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efforts. These types of programs can potentially provide public demonstration opportunities for
emerging onsite metal analysis. The remainder of the final working session was spent openly
discussing the consensus recommendations derived from the working group discussions.
THE IMPORTANCE OF AND CHALLENGES FOR METAL MONITORING IN WATER
QUALITY PROGRAMS
Metal elements are ubiquitous and diverse components of the earth's geochemistry and play
critical roles in ecosystem function. While several metals (e.g., Cu, Co, Fe, Mn, Mo, Zn) are
essential micronutrients at sub-nanomolar concentrations, some (e.g., Cu, Zn) at higher
concentrations can become potent toxins to a range of biological systems. Metals are highly
active in redox chemistry, contributing to their dual biological function, but this inherent reactive
character also presents significant challenges to the measurement of biologically active species
and interpretation of bulk metal concentration data. Complexation reactions control the
bioavailability of metal cations:
M
+n
+ L
-n
]
ML
and
[M]
tot
= M
+n
+ ML
where
M
+n
represents the free metal species of interest and
L
-n
represents a complexing agent
(dissolved inorganic anions, oxides, dissolved organics, detrital surfaces, mineral particles). The
study of environmental metal distributions becomes even more daunting when one considers
biologically catalyzed covalent modifications and metallo-organic compounds produced by
human industrial activities (e.g., methyl mercury and tributyl tin). Indeed, even the act of
sampling can induce rapid transitions in metal species distributions (see for example, Nuzzio et
al, 2002) and at minimum requires simultaneous measurement of ambient physiochemical
properties (pH, salinity, DO, temperature, alkalinity) for rigorous interpretation of offsite
measures. For these reasons metals represent an important target for development of
in situ
analytical tools.
The distribution of metals in the environment has changed continuously since their routine
adoption into human society and even more dramatically with global spread of industrialization.
The changes in metal availability have had pronounced effects on plant and animal distributions.
In large part, they are determined by the organisms' capacity to acclimate to either limiting or
superabundant metal availabilities. Water quality and underlying sediment habitat quality have
also been significantly impacted by the anthropogenic changes in metal distributions. The U.S.
Clean Water Act (CWA) and similar international regulations mandate federal and state agencies
to develop programs to protect the chemical, physical and biological integrity of the nation's
waters. To date, the USEPA has published (EPA-8230R-03-010 2003) 165 water quality criteria.
Of these 165 criteria, 10% define upper limits to the specific metal content of fresh and saltwater
habitats. Recommended metal criteria maximum concentrations, above which which significant
O
VERVIEW
ACT Workshop on Trace Metal Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5