ABSTRACT: Chemical weathering and soil removal rates are responsible for the Earth’s landscape, composition of surface and ground-
water, producing the soils and buffering the composition of the atmosphere. This study aimed to assess the chemical weathering and
soil removal rates in the Sorocaba River basin, São Paulo State, Brazil, allowing answering the questions about the dynamics of fluvial
transport of dissolved and suspended solids, the chemical weathering processes and associated atmospheric/soil CO2 consumption, and
the relationship between chemical weathering and soil erosion rates. The annual specific flux of total suspended solids and total dissolved
solids were 49.59 and 60.97 t/km2/yr. The chemical weathering process dominant in the Sorocaba River basin was the monosiallitization
(RE = 2.4), with an associated atmospheric/soil CO2 consumption of 2.3 × 105 mol/km2/yr. The chemical weathering and soil removal
rates were 7.2 and 29.8 m/Myr, respectively, indicating a soil thickness reduction. Finally, the soil removal rate in the Sorocaba River basin
is almost 3-fold higher than the Cenozoic soil removal rates, being this difference related to the current land use which increased the soil
removal processes.
KEYWORDS: Fluvial geochemistry; disturbed watershed; water-rock interactions; rainwater and anthropogenic influences.
DOI: 10.1590/2317-4889202020190030
ARTICLE
INTRODUCTION
Chemical weathering is typically a destructive process,
which allows the development of new minerals from the
weathering of primary minerals. In addition, water-rock inter-
actions are responsible for the Earth’s landscape, composition
of surface and groundwater, producing the soils and buffering
the composition of the atmosphere, being this process one of
the main mechanisms of atmospheric CO2 removal and con-
sequent deposition of carbonates Ca2+ and Mg2+ in oceans,
playing an important role in moderating terrestrial climate
(Gaillardet et al. 1999, Millot et al. 2002). Residual products
are subject to other processes of the supergene cycle, such as
erosion, transport, and sedimentation, which ultimately lead
to continental denudation, with consequent flattening on the
relief (Teixeira et al. 2000).
Pioneering studies to investigate the nature and composition
of the dissolved and suspended load transported by rivers were
performed in the 1960-70s (Barth 1961, Johnson et al. 1968,
Gibbs 1970, Tardy 1971, Martin and Meybeck 1979). Since
then, many studies have been carried out to assess chemical
weathering and soil erosion rates using mass-balance models
adjusted to atmospheric and anthropogenic (mainly originating
from domestic sewage and industrial and agricultural activities)
contributions, once the total river fluxes integrate the contribu-
tions of these different sources (Probst 1986, 1992, Meybeck
1987, Lasaga et al. 1994, White and Blum 1995, Boeglin and
Probst 1996, 1998, Boeglin et al. 1997, Gaillardet et al. 1999,
Semhi et al. 2000, Millot et al. 2002, Meybeck et al. 2003,
Walling and Fang 2003, Riebe et al. 2004, Chakrapani 2005,
Weijden and Pacheco 2006, Louvat et al. 2008, Gurumurthy
et al. 2012, Laraque et al. 2013, Li et al. 2014). The interest in
assessing the chemical weathering and soil removal rates in
watersheds under different geological and climatic setting also
occurred in Brazil (Stallard and Edmond 1981, 1983, 1987,
Moreira-Nordemann 1980, 1984, Mortatti et al. 1997, 2008,
Gaillardet et al. 1997, Bortoletto Junior et al. 2002, Conceição
and Bonotto 2003, 2004, Mortatti and Probst 2003, Bonotto
et al. 2007, Sardinha et al. 2010, Fernandes et al. 2012, 2016a,
Conceição et al. 2015, Couto Júnior et al. 2019, Spatti Júnior
et al. 2019).
The state of São Paulo established 21 units of Water
Resources Management (UGRHI), according to Law
No. 7,663, published in December 30th, 1991 (São Paulo
1991). The Sorocaba River basin belongs to UGRHI-10
(Médio Tiête — Sorocaba), presents well-defined climatic
seasonality (tropical climate) and a diverse geological and
geomorphological context. Successive cycles of development
and diversification of human activities have occurred since
its occupation in the seventeenth century. Nowadays, this
watershed covers 18 municipalities (1,212,376 inhabitants),
an important industrial park, with over 1,850 enterprises and
Hydrochemistry applied to assess the chemical
weathering and soil removal rates in the Sorocaba
River basin, São Paulo State
Alexandre Martins Fernandes1 , Fabiano Tomazini da Conceição1* , Jeferson Mortatti2
Brazilian Journal of Geology
SO
CI
ED
AD
E BRASILEIRA DE GEOLO
G
IA
DESDE 1946
BJGEO
© 2020 The authors. This is an open access article distributed
under the terms of the Creative Commons license.
1Universidade Estadual Paulista “Júlio de Mesquita Filho” – Rio Claro (SP),
Brazil. E-mails: alefernandes1966@yahoo.com.br, ftomazini@rc.unesp.br,
fabiano.tomazini@unesp.br
2Universidade de São Paulo – Piracicaba (SP), Brazil.
*Corresponding author.
1
http://orcid.org/0000-0002-7059-3142
http://orcid.org/0000-0002-3625-4788
http://orcid.org/0000-0002-5002-7928
mailto:alefernandes1966@yahoo.com.br
mailto:ftomazini@rc.unesp.br
mailto:fabiano.tomazini@unesp.br
large agricultural areas (IBGE 2010). Approximately 65% of
the demands for public supply in the Sorocaba River basin are
supplied by Itupararanga Reservoir (IPT 2006). Despite its
importance, few studies have been conducted in the Sorocaba
River basin related to the rainwater chemical composition and
annual atmospheric deposition (Conceição et al. 2011, 2013),
the chemical weathering rates in the Upper Sorocaba River
basin (Sardinha et al. 2010, Fernandes et al. 2016a), the water
quality of the Itupararanga Reservoir (Pedrazzi et al. 2013,
2014), and the origin and flux of trace elements and isotopic
composition of particulate organic matter in suspended sed-
iment (Fernandes et al. 2012, 2016b).
Thus, this study aims to assess the chemical weathering
and soil removal rates in the Sorocaba River basin, allowing
answering the following questions:
• What are the dynamics of fluvial transport of dissolved
and suspended solids?;
• What are the chemical weathering processes and associ-
ated atmospheric/soil CO2 consumption?;
• What is the relationship between the chemical weathering
and soil removal rates?
STUDY AREA
The Sorocaba River basin is located in the southeast-
ern portion of São Paulo State, Brazil, between latitudes 23
and 24ºS and longitudes 47 and 48ºW, and occupies an area
of 5,269 km2. Considered the most important tributary of
the left bank of Tietê River, Sorocaba River travels 227 km
before flowing into Tiete River, in Laranjal Paulista munic-
ipality (IPT 2006). This watershed is inserted into two
main geomorphological units: Atlantic Plateau and Paulista
Peripheral Depression (Ross 1996 — Fig. 1). The Atlantic
Plateau presents metamorphic rocks belonging to the São
Roque Group and Embu Complex, with associated granitic
rocks (Godoy et al. 1996). The relief is comprised of hills
shapes with convex tops and deep valleys with altitudes that
range between 800 and 1,000 m a.s.l. and slope above 20%
(Ross 1996, Perrota et al. 2005). In the Paulista Peripheral
Depression outcrop the sedimentary rocks belonging to the
Parana Sedimentary Basin (Paleozoic-Mesozoic), i.e., Itararé
Group (diamictic, sandstones, mudstones, and rhythmites),
Guatá Group (siltstones and sandstones), and Passa Dois
Group (siltstones, mudstones, and shales) (Conceição and
Bonotto 2004, IPT 2006). The relief presents hills with tab-
ular and large convex tops, prevailing altitudes between 600
and 700 m a.s.l. and slopes ranging from 5 to 10% (Ross
1996, Perrota et al. 2005).
The predominant soils in the study area are Red Argisol
(49%), Red Latosol (38%), and Red-Yellow Latosol (9%),
according to the Brazilian soil classification (EMBRAPA 2013,
Oliveira et al. 1999), corresponding to Ultisols and Oxisols in the
USDA nomenclature (USDA 1999), respectively. Forests, fields,
and Savanna characterized the original vegetation. Currently,
with the agricultural occupation and the urbanization pro-
cesses, land use is characterized by the predominance of the
Figure 1. Geological map of Sorocaba River basin with location of the fluvial sampling point at the Tatuí municipality, and the pluviometric
and fluviometric stations (E4-019 and 4E-004, respectively).
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Braz. J. Geol. (2020), 50(1): e20190030
pastures and fields (77%), followed by areas with agricultural
crops (14%), reforestation areas (3%), original vegetation
cover (2%), and urban areas (4%) (IPT 2006).
The climate is Cwa type, according to the Köppen classi-
fication (Köppen 1948), characterized by the predominance
of rainfall in summer and dryness in winter, with an average
annual temperature of 18 to 22ºC (IPT 2006). Figure 2A shows
the monthly averages of rainfall and discharge in the Sorocaba
River basin from 1979 to 2008, calculated from the monthly
historical data of the Pluviometric station E4-019 (23º20’S,
47º41’W) and the Fluviometric station 4E-004 (23º19’S,
47º46’W) (DAEE 2010), respectively. During this period,
the average annual rainfall was 1,276 mm, where January and
August were the months with the highest and lowest rainfall
values (236 and 35 mm, respectively). In the same historical
period, the average annual discharge was 53.8 m3/s, with the
highest monthly average in February (98.3 m3/s) and lowest
in August (33.7 m3/s). Figure 2B shows a significant positive
linear correlation between the average monthly values of rain-
fall and discharge of these 30 years.
MATERIALS AND METHODS
Sampling and analytical methods
The river sampling point was established approximately
500 m upstream from the confluence of the Sorocaba and
Tatuí rivers, in the municipality of Tatuí (Lat. 23º19’09”S,
Long. 47º46’44”W), as can be seen in Figure 1, covering an
area of 3,942 km2, i.e., 74.8% of the total area of the Sorocaba
River basin, with a total population of 1,061,023 inhabitants
(IBGE 2010) and the urban sewage treatment percentage esti-
mated at 17.5% (IPT 2006). This sampling point was chosen
due to there being a fluviometric station installed (limnimet-
ric ruler and an automatic limnigraph), managed by DAEE/
CTH, with daily discharge data since 1940. These data were
used to validate the discharge measurements performed during
the sampling period.
Twelve fluvial water sample collections were carried out at
the Sorocaba River, covering one complete hydrological cycle
( Jun/2009 to Jun/2010). Sorocaba River waters (1,000 mL)
were collected in each sampling at 1.5 m deep, using a sin-
gle-stage punctual sampler. The samples were separated into
two 500 mL aliquots, one crude and the other preserved with
0.1 mL of concentrated H2SO4. Both aliquots were stored in
identified polyethylene bottles and kept at 4ºC until labora-
tory processing.
Discharge (Q), hydrogenionic potential (pH), electrical
conductivity (EC), and temperature (T) were characterized
in the field using direct reading equipment. The discharge was
represented by the product of the wet river channel cross-sec-
tion area (m2), obtained by bathymetry, and the average veloc-
ity of the water flow in this section (m/s) quantified using a
Digital Micromolinete Global Water FP 101. The pH values
were determined using a DM2 Digimed portable pHmeter,
with a relative accuracy of 0.01% and calibrated with stan-
dard solutions DM-S1B (pH 4.01) and DM-S1A (pH 6.86).
In addition, EC and T were quantified using the Digimed DM3
sensor, with a resolution of 0.01 mS/cm, relative accuracy of
0.05% and automatic temperature compensation, previously
calibrated with conductivity standard solutions DM S6A
(1,412 mS/cm and DM S6B (146.9 mS/cm).
Crude fluvial water samples were filtered through cellulose
membrane filters (0.45 mm), previously dried and weighed.
These filtered samples were analyzed by ion chromatography
Dionex ICS-90 equipped with analytical columns IonPac® CS12A
4x250 mm and IonPac® AS14A 4x250 mm, for the quantification
of dissolved ions (Na+, K+, Ca2+, Mg2+, Cl-, SO4
2-, PO4
3-, and NO3
-
), with a detection limit of 0.001 mg/L (Dionex Corporation
2004) and quantification limit of 0.01 mg/L (Ribani et al.
2004). The HCO3
- was represented by the alkalinity content
and was quantified by the Gran method (Edmond 1970). The
preserved fluvial water samples were filtered through a glass
fiber membrane filter (0.3–0.6 mm) and used in the quantifi-
cation of dissolved Si4+ concentration by optical emission spec-
trometry with inductively coupled argon plasma, ICP-OES
Optima 3000 DV, with a detection limit of 0.02 mg/L, and the
result was expressed in terms of SiO2. The total dissolved sol-
ids (TDS) correspond to the sum of dissolved cations, anions
and silica. The total suspended solids (TSS) was quantified
by gravimetry (APHA 1999), considering the retained mate-
rial in the cellulose membrane filter after drying in a stove at
A B
Figure 2. (A) Monthly average rainfall and discharge for a 30-year period (1979–2008) in the Sorocaba River basin, and (B) relationship
between the monthly average rainfall and discharge for the same period.
3
Braz. J. Geol. (2020), 50(1): e20190030
60ºC to constant weight. The analysis of the river water sam-
ples was performed at Stable Isotope Laboratory (dissolved
ions, HCO3
- and TSS) and Analytical Chemistry Laboratory
(dissolved silica), both located at CENA/USP.
Theoretical background
The fluvial fluxes (FW, in t/km2/yr) of dissolved chemical
species, TDS and TSS related to chemical weathering and soil
removal processes, were calculated using a mass balance model
expressed in Equation 1 (White and Blum 1995), consider-
ing negligible the fluxes from the biomass change and derived
from the ionic exchange sites in clay minerals.
FW = Friver – Frainfall – Fanthropogenic (1)
In which:
Friver = the measured river flux (t/km2/yr);
Frainfall = the atmospheric inputs (t/km2 /yr);
Fanthropogenic = the anthropogenic influences (t/km2/yr).
The RE index can be used to determine the predominant
process of chemical weathering of rocks in a drainage basin.
Initially proposed by Tardy (1971), this index is equiva-
lent to the molecular ratio (SiO2)/(Al2O3) of secondary
minerals neoformation within the soil profile. Boeglin and
Probst (1998) modified the RE index, being expressed by
the molar ratio of chemical dissolved species in the surface
waters (Eq. 2).
RE= 3K + 3Na + 2Ca + 1.25Mg – SiO2
0.5K + 0.5Na + Ca + 0.75Mg
(2)
The atmospheric/soil CO2 consumption during chemical
weathering processes (FCO2 – in mol/km2/yr) was estimated
by the sum of corrected fluxes of Na+, K+, Ca2+, and Mg2+ (F(ion)
sil - in mol/km2/yr), according to Equation 3 (Gaillardet et al.
1999, Gurumurthy et al. 2012).
FCO2 = FNa sil + FK sil + 2 FCa sil + 2 FMg sil (3)
The chemical weathering of rocks (IQ – in t/km2/yr) can
be estimated through the sum of the corrected annual fluvial
flux of Na+, K+, Ca2+, Mg2+, and SiO2 (FW(ion) - in t/km2/yr),
i.e., after correction of atmospheric inputs and anthropogenic
contributions, according to Equation 5 (Probst 1992). The
ratio among the IQ and the average density of rocks for the
watershed represent the chemical weathering rate (Wq - in m/
Myr), as expressed in Equation 5.
IQ = FW(Na+) + FW(K+) + FW(Ca2+) + FW(Mg2+) + FW(SiO2) (4)
Wq=
IQ
ρ
(5)
The soil removal rates (Wm in m/Myr) can be calculated
through Equation 5; however, the use of corrected TSS annual
flux (t/km2/yr) and the average soil density (g/cm3) is nec-
essary instead of IQ and average density of rocks, respectively
(Mortatti et al. 1997, Boeglin and Probst 1998).
RESULTS
Table 1 shows the results of Q, pH, EC, T and the concen-
trations of dissolved ions and SiO2, TDS, and TSS, with their
respective discharge weighted average for the study period.
The discharge showed seasonal variation in consonance with
the historical data of the monthly average (Fig. 2A), with the
highest value obtained in Jan/2010 (230.40 m3/s) and the low-
est in Jun/2009 (28.77 m3/s). Despite the similar seasonality,
Table 1. Physical and chemical parameters for the Sorocaba River surface waters.
Parameter Unit
Sampling date
CWAVJun/09 Jul/09 Aug/09 Sep/09 Nov/09 Dec/09 Jan/10 Feb/10 Mar/10 Apr/10 May/10 Jun/10
Q m3/s 28.77 32.60 81.35 67.01 110.03 228.58 230.49 118.64 98.56 71.79 48.85 31.64 95.69
pH 6.8 6.9 6.9 6.9 6.7 6.5 6.6 6.8 6.8 6.9 6.8 6.9 6.7
EC mS/cm 136.9 141.8 108.6 110.9 82.2 70.9 73.7 98.1 89.5 99.0 115.0 128.0 104.6
T ºC 16.7 17.0 16.5 19.5 26.3 26.2 27.5 25.0 26.8 25.5 22.0 20.3 22.4
SiO2 mg/L 34.00 28.00 13.72 13.66 14.00 9.91 9.53 11.70 11.58 12.07 16.00 28.00 13.05
Ca2+ mg/L 17.92 16.90 9.20 10.40 9.00 5.69 6.80 10.40 11.70 13.41 13.08 15.00 9.43
Mg2+ mg/L 1.32 1.02 0.90 0.97 0.86 0.80 0.87 0.92 1.00 1.10 1.10 1.40 0.93
Na+ mg/L 19.15 15.92 10.59 11.41 6.38 5.64 5.61 7.22 8.03 8.65 10.94 13.82 8.03
K+ mg/L 2.26 1.90 1.70 1.83 1.65 1.47 1.70 1.75 1.90 2.00 2.10 2.30 1.75
HCO3
- mg/L 51.31 45.05 36.03 38.59 35.21 25.77 30.26 38.00 36.20 37.99 39.86 43.41 34.25
Cl- mg/L 18.90 15.20 7.86 8.62 4.26 3.63 4.40 5.48 6.28 7.50 8.64 10.75 6.22
SO4
2- mg/L 7.74 6.56 4.47 4.82 4.08 2.71 2.37 2.72 4.40 5.28 5.99 9.25 3.88
NO3
- mg/L 4.82 3.10 2.06 2.29 1.53 0.66 0.87 1.45 3.40 4.00 5.38 5.61 2.02
PO4
3- mg/L 0.13 0.16 0.07 0.04 0.07 0.02 0.03 0.05 0.16 0.16 0.19 0.21 0.07
TDS mg/L 157.55 133.81 86.61 92.63 77.05 56.29 62.45 79.69 84.64 92.16 103.28 129.74 79.64
TSS mg/L 19.50 31.00 70.33 50.83 105.33 74.00 66.33 25.83 66.67 41.67 18.33 11.83 59.56
CWAV: weighted average element/compound concentration for the study period; Q: discharge; EC: electrical conductivity; T: temperature; TDS: total dissolved
solids; TSS: total suspended solids.
4
Braz. J. Geol. (2020), 50(1): e20190030
the average discharge for the study period (95.69 m3/s) was
1.8 times higher than the historical annual average for the
period of 1979–2008 (53.8 m3/s). This is justified by the fact
that the rainfall in the study period (2,101 mm) was higher
than the historical average (1,276 mm), with a direct impact
on the discharge values. During the historical period, a sim-
ilar occurrence was observed only in 1983, with an annual
rainfall of 2,054.0 mm and average discharge of 143.49 m3/s.
The Sorocaba River waters presented a pH close to neutral,
ranging from 6.5 to 6.9. The EC showed a significant seasonal
variation (annual average of 104.6 mS/cm), with values below
74 mS/cm in the months of highest rainfall and discharge, and
values above 135 mS/cm in Jun and Jul/2009. During the dry
season (May to October), EC values were higher than the
expected limit for natural waters, i.e., 100 mS/cm (Hermes
and Silva 2004). The T followed the seasonal variation, with
the lower values in winter (16.5ºC in Aug/2009) and higher
in summer (27.5ºC in Jan/2010).
The concentration of [TSS] was directly related to the
discharge (Fig. 3A). According to Probst (1986), for most
world rivers the model obtained for the relationship between
[TSS] and Q ([TSS] = a.Qb) presents positive b exponent
with values between 1 and 2, indicating that the increase in
[TSS] is a function of the discharge increase. This exponent
in the model established for the Sorocaba River was 0.7039,
indicating that the [TSS] was also influenced by rainfall. This
influence is highlighted in the November and December 2009,
when the fluvial water sampling was performed after two days
of significant precipitation, with accumulated volumes of 45.8
and 25.9 mm, respectively.
On the other hand, the relationship between [TDS] and
discharge was inverse and significant (Fig. 3B), which charac-
terizes the dilution process with increasing discharge. Among
the dissolved chemical species that composes the TDS, eval-
uable on a molar basis of CWAV, the anionic predominance of
HCO3
- (33.1%) was verified, followed by Cl-, SO4
2-, NO3
-,
and PO4
3-, while for the cations the greatest participation was
Na+, with 20.6%, followed by Ca2+, Mg2+, and K+, respectively,
and the SiO2 represented 12.8% of the TDS. The relationship
“sum of cation vs. sum of anion” (Probst 1992), in meq/L,
indicated a deficit of anionic charge in the charge balance
(Fig. 3C). It can be attributed to the presence of dissolved
organic anions not counted in this study, such as dissolved
organic carbon (Probst et al. 1992, Boeglin and Probst 1996,
Laraque et al. 2013).
DISCUSSION
Dynamics of fluvial transport in the
Sorocaba River basin
The fluvial fluxes integrate the contributions of the chem-
ical weathering and soil removal processes that occur in nat-
ural watersheds. However, nowadays it is also necessary to
consider the atmospheric inputs and anthropogenic influ-
ences in the fluvial dynamics (Stallard and Edmond 1981,
Mortatti et al. 1997, Semhi et al. 2000, Bortoletto Junior et al.
2002, Conceição and Bonotto 2004, Weijden and Pacheco
2006, Mortatti et al. 2008, Conceição et al. 2010, Hissler
et al. 2015, 2016).
The Friver of dissolved chemical species, TDS and TSS were
quantified in the specific transport form, the result of the prod-
uct between CWAV and average discharge of the study period
weighted by surface of study area, according to the stochastic
methodology proposed by Probst (1992). Frainfall was repre-
sented by the specific input of solute, obtained from the total
precipitation in the study period (2,101 mm) and the aver-
age concentration of dissolved chemical species obtained by
Fernandes (2012).
The Fanthropogenic for dissolved chemical species, TDS
and TSS were obtained using secondary data, despite the
uncertainties associated with these data regarding the real-
ity of the studied basin. In relation to dissolved load, it was
considering the per capita values of the dissolved chemical
species present in untreated domestic effluents discharged
directly in the river (g/hab/day) established by Mortatti
et al. (2008, 2012) for the Médio Tietê basin (SiO2 = 0.84,
Ca2+ = 7.50, Mg2+ = 1.3, Na+ = 13.1, K+ = 2.6, HCO3
- =
42.0, Cl- = 7.1, and SO4
2- = 12.5), and the total population
upstream of the sampling point (1,061,023 inhabitants).
The anthropogenic contribution of SiO2 was considered
negligible, such as reported in other studies (Mortatti
et al. 2008, 2012). On the other hand, the Fanthropogenic asso-
ciated to suspended sediment load was represented by the
per capita TSS load contained in untreated urban sewage
(0.022 kg/hab/day), obtained from average production
of untreated urban sewage (100 L/hab/day) and respec-
tive TSS average concentration (220 mg/L), both global
references data published by Tchobanoglous and Burton
(1991), the total population upstream of the sampling point
and the respective percentage of urban sewage treatment
(17.5%) (IBGE 2010).
A B C
Figure 3. Relationships (A) between discharge and [TSS] and (B) between discharge and [TDS], (C) and charge balance in the Sorocaba
River in the study period, with S+ and S- corresponding to total dissolved cations and anions, respectively.
5
Braz. J. Geol. (2020), 50(1): e20190030
http://a.Qb
The fluxes of cations, anions, silica, TDS, and TSS in the
Sorocaba River basin are shown in Table 2. The total fluvial
flux of TDS was 33% higher than that observed to TSS flux.
Among the dissolved chemical species, the HCO3
- presented
the highest fluvial flux, corresponding to 43% of TDS, fol-
lowed by SiO2 (16.4%), Ca2+ (11.8%), Na+ (10.1%), and
Cl- (7.8%), while the fluvial flux presented by SO4
2-, NO3
-,
K+, Mg2+ and PO4
3- were lower than 5 t/km2/yr and together
represented the remaining 10.9% of TDS. The atmospheric
inputs account for 17.3% of the total specific flux of TDS in
the Sorocaba River. Regarding the anthropogenic inputs,
there was a higher contribution to the dissolved load (ca.
14% of the fluvial TDS) than to the suspended solids load
(ca. 4% of the fluvial TSS).
Assuming that the suspended load represents approx-
imately 90% of the total sediment river flux (Walling and
Fang 2003), the specific flux of the total suspended solids
exported by the Sorocaba River was estimated at 45.59 t/
km2/yr. After correction of the anthropogenic contribu-
tions, the specific flux related to the soil removal (Fw) was
43.81 t/km2/yr. According to the classification proposed
by Meybeck et al. (2003) for the world’s rivers, from very
low to extremely high, the soil removal in the Sorocaba
River basin was considered as medium-specific sediment
flux (range from18.25 to 73 t/km2/yr).
Chemical weathering processes and
atmospheric/soil CO2 consumption
The weathering process is characterized according to the
classification proposed by Pedró (1966), where RE ≈ 0 char-
acterizes the total hydrolysis process called allitization, with
only aluminum and iron fixed as insoluble hydroxides; when
RE ≈ 2, the process is called partial hydrolysis with monosial-
litization, occurring the kaolinite formation; and to RE ≈ 4
the predominant process is the partial hydrolysis with bisial-
litization and is related to the formation of mineral 2:1, such
as montmorillonite.
The predominant process of chemical weathering of rocks
in the Sorocaba River basin, was determined using the RE
index (Eq. 2) and corresponded to 2.4, value that character-
izes the predominance of partial hydrolysis with a tendency to
monosiallitization, i.e., to the kaolinite stability domain, sim-
ilar to that observed in the Amazonas River basin (Mortatti
and Probst 2003). However, in two watersheds (Tietê and
Piracicaba river basins) located in the same region of the
Sorocaba River, a different situation was verified, i.e., a ten-
dency to the bisiallitization domain, probably due to extensive
agricultural areas with a high degree of soil tillage, fact that
may influence the remobilization of major ions instead of sil-
ica (Bortoletto Junior 2004).
According to Conceição and Bonotto (2004) and Fernandes
et al. (2016a), the main minerals found in the igneous and
metamorphic rocks of the Sorocaba River basin are bio-
tite (K(Mg,Fe)3(Si3Al)O10(OH)2), muscovite (KAl2(Si3Al)
O10(OH)2), sillimanite (Al2SiO5), quartz (SiO2), microcline
(KAlSi3O8), oligoclase ((Na,Ca)(Si,Al)4O8), and hornblende
(Ca2Na(Mg,Fe)4(Al,Fe,Ti)AlSi8AlO22(OH)2). For sedimen-
tary rocks, quartz, albite (NaAlSi3O8), microcline, kaolinite
(Al2Si2O5(OH)), and illite (K0.9Al2Si4O10(OH)2) were high-
lighted. Theoretically, the weathering reactions involving the
mineral rock of the Sorocaba River basin indicate that the Na+
has its origin in the hydrolysis of albite, hornblende and pla-
gioclase; K+ ions are derived from the hydrolysis of muscovite,
microcline, biotite and illite; the Ca2+ can be attributed to the
hydrolysis of hornblende and plagioclase; and Mg2+ can be
released by the hydrolysis of hornblende and biotite. In addi-
tion, the Sorocaba River basin does not contain volumetri-
cally significant Cl–, NO3
–, PO4
3– or SO4
2– bearing minerals.
Therefore, only small inputs of these anions are expected in
the rivers due to water-rock interactions. Quartz and kaolin-
ite are not weathered and remain in the soil profile, as well as
the supergene minerals, i.e., kaolinite, goethite (FeOOH),
and rutile (TiO2).
The atmospheric/soil CO2 consumption during the
chemical weathering processes in the Sorocaba River
basin was obtained using Equation 3 and corresponded
to 2.3 × 105 mol/km2/yr. This value was lower than that
observed in the Tietê River basin (3.8 × 105 mol/km2/yr,
Bortoletto Junior 2004). However, it was higher than other
Brazilian watersheds, such as the Amazonas Basin (0.3 ×
105 mol/km2/yr, Mortatti and Probst 2003) and Jamari
and Jiparana basins (0.8 × 105 and 1.4 × 105 mol/km2/yr,
respectively, Mortatti et al. 1992) in northern region, or in the
Paraná Basin (0.9 × 105 mol/km2/yr, Gaillardet et al. 1999)
and Piracicaba Basin (1.4 × 105 mol/km2/yr, Bortoletto Junior
2004), both in the Southeastern Brazilian region.
Chemical weathering
and soil removal rates
The IQ value in the Sorocaba River basin, obtained
using Equation 4 and the data of Table 2, corresponded
to a flux of 19.1 t/km2/yr, representing 31.4% of TDS flux
at the river. The Amazonas and Tietê River basins showed
higher fluxes (IQ) than that observed for the Sorocaba River
*Data reported in Fernandes (2012).
Table 2. The annual flux (t/km2/yr) of total suspended solids (TSS), total dissolved solids (TDS), dissolved silica, cations and anions in the
Sorocaba River basin.
Species TSS TDS SiO2 Ca2+ Mg2+ Na+ K+ HCO3
- Cl- SO4
2- NO3
- PO4
3-
Friver 45.59 60.97 9.99 7.22 0.71 6.15 1.34 26.22 4.76 2.97 1.54 0.06
Frainfall
* --- 10.57 --- 2.99 0.12 0.40 0.28 3.60 0.71 1.24 1.18 0.05
Fantropogenic 1.78 8.54 0.08 0.74 0.13 1.29 0.26 4.13 0.70 1.23 --- ---
Fw 43.81 41.85 9.91 3.49 0.47 4.46 0.80 18.49 3.35 0.50 0.36 0.01
6
Braz. J. Geol. (2020), 50(1): e20190030
basin, with 32.2 e 41.4 t/km2/yr, respectively (Mortatti
and Probst 2003, Bortoletto Junior 2004). The chemical
weathering rate (Wq) for the Sorocaba River basin was
calculated using Equation 5 and the regional value of the
mean density of rocks (2.65 g/cm3 — Brasil 1983) and
corresponded to 7.2 m/Myr. This rate was 22 and 54%
higher than those obtained for the Tietê (5.9 m / Myr)
and Piracicaba (4.7 m / Myr) river basins, respectively
(Bortoletto Junior 2004).
The soil removal rate in the Sorocaba River basin was
29.8 m/Myr, considering that the average soil density is 1.47
g/cm3 (Fernandes et al. 2012). This rate was lower than that
observed in the Amazonas River basin (123 m/Myr, Mortatti
and Probst 2003) and higher than that in the Jamari and
Jiparaná river basins (6.5 m/Myr in both basins, Mortatti
et al. 1992). The Tietê and Piracicaba river basins, located
in the same geographical region as the Sorocaba River, pre-
sented higher rates when compared to those obtained in this
study, i.e., 42.6 and 37.0 m/Myr, respectively (Bortoletto
Junior 2004). Considering the chemical weathering and soil
removal rates in the Sorocaba River basin (7.2 and 29.8 m/
Myr, respectively), in the present climatic setting, there is a
soil thickness reduction.
The cooling/denudation crustal rates quantified using
apatite fission track (AFT), apatite (U-Th)/He (AHe) and
in situ cosmogenic 10Be could be used to compare the pres-
ent soil removal rates obtained by a fluvial mass-balance
with the Cenozoic soil removal rates. Values of past denu-
dation obtained in southeast Brazil ranging from 8.8 to
15.7 m/Myr, using in situ cosmogenic 10Be (Cherem et al.
2012). Hackspacher et al. (2004) used AFT ages to indicate
a cooling/denudation rate of 11 m/Myr at the boundary
between the Paraná Sedimentary Basin and the basement
rocks. The soil removal rate in the Sorocaba River basin
(29.8 m/Myr) is almost 3-fold higher than the estimates of
Cenozoic denudation reported Cherem et al. (2012) and
Hackspacher et al. (2004). This difference can be explained
by the present land use in the Sorocaba River basin, where
the replacement of the original vegetation by agricultural
and livestock activities increased the erosion processes and,
consequently, the present denudation rates, even though
the study region has remained roughly in the same latitude
during the drift to west of South America, since the time
of the separation of continents and the basalt eruptions of
the Serra Geral Formation.
Couto Júnior et al. (2019) evaluated three different sce-
narios from land use changes and how they have affected
soil loss in a watershed located in the PPD, using the USLE
model. Similar to Sorocaba River basin, the main types of
soils occurring in the studied area were Ultisol and Oxisol.
The authors verified a similar soil removal rate, it was almost
3-fold higher than the long-term denudation rates suggested
by the literature for the Peripheral Depression, and reinforced
that the increase in the denudation rate is mainly related to
land use/land cover changes than to the soil type present in
the studied area.
CONCLUSION
This study aimed to evaluate the chemical weathering of
rocks and soil removal processes that occur in the Sorocaba
River basin and allowed a better understanding of the dynam-
ics of fluvial transport of dissolved and suspended solids, of
the chemical weathering processes and the atmospheric/soil
CO2 consumption and of the relationship between chemi-
cal weathering and soil removal rates. The TSS concentra-
tion was directly related to the discharge and influenced by
rainfall, with higher concentrations recorded after rainfall
events. However, the TDS concentration showed dilution
behavior in a wet period. The annual specific flux of TDS
was 60.97 t/km2/yr, but after the atmospheric inputs and
anthropogenic contributions (ca. 17 and 14%, respectively)
this value was corrected to 41.85 t/km2/yr and represents
the fluvial flux related to the chemical weathering of rocks.
The total annual specific flux of TSS was 45.59 t/km2/yr,
with a small portion derived from the anthropogenic contri-
butions (ca. 4%). The chemical weathering process showed
a tendency to monosiallitization (RE = 2.4), with an atmo-
spheric/soil CO2 consumption rate of 2.3 × 105 mol/km2/yr.
The chemical weathering and soil removal rates were 7.2
and 29.8 m/Myr, respectively, indicating a soil thickness
reduction. The present soil removal rate in the Sorocaba
River basin was almost 3-fold higher than the Cenozoic soil
removal rates, reinforcing that the human-landscape sys-
tems are complex and affect the natural denudation rates,
and, consequently, the present landscape evolution in the
State of São Paulo.
ACKNOWLEDGMENTS
The authors are grateful to Fundação de Amparo à
Pesquisa do Estado de São Paulo (FAPESP) (Process No.
08/57104-4 and 08/09369-9) and Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq) (Process
No. 134169/2009-3), for financial support. The authors
would also like the Stable Isotope Laboratory of the Center
for Nuclear Energy in Agriculture (LIE-CENA/USP), São
Paulo, Brazil, for the research infrastructure. A. M. Fernandes
is also grateful to the Graduate Program of the Faculty of
Civil Engineering of UNESP Bauru, for the Postdoctoral
grant. Specially, Dr. Claudio Riccomini (Editor-in-Chief )
and two anonymous referees are thanked for their detailed
and insightful review comments, which helped to improve
the manuscript.
ARTICLE INFORMATION
Manuscript ID: 20190030. Received on: 04/30/2019. Approved on: 12/16/2019.
All authors wrote the first and final drafts of the manuscript and prepared all figures and tables.
Competing interests: The authors declare no competing interests.
7
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