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Sistema de Información Científica
Red de Revistas Científicas de América Latina y el Caribe, España y Portugal
Rev. Int. Contam. Ambient. 26 (2) 119-127, 2010
TETRAETHYL LEAD RELEASE FROM SEDIMENTS IN A MINE-IMPACTED
TROPICAL RIVER
Flor ARCEGA-CABRERA
1
, Silvia E. CASTILLO-BLUM
2
and Ma. Aurora ARMIENTA
3
1
Facultad de Química, Unidad de Química Sisal, Universidad Nacional Autónoma de México, Puerto de Abrigo
s/n, Sisal, Yucatán, 97355, México. E-mail: farcega@unam.mx
2
Depto. de Química Inorgánica, Facultad de Química, Universidad Nacional Autónoma de México, Circuito
Exterior s/n, Ciudad Universitaria, D.F., México 04510, México
3
Instituto de Geofísica, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria,
D.F., México 04510 México
(Recibido Junio 2008, aceptado agosto 2009)
Key words: México, Taxco, tetraethyl lead, kinetics, mining pollution, factor analysis
ABSTRACT
Desorption kinetics of tetraethyl lead, under changing pH and controlled temperature,
Eh and dissolved oxygen, in a mining-impacted Mexican tropical river, was examined
using samples placed in a closed system. The objective of this study was to determine
the potential environmental risk for local residents from the tetraethyl lead present in
the water column. Statistical and geochemical methods elucidated the potential envi-
ronmental threat of free tetraethyl lead (TEL). Speci±cally, analysis of water samples
from three stations in the Taxco River basin indicated that altering pH caused signi±-
cant changes in the concentration of TEL in the water column. Since this study was
performed on a closed controlled system that contained sediments and water from the
river, desorption from sediments caused by changing pH is the mechanism by which TEL
increases in the river water. Since the samples used for this study are natural samples
collected in the river, it is assumed that this behavior could be expected in the river if
a suddenly decrease in pH occurs. It is probable that the processes of TEL desorption
depend on several variables, since ±tted curves, for changes in TEL release with time,
go from linear (direct relationship) to polynomial (co-linearity) suggesting a complexity
of interacting factors. Therefore, factor analysis was used to tease out how the geo-
chemical characteristics of sediments and pH interact to control TEL concentration’s
changes. Factor analysis showed that the most probable sources of TEL present in the
Taxco River basin, are wet deposition, run-off, and leaching of the hydrological basin.
TEL in situ microbial formation is probably not a source in this particular area. In the
sample taken from a station near a tailing and an active mine, TEL concentrations at pH
6.0 exceeded the recommended safe limit established by the US EPA (4.6
x
10
-
7
mol
m
-
3
) and as a result could represent a potential environmental risk for local residents.
Palabras clave: México, Taxco, tetraetilo de plomo, cinética, contaminación por minería, análisis de factores
RESUMEN
Se estudió la cinética de desadsorción del tetraetilo de plomo (TEL) dentro de un sistema
cerrado, bajo condiciones de pH variable y con temperatura, Eh y oxígeno disuelto esta-
bles, utilizando muestras naturales de un río tropical mexicano impactado por desechos
F. Arcega-Cabrera
et al.
120
mineros. El objetivo de este estudio fue determinar el riesgo ambiental potencial que el
tetraetilo de plomo representa para los habitantes del área, utilizando como herramientas
de análisis la geoquímica y la estadística multivariada. El análisis de la muestra de agua
de tres estaciones en la cuenca del río Taxco indicó que las variaciones de pH causaron
cambios signi±cativos en la concentración de TEL presente en la columna de agua. El
utilizar un sistema cerrado con muestras naturales, permitió mostrar que el mecanismo
a través del cual la concentración de TEL incrementa en el agua al disminuir el pH
probablemente es la desadsorción desde los sedimentos. Debido a que las muestras
utilizadas para este estudio son muestras naturales, se puede esperar que este mismo
comportamiento se repita en el caso de presentarse una disminución de pH en el agua
de la cuenca. Es probable que el incremento en la concentración de TEL en el agua para
un tiempo determinado dependa de una compleja interacción de diferentes variables, ya
que los ajustes de curvas realizados van de lineales (interacción directa) a polinomiales
(colinealidad). Por lo anterior, se llevó a cabo un análisis de factores para elucidar cómo
interactúan las características geoquímicas de los sedimentos y el pH para controlar los
cambios de concentración del TEL. Así también, el análisis de factores mostró que las
probables fuentes de TEL en la cuenca del río Taxco son la depositación húmeda, esco-
rrentía y lavado de la cuenca hidrológica. La formación in situ de TEL parece no ser una
fuente en esta área. En la muestra tomada en la estación cercana a los jales mineros y a
la mina activa, las concentraciones de TEL a pH 6.0 excedieron los límites recomenda-
dos establecidos por la US EPA (4.6
x
10
-
7
mol m
-
3
) y representan un riesgo potencial
ambiental para los residentes de la zona.
INTRODUCTION
The impact of mining in México comes mainly
from solid and liquid wastes released into the envi-
ronment (Armienta
et al
. 2003). Acid mine drainage
(AMD) is a liquid waste formed in tailings containing
high concentrations of metals in solution. It may be
counted as a principal potential pollutant in aquatic
systems because it promotes a decrease in natural
pH, alters Eh (oxidation/reduction potential), and
affects conductivity (Ritcey 1989, Sangupta 1993,
Vanderlinden
et al
. 2006). Mines and tailings are
weathered over time, releasing potential pollutants
such as organic lead compounds (Rhue
et al
. 1992).
In a mining area, expected lead concentrations in soils
are high (above 150 x 10
-
3
g kg
-
1
according to RAIS
2000). Under appropriate conditions, alkylation of
lead through purely chemical processes occurs (e.g.,
Jarvie
et al
. 1975, Walton
et al
. 1988, Rhue
et al
. 1992).
Lead alkylation via microorganisms such as sulfate
reducing bacteria, present in these types of environ-
ments, is also expected since they are able to convert
inorganic forms to organic derivatives (Schmidt and
Huber 1976, Huber and Kirchmann 1978, Jarvie
et
al
. 1983a, Gilmour and Henry 1991, Pongratz and
Heumann 1999). The reverse process, in which organic
forms of lead are converted into inorganic forms, has
been reported by Gallert and Winter (2004). A common
product of chemical and biological alkylation of mine
wastes is tetraethyl lead (TEL), a highly hydrophobic
compound. It is considered toxic because it can pen-
etrate membranes and skin, and it attacks the nervous
and endocrine systems (Gallert and Winter 2002);
although some bacteria can survive and even ²ourish
in the presence of TEL (Teeling and Cypionka 1997).
In situ
formation of TEL in natural systems has
been reported (Jarvie
et al
. 1983b, Hewitt and Harrison
1986), but it remains an issue of debate in the literature
(e.g., Pelletier 1995). Radojevic and Harrison (1987)
demonstrated that R
4
Pb compounds are degraded to
R
3
Pb
+
and R
2
Pb
2+
, and in aquatic systems the alkyl-
ated species include methyl, ethyl, or both groups.
The principal factors involved in lead methylation in
soils and sediments, are the acidity of the environment,
the organic carbon content, and the sulfate concentra-
tion. The main intracellular donors used by organisms
to accomplish methylation are methyl cobalamine,
methionine, dimethyl
b
-propiothein, tetrahydrofolic
acid, and S-adenosyl methionin. In addition, the main
intracellular donors include polyfunctional halo-
carbon centers, and products of cellular excretion
such as acetate, propionate, and other inorganic acids
(Brinckmann and Olson 1987).
Although the use of leaded gasoline, in which
the main anti-knocking agents were tetraethyl lead
(TEL) (Mulroy and Li-Tse 1998) and tetramethyl
lead (Nevenka and Branica 1994), was prohibited
worldwide (in México in the 1990s), their degra-
dation products such as triethyl lead and trimethyl
lead are still ubiquitous in the environment (Blais
TETRAETHYL LEAD RELEASE FROM SEDIMENTS
121
and Marshall 1986, Radojevic and Harrison 1987,
Van Cleuvenbergen
et al
. 1990, Unob
et al
. 2003).
Wet deposition of atmospheric TEL and its posterior
leaching to the river could be an additional source
in the study area.
According to a number of researchers (e.g.,
Rashid and Leonard 1973, Wilkins 1974, Sholkow-
itz 1978, Rendell
et al
. 1980, Förstner
et al
. 1989,
Wilkins 1991, Coker 1995, Clevenger and Samir
1997), kinetic studies have been highly successful
in the evaluation of the potential pollution of rivers
by metals. It has been possible, through the results
of these studies, to predict metal behavior in natural
systems and to prevent possible threats to the envi-
ronment and human populations. Therefore, kinetics
was applied here to study both the release of TEL
from sediments to the water column under controlled
conditions (T, pH) and the factors that affect that re-
lease, speci±cally pH. The geochemical parameters
involved in the kinetic behavior and the contribution
of biogenic TEL to the environment were determined
with factor analysis. Finally, these results were used
to predict the potential toxicity of the Cacalotenango
and Taxco rivers to the inhabitants of the study area.
Data generated during this study will allowed other
scientist or health of±cials to determine the potential
risk that water consumers may face.
MATERIALS AND METHODS
The hydrological basin studied here (
Fig. 1
) is
located in northern Guerrero, México, between 18º
30’ and 18º 33’ N and 99º 36’ and 99º 40’ W 10 km
southwest of the city of Taxco de Alarcón. It consists
of two main rivers, Cacalotenango (RCA) and Taxco
(RTX), that receive mine waste from two tailings
(El Fraile and La Concha), one closed mine (Jesús),
one active mine (La Concha-RLC), and urban waste
from the towns of Dolores, Santa Rosa, El Fraile, and
Cacalotenango.
Samples from three out of twelve river stations
(La Concha River station [RLC], Cacalotenango
River station [RCA], and Taxco River station [RTX])
were selected for the kinetic tests because total lead
concentrations in sediments were 8 to 30 times
higher (RLC 5.8 g kg
-
1
, RCA 1.4 g kg
-
1
and RTX
0.9 g kg
-
1
)
than average concentrations in pristine
sediments taken from the study area (Arcega-Cabrera
et al
. 2005, Arcega-Cabrera 2006, Arcega-Cabrera
et
al
. 2009). They have been previously identi±ed as a
potential risk to the environment (Arcega-Cabrera
et
al
. 2009), and according to Jarvie
et al
. (1975), Wal-
ton
et al
. (1988), and Rhue
et al
. (1992) alkylation of
lead is favored at sites with high lead concentrations.
Samples were collected during the post-rainy season,
because TEL could reach its highest concentrations
during this period. During the rainy and post-rainy
seasons, TEL could be released into the river’s water
column via leaching and run-off from the hydrologi-
cal basin (Nevenca and Branica 1994).
Four water-sediment samples were collected at
every station (RLC, RCA and RTX) (
Fig. 1
) follow-
ing the methods proposed by Rubio and Ure (1993)
and Loring and Rantala (1992). Every sample col-
lected during the ±eld study, consisting of 1 kg of
RLC
RCA
RTX
Taxco river
La Concha
El Fraile
Taxco city
Cacalotenango river
30
27
24
21
10
15
89
92
95
98
101
104
107
114
115
116
Golfo de México
Océano Pacífico
GUERRERO
Chilapa
Chilpancingo
Acapulco
Zihuatanejo
Iguala
C. Altamirano
Taxco
Fig. 1.
Study area showing the location of the sediment and water sampling sites (
RLC-La Concha, RCA-Cacalotenango, and RTX-Taxco river stations), and main
relevant features on the area:
Rockwaste dump,
El Fraile town,
Santa Rosa town,
Dolores town
Tailings,
Jesús mine,
rivers.
F. Arcega-Cabrera
et al.
122
sediment and 3 L
of water, was placed in a plastic
container and rapidly transported to the laboratory
(transportation time <2 h) in order to avoid signi±cant
changes in the
in situ
physicochemical conditions
(
Table I
).
Once in the laboratory, samples were placed in
closed containers, and air was bubbled through the
samples to maintain water circulation and oxygen
levels similar to ±eld conditions (
Table I
and
II
).
Approximate river ²ow (
Table II
) was measured
at the river stations where and when water samples
were collected (RCA, RLC, and RTX) based on the
method of Ficklin and Mosier (2002) assuming a
²at river bottom. Temperature was kept constant and
similar to ±eld conditions (
Table I
). The pH was ±xed
and kept constant at 6.0, 7.0, 8.0, and at ±eld values.
Aliquots (5
x
10
-
3
L) of the overlaying water were
sampled at 0, 0.5, 1, 2, 3, 4, and 6 hours and placed in
vials at 4 ºC in the dark to avoid further degradation
via photolysis or TEL formation via microorganisms.
Sediment samples were analyzed for Li, carbon-
ates and organic matter content according to the
methods described by Loring and Rantala (1992).
Temperature (T), pH, and conductivity were mea-
sured using a Conductronic PC18 potentiometer/
conductivity meter calibrated with pH=4 (J. T. Baker,
potassium acid phthalate buffer, NIST as RS), pH=7
(J. T. Baker, phosphate buffer, NIST as RS), and
pH=10 (J. T. Baker, borate buffer, NIST as RS) buf-
fers, and with a solution of 1000 mg/L of KCl (J. T.
Baker, ACS) corresponding to a conductivity value
of 1.99 x 10
-
3
S/cm. Eh was measured with an In-
strulab ORP meter corrected using a Zobell solution
(potassium ferrocyanide, and potassium ferricyanide,
J. T. Baker, ACS) prepared as indicated by APHA
(American Public Health Association 1992). These
parameters were continuously monitored to ensure
the constant experimental conditions.
Determination of tetraethyl lead concentration was
done in a diode array Hewlett-Packard UV-Visible
spectrophotometer model 8453 equipped with quartz
cells (1 cm pathlength). The spectrophotometer was
calibrated with a tetraethyl lead standard solution
(AccuStandard, Inc. UHP) using a wavelength range
of 191
x
10
-
9
m to 209 x 10
-
9
m; behavior was linear
from 0.1
x
10
-
7
mol m
-
3
to 8.9
x
10
-
7
mol m
-
3
. The
calibration curve was prepared by taking aliquots
of the standard and diluting with deionized water to
avoid differences between the samples and the ana-
lytical curve’s background signals. A relative standard
deviation below 10 % was achieved by repeating the
calibration curve 6 times. After samples had settle
down, the water without suspended solids was placed
in the quartz cells to measure TEL concentrations.
This measure was repeated 3 times in 10 randomly
selected samples (12 % of the total population) and a
relative standard deviation below 10 % was achieved.
Detection limit was 0.8 x 10
-
8
mol m
-
3
.
RESULTS AND DISCUSSION
In situ
conditions of pH, Eh, conductivity, organic
matter, and carbonate content are shown in
table I
.
TABLE I
. FIELD CONDITIONS (FC) AT STATIONS RLC (LA CONCHA RIVER), RCA (CACALOTE-
NANGO RIVER) AND RTX (TAXCO RIVER)
pH
Conductivity
(S/m)
Eh (V)
Temperature
(ºC)
Dissolved Oxygen
(mg/L)
OM
(g/kg)
Carbonates
(g/kg)
FC RLC
8.08
463*10
-
4
0.312
20.5
3.0
29
220
FC RCA
8.10
323*10
-
4
0.239
20.8
4.2
24
147
FC RTX
8.27
630*10
-
4
0.215
21.0
4.3
44
90
OM= Organic matter in sediments
TABLE II
. FLOW AND VELOCITY CONDITIONS IN TAXCO (RTX),
LA CONCHA (RLC) AND CACALOTENANGO (RCA)
RIVERS IN POST-RAINY SEASON
River
Altitude
(m)
Depth
(m)
Width
(m)
Velocity
(m s
-
1
)
Flow
(m
3
s
-
1
)
La Concha
1483
3.0
0.5
0.3
0.5
Cacalotenango
1400
1.2
7.5
5.3
3.0
Taxco
1115
2.9
11.0
0.5
16.0
TETRAETHYL LEAD RELEASE FROM SEDIMENTS
123
The role these parameters played in the observed TEL
release behavior is discussed later.
The average range of TEL in other regions im-
pacted by mining is 6.1 x 10
-
8
to 8.58 x 10
-
7
mol
m
-
3
(Nevenca and Branica 1994). In the Taxco river
basin, the range observed at the three sampling sta-
tions was 0.3 x 10
-
7
mol m
-
3
to 8.5 x 10
-
7
mol m
-
3
.
Results of the kinetic experiments are shown in
fgures 2
,
3
, and
4
. Signi±cant changes in released
TEL were observed among samples and experimental
pH values. For example in samples from station RLC
(
Fig. 2
), water column concentrations increased with
decreasing pH. This behavior could be explained by
the low content of organic matter (29 g kg
-
1
) pres-
ent in the sediments. Organic matter functions as a
stabilizer of the highly hydrophobic TEL molecules,
and it is probable that TEL can only be adsorbed
onto a limited number of lithogenic particles. This
adsorption is relatively weak and can be easily af-
fected by slight changes in pH and redox potential.
Importantly, methylation increases the amount of
metal compounds dissolved in water because it
weakens the adsorption of the metal compound onto
the substrate (Thayer 1987).
In samples from RLC, TEL concentrations ex-
ceed the US EPA’s recommended safe limit of 0.46
m
mol m
-
3
at pH 6.0 (
Fig. 2
) (Irwin
et al
. 1997).
Therefore, an acid mine drainage release (AMD)
from La Concha mine would increase acidity and
TEL concentrations in the water column. Such a
concentration increment would have the potential to
increase toxicity and risk for the local population.
In contrast,
Fig. 3
shows that at RCA the released
concentrations of TEL are lower, and at
in situ
condi-
tions TEL concentrations in overlying water are not
detectable. This could show that TEL concentrations
are lower
per se
at RCA than those at RLC. Only at
pH 6.0 and after 2 h did the concentrations reach and
exceed the US EPA’s safe limits (Irwin
et al
. 1997).
Both the organic matter content (24 g kg
-
1
) and
0
1
2
3
4
5
6
0,0
0,2
0,4
0,6
0,8
Tetraethyllead,
μ
mol m
3
Time, h
pH 6.0
pH 7.0
pH 8.0
pH in situ 8.08
pH
8.0
Linear
y = A+B*x
R = 0.9885 SD=0.01664
pH
7.0
Boltzmann
y=[(A
1
-A
2
)/1+
e
(x0-x)/dx
]+A
2
r
2
=
0.8996
pH
6.0
Boltzmann
y=[(A
1
-A
2
)/1+
e
(x0-x)/dx
]+A
2
r
2
=
0.9603
Fig. 2.
Release of TEL to the water column as a function of time at different pH values at RLC station
Tetraethyllead,
μ
mol m
3
0
1
2
3
4
5
6
0,0
0,2
0,4
0,6
Time, h
pH 8.1
in situ
N.D.
pH 6.0
pH 7.0
pH 8.0
pH
8.0
Linear
y = A+B*x
r
2
= 1 SD=0.0023
pH
7.0
Boltzmann
y=[(A
1
-A
2
)/1+
e
(x0-x)/dx
]+A
2
r
2
=
0.8826
pH
6.0
Boltzmann
y=[(A
1
-A
2
)/1+
e
(x0-x)/dx
]+A
2
r
2
=
0.9036
Fig. 3.
Release of TEL to the water column as a function of time at different pH values at RCA station.
F. Arcega-Cabrera
et al.
124
TEL concentrations are lower at RCA than at RLC.
Lower TEL concentrations could be explained by:
1) the presence of a higher number of active sites in
the RLC sediments, which may prevent TEL release;
2) the organic matter content in the suspended sol-
ids (OMSS >850 g kg
-
1
) at RCA may be capturing
TEL present in the water column since, according to
Mulroy and Li-Tse (1998), a high concentration of
organic matter or hydrocarbons in particulate mate-
rial capture and isolate TEL from direct contact with
the water column; or 3) lower TEL concentrations in
RCA sediments than in RLC. Therefore, an initially
lower concentration of TEL at RCA and the in±u-
ence of particulate matter, as well as organic matter
content in the sediments could explain the differences
between the two stations.
At RTX (
Fig. 4
), TEL release rates from the sedi-
ments are similar to those found at RCA. At pH 8.0 the
concentration remains constant without a signi²cant
TEL release. Here, the higher organic matter content in
the sediments (44 g kg
-
1
) may be functioning as stabi-
lizer of TEL. Nevertheless, at pH 6.0 the concentration
at RTX is higher than at RCA, which contradicts the
expected behavior if the content of organic matter in
sediments is taken as an indirect measure of the sedi-
ments’ adsorption strength toward TEL.
Any explanation of these data must take into ac-
count our limited understanding of the distribution
of the alkyl compounds between the dissolved and
the particulate phases in natural aquatic systems. The
presence of urban wastes (labile organic material with
more active adsorption sites) in the Taxco river results
in a signi²cant amount of TEL being adsorbed to the
organic covers of the particulate material (Nevenca
and Branica 1994). Consequently, TEL at this station
could be released from sediments but more signi²-
cantly from the particulate matter.
Finally, factor analysis (
Table III
) indicates which
geochemical characteristics of the sediments affect
TEL behavior. Four factors explain 93 % of the total
variability. The ²rst factor groups TEL concentrations
only. In the second factor, suspended solids (SS) and
organic matter in sediments (OMsed) show an inverse
relationship with the ²rst factor. This relationship
demonstrates that an increase in the organic matter
content in the sediments diminishes TEL release and
corroborates the previous discussion.
The relationship of SS with the third factor, which
includes total Li, OM in the suspended solids, CaCO
3
in sediments, and an inverse relationship with dis-
solved oxygen, suggests that Li concentration in the
sediments is directly related to TEL concentrations,
suggesting transport of TEL via runoff. Moreover,
TABLE III
. FACTOR ANALYSIS OF THE TETRAETHYL
LEAD RELEASE
Variable
Factor 1
Factor 2
Factor 3
Factor 4
Total Li
0.1
-
0.3
0.93
-
0.15
SS
-
0.01
-
0.94
-
0.28
-
0.13
OMSS
0.08
-
0.64
0.73
-
0.18
OMsed.
0.0
-
0.97
-
0.12
-
0.15
CaCO
3
sed
0.09
0.39
0.92
-
0.03
X
TEL
0.97
0.06
0.13
0.21
X
Eh
0.19
0.0
-
0.12
0.94
X
Cond
-
0.13
-
0.63
0.28
-
0.67
X
T ºC
0.07
0.11
0.03
0.37
X
O
2
-
0.06
-
0.2
-
0.97
0.12
Total Prop.
0.34
0.19
0.18
0.22
SS= suspended solids; OMSS= organic matter in suspended
solids; OMsed= organic matter in sediments; CaCO
3
sed= car-
bonates percentage in sediments;
X
TEL= average tetraethyl lead
concentration,
X
Eh= average redox potential,
X
Cond= average
conductivity,
X
T ºC= average temperature;
X
O
2
= average dis-
solved oxygen concentration; Total Prop.= explained variability.
Fig. 4
. Release of TEL to the water column as a function of time at different pH values at RTX station
Tetraethyllead,
μ
mol m
3
0
1
2
3
4
5
6
0,0
0,2
0,4
0,6
Time, h
pH 8.27
in situ
N.D.
pH 6.0
pH 7.0
pH 8.0
pH
8.0
Linear
y = A+B*x
r = 1 SD=0.0004
pH
7.0
Boltzmann
y=[(A
1
-A
2
)/1+
e
(x0-x)/dx
]+A
2
r
2
= 0.8854
pH
6.0
Boltzmann
y=[(A
1
-A
2
)/1+
e
(x0-x)/dx
]+A
2
r
2
=
0.9727
TETRAETHYL LEAD RELEASE FROM SEDIMENTS
125
there is an inverse relationship with SS because most
of the suspended solids are formed by inorganic
material leached from the hydrological basin in
the post-rainy season. This signi±cant relationship
suggests that an important source of TEL could
be the hydrological basin leaching. This is cor-
roborated by the signi±cant relationship of OMSS
with TEL, which suggests that TEL could be pres-
ent and co-transported on the organic particles of
suspended material present in the water column.
It could also remain suspended depending on the
river ²ow conditions. The relationship between
carbonate (CaCO
3
) content in the sediments and
TEL concentrations shows a signi±cant dissolution
of carbonates with a decreasing pH (Arcega-Cabrera
2005), which releases TEL to the water column.
Brinckmann and Olson’s (1987) ±nding that acid-
ity is a primary factor in promoting the release of
alkyl compounds to the water column lends further
support to this conclusion.
The direct relationship between factor 4 (which
groups Eh and conductivity) and factor 1 indicates
that redox changes may affect TEL concentration.
Under oxidizing conditions, TEL could be released
from the water column because fewer organic-matter
bonding sites are available for TEL adsorption in
an oxidized sediment. Tetraethyl lead’s inverse re-
lationship with conductivity in factor 4 evidences
a competition for adsorption sites in the particulate
matter between TEL and other ions present in the
water. Consequently, if conductivity increases, TEL
will tend to stay in the sediments.
Temperature and oxygen concentration are in-
direct parameters that re²ect microbial activity in
relation to TEL contents in sediments. The lack of
a relationship between TEL and temperature and its
inverse relationship with oxygen show that microor-
ganisms probably did not participate in the formation
of TEL. Thus, if an
in situ
formation of TEL happens
in the area it could be due to chemical reactions
forced by changes in pH.
In conclusion, these data showed that in a closed
controlled natural system, under changing pH con-
ditions, there is a release of TEL from the bed river
sediments to the water column. In the sampling
station down river of an active mine and a tailing,
the released TEL at pH 6 or below, almost instantly,
exceeded the US EPA’s recommended safe levels
and, hence, is a potential risk to area residents that
use this water. The data also illustrate that the com-
plex geochemical characteristics of the sediment and
the pH of the water column interact to control the
behavior of TEL. It is possible that TEL enters the
water column via wet deposition, run-off, and leach-
ing of the hydrological basin. Although this study
evidenced an increase of tetraethyl lead concentration
in the water column under pH changing conditions,
further research is needed to evaluate tetraethyl lead
concentrations in sediments and possibly in biota.
ACKNOWLEDGMENTS
The authors thank Dr. Óscar Talavera Mendoza
and the Escuela de Ciencias de la Tierra, Universi-
dad Autónoma de Guerrero for allowing us to use
their laboratory facilities and Dr. Juventino García
Alejandre for his help in treating samples. F.A.C.
thanks CONACyT for a scholarship that helped
fund this research. Funding was also provided by the
CONACyT-SEMARNAT C01-0017-2002 project.
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