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Hydrogeochemical responses to rainfall inputs in a small rainforest basin: Rio de Janeiro, Brazil
Phys. Chew. Earth (A), Vol. 24, No. 10, pp. 871-879, 1999 0 1999 Elsevier Science Ltd All rights reserved 1464- 1895/99/$ - see front matter
Pergamon
PII: S1464-1895(99)00129-5
Hydrogeochemical Responses to Rainfall Inputs in a Small Rainforest Basin: Rio de Janeiro, Brazil C. S. Silveira
and A. L. Coelho Netto
GEOHECO/Laboratory of Geo-Hydroecology, Department of Geography, Federal University of Rio de Janeiro (UFRJ), Ilha do Fundgo. CEP: 21 941-590 Rio de Janeiro, Brazil Received 07 April 1998; revised 02 September 1998; accepted 30 May I999 Abstract. This work provides basic information about hydrogeochemical contributions to different pathways of water in a small tropical rainforest catchment (3.5 km2) in Rio de Janeiro, Brazil. Precipitation and discharge measurements were conducted at the basin outlet, and at five other stream sites that were selected according to their hydrological behaviour and lithology. Precipitation and streamflow samples were collected weekly, over seven months, and continuously during one storm event. These streamflow samples were analyzed for Ca”, Mg” Na’, K’ and SiOZ. Precipitation, throughfall and topsoil water samples were also collected after various storm events, over six months, within a small sub-catchment and were analysed for Ca”, Mg”, K’, Na’ and pH. Results from this research showed that rainfall was acid, with low cation content. Throughfall was neutral and cations enriched. Analysis of topsoil water indicated that litter was an important source of Cal’ and K’ by means of saturation overland flow. In relation to throughfall, streamflow output was characterized by lower concentrations of K’, Ca”, and Mg”. Biotitc gneiss weathering was the main source of Mg2+ to streamflow. During stormflow periods SiOZ and Na’ concentrations decreased due to Hortonian overland flow contributions to the stream channel. The increasing concentrations of Ca2’ and K+ were due to litter leaching, especially in the zones producing saturation overland flow. Streamflow hydrogeochemistry was not only a product of weathering processes, but also of biotic processes. 0 1999 Elsevier Science Ltd. All rights reserved 1 Introduction Surface and subsurface pathways of water reflect the interactions between climate, biota, soil, relief and weathering processes, There is a strong correlation between soil water residence time and the evolution of weathering processes (Ollier, 1969; Colman, 1981; Colman and Dethier, 1986). Hydrogeochemical studies can therefore be used to understand weathering processes Hewlett and Hibbert (1967) proposed the concept of delayed flow and quickflow in order to clarify soil water residence time. In the same way Dunne and Leopold (1978) developed the concept of baseflow and stormflow. The
concept of “old water” and “new water” was more recently introduced by Pearce et al. (1986), to identify the contribution of stored water and rainwater to channels and to obtain an hydrological knowledge of a basin using an hydrogeochemical approach. However, hydrogeochemical studies are still scarce in tropical regions (Mosley, 1979; Bittencourt, 1980; Ovalle. 1985; Stahard, 1985: Pearce et al. 1986; Lesack 1993a; Lesack, 1993b; Ramirez and Andara, 1993). Experimental studies with conservative tracers and stable isotopes showed that stormflow was mainly composed by old water (Pearce et al., 1986: Hooper and Shoemaker, 1986; Neal et al. 1990; Neal et al. 1992; Lesack, 1993b; Robson et al, 1993; Anderson et al. 1997). However, working in a tropical rainforest, Mosley ( 1979) suggested that stormflow occurred as macropore flow. and therefore reflected rainwater contribution. Based on these results stormflow complexity reflects runoff generation processes that take place within a basin. Dunne and Black (1970) indicated that the final runoff of a basin was the result of partial area contributions, its extension and its location (the “variable source area” concept). Although tropical rainforest hydrology has a great complexity (Bonell, 1993), Coelho Netto (1985) developed an empirical model of distinct runoff producing zones in our study area in the Upper Cachoeira river basin. In this model roads, tracks and parking lots (cleared sites) produce Hortonian overland flow, which is thought to be responsible for the first sharp peak in a typical hydrograph of our basin. A second smoother and delayed hydrograph peak results from hollows with thick soil mantles that produce subsurface flow. Rock escarpments enhance soil moisture so that in wetter periods, when surrounded by extensive escarpments, the hollows can produce saturation overland flow, specially those with a shallow soil mantle. Saturation overland flow also contributes to the first hydrograph peak (Coelho Netto, 1985). Based on this empirical runoff generation model for our study area. detailed research on solute fluxes was conducted in order to characterize the hydrogeochemical contribution of different water pathways in a tropical mountainous rainforest catchmcnt. ldcntifying hydrogeochemical flow pathways is one approach to understand weathering processes in this environment.
871
872
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
2 Study Area Our study area was a small catchment (3.5 km*) in the southern slopes of the Tijuca massif, Upper Cachoeira River Basin Experimental Station - GEOHECO - UFRJ, Rio de Janeiro city, SE Brazil (Fig. 1). Previous studies at this basin provided knowledge of its hydrological processes (Coelho Netto, 1985; Coelho Netto, 1987; Castro Jr., 1991; Miranda et al., 1991; Nunes et al. 1992; Freire AllemBo,
BIUZU.
CACHOEIRA
RIVER
BASIN
Ai+ 0-w
Responses to Rainfall in a Small Rainforest Basin gradients above 58” and represent 1.5% of the area. Elevation ranges from 460 m to 1022 m (Coelho Netto et al., 1980). The Precambrian bedrock is composed of biotite gneiss, plagioclase-microcline gneiss and microcline granitoid (Heilbron et al., 1993) (Fig. 2). Plagioclase-microcline gneiss has a mineralogy mainly of quartz, plagioclase, microcline, biotite and minor hornblend. Biotite gneiss mineralogy is dominantly quartz, plagioclase, biotite and minor garnet and sillimanite. Microcline granitoid has a mineralogy dominantly of K-feldspar, quartz, plagioclase and biotite (Heilbron et al., 1993). Biorite gneiss is responsible for most of the low relief features in the area (Coelho Netto, 1985). The regolith mantle includes saprolite of variable thickness, which normally decreases on steeper slopes; on the other hand hillslope deposits (talus and colluvium) tend to get thicker towards the hollows axis. Pedogenetic horizons are poorly differentiated by leaching processes. Latosols with deep profiles predominate and Cambisols occur in the footslope of rock escarpments (Coelho Netto et al. 1980). Colluvial deposits consist of large boulder to pebbled particles in a sandy-clayey, poorly sorted and nonstratified matrix (Coelho Netto et al. 1980). Because of the mineralogical composition (mainly kaolinite and gibbsite; minor quartz, goethite, illite and vermiculite) soils have a low cation exchange capacity (Rosas et al, 1991). Interception by tree canopies ranges from 17% to 24% (Coelho Netto, 1985; Miranda et al., 1991). Litter water retention capacity ranges from 130% to 330% by weight (Vallejo, 1982; Coelho Neto, 1987; Miranda et al. 1991). The uppermost soil layers, including the litter and the sandy-clayey topsoil, provides high infiltration capacity. In addition biogenic processes play an important role in controlling soil macroporosity (Castro Jr., 1991) and the tree root systems favour rainwater percolation deep into the soil (Freire Allemgo, 1997).
Atlwlk_
-
Fig. 1. Study am location: GEOHECO - UFRJ Experimental Station at Upper Cachoeira river basin, Tijuca Massif, Rio de Janeiro city, Brazil.
The
present
study
area
is
part
of
the
tropical
Mean annual discharge at the basin outlet was around 0.072 m’/s, ranging from 0.023 m’/s to 0.144 m’/s in the period 1976-1996. Average monthly baseflow usually varied from 0.033 m’/s to 0.052 m’/s with lower discharge values in August/September, after the drier season. During stormflow, discharge could reach more than 2.0 m’/s. Rainfall below 10 mm did not affect baseflow conditions and separation of baseflow was difficult due to delayed subsurface flow occurrence, specially after severe rainfalls. On average, 30% of rainfall, left the basin as streamflow.
mountainous climatic zone of Rio de Janeiro city, with an average monthly temperature ranging from 19°C (July) to
3 Methods
25°C (February). Mean annual precipitation is around 2300 mm, with the summer months (December - April) being the wettest (Coelho Netto Silva and Santos, 1979). The vegetation is a secondary tropical rainforest (Tijuca National Park) that is still preserved despite surrounding urban pressure. The forest was formed over 1.50 years ago by natural succession processes (Oliveira, 1992). The average slope gradient is 18” with 50% of the slope areas ranging from 12” to 22”. Rock escarpments have slope
Since 1976 a raingauge station and a streamgauge station (the Upper Cachoeira Experimental Station), at the basin outlet, provided continuous records of precipitation and discharge. Streamflow was measured in a broad-crested weir (contracted rectangular form), with a standard vertical staff gauge and a vertical float counterweight. Rainfall was measured with a recording 0.2 mm tipping-bucket iaingauge, and also a non-recording gauge.
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
Responses to Rainfall in a Small Rainforest Basin
873
Legend .# stmamtlmsampling
site + rainfall
sampling site
H throughfau end topsoil wabx sampling site + rock sampling site
m
Biotito
[
Micmcl. eoid
m
Pla+cl. gneiss
.
??cl--h
gnaiss Mirmcl.
N
‘,_ main roads
sites 0 --
Fig. 2. Upper
Cachoarn river basin: lithological
5OOfTl
background and sampling sites location.
To conduct this study, water samples were collected in during two separate periods. During the first period six channel sites and one rainfall site were selected to monitor rainfall and streamflow based on their hydrological behaviour and lithological differences (Fig. 2). Samples were collected weekly over seven months (September 1995 - March 1996), and also at variable time intervals (observing discharge variations) during a storm event, simultaneously at four stream sites (1, 2, 4 and 6 - see Fig. 2). Stormflow samples were collected to evaluate the relationship between stormflow discharge and solute fluxes. Precipitation was also sampled weekly using a polyethylene funnel (30 cm in diameter) connected by a tube to a polyethylene bottle. This sampler was set up outside the forest. These samples were analysed for Ca”, Mg”‘, Na’, K’ and SiO,. The second period lasted over six months (April 1996 September 1996), during which an intensive sampling on a small sub-catchment (0.6 ha) was conducted. Throughfall and topsoil waters were sampled at two different sites of the sub-catchment, one near the watershed divide and the other at the bottom of the middle valley) to evaluate the chemical input. Two throughfall polyethylene collectors were installed at each site mentioned above to get data variability. Topsoil water was collected using a polyethylene trough connected by a tube to a polyethylene bottle. Troughs were horizontally inserted into the soil 5 cm, below the litter layer. After a storm event, samples were collected and the collectors were rinsed with
deionized water. Samples from this sub-catchment analysed for Cal’, Mg2’, K’, Na’ and pH.
were
All samples were collected in polyethylene bottles (120 ml), filtered through 0.45 pm pore-size membrane filters in the laboratory and promptly analysed. Ca” and Mg” were measured by flame atomic absorption spectrophotometry (Perkin Elmer 3 100 spectrophotometer calibrated with standard solutions of 0.0 mg/l, 1.0 mg/l, 5.0 mg/l and 10.0 mg/l) after addition of 10% La. Nd and K’ were measured by flame emission spectrometry (Micronal B262 spectrometer calibrated with Na’/K’ standard solutions of O.O/O.O mg/l, 7.5/2.5 mg/l and lO.O/S.O mg/l). The molybdenum blue method was used for cletermining SiO, (Beckman DU7 colourimeter calibrated with 0.0 mg/l, 3.0 mg/l, 5.0 mgll and 10.0 mg/l standard solutions). A detailed description of the analytical methods used is given by the United States Geological Survey (1979). Duplicate analyses indicated a precision of 0.07 mgll for Ca” and Mg” and 01 0.1 mgll for Na’, K’ and SiO,. pH was analysed in laboratory with a Digimed DMPH2 pH meter calibrated with buffers of pH 4 and pH 9. In total, 37 samples of rainfall, 40 samples of throughfall, 20 samples of topsoil water and 20.5 samples of streamflow were analysed. In addition to water samples this research called for an analysis of rock. One sample from fresh rock and one from a corresponding saprolite of biotite gneiss and plagioclasemicrocline gneiss were collected and sent to a laboratory for geochemical analyses (CaO, MgO, Na,O, 40, SiOZ and A&O,). The difference in concentration between elements analysed in rock and 100% were considered to be caused
814
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
Responses to Rainfall in a Small Rainforest Basin
by FeO, Fe,O, and H,O. The last two are important in soil and weathering profiles as part of oxided compounds and clay minerals (Nahon, 1986).
probably due to the formation of illite - (K, H,O) Al, (Si, Al), 0,” (OH),. Silica was also depleted from the weathering profile, but much less than alkaline elements.
The results found were compared with earlier studies of soil chemistry in the area (Ovalle, 1985; Rosas et al. 1991; Clevelkio Jr, 1996).
discharge (ma/s) 0.2
-
Average monthly baseflow
----*----
Baseflow in the study period
0.18
4 Results and Discussion
0.16
The comparison between monthly rainfall distribution during the first study period and the average monthly rainfall regime, indicated that rainfall was greater than the average in September-October-1995 and also from January to March-1996 (Fig. 3). This increase in rainfall amount was caused by an increased number of extreme rainfall days (> lOOmm/day). Such phenomena have been observed before in the area (Coelho Netto Silva and Santos, 1979). Based in this information the study period could be characterized as very wet with daily events of high intensity. monthly rainfall in the study area rainfall (mm)
-
average monthly rainfall
800
600
:
0.14 0.12
:
.*. 8’ :
i :
:
: :
:
I
: i
Fig. 4. Distribution of mean monthly baseflow discharge for the period: 1977-1982 (Coelho Netto, 1985) and monthly baseflow discharge of the study period (Silveira, 1997) both at the basin outlet of the Experimental Station.
Based on these analyses, the solubility relationship found for alkaline elements in plagioclase-microcline gneiss is calcium = sodium > potassium > magnesium, whereas for biotite gneiss the relationship is: calcium = sodium > magnesium > potassium. Therefore, release of magnesium was easier in biotite gneiss than in plagioclase microcline gneiss, and the reverse was true for potassium. plagioclase-microcline
gneiss (%)
~~
Fig. 3. Average monthly rainfall period (1977.1990 and 1992) at the basin outlet (Cap& Mayrink Station - SERLAKEOHECO) and monthly rainfall in the study period.
cao
1.30 #
0.10
I.20
0.10 I
I
1 10.60
[
I
-I
The rainfall regime during the study period was reflected in the behaviour of the baseflow discharge (Fig. 4). There was a higher water storage in the basin during the study period. However, we can not exclude the ocurrence of subsurface stormflow masking the real baseflow condition. I”othersq
8.90
3.75
[
10.10
1
4.1 Geochemistry of bedrock Biotite - K(Mg, Fe), (Al Si, 0,,) (OH), - is probably the most important source of magnesium in fresh biotite gneiss. showed higher Plagioclase-microcline gneiss concentrations of potassium and aluminum, which could be attributed to the presence of microcline - K (Al Si, 0,) (Table 1). Data from saprolite samples indicated that leaching processes take away most of the alkaline elements (sodium, calcium, magnesium and potassium) from fresh rock (Table 1). Potassium was the only alkaline element that remained in significative concentration in saprolite. This was
Table 1. Chemical composltion basin.
of fresh rock and saprolite m the study
4.2 Input-output interactions of solute fluxes Throughfall pH and ionic concentrations were higher than rainfall (Table 2), due to leaching from the vegetation canopies, a normal process in tropical rainforests (Silva Filho et al., 1984; Trudgill, 1988). Streamflow at the basin outlet was characterized by an increase in Na’ in relation to throughfall, showing that it
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical was probably not used much by the vegetation (Fig. 5). Soil retention of Ca*‘, Mg” and K’ can he associated with biotic processes since these elements are important nutrients. The retention of K’ can he associated with the formation of illite in the soils. This uptake of K’ from the soil solution is termed “illitization” (Appelo and Postma, 1994). Higher values of SiO, in the streamflow were due to weathering processes. Rainfall (n=lll)
Throughfall (n=40)
Throughfall /rainfall ratio (~10)
Topsod water
near watershed divide (“=I())
C””
Na’
031 (S&20) 2.5
0.95 (S=O.S4)
3.0 (S=l.2)
4.8
3.9
0.56 (kO.21)
bottom of the middle Valley (n= IO)
875
water samples at the bottom of the middle valley sue had higher concentrations of dissolved elements than near the watershed divide (Table 2). These results point out an effect of variability on the water residence time within different soil compartments, being higher at the bottom of the middle valley th& near the watershed divide. The lower concentrations of dissolved elements near the watershed divide than at the bottom of the middle valley can he associated with the abundance of fine root biomass near the watershed divide (Filgueira Leite et al., 1997). Roda et al. (1990) have already stressed the importance of biological uptake and cation exchange processes in soil water composition. In our study area litter and forest leaves were found to he important sources of Cal’. K* and Mg”’ (Clevelario Jr, 1996). Although the composition of topsoil water was quote different from that of throughfall and streamtlow, previous studies have indicated that litterflow occurred over litter and within the root-litter mat, being discontmuous and negligible compared with the stream (Coelho Netto, 19871.
19.4x (SG7.74)
2.9
Responses to Rainfall in a Small Rainforest Basin
6.6
4.3 Flow pathways
Table 2. Average and standard deviation (S) of ionic concentrattonb (mg/l) and pH; in rainfall, throughfall and topsoil water samples. Throughfall/ ramfall ratios are an average of throughfall/ralnfall ratios at each storm event analysed.
0
10
0
IO
0
10
A comparison of streamflow samples taken at different sites reflected the influence of lithology. Higher Mg” concentrations in the streamflow in hiotite gneiss areas compared to the other sites suggest that weathering of this rock was an important source of Mg2’(Table 3). The lowest concentration of SiOz in those sites suggests that alkaline cations are being more efficiently leached in this lithology (Table 3) Another possible explanation is that biotite gneiss weathering processes generate only kaolinite, with little or no residue of SiO,. On the other hand plagioclasemicrocline gneiss constitute a source of SiO?. Although K’ is a major element in this lithology, the low values at streamflow are probably because of its retention in the soil as illite. Ca” and K’ streamflow concentrations also did not show a significant difference between different sites. At the basin outlet (site I) streamflow is from a mixture of different source areas (Table 3).
Na’
SiO,
?? rainfal ?? throughfal streamfbw concentrations in mgl
Fig. 5. Average concentrattons of Ca”, Mg”. K’, Na* and SiO, in mmfall, throughfall and streamflow at site I (basin outlet) -- SiO, was not analysed in throughfall.
6 (~12)
0.59 0.60 (S=O.O9) (S=O.l I)
5.3 (S=O.4)
08 (S=O I)
2.7 (S=O 2)
The root mat was also found to exert an important influence in the uptake of water and nutrients. The results from the sub-catchment study area indicated that topsoil
Table 3. Average and standard deviation (S) of iomc concentrations (mgll) at SIX samphng sites in the Upper Cachoeirn river basin (for location of each site see Fig. 1).
816
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
We made an analysis of streamflow discharge and solute concentration to better understand the hydrological sources of the studied elements. Despite the existence of a certain concentration of Ca” and Mg” in the baseflow, the weak positive correlations with discharge suggested that stormflow could also be an important source of these elements. On the other hand the main source of SiO, and Na’ could be associated with groundwater and delayed subsurface flow contributions to channel (Fig. 6).
Responses to Rainfall in a Small Rainforest Basin The contribution of throughfall could have increased Ca*+ and K’ concentrations (Fig. 7).
0
ZO
50
100
150
200
.E, 1 20 E ‘Z 40 discharge at the basin
: 0
50
100
150
200
time (min)
site 1 - basin outlet
time (min) 7 ‘,
. :““:*
0
“,
50
: ’ ‘*
100
150
+ *
0.14.. 0
Fig. 6. Relationship between streamflow discharge and Ca”, Mg’*, Na*, SiO, and K*concentmtions at site number 1 (n =26).
Because of the rapid hydrological response of the basin we decided to investigate the hydrogeochemical behaviour during a stormflow period generated by a 1OOmm storm event, 80 minutes in duration, on 1 March, 1996. The hydrogeochemical response of the basin proved to be rapid, with concentration values changing at the beginning of the rainfall and returning to their initial values just after the rainfall has ceased (Fig. 7). Lagtime to initial hydrograph rise was 35 minutes. The first hydrograph peak was associated with Hortonian overland flow. The smooth peak that occurred after the rainfall has ceased (70-100 minutes) could be associated with subsurface contributions. Hortonian flow for this rainfall had low concentrations of SiO,, Nd and Mg2’, because its main source was rainfall.
‘01
‘.
:. 50
. . . ; “. 100
‘I 200
Mg
-A--u
Ca Na+ K+
u
SiOz
’ ; ” ‘. 1 150 200 time (min)
site 6
8
time (min) 0.1
““;““;““;““I 0 50
100
150
200
Fig. 7. Rainfall intensity (mm / lOminutes), streamflow discharge (m3k) at the basin outlet, and solute concentrations (mg/l) at site 1, site 4 and site 6 during a 1OOmm storm event.
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical The decrease of Na’ and SiO, during rainfall was a common pattern for all sites (Fig. 8a and Fig. 8b). As the recession limb progressed in response to the end of the rainfall, Na’ and SiO, concentrations tended to increase, as evidence of the contribution of subsurface flow. Ovalle (1985) and Harriman et al. (1990) also described a dilution in SiO, concentration in stormflow. 0
50
150
100
200
0 P 2
5 10
z.5
15
L.
20
time (min.)
25 1 6
r6 xl ?? 4 +2
Responses to Rainfall in a Small Rainforest Basin
877
there was a large increase in Ca*’ and K’ concentrations (Fig. 7). This was probably related to the contribution of saturation overland flow, which was responsible for leaching these elements from the litter layer. The decrease of rainfall intensity was followed by a decrease in the concentration of the two ions cited above. This fact reinforces the importance of saturation overland flow source areas in controlling Ca2’ and K’ concentrations at site 6. An analysis of site 4 showed a delayed increase of K’, Ca”, Mg*’ and SiO, concentrations relative to the rainfall peak. (Fig. 7). This behaviour could be associated with the delayed contribution of subsurface flow paths, which are dominant in this area. Considering that litterflow contributions of Cal’ and K’ were negligible to the streamflow composition, we suggest that a probable source of these elements is related to weathered feldspars, both in the colluvium and in the underlying plagioclase-microcline saprolite. Stormflow seemed to be greatly influenced by the forest vegetation, which changed the composition of Hortonian and saturation overland flow. 5 Conclusions Weathering processes of biotite gneiss were responsible for higher concentration values of Mg” in streamflow. Streamflow chemistry, however, was not only the result of weathering processes but also of biotic processes of the forest.
o!““:“‘.:““:““’ 0
50
150
100
200
The basin showed a quick hydrological and geochemical response to rainfall inputs. We found that throughfall increased the concentration of all elements, specially K’, and pH values, in relation to rainfall, which was probably because of chemical interactions with the vegetation. Topsoil water incorporated cations from the litter, and from biotic processes, but its contribution to channel was insignificant.
time (min.)
A
u
rainfal
throughfal
topsoil water ~~ 0
50
100
150
200
time (min.) Fig. 8. Rainfall intensity (mm/l0 minutes) at the basin outlet; Na* (A) and StO, (B) concentrations at sites number I, 2, 4 and 6 during a storm event of IOOmm
The hydrogeochemical responses varied in space and trme within the study basin. The lagtime at different stream sates was probably due to the variability of hydrological processes in the contributing areas (Fig. 7). An analysis of site 6, which included a saturation overland flow producing /one, indicated that, just after the beginning of the rainfall,
*
Na’, SiO, 4
groundwater SiO,. Na’
Fig. 9. Sketch of hydrogeochemical contributions to streamflow in a mountainous rainforest catchment. Arrow sne is proportional to solutes contnbution.
During the rising limb of the hydrograph, Hortonian overland flow promoted a dilution of elements, specially the ones that do not have a biotic source (Na’ and SiO,).
878
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
However, saturation overland flow contributions increased Ca*‘and K’ concentrations due to litter leaching. Since SiO, and to a lessor extent Na’, do not have a significant biotic source they showed a decreasing concentration behaviour. As rainfall ceased, Hortonian overland flow and saturation overland flow contributions also decreased and contributions from weathered profiles acquired greater importance. At this point, however, concentrations of Ca”, K+ and Mg”’did not increase as much as they do during the rising limb of the hydrograph. This fact could be associated with the highly leached weathering profiles. As the recession limb progressed and subsurface flow contribution prevailed, concentrations of Nd and SiO, increased. The increase in Nd was probably due to its high solubility and the increase in SiO, was probably due to weathering processes. Therefore Na’ and SiO, concentrations are proportional to delayed subsurface flow and groundwater contribution (Fig. 8). Based on these results we suggest that stormflow could either be mainly composed of “new water” or, at least, of “old water” masked by biotic factors. The great complexity of weathering and biotic hydrogeochemical contributions in mountainous tropical rainforest still demands more detailed studies for a better comprehension of this environment. Acknowledgemenrs: This work was financed by grants from CAPES, CNPq and FUJB; IBAMA allowed us to set up our samplingequipment in Tijuca National Park; SERLA provided hydrological data; Special thanks to Prof. Dr. JoPo Alfred0 Medeiros and all the staff of LAM (Mineral and Environmental Analysis Laboratory) for assistance during water samples analyses; Ana Valtria Freire AllemBo, Adriana Filgueira L&e, RogCrio Lafayette Pinto and Claudia Valdetaro Madeira gave precious help during field work and late discussions; Claudia Marques Freire, Jose Arruda Freire and William Zamboni de Mello made valious suggestions on the manuscript; Comments and suggestions of Physics and Chemistry of the Earth referees led to improvement of the manuscript.
References Anderson, S.P.; Dietrich, W.E.; Tones, R.; Montgomery, D.R.; Loague, K., Concentration-discharge relationships in runoff from a steep, unchanneled catchment. Water Resources Research, 33: 21 l-225, 1997. Appelo, CA. J. and Postma, D., Geochemistry, pollution. Rotterdam. Balkema. 5368, 1994.
groundwater
and
Bittencourt. A.V.L., Aspectos hidrogeoquimicos da altera@ intempdrica de basaltos da bacia do Paranti bacia hidrogtifica do Jacutinga (PR). Rev. Bras. Geoc.. 10: 202-212, 1980.
Responses to Rainfall in a Small Rainforest Basin Coelho Netto, A.L.; Santos. A.A.M.; Meis, M.R.M., OS solos e a hidrologia das encostas do alto rio Cachoeira, RJ - Estudo preliminar. Rev. &us. Geograjiu, 42: 585-611, 1980. Coelho Netto. A.L., Surface hydrology and soil emsion in a tropical mountainous rainforest drainage basin, Rio de Janeiro. PhD Thesis, Katholieke Universit Leuven, Belgium. 181p, 1985.
Coelho Netto, A.L., Overland flow production in a tropical rainforest catchment: the role of litter cover. Catena, 14: 213-231, 1987. Colman, S.M., Rock weathering Research, 15: 250.264, 1981.
rates as function of time. Quaremary
Colman, SM. and Dethier, D.P., An overview of rates of chemical weathering. In: Dethier, D. P. (ed.) Rates of chemical weathering of rocks and minerals. p:I-18, 1986. Dunne, T. and Black, R.D., Partial area contributions to storm runoff in a small New England watershed. Water Resources Research, 6: 12961311, 1970. Dunne, T. and Leopold, L.B., Water in environmental lorque. W.H.Freeman and Company. 818p, 1978.
planning.
Nova
Filgueira L&e, A.; Silveira, C.S.; F&e AllemBo, A.V.; Lafayette Pinto, R.; Coelho Netto, A.L., Atua@o da biomassa de raizes finas na hidrogeoquimica de uma bacia montanhosa florestada - Parque National da Tijuca, RJ. In: Congress0 Brasileiro de Geoquimica, 6. Salvador, 1997. Anuis... p.34.38, 1997. Freire Allemlo. A.V.. Recarga e drenagem em solos florestados: o papel dos sistemas radiculares. Tese de Mestrado - UFRJ. 136p, 1997. Harriman, R.; Gillespie, E.; King, D.; Watt, A.W.; Christie, A.E.G.: Cowan, A.A.; Edwards, T., Short-term ionic responses as indicators of hydrochemical processes in the Allt a’ Mharcaidh catchment, western Caimgonns, Scotland. Journal ofHydrology, 116: 267-285, 1990. Hewlett, .J.D. and Hibbert, A.R., Factors affecting the response of small watersheds to precipitation in humid regions. In: Sopper, W.E. and Lull, H.W. (eds.) Foresf Hydrology. Pergamon Press, Oxford. p. 275290. 1967. M.; Vakxiano, CM.; Pires, F.R.M.; Bessa, M.P.. Heilbron. Litoestratigratia, evolu@o tectono-metam6riica e magmatismo no pti cambriano do setor sudeste do municipio do Rio de Janeiro. In: SIMP. GEOL. SE, 3. Rio de Janeiro, 1993. Bol. de rewmo.~ Rio de Janeiro, SBG. p.53, 1993. Hooper, R.P. AND Shoemaker, C.A., A comparison of chemical and isotopic hydrograph separation. Water Resources Research 22: 14441454, 1986. Lesack, L.F.W., Water balance and hydrologic characteristics of a rain forest catchment in the central Amazon basin. Water Resources Research 29: 759-773, 1993a. Lesack, L.F.W., Export of nutrients and major ionic solutes from a rain forest catchment in the central Amazon basin. Water Resources Research 29: 743-758, 1993b.
Bonell, M., Progress in the understanding of runoff generation dynamics in forests. Journnl ofHydrology, 150: 217-275, 1993.
Miranda, J.C.; Freire AllemHo, A.V.; Nunes. V.M.; Coelho Netto, A.L., Distribui@o de chuvas e intercep@o pela vegeta@o florestal: Parque National da Tijuca. RJ. In: SIMP. GEOG. FfS. APL., 4. Curitiba, 1991. An&... Curitiba, AGB. p. 141-147, 1991.
Castro Jr.. 0 papal da fauna endopedanica na estrutura@o ffsica do solo e seu significado para a hidrologia de superficie em regib montanhosa florestada, PNT-RJ. Tese de mestrado. UFRJ. 150~. 1991.
Mosley, M.P., Streamflow generation in a forested watershed. Zealand. Water Resources Research 15: 795-806, 1979.
Cleveltio Jr., J., DistribuiGb de carbon0 e de elementos minerais em urn ecossistema florestal tropical timid0 baixo-montano. Tese de Doutorado. Universidade Federal de Vicosa. 135p.. 1996. Coelho Netto Silva, A.L. and Santos, AC., Anelise da frequ&ncia de chuvas no macico da Tijuca. Rev. Hidrol. Rec. Hidr.. 2: 3-18, 1979.
New
Nahon, D.B.. Evolution of iron crusts in tropical landscapes. In: Dethier, D.P. (ed.) Rates of chemical weathering. p:169-191, 1986. Neal, C.; Robson, A.; Smith, C.J., Acid neutralization capacity variations for the Hafren forest stream, Mid-Wales: inferences for hydrological processes. Joumrrl r$Hydrulogy, 121: 85-101, 1990.
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
Responses to Rainfall in a Small Rainforest Rasin
879
Neal, C.; Neal, M.; Wanington, A.; Avila, A.; Pifml. J.; Rod%. F., Stable hydrogen and oxygen isotope studies of rainfall and streamwaters for two contrasting helm oak areas of Catalonia, northeastern Spain. Journul r,fHy&olo~y, 140: 163-178, 1992.
Row. R.O.; Perein. J.L.G.; Coelho Netro, A.L., Estudos dos solos de uma pequena bacia florestada. Parque Naclonal da Tijuca RJ. In: Simp. Grog. Fis. Apl.. 4. Curitiba. 1991. Anais... Curitiba. AGB. p. 34. 41, 1991
Nunes. V.M.: Allem?io, A.V.F.; Miranda, J.C.: Castro JR, E.; Coelho Netto, A.L., Sisremas radiculares e hidrologia de encostas florestadas: subsidies i%a&se de estabilidade. In: COBRAE. 1. Rm de Janeiro, 1992. Awis... Rio de Janeiro. p, 781-797, 1992.
Silva Filho, E.V.; Ovalle, A.R.; Brown. I.F., Estudo biogeoquimnzo dab entradas atmosf&icas de Na. K, Ca e Mg na bacia do alto rio Cachoeira. Parque National da Tijuca, Rio de Janeiro. In Congr. Bras Ceol., 33. Rio de Janeiro. 1984. Anuts...Rio de Janeiro, SBG p 47294737, 1984.
Oliveira, R.R., Aspectos ecol6gicos In: Cruz, P.O.; Cezar, P.B.; Oliveira R.R. (eds.) A Florato da Tijucu e a cidade do Rio de Juneim. Rio de Janeiro. NovaFronteira. p.141.151, 1992. Oilier. C.D.. WeorherinR.
I.ed. Edinburgh, Oliver and Boyd. 304~. 1969.
Ovalle. A.R., Estudo geoquimico de &guas fluviais da bacia do alto rio Cachoeira, Parque National da Tjuca, RJ. Tese de mestndo, UFF 60~. 1985. Pearce, A.J.; Stewart, M.K.; Sk&h, M.G.. Storm runoff generation m humid headwater catchments, 1. Where does the water come from’? W&r Resources Research 22: 1263-1212, 1986. Ram&z, A.J. and Andam, A., Water chemistry and chemical weathering in northern Venezuelan drainages. Chemical Geology, 107: 3 17-3 18, 1991. Robson. A.J.; Neal, C.; Smith, C.J.. Linking variations in short- and me&urn-term stream chemistry to rainfall inputs - some observations at Plynlimon, Mid-Wales. Journul ofHydrology, 144: 291-3 IO, 1993. Rod&, F.; Avila. A.; Bonilla. D., Precipitation, throughfall. and stream-water chemistry m a Helm-Oak (Quercus Journul ofHydmlojiy, 116: 167.183, 1990.
soil solution ilen) Forest.
Silveira, C.S.. Hidrogeoquimica em sistema de drenagem montnhosoflorestal: subsidio g compreens?io do process0 de intempensmo Parque National da Tijuca, RJ. Tese de Mestrado UFRJ. 157~. 1997. Stallard, R.F.. River Chemistry, geology, geomorphology, and soils in the Amazon and Onnoco basins. In: Drever, J.I. (ed.) 7’he chemi.vrT of weurhering. p: 293.316. 198% Trugill, S.T., Soil und vqerarion Press 21 Ip, 1988.
sysremr. Nova York. Oxford University
United States Geological Survey. Methods fordekrmmotron oj morpmc subsmnces in wufer andj7uvinl sediments. Skougstad, M.W.; Fishman. M.J.: Friedman, L.C.; Erdman. DE; Duncan. S.S. (eds.). Techniques of Water Resources Investigations of the United States Geological Survey. Book 5 Capt. Al. 620~. 1979. Vallejo, R.R. A intluencia do “litter” florestal na distribui@o pluviais. Tese de Mestrado. IGEO-UFRJ. 120~. 1982
de @as
Pergamon
PII: S1464-1895(99)00129-5
Hydrogeochemical Responses to Rainfall Inputs in a Small Rainforest Basin: Rio de Janeiro, Brazil C. S. Silveira
and A. L. Coelho Netto
GEOHECO/Laboratory of Geo-Hydroecology, Department of Geography, Federal University of Rio de Janeiro (UFRJ), Ilha do Fundgo. CEP: 21 941-590 Rio de Janeiro, Brazil Received 07 April 1998; revised 02 September 1998; accepted 30 May I999 Abstract. This work provides basic information about hydrogeochemical contributions to different pathways of water in a small tropical rainforest catchment (3.5 km2) in Rio de Janeiro, Brazil. Precipitation and discharge measurements were conducted at the basin outlet, and at five other stream sites that were selected according to their hydrological behaviour and lithology. Precipitation and streamflow samples were collected weekly, over seven months, and continuously during one storm event. These streamflow samples were analyzed for Ca”, Mg” Na’, K’ and SiOZ. Precipitation, throughfall and topsoil water samples were also collected after various storm events, over six months, within a small sub-catchment and were analysed for Ca”, Mg”, K’, Na’ and pH. Results from this research showed that rainfall was acid, with low cation content. Throughfall was neutral and cations enriched. Analysis of topsoil water indicated that litter was an important source of Cal’ and K’ by means of saturation overland flow. In relation to throughfall, streamflow output was characterized by lower concentrations of K’, Ca”, and Mg”. Biotitc gneiss weathering was the main source of Mg2+ to streamflow. During stormflow periods SiOZ and Na’ concentrations decreased due to Hortonian overland flow contributions to the stream channel. The increasing concentrations of Ca2’ and K+ were due to litter leaching, especially in the zones producing saturation overland flow. Streamflow hydrogeochemistry was not only a product of weathering processes, but also of biotic processes. 0 1999 Elsevier Science Ltd. All rights reserved 1 Introduction Surface and subsurface pathways of water reflect the interactions between climate, biota, soil, relief and weathering processes, There is a strong correlation between soil water residence time and the evolution of weathering processes (Ollier, 1969; Colman, 1981; Colman and Dethier, 1986). Hydrogeochemical studies can therefore be used to understand weathering processes Hewlett and Hibbert (1967) proposed the concept of delayed flow and quickflow in order to clarify soil water residence time. In the same way Dunne and Leopold (1978) developed the concept of baseflow and stormflow. The
concept of “old water” and “new water” was more recently introduced by Pearce et al. (1986), to identify the contribution of stored water and rainwater to channels and to obtain an hydrological knowledge of a basin using an hydrogeochemical approach. However, hydrogeochemical studies are still scarce in tropical regions (Mosley, 1979; Bittencourt, 1980; Ovalle. 1985; Stahard, 1985: Pearce et al. 1986; Lesack 1993a; Lesack, 1993b; Ramirez and Andara, 1993). Experimental studies with conservative tracers and stable isotopes showed that stormflow was mainly composed by old water (Pearce et al., 1986: Hooper and Shoemaker, 1986; Neal et al. 1990; Neal et al. 1992; Lesack, 1993b; Robson et al, 1993; Anderson et al. 1997). However, working in a tropical rainforest, Mosley ( 1979) suggested that stormflow occurred as macropore flow. and therefore reflected rainwater contribution. Based on these results stormflow complexity reflects runoff generation processes that take place within a basin. Dunne and Black (1970) indicated that the final runoff of a basin was the result of partial area contributions, its extension and its location (the “variable source area” concept). Although tropical rainforest hydrology has a great complexity (Bonell, 1993), Coelho Netto (1985) developed an empirical model of distinct runoff producing zones in our study area in the Upper Cachoeira river basin. In this model roads, tracks and parking lots (cleared sites) produce Hortonian overland flow, which is thought to be responsible for the first sharp peak in a typical hydrograph of our basin. A second smoother and delayed hydrograph peak results from hollows with thick soil mantles that produce subsurface flow. Rock escarpments enhance soil moisture so that in wetter periods, when surrounded by extensive escarpments, the hollows can produce saturation overland flow, specially those with a shallow soil mantle. Saturation overland flow also contributes to the first hydrograph peak (Coelho Netto, 1985). Based on this empirical runoff generation model for our study area. detailed research on solute fluxes was conducted in order to characterize the hydrogeochemical contribution of different water pathways in a tropical mountainous rainforest catchmcnt. ldcntifying hydrogeochemical flow pathways is one approach to understand weathering processes in this environment.
871
872
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
2 Study Area Our study area was a small catchment (3.5 km*) in the southern slopes of the Tijuca massif, Upper Cachoeira River Basin Experimental Station - GEOHECO - UFRJ, Rio de Janeiro city, SE Brazil (Fig. 1). Previous studies at this basin provided knowledge of its hydrological processes (Coelho Netto, 1985; Coelho Netto, 1987; Castro Jr., 1991; Miranda et al., 1991; Nunes et al. 1992; Freire AllemBo,
BIUZU.
CACHOEIRA
RIVER
BASIN
Ai+ 0-w
Responses to Rainfall in a Small Rainforest Basin gradients above 58” and represent 1.5% of the area. Elevation ranges from 460 m to 1022 m (Coelho Netto et al., 1980). The Precambrian bedrock is composed of biotite gneiss, plagioclase-microcline gneiss and microcline granitoid (Heilbron et al., 1993) (Fig. 2). Plagioclase-microcline gneiss has a mineralogy mainly of quartz, plagioclase, microcline, biotite and minor hornblend. Biotite gneiss mineralogy is dominantly quartz, plagioclase, biotite and minor garnet and sillimanite. Microcline granitoid has a mineralogy dominantly of K-feldspar, quartz, plagioclase and biotite (Heilbron et al., 1993). Biorite gneiss is responsible for most of the low relief features in the area (Coelho Netto, 1985). The regolith mantle includes saprolite of variable thickness, which normally decreases on steeper slopes; on the other hand hillslope deposits (talus and colluvium) tend to get thicker towards the hollows axis. Pedogenetic horizons are poorly differentiated by leaching processes. Latosols with deep profiles predominate and Cambisols occur in the footslope of rock escarpments (Coelho Netto et al. 1980). Colluvial deposits consist of large boulder to pebbled particles in a sandy-clayey, poorly sorted and nonstratified matrix (Coelho Netto et al. 1980). Because of the mineralogical composition (mainly kaolinite and gibbsite; minor quartz, goethite, illite and vermiculite) soils have a low cation exchange capacity (Rosas et al, 1991). Interception by tree canopies ranges from 17% to 24% (Coelho Netto, 1985; Miranda et al., 1991). Litter water retention capacity ranges from 130% to 330% by weight (Vallejo, 1982; Coelho Neto, 1987; Miranda et al. 1991). The uppermost soil layers, including the litter and the sandy-clayey topsoil, provides high infiltration capacity. In addition biogenic processes play an important role in controlling soil macroporosity (Castro Jr., 1991) and the tree root systems favour rainwater percolation deep into the soil (Freire Allemgo, 1997).
Atlwlk_
-
Fig. 1. Study am location: GEOHECO - UFRJ Experimental Station at Upper Cachoeira river basin, Tijuca Massif, Rio de Janeiro city, Brazil.
The
present
study
area
is
part
of
the
tropical
Mean annual discharge at the basin outlet was around 0.072 m’/s, ranging from 0.023 m’/s to 0.144 m’/s in the period 1976-1996. Average monthly baseflow usually varied from 0.033 m’/s to 0.052 m’/s with lower discharge values in August/September, after the drier season. During stormflow, discharge could reach more than 2.0 m’/s. Rainfall below 10 mm did not affect baseflow conditions and separation of baseflow was difficult due to delayed subsurface flow occurrence, specially after severe rainfalls. On average, 30% of rainfall, left the basin as streamflow.
mountainous climatic zone of Rio de Janeiro city, with an average monthly temperature ranging from 19°C (July) to
3 Methods
25°C (February). Mean annual precipitation is around 2300 mm, with the summer months (December - April) being the wettest (Coelho Netto Silva and Santos, 1979). The vegetation is a secondary tropical rainforest (Tijuca National Park) that is still preserved despite surrounding urban pressure. The forest was formed over 1.50 years ago by natural succession processes (Oliveira, 1992). The average slope gradient is 18” with 50% of the slope areas ranging from 12” to 22”. Rock escarpments have slope
Since 1976 a raingauge station and a streamgauge station (the Upper Cachoeira Experimental Station), at the basin outlet, provided continuous records of precipitation and discharge. Streamflow was measured in a broad-crested weir (contracted rectangular form), with a standard vertical staff gauge and a vertical float counterweight. Rainfall was measured with a recording 0.2 mm tipping-bucket iaingauge, and also a non-recording gauge.
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
Responses to Rainfall in a Small Rainforest Basin
873
Legend .# stmamtlmsampling
site + rainfall
sampling site
H throughfau end topsoil wabx sampling site + rock sampling site
m
Biotito
[
Micmcl. eoid
m
Pla+cl. gneiss
.
??cl--h
gnaiss Mirmcl.
N
‘,_ main roads
sites 0 --
Fig. 2. Upper
Cachoarn river basin: lithological
5OOfTl
background and sampling sites location.
To conduct this study, water samples were collected in during two separate periods. During the first period six channel sites and one rainfall site were selected to monitor rainfall and streamflow based on their hydrological behaviour and lithological differences (Fig. 2). Samples were collected weekly over seven months (September 1995 - March 1996), and also at variable time intervals (observing discharge variations) during a storm event, simultaneously at four stream sites (1, 2, 4 and 6 - see Fig. 2). Stormflow samples were collected to evaluate the relationship between stormflow discharge and solute fluxes. Precipitation was also sampled weekly using a polyethylene funnel (30 cm in diameter) connected by a tube to a polyethylene bottle. This sampler was set up outside the forest. These samples were analysed for Ca”, Mg”‘, Na’, K’ and SiO,. The second period lasted over six months (April 1996 September 1996), during which an intensive sampling on a small sub-catchment (0.6 ha) was conducted. Throughfall and topsoil waters were sampled at two different sites of the sub-catchment, one near the watershed divide and the other at the bottom of the middle valley) to evaluate the chemical input. Two throughfall polyethylene collectors were installed at each site mentioned above to get data variability. Topsoil water was collected using a polyethylene trough connected by a tube to a polyethylene bottle. Troughs were horizontally inserted into the soil 5 cm, below the litter layer. After a storm event, samples were collected and the collectors were rinsed with
deionized water. Samples from this sub-catchment analysed for Cal’, Mg2’, K’, Na’ and pH.
were
All samples were collected in polyethylene bottles (120 ml), filtered through 0.45 pm pore-size membrane filters in the laboratory and promptly analysed. Ca” and Mg” were measured by flame atomic absorption spectrophotometry (Perkin Elmer 3 100 spectrophotometer calibrated with standard solutions of 0.0 mg/l, 1.0 mg/l, 5.0 mg/l and 10.0 mg/l) after addition of 10% La. Nd and K’ were measured by flame emission spectrometry (Micronal B262 spectrometer calibrated with Na’/K’ standard solutions of O.O/O.O mg/l, 7.5/2.5 mg/l and lO.O/S.O mg/l). The molybdenum blue method was used for cletermining SiO, (Beckman DU7 colourimeter calibrated with 0.0 mg/l, 3.0 mg/l, 5.0 mgll and 10.0 mg/l standard solutions). A detailed description of the analytical methods used is given by the United States Geological Survey (1979). Duplicate analyses indicated a precision of 0.07 mgll for Ca” and Mg” and 01 0.1 mgll for Na’, K’ and SiO,. pH was analysed in laboratory with a Digimed DMPH2 pH meter calibrated with buffers of pH 4 and pH 9. In total, 37 samples of rainfall, 40 samples of throughfall, 20 samples of topsoil water and 20.5 samples of streamflow were analysed. In addition to water samples this research called for an analysis of rock. One sample from fresh rock and one from a corresponding saprolite of biotite gneiss and plagioclasemicrocline gneiss were collected and sent to a laboratory for geochemical analyses (CaO, MgO, Na,O, 40, SiOZ and A&O,). The difference in concentration between elements analysed in rock and 100% were considered to be caused
814
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
Responses to Rainfall in a Small Rainforest Basin
by FeO, Fe,O, and H,O. The last two are important in soil and weathering profiles as part of oxided compounds and clay minerals (Nahon, 1986).
probably due to the formation of illite - (K, H,O) Al, (Si, Al), 0,” (OH),. Silica was also depleted from the weathering profile, but much less than alkaline elements.
The results found were compared with earlier studies of soil chemistry in the area (Ovalle, 1985; Rosas et al. 1991; Clevelkio Jr, 1996).
discharge (ma/s) 0.2
-
Average monthly baseflow
----*----
Baseflow in the study period
0.18
4 Results and Discussion
0.16
The comparison between monthly rainfall distribution during the first study period and the average monthly rainfall regime, indicated that rainfall was greater than the average in September-October-1995 and also from January to March-1996 (Fig. 3). This increase in rainfall amount was caused by an increased number of extreme rainfall days (> lOOmm/day). Such phenomena have been observed before in the area (Coelho Netto Silva and Santos, 1979). Based in this information the study period could be characterized as very wet with daily events of high intensity. monthly rainfall in the study area rainfall (mm)
-
average monthly rainfall
800
600
:
0.14 0.12
:
.*. 8’ :
i :
:
: :
:
I
: i
Fig. 4. Distribution of mean monthly baseflow discharge for the period: 1977-1982 (Coelho Netto, 1985) and monthly baseflow discharge of the study period (Silveira, 1997) both at the basin outlet of the Experimental Station.
Based on these analyses, the solubility relationship found for alkaline elements in plagioclase-microcline gneiss is calcium = sodium > potassium > magnesium, whereas for biotite gneiss the relationship is: calcium = sodium > magnesium > potassium. Therefore, release of magnesium was easier in biotite gneiss than in plagioclase microcline gneiss, and the reverse was true for potassium. plagioclase-microcline
gneiss (%)
~~
Fig. 3. Average monthly rainfall period (1977.1990 and 1992) at the basin outlet (Cap& Mayrink Station - SERLAKEOHECO) and monthly rainfall in the study period.
cao
1.30 #
0.10
I.20
0.10 I
I
1 10.60
[
I
-I
The rainfall regime during the study period was reflected in the behaviour of the baseflow discharge (Fig. 4). There was a higher water storage in the basin during the study period. However, we can not exclude the ocurrence of subsurface stormflow masking the real baseflow condition. I”othersq
8.90
3.75
[
10.10
1
4.1 Geochemistry of bedrock Biotite - K(Mg, Fe), (Al Si, 0,,) (OH), - is probably the most important source of magnesium in fresh biotite gneiss. showed higher Plagioclase-microcline gneiss concentrations of potassium and aluminum, which could be attributed to the presence of microcline - K (Al Si, 0,) (Table 1). Data from saprolite samples indicated that leaching processes take away most of the alkaline elements (sodium, calcium, magnesium and potassium) from fresh rock (Table 1). Potassium was the only alkaline element that remained in significative concentration in saprolite. This was
Table 1. Chemical composltion basin.
of fresh rock and saprolite m the study
4.2 Input-output interactions of solute fluxes Throughfall pH and ionic concentrations were higher than rainfall (Table 2), due to leaching from the vegetation canopies, a normal process in tropical rainforests (Silva Filho et al., 1984; Trudgill, 1988). Streamflow at the basin outlet was characterized by an increase in Na’ in relation to throughfall, showing that it
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical was probably not used much by the vegetation (Fig. 5). Soil retention of Ca*‘, Mg” and K’ can he associated with biotic processes since these elements are important nutrients. The retention of K’ can he associated with the formation of illite in the soils. This uptake of K’ from the soil solution is termed “illitization” (Appelo and Postma, 1994). Higher values of SiO, in the streamflow were due to weathering processes. Rainfall (n=lll)
Throughfall (n=40)
Throughfall /rainfall ratio (~10)
Topsod water
near watershed divide (“=I())
C””
Na’
031 (S&20) 2.5
0.95 (S=O.S4)
3.0 (S=l.2)
4.8
3.9
0.56 (kO.21)
bottom of the middle Valley (n= IO)
875
water samples at the bottom of the middle valley sue had higher concentrations of dissolved elements than near the watershed divide (Table 2). These results point out an effect of variability on the water residence time within different soil compartments, being higher at the bottom of the middle valley th& near the watershed divide. The lower concentrations of dissolved elements near the watershed divide than at the bottom of the middle valley can he associated with the abundance of fine root biomass near the watershed divide (Filgueira Leite et al., 1997). Roda et al. (1990) have already stressed the importance of biological uptake and cation exchange processes in soil water composition. In our study area litter and forest leaves were found to he important sources of Cal’. K* and Mg”’ (Clevelario Jr, 1996). Although the composition of topsoil water was quote different from that of throughfall and streamtlow, previous studies have indicated that litterflow occurred over litter and within the root-litter mat, being discontmuous and negligible compared with the stream (Coelho Netto, 19871.
19.4x (SG7.74)
2.9
Responses to Rainfall in a Small Rainforest Basin
6.6
4.3 Flow pathways
Table 2. Average and standard deviation (S) of ionic concentrattonb (mg/l) and pH; in rainfall, throughfall and topsoil water samples. Throughfall/ ramfall ratios are an average of throughfall/ralnfall ratios at each storm event analysed.
0
10
0
IO
0
10
A comparison of streamflow samples taken at different sites reflected the influence of lithology. Higher Mg” concentrations in the streamflow in hiotite gneiss areas compared to the other sites suggest that weathering of this rock was an important source of Mg2’(Table 3). The lowest concentration of SiOz in those sites suggests that alkaline cations are being more efficiently leached in this lithology (Table 3) Another possible explanation is that biotite gneiss weathering processes generate only kaolinite, with little or no residue of SiO,. On the other hand plagioclasemicrocline gneiss constitute a source of SiO?. Although K’ is a major element in this lithology, the low values at streamflow are probably because of its retention in the soil as illite. Ca” and K’ streamflow concentrations also did not show a significant difference between different sites. At the basin outlet (site I) streamflow is from a mixture of different source areas (Table 3).
Na’
SiO,
?? rainfal ?? throughfal streamfbw concentrations in mgl
Fig. 5. Average concentrattons of Ca”, Mg”. K’, Na* and SiO, in mmfall, throughfall and streamflow at site I (basin outlet) -- SiO, was not analysed in throughfall.
6 (~12)
0.59 0.60 (S=O.O9) (S=O.l I)
5.3 (S=O.4)
08 (S=O I)
2.7 (S=O 2)
The root mat was also found to exert an important influence in the uptake of water and nutrients. The results from the sub-catchment study area indicated that topsoil
Table 3. Average and standard deviation (S) of iomc concentrations (mgll) at SIX samphng sites in the Upper Cachoeirn river basin (for location of each site see Fig. 1).
816
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
We made an analysis of streamflow discharge and solute concentration to better understand the hydrological sources of the studied elements. Despite the existence of a certain concentration of Ca” and Mg” in the baseflow, the weak positive correlations with discharge suggested that stormflow could also be an important source of these elements. On the other hand the main source of SiO, and Na’ could be associated with groundwater and delayed subsurface flow contributions to channel (Fig. 6).
Responses to Rainfall in a Small Rainforest Basin The contribution of throughfall could have increased Ca*+ and K’ concentrations (Fig. 7).
0
ZO
50
100
150
200
.E, 1 20 E ‘Z 40 discharge at the basin
: 0
50
100
150
200
time (min)
site 1 - basin outlet
time (min) 7 ‘,
. :““:*
0
“,
50
: ’ ‘*
100
150
+ *
0.14.. 0
Fig. 6. Relationship between streamflow discharge and Ca”, Mg’*, Na*, SiO, and K*concentmtions at site number 1 (n =26).
Because of the rapid hydrological response of the basin we decided to investigate the hydrogeochemical behaviour during a stormflow period generated by a 1OOmm storm event, 80 minutes in duration, on 1 March, 1996. The hydrogeochemical response of the basin proved to be rapid, with concentration values changing at the beginning of the rainfall and returning to their initial values just after the rainfall has ceased (Fig. 7). Lagtime to initial hydrograph rise was 35 minutes. The first hydrograph peak was associated with Hortonian overland flow. The smooth peak that occurred after the rainfall has ceased (70-100 minutes) could be associated with subsurface contributions. Hortonian flow for this rainfall had low concentrations of SiO,, Nd and Mg2’, because its main source was rainfall.
‘01
‘.
:. 50
. . . ; “. 100
‘I 200
Mg
-A--u
Ca Na+ K+
u
SiOz
’ ; ” ‘. 1 150 200 time (min)
site 6
8
time (min) 0.1
““;““;““;““I 0 50
100
150
200
Fig. 7. Rainfall intensity (mm / lOminutes), streamflow discharge (m3k) at the basin outlet, and solute concentrations (mg/l) at site 1, site 4 and site 6 during a 1OOmm storm event.
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical The decrease of Na’ and SiO, during rainfall was a common pattern for all sites (Fig. 8a and Fig. 8b). As the recession limb progressed in response to the end of the rainfall, Na’ and SiO, concentrations tended to increase, as evidence of the contribution of subsurface flow. Ovalle (1985) and Harriman et al. (1990) also described a dilution in SiO, concentration in stormflow. 0
50
150
100
200
0 P 2
5 10
z.5
15
L.
20
time (min.)
25 1 6
r6 xl ?? 4 +2
Responses to Rainfall in a Small Rainforest Basin
877
there was a large increase in Ca*’ and K’ concentrations (Fig. 7). This was probably related to the contribution of saturation overland flow, which was responsible for leaching these elements from the litter layer. The decrease of rainfall intensity was followed by a decrease in the concentration of the two ions cited above. This fact reinforces the importance of saturation overland flow source areas in controlling Ca2’ and K’ concentrations at site 6. An analysis of site 4 showed a delayed increase of K’, Ca”, Mg*’ and SiO, concentrations relative to the rainfall peak. (Fig. 7). This behaviour could be associated with the delayed contribution of subsurface flow paths, which are dominant in this area. Considering that litterflow contributions of Cal’ and K’ were negligible to the streamflow composition, we suggest that a probable source of these elements is related to weathered feldspars, both in the colluvium and in the underlying plagioclase-microcline saprolite. Stormflow seemed to be greatly influenced by the forest vegetation, which changed the composition of Hortonian and saturation overland flow. 5 Conclusions Weathering processes of biotite gneiss were responsible for higher concentration values of Mg” in streamflow. Streamflow chemistry, however, was not only the result of weathering processes but also of biotic processes of the forest.
o!““:“‘.:““:““’ 0
50
150
100
200
The basin showed a quick hydrological and geochemical response to rainfall inputs. We found that throughfall increased the concentration of all elements, specially K’, and pH values, in relation to rainfall, which was probably because of chemical interactions with the vegetation. Topsoil water incorporated cations from the litter, and from biotic processes, but its contribution to channel was insignificant.
time (min.)
A
u
rainfal
throughfal
topsoil water ~~ 0
50
100
150
200
time (min.) Fig. 8. Rainfall intensity (mm/l0 minutes) at the basin outlet; Na* (A) and StO, (B) concentrations at sites number I, 2, 4 and 6 during a storm event of IOOmm
The hydrogeochemical responses varied in space and trme within the study basin. The lagtime at different stream sates was probably due to the variability of hydrological processes in the contributing areas (Fig. 7). An analysis of site 6, which included a saturation overland flow producing /one, indicated that, just after the beginning of the rainfall,
*
Na’, SiO, 4
groundwater SiO,. Na’
Fig. 9. Sketch of hydrogeochemical contributions to streamflow in a mountainous rainforest catchment. Arrow sne is proportional to solutes contnbution.
During the rising limb of the hydrograph, Hortonian overland flow promoted a dilution of elements, specially the ones that do not have a biotic source (Na’ and SiO,).
878
C. S. Silveira and A. L. Coelho Netto: Hydrogeochemical
However, saturation overland flow contributions increased Ca*‘and K’ concentrations due to litter leaching. Since SiO, and to a lessor extent Na’, do not have a significant biotic source they showed a decreasing concentration behaviour. As rainfall ceased, Hortonian overland flow and saturation overland flow contributions also decreased and contributions from weathered profiles acquired greater importance. At this point, however, concentrations of Ca”, K+ and Mg”’did not increase as much as they do during the rising limb of the hydrograph. This fact could be associated with the highly leached weathering profiles. As the recession limb progressed and subsurface flow contribution prevailed, concentrations of Nd and SiO, increased. The increase in Nd was probably due to its high solubility and the increase in SiO, was probably due to weathering processes. Therefore Na’ and SiO, concentrations are proportional to delayed subsurface flow and groundwater contribution (Fig. 8). Based on these results we suggest that stormflow could either be mainly composed of “new water” or, at least, of “old water” masked by biotic factors. The great complexity of weathering and biotic hydrogeochemical contributions in mountainous tropical rainforest still demands more detailed studies for a better comprehension of this environment. Acknowledgemenrs: This work was financed by grants from CAPES, CNPq and FUJB; IBAMA allowed us to set up our samplingequipment in Tijuca National Park; SERLA provided hydrological data; Special thanks to Prof. Dr. JoPo Alfred0 Medeiros and all the staff of LAM (Mineral and Environmental Analysis Laboratory) for assistance during water samples analyses; Ana Valtria Freire AllemBo, Adriana Filgueira L&e, RogCrio Lafayette Pinto and Claudia Valdetaro Madeira gave precious help during field work and late discussions; Claudia Marques Freire, Jose Arruda Freire and William Zamboni de Mello made valious suggestions on the manuscript; Comments and suggestions of Physics and Chemistry of the Earth referees led to improvement of the manuscript.
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