20th century urban changes in the land-toocean fluvial transport of trace metals

AutorMiguel Ángel Álvarez-Vázquez - Elena De Uña-Álvarez - Ricardo Prego
Miguel Ángel Álvarez-Vázquez a, b, Elena De Uña-Álvarez b
 Ricardo Prego a
a Marine Biogeochemistry Group, Instituto de Investigaciones Marinas (CSIC),
C/ Eduardo Cabello 6, 36208 Vigo, Spain.
b Area of Physical Geography, GEAAT Research Group, University of Vigo,
Campus As Lagoas s/n, 32004 Ourense, Spain
mianalva@uvigo.es – malvarezv@iim.csic.es
In 1972, The United Nations Conference on the Human Environment
acknowledges that humankind has reached a development stage with the
capacity to transform the environment ‘in countless ways and on an
unprecedented scale’ (United Nations, 1972). This assumption has led to
include the environmental sustainability between the Millennium
Development Goals (United Nations, 2000), nowadays specified in the
2030 Agenda for Sustainable Development goals and targets (United
Nations, 2015). The aforementioned ‘countless ways’ are characterized
mainly by numerous local events combined in a global impact, particularly
after the mid-20th century when ‘is there clear evidence for fundamental
shifts in the state and functioning of the Earth System that are beyond
the range of variability of the Holocene and driven by human activities’
(Steffen et al., 2015: 81).
The small drainage networks in the NW of the Iberian Peninsula is
under different sources of human pressure, typically affected by one, or a
few, major anthropogenic factors; for instance, the environmental pressure
related to urban areas. It makes the small rivers of this area suitable for
the study of human impacts with the isolation of their sources. In this
way, the urbanization phenomena is coupled with an increased land-to-
ocean transport of potentially toxic trace elements and other contaminants
(Wolanski et al., 2019); being wastewater treatment a critical factor
controlling the magnitude of the human disturbance (Aktar, 2009).
Continental inputs of potentially toxic elements, related to urbanization
phenomena, was reported to be negligible when a correct wastewater
treatment, but the lack of treatment can increase the continental transport,
overpassing by far the natural background (Meybeck and Vörösmarty,
2005: 107).
Potentially toxic elements (Thornton et al., 2001: 12-63) are generally
included under the general denomination of trace elements, defined by
the International Union of Pure and Applied Chemistry as ‘any element
having an average concentration of less than about 100 parts per million
atoms (ppma) or less than 100 mg g-1’ (IUPAC, 2014: 1551). This is not
a definition dependent on intrinsic characteristics of the elements, but
linked with their concentration or content in a given matrix. This fact
makes that a given element may be considered or not as a trace depending
on the studied matrix. For example, Aluminum is a trace element in fluvial
waters but a major constituent of sediment. This low concentrations in
the environment provides trace elements with a huge sensitivity to human
inputs, being very good human pressure tracers and environmental
indicators. Moreover, trace elements are well supported by more than four
decades of reliable data, available for comparison, because analytics ‘has
changed little’ since the generalization of inductively coupled plasma
techniques for analysis, around the 1990s (Callender, 2003: 68).
Richir and Gobert (2016) pinpoint as trace elements in water
ecosy stems th e followi ng: Ag, Al, As , Be, Bi, Cd, C o, Cr, Cu, Fe, Mn, Mo,
Ni, Pb, Sb, Se, Sn, V and Zn; certainly, considering the IUPAC definition
and the 92 naturally occurring elements, the list should be much higher.
A number of trace elements are micronutrient elements (Goldman, 2009:
378) which have a metabolic role and living things needs to intake them
for living maintenance. However, they follow the maxima given by
Paracelsus ‘sola dosis facit venenum’ or ‘all things are poison, and nothing
is without poison, the dosage alone makes it so a thing is not a poison’; i.e.
while low concentrations fosters the life, higher concentrations can impair
the living metabolism. This is the basis of the definition of ‘potentially toxic
elements’: elements with capacity to produce toxicity. Thornton et al.
(2001: 9) includes Cd, Cr, Cu, Fe, Hg, Ni, Pb and Zn in between

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