8273

• UCREFRP Catalog Number: 8273
• Author: U.S. Bureau of Reclamation, U.S. Fish and Wildlife Service, U.S. Geological Survey and U.S. Bureau of Indian Affairs.
• Title: Guidelines for Interpretation of the Biological Effects of Selected Constituents in Biota, Water, and Sediment.
• USFW Year: 1998.
• USFW - Doc Type: \
• Copyright Material: NO

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Introduction

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Concentrations in any solid medium (such as
organic tissues, sediment, or animal feed) may
be measured on either a dry-weight (dw) basis
or a wet-weight (ww) basis. The resulting
values are markedly different, and the dw
value is invariably higher. In fish and animal
tissues, the dw concentration is generally in
the range of 3 to 5 times the ww value, but
there is no set conversion factor. The ratio
between dw and ww depends on the water
content of the tissue, which varies between
species and between organs, and even varies
within individual organs over time. Criteria
based on wet-weight measurements should
not be used to assess the toxicity of dry-
weight concentrations, and vice versa.

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"Fresh weight" describes a wet-weight
measurement that is made either in the field
or within a few hours after collection. Media
such as eggs and animal tissue may begin
losing water as soon as they are collected,
which results in higher wet-weight
concentrations of most other constituents if
they are not analyzed promptly.

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Many chemical elements have two or three
different valences or oxidation states that are
common in the environment, and the toxicity
of these varying forms can differ greatly.
Arsenic (III), for instance, is much more toxic
than arsenic (V), yet some tests do not
differentiate between these forms and report
only "total arsenic." A criterion established
using arsenic (III) would be misleadingly low
in most natural settings, for arsenic (V) is
usually more abundant.

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Even where the valence state doesn't vary, the
various compounds an element makes with
other elements can greatly affect toxicity.
Dimethyl mercury (C2H6Hg), for instance, is
far more poisonous than mercuric sulfide
(HgS), even though both of them are based
on mercury (II). It is common for organic
(carbon-based) compounds to be more toxic
than others because they are more readily
taken up in the metabolism of living
organisms.

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Concentrations of elements or compounds in
water may be measured in two different ways.
Under one method, water samples are filtered
before analysis to remove all microorganisms
and other suspended particles. The resulting
measurement is called a total dissolved concen-
tration. In the other method, no filtering is
done, and the resulting measurement is a total
recoverable concentration. The difference
between these figures can be strongly
influenced by the overall biotic productivity
of a water body. In highly productive waters,
both nutrients and toxins are quickly taken up
by microorganisms, leaving only small
amounts of these dissolved in the water
column. Thus, a measurement showing only
dissolved constituents may miss significant
amounts of toxins that are nonetheless present
in the water column and available through the
food chain. Where productivity is low, the
dissolved concentration will be very close to
the total recoverable concentration.

Many reports give chemical concentrations in
either parts per million (ppm) or parts per
billion (ppb). A few use the ambiguous
abbreviation "ppt," which may stand for either
parts per thousand or parts per trillion.
Obviously, in reading such reports, it is
important to know which meaning of "ppt"
was intended. In accordance with principals
of the International System of Units, most
concentrations in this volume are expressed in
units of either weight per weight (for solid
media) or weight per volume (for liquids).
Here is a brief list of equivalents that clarify
how these units relate to one another:

Parts per thousand = g/kg or g/L
(ppt or per mil or%o)

Parts per million (ppm) = mg/kg or mg/L

Parts per billion (ppb) = l1g/kg or I1g/L

Parts per trillion (ppt) = ng/kg or ng/l

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