Evaluation of the Siltation of River Taquari , Pantanal , Brazil , through 210 Pb Geochronology of Floodplain Lake Sediments

The Taquari River basin (TRB) drains an area of 28,000 km in the plateau adjacent to the so-called Pantanal, a large floodplain area in midwest Brazil. It is characterized by sedimentary soils, primarily sandy soils, with areas of accentuated declivities and high erosive potential. These characteristics of TRB, associated with its intensive use and the mishandling of the soil by agricultural activities during the last 25 years have intensified erosion processes. This is evidenced by the presence of recent gullies and by the siltation of rivers of the TRB. The intensification of these processes causes the instability of the riverbed, and consequent flooding of large areas, leading to environmental, social and economic problems. These processes have already been reported by several authors. Direct estimates of sediment production related to agricultural activity are difficult to obtain for such a large and diversified area as the TRB, and no data are available at present. Historical records of suspended sediment loads for the Taquari River are scarce, although attempts have been made to compare such information. The lack of former studies for the characterization of this environment, prior to the expansion of agricultural activities in the plateau of the upper Taquari River region, makes it difficult to evaluate and compare past and present environmental impacts. Such evaluation can however be undertaken using Evaluation of the Siltation of River Taquari, Pantanal, Brazil, through 210Pb Geochronology of Floodplain Lake Sediments


Introduction
The Taquari River basin (TRB) drains an area of 28,000 km 2 in the plateau adjacent to the so-called Pantanal, a large floodplain area in midwest Brazil.It is characterized by sedimentary soils, primarily sandy soils, with areas of accentuated declivities and high erosive potential.These characteristics of TRB, associated with its intensive use and the mishandling of the soil by agricultural activities during the last 25 years have intensified erosion processes.This is evidenced by the presence of recent gullies and by the siltation of rivers of the TRB.The intensification of these processes causes the instability of the riverbed, and consequent flooding of large areas, leading to environmental, social and economic problems.][3][4][5][6][7][8][9] Direct estimates of sediment production related to agricultural activity are difficult to obtain for such a large and diversified area as the TRB, and no data are available at present.Historical records of suspended sediment loads for the Taquari River are scarce, although attempts have been made to compare such information. 8The lack of former studies for the characterization of this environment, prior to the expansion of agricultural activities in the plateau of the upper Taquari River region, makes it difficult to evaluate and compare past and present environmental impacts.Such evaluation can however be undertaken using a methodology that provides information on past sedimentation rates.
Usually, 210 Pb sediment dating is validated using data on 137 Cs in fallout from atomic tests or from accidents such as Chernobyl.However, both contribute very little to the pool of 137 Cs in the Southern Hemisphere, thereby limiting the use of this type of data for dating purposes. 101] In the Amazon and Pantanal regions, the sedimentary records of Hg offer potential opportunity for recent sediment dating, due to the gold rush that took place in Brazil and other neighboring countries in the 1980's. 12Small scale primary gold mining, using Hg amalgamation for gold recovery, is a practice that started as early as the 18 th century, in areas such as Poconé, on the border of the Pantanal and the surrounding lateritic soil plateaus. 13However, as in other gold mining areas, the link between Hg emissions to the environment from gold mining and Hg in fish could not be demonstrated, and the latter were extremely low even in piscivorous fish species sampled near Poconé. 14In contrast, high natural Hg levels have been found in most tropical soils and soil erosion was shown to be the main Hg source to aquatic systems in different drainage basins in the Amazon.Roulet et al 15 and.Lechler at al 16 have shown that while lateritic and other forest soils along the Madeira River were enriched in Hg, recent floodplain soils along the Jamari River were only slightly enriched in Hg and the channel bed sediments were very low in Hg.
Hylander et al 13 have carried out a survey of total Hg in surface sediments from floodplain lakes covering the north of the Pantanal, near Cáceres and Barão de Melgaço, and, at the South, in the region near the confluence of the Cuiabá and Paraguay rivers.They have found a mean value of 33.2 ng g -1 (first quartile 18.4 and third quartile 46.8 ng g -1 ).The Hg content of fine bottom sediments from water courses in the immediate vicinity of present or former gold mining sites was higher than in the lake sediments.The results reported by them, for both lakes and river sediments, were lower than those previously reported [17][18][19][20] for surface sediments.Leady and Gottgens 21 observed similar values for Hg in lake sediments from a non-contaminated reference area in Northern Pantanal (29.1±0.7 ng g -1 ).
The lakes included in the present study are situated in the middle Taquari drainage basin area.Gold mining areas are absent in the upper and middle Taquari drainage basin, but the middle Taquari is considered a deposition region for the lateritic soil eroded from the upper basin, a process enhanced by the agricultural development in the Taquari river highlands.The upper land is considered a natural source of Hg to the Pantanal area 13 and therefore if an increase in the mass sedimentation rates has occurred in recent decades, due to an increase in soil erosion at the upper TRB, an increase in the Hg flux to the sediment column must have ocurred .This Hg flux should correlate more closely with the mass sedimentation rate than with the Hg concentration in sediment layers of floodplain lakes from the middle Taquari region.On the other hand, these Hg concentrations in sediments should be relatively constant.
These lakes are oxbow lakes, formed by abandoned river meanders.They are presently separated from the river and receive river water mainly during the flood season, when large amounts of sediment in suspension are transported by the river.The lakes were selected using aerial photographs from 1966, at the 1:60,000 scale and Landsat-TM satellite images (bands 3, 4 and 5 composition), at the 1:100,000 scale, from 1995.Since the aerial photographs from 1966 were taken during the drought period of 1958-1972, the chosen lakes were those older than 30 years and with a probability of receiving water even during that dry period.
A 20kg gravity corer with PVC tubes of 100 cm length and 5.0 cm internal diameter was used to collect sediment cores from study lakes.A first sampling campaign was carried out during October 96, and another one year later.Three cores were collected from Lake 1, three from Lake 2 and one from Lake 3. The cores were sectioned into 2cm slices, and the central part of each slice was reserved for mercury determination.Each slice was dried at 105°C, ground, a 5 gram aliquot taken and the excess 210 Pb content was determined by HBr leaching as described by Godoy et al 22 .A low-background proportional counter (EG&G Prof. Berthold LB 750) was used and the detection limit for 5 g dry material and 24,000 s counting time was 3 mBq g -1 .
The results were evaluated according to Appleby and Oldfield. 23or the total Hg determination, sediment were dried at 50 o C to constant weight and samples of up to 2 g were digested for 5 min in a hot bath at 60 o C, in 5 mL of Milli-Q water and 5 mL of HCl:HNO 3 (3:1).After addition of 50 mL of Milli-Q water and 15 mL of 5 % KMnO 4 , samples were further digested for 30 min at 60 o C and left overnight at room temperature.A few drops of 12 % NH 4 OCl were added, the extracts were filtered through Whatman 42 paper

Results and Discussion
Figures 2a-2c show the changes on the mass sedimentation rates (mg cm -2 year -1 ) found in Lake 1. Sampling point 1 was located closer to the river than the sampling point 2, and point 2 was sampled again during the 1997 sampling campaign (Figure 2c).All the profiles show a similar picture, with a peak during the 1970s to 1980s coinciding with the end of the dry period of 1958-1972.Preliminary evaluation of the existing rainfall data for the plateau area indicated that the rainfall erosivity was larger during the period from 1974 to 1994 than that from 1965 to 1973. 8In the 1970s, agricultural activities on the plateau underwent a marked expansion, intensifying the erosion processes, over the last 25 years.The values of the mass sedimentation rate corresponding to the present decade also show a period of higher sedimentation rates at present.For all the three cores, the mean mass sedimentation rates for the 1990s are significantly higher (p<0.001)than those for the 1920-1950 period: (395±44) and (141±44) mg cm -2 year -1 for the sampling point 1, (261±54) and (125±17) mg cm -2 year -1 and (458±85) and (143±27) mg cm -2 year -1 for the first and second cores of sampling point 2. A comparison between the results of these cores is shown in Figure 3, illustrating the similarity of the two cores taken at point 2, one during 1996 and the other during 1997.
Similar results were observed in Lake 2 for the sampling points 1 and 2 (Figures 4a and 4b).Both points are located in the inlet channel of the lake, one at its left and the other at its right side, almost one in front of the other.The increase in mass sedimentation rates, observed since the late 1970s, was confirmed and the recent rates are substantially greater than those for the 1920-1950 period: (511±76) and (105±21) mg cm -2 year -1 and (525±81) and (93±18) mg cm -2 year -1 for the sampling points 1 and 2, respectively.The close concordance between the results of both sampling points is illustrated in Figure 4c.
Forsberg et al 24 have reported an analogous case to lakes 1 and 2 situation in floodplain lakes near the Samuel hydro-electrical power station, Rondonia state, Northern Brazil A close correlation was observed between mass sedimentation rate and land denudation and cassiterite artisanal mining activities.The mass sedimentation rate has increased from original values around 50 mg cm -2 year -1 up to 800 mg cm -2 year -1 during the seventies.
A third point was sampled during the first campaign.It was located at the end of the lake.There were several curves and straight channels between the lake entrance and this sampling point, as a consequence, the obtained results showed a characteristic pattern of a location with constant sedimentation rates (Figure 5a), and a much lower mass sedimentation rate was observed (63 mg cm -2 year -1 ).Lake 3 is a small lake, located downstream the two others.Between the lake and the river, there was a buffer  zone, wider than 100 m, with a dense aquatic vegetation mat.This promotes deposition of the suspended material before the lake itself, leading to the observed constant sedimentation rates (Figure 5b), and also to a low mass sedimentation rate (52 mg cm -2 year -1 ).
Based on satellite images, Oliveira et al 25 have shown that the area of the upper Taquari River drainage basin covered with native vegetation has decreased from 96.2% in 1977, to 57.0% in 1984 and 42.5% in 1991, the deforested area being used for both annual crops and cattle raising.Using the mass sedimentation rates observed for sampling points 1 and 2 in Lake 2, the mean value has increased from 142 mg cm -2 year -1 in 1977 to 306 mg cm -2 year -1 in 1984 and 523 mg cm -2 year -1 in 1991.As the mass sedimentation rate observed for the period prior to 1950 was about 100 mg cm -2 year -1 , it becomes clear that the land denudation occurring on the upper drainage basin area of the Taquari River had a significant impact on sedimentation rates.Mercury levels were determined in a core from Lake 1 (sampling point 2/1997) and Lake 2 (sampling point 2/1997).The results obtained are shown in Figures 6a and 6b.As expected, the total Hg concentration, despite some fluctuations, are, after the 40's, quite constant and similar for both lakes (Lake 1: 52±10 ng g -1 , N=20, and Lake 2: 47.0±5.7 ng g -1 , N=15).These values are in the same range of that reported by Hylander et al 13 for surface sediments from other lake in the Pantanal region.The mean value of the Hg content before 1940 is also similar for both lakes (21 ng g -1 and 23 ng g -1 , for lakes 1 and 2 respectively) and coherent with values reported by Lacerda et al 17 and Leady and Gottgens 21 for mercury in lacustrine sediments in remote areas of the Pantanal wetlands (20 ng g -1 and 29 ng g -1 , respectively).
In contrast, the Hg flux has steadily increased dominated by the mass sedimentation, showing the same pattern in both lakes.These findings show that an increase on the soil erosion at the upper TRB has occurred and that the eroded material has reached the middle Taquari river region.The Hg fluxes at the bottom of both cores (12-15 mg cm -2 year -1 ) are comparable to those of remote areas between 10 o N and 10 o S 26 and with values observed around the turn of the century in similar environments as the Everglades, Florida. 27These fluxes, due to the increase of

Figure 1 .
Figure 1.Localization of the studied lakes, with the Brazilian Pantanal area enhanced.

Figure 3 .
Figure 3.Comparison between the results obtained for the different cores taken from Lake 1.

Figure 6 .
Figure 6.Mercury concentration and annual flux and mass sedimentation rate time profile for: (a) Sampling point 2, Lake 1, October 1997 and (b) Sampling point 2, Lake 2, October 1997.