluctuated between very high (hematite) and very low (greenalite) without spending much time in the middle range. But there may be another explanation. There is something the authors are not saying about the conditions for this diagram. The other diagram is one for copper in the presence of carbonate as well as sulfide. In the oxic zone is a big field for malachite that almost completely displaces tenorite (CuO). The remainder of the tenorite field is filled by a field for the soluble complex CuO₂^2–. This diagram does not indicate what all of the concentrations are. Whenever you look at an E_h-pH diagram, ask yourself what exactly the author’s assumptions are.
3. An episode of poisoning from smelter outfall I did not mention when we discussed the effects of mining was at the town of Mill Creek, near Anaconda. What sort of poisoning would you expect from a copper smelter? It wasn’t copper poisoning. Arsenic trioxide is a by-product of smelting the high-arsenic ore from Butte, and there was significant outfall from the Washoe Smelter stack onto Mill Creek. Eventually (during the 1970’s or 80’s) the town was evacuated and all the residents were bought out.

Acute toxicity in trout was discussed in the last lecture, as was the way trout and other fish respond to metals in their water.

Metal and Metalloid Uptake into Organisms

This is dependent on the individual metal and the individual organism. It is a measure of the metal’s bioavailability, since it depends on the chemical form of the metal. It also gives us a measure of food-chain concentration (biomagnification).

Research has been done on copper uptake by Mytilus edulis, a marine mussel. Copper uptake was enhanced by the presence of lead, zinc, and/or silver. In other studies, manganese, iron (especially in sediments) and some organics suppressed copper uptake.

Data from whole fish are useful, but sometimes data from specific organs can be more useful. Certain metals, for instance, concentrate in the liver. (Recall the Blackfoot River study.)

Sediment Toxicity

There are two ways to approach this: bioassays (laboratory tests) and bioassessment (field-based analysis). A bioassay uses an aquarium population that can be exposed to a substance or mixture under controlled conditions. A bioassessment is like the study that was done on the Blackfoot River.

There are a large number of procedures.

a. Background/reference site concentrations are measured in water, organisms, soil, air, or whatever.

• These can then be compared to "contaminated" sites.
• They serve as a benchmark for measuring enrichment factors.
• But this tells us nothing about how the enrichment affects the organisms.

b. The Apparent Effects Threshold (AET)

• This is the concentration of the contaminant of interest above which there is a statistically measurable effect on the organism(s) of interest. (This effect can be a decrease in population, a decrease in diversity, increased incidence of some disease, occurrence of tumors, etc.)
• The assumption is made that, if a sediment has a higher concentration than the AET, there will be some sort of biological ill effect.
• These are determined empirically in the field. This is an advantage, since the data are from the real world.
• A disadvantage is that the AET is always for one substance or element. Separating out the effects of different contaminants is hard.

c. Screening Level Concentrations (Long and Morgan (NOAA), 1991)

They got AET’s out of about 85 papers and reduced the data to two values:

• ER-L (a low-range AET): the level of first damage to a diverse assemblage.
• ER-M: the level at which most of the organisms in the assemblage start to suffer some damage.

The second handout has a table of data for a number of elements as well as for a large number of organic pollutants.

d. Sediment Quality Criteria

• These are not very well or universally developed.
• They are developed by summarizing very many different experiments and observations done by many different procedures.
• The first sediment quality criteria in the US were established in 1973 for 7 pollutants:

The handout has a list of SQC’s established for various elements in sediments by USEPA, the Wisconsin Department of Natural Resources, and the Ontario Ministry of the Environment, as well as some guidelines developed by a private consulting company.

e. The Equilibrium Partitioning (EP) Approach

This is a way to compare sediment chemistry to water quality criteria. Numerical modeling is used to calculate what the equilibrium concentrations in pore water would be at a given sediment concentration. These calculated pore water concentrations are then compared to water quality criteria.

This is an increasingly common approach these days. It has some problems:

• It assumes equilibrium.
• It assumes we know and understand all the equilibria.
• Redox environments may change — what then?
• This is a shaky approach, for the above reasons. The water quality criteria themselves are shaky. That makes the EP approach doubly shaky!
• Pore water isn’t everything, since many animals eat the sediment itself.

Nevertheless, EP is a really big deal to a lot of people, because it yields predicted toxicities lower than those obtained from other approaches.

f. Spiked Bioassays

This is more experimental, less empirical. It is a laboratory method. One contaminant of interest is added to a microcosm. A parameter, such as the 96 hour LC₅₀ (the concentration resulting in 50% mortality in 96 hours), is read. Note that this is a short-term, single-contaminant test. It has little to do with the real world.

Yet many of the sediment criteria come from this kind of approach.

How Bioassays are Done

You pick an organism to use.

a. Bacteria are popular. These tests are rapid, but there are problems with them.

• Bacteria are tough and are often more tolerant of contaminants that eukaryotes are.
• Many factors other than the contaminant can affect bacterial growth, and they can be confounded with the toxicity of the contaminant.

b. BIOTOX ¨ is a test kit from Beckman. It is based on a photobacterium. The bioluminescence decreases when the bacteria are stressed. (But, again, these are bacteria.)

c. Algae, especially diatoms, are used.

d. Midge larvae (benthic organisms)

e. Fish

• Minnows
• Baby trout

Now let’s look at a real river (the Clark Fork). We will use Apparent Effects Thresholds to calculate a contamination index for various elements.

Assumptions Made

1. All metals act independently.
2. There is nothing else that is significantly toxic.
3. Clark Fork biota are essentially similar to those in the published work. (But most of the published work is based on marine organisms!)

As we move upstream on the Clark Fork, the Contaminant Index increases, and the number of taxa in the river decreases. However, there are confounding factors to keep in mind:

1. Climate differences creep in (with increase in altitude, for one thing).
2. The stream gets smaller as we go upstream.
3. The geology is different.
4. Etc.

To previous lecture: Lecture No 15. Photosynthesis-Linked Periodic Variations; Trace Metal Toxicity

To next lecture: Lecture No 17. Biomethylation

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