THEMATIC CONTRIBUTION

Arsenic in the rivers of the Amazon Basin

Wilson Scarpelli Mineral Exploration Consultant [email protected]

ABSTRACT The rivers flowing from the bring more than 5 tons of arsenic per year to the Atlantic as part of their load of transported sediments and also dissolved in the waters. The combined arsenic content, of about 1 mg/m3, is close to the 10 mg/m3 maximum acceptable for potable waters in most of the countries. Following their deposition in the Amazon Fan area, the most important carriers of arsenic, iron oxides and hydroxides, are diagenetically reduced to sulfides, carbonates and phosphates, liberating additional soluble arsenic which causes higher concentrations of arsenic along the shore.

KEYWORDS Arsenic, arsenic in Amazon Basin waters, Amazon Basin, Andean rivers, iron oxyhydroxide reduction

Introduction The Amazon Basin, defined as the area drained by the rivers that contribute to the , covers about 6,110,000 km2, over large portions of the territories of , , , , and . As shown in Fig. 1, the basin expands in two quite different geomorphological areas, and each one contributes in a different way to its development. In the west, for 3,200 kilometers the basin drains the eastern cordilleras of the Andes, from south of La Paz, in Bolivia, to near Bogota, in Colombia. This area has high elevations and is made up of very long ridges, with elevations greater than 4,000 meters, steep slopes, a thin cover of vegetation, and a very intense rate of erosion. The rivers leaving it contain a high bedload of sediments, which gives them a distinctive brown color. To the east of the Andes, surface elevations rarely surpass a few hundred meters, rainfall is intense, vegetation cover is thick, the rocks are deeply weathered and the rate of erosion is relatively low. The rivers draining this portion of the basin transport a small bedload of sediments, and the waters are relatively free of sediments and are occasionally dark due to accumulations of organic matter.

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Figure 1 – The western portion of the Amazon Basin, shown Bogotá Boa Vista in light gray, covers the Flow of 209,000 m33 /sec eastern cordilleras of the 600,000 tpy solids Macapá Andes, where the erosion Solimões River Quito 65% of the solids is intensive and Óbidos responsible for 97% of the Santarém Belém sediment which is transported seaward. The DEEP WEATHERING eastern portion, shown in variations of green, is Cruzeiro do Sul characterized by deep 32% of the solids weathering and low levels of erosion

Araca Cuiabá Intesive La Paz Brasília Erosion 2,000 km

Negro River Manaus

Solimões River

Figure 3 – The encounter of the brown waters of the Amazon River with Madeira River the clean waters of the Tapajós river, at Santarém, Pará

Figure 2 – The encounter of the brown waters of the Andean Solimões River with the dark clean waters of the in the north. The Andean Madeira River is shown in the lower right

As result of the contrasting bedloads of the zon Basin. As mentioned later, the paper indicated rivers draining the two areas, a characteristic of the high values of arsenic in both the Amazon and the Amazon Basin are the “encounters of waters”, as Guamá Rivers at the mouth of the Amazon. those of Manaus and of Santarém (Figs. 2 and 3), High quality information regarding water flow when the brownish and sediment loaded waters volume and the transported materials is available coming from the Andes encounter and mix with from the sampling, measurements, and assay work the dark clean waters of the other rivers. done by ANEEL (the national agency of electrical energy) at its 60 monitoring stations located along ANEEL sampling the most important rivers of the Brazilian portion of the basin. An unpublished paper by S.F.P. Pereira, of the The information is being compiled and com- Laquanam (the environmental and analytical chem- prehensively studied through the HiBAM Project istry laboratory of the University of Pará) triggered (Hidrology and Geochemistry of the Amazon the interest to search through the literature to find Basin), a joint program made up of an association out about the distribution of arsenic in the Ama- of the CNPq (Research National Council, of

21 W. Scarpelli TERRÆ 2(1-2):20-27, 2005

Brazil) with the IRD (institute of S E

research for the development, of 5 D Atlantic France). The activities of HiBAM Ocean N COLOMBIA Fr. G. <0.1 Trombetas are carried out with the partici- A <0.1

Branco pation of ANA (the national wa- <0.1 <0.1 0 Negro <0.1 0.3 0.1 0.7 ter agency), ANEEL, UnB (Uni- 0.5 1.0 <0.1 0.2 1.4 0.9 0.1 0.7 versity of Brasília), and the IRD. 0.8 0.2 0.8 0.4 1.5 0.6 0.7 Among the many papers pres- -5 Solimões 0.4 Tapajós Purus adeira enting the information compiled PERU M by the HiBAM Project, Seyler BRAZIL Xingu and Boaventura (2001) pres- 0.6 0.6 -10 ANDES 0.6 ented tables with good quantita- Araguaia tive information obtained at the 0.8 ANEEL sample stations. They

-15 show that from 1965 to 1990 the BOLIVIA flow of the Amazon River avera- Pacific 3.8 0.6 3 Ocean ARSENIC IN SOLUTION ged 209,000 m per second at the 3 Andean Drainage Non-Andean Drainage (mg/m ) -20 Óbidos station, in the state of Pa- (and sampled points) (and sampled points)

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70 65 60 55 rá. This rate of flow corresponds 50 45 to 6,500,000,000 m3 annually. They also show that the river car- S VENEZUELA E GUYANA

5 ried 600,000,000 tons of sedi- D SURINAME ments to the ocean each year, and N COLOMBIA Fr. G. <0.1 Trombetas A that 62% of this tonnage arrives <0.1

Branco <0.1 0 from the Solimões River, 35% <0.1 Negro 0.4 <0.1 0.2 0.1 O.6 from the Madeira River, and only 0.4 <0.1 0.2 1.2 0.2 3.8 0.1 0.6 3% from all of the other rivers. <0.1 0.4 0.7 2.4 0.1 0.1

-5 These numbers explain quite Solimões 1.2 Tapajós Purus adeira well the reasons for the brown- PERU M Xingu ish color of the rivers with wa- BRAZIL 0.6

ters coming from the Andes, like -10 4.2 4.9 ANDES Araguaia the Beni, the Madeira, the So- 18.6 limões, the Amazon and others.

They also explain the reason for -15 the contrast of color of the wa- BOLIVIA Pacific ters of these rivers and the wa- 3.8 0.6 Ocean ARSENIC IN SUSPENSION ters of rivers that originate in the Andean Drainage Non-Andean Drainage -20 (mg/m3 ) Brazilian Shield, like the Negro, (and sampled points) (and sampled points)

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70 65 60 55 the Tapajós, and others. 50 45 In their tables, Seyler and Figure 4 – Arsenic content of the rivers of the Amazon Basin as Boaventura (2001) also present determined by sampling at ANEEL monitoring stations. Andean rivers are shown in brown to represent their high load of figures of the amount of solids sediments. They show higher concentrations of arsenic in both being transported, and of the ar- soluble (above) and suspended (below) forms senic content in both the trans- ported solids and dissolved sol- ids in the waters. Their numbers are shown in the HiBAM data on arsenic in the Amazon Basin columns on the left hand side of Table 1. They made it possible to calculate, for each sampling sta- The concentrations of arsenic in the rivers are tion, the percentage of arsenic transported as sol- shown in Figures 4 and 5, on which the rivers with ids, the total concentration of arsenic in the rivers, Andean waters are drawn in brown. The illustra- and the total mass of arsenic being transported in tions show the contents of arsenic in soluble form, tons per day. in suspended solids, the total concentration of

22 TERRÆ 2(1-2):20-27, 2005 W. Scarpelli

Table 1 – Arsenic transported in the rivers of the Amazon Basin as observed with samples collected under the framework of the HiBAM project at the network of stations operated by ANEEL in the Brazilian Amazon. Rivers with Andean waters and sediments are shown in negrite (ton/day) (ton/day) As total mass As total mass 3 3 (mg/m ) (mg/m ) As total grade As total grade 3 3 Calculated Values Calculated Values As in the As in the sediments (mg/m ) sediments (mg/m ) 3 3 (mg/m ) (mg/m ) As in solution As in solution 3 3 (mg/m ) (mg/m ) As in solution As in solution As in the As in the sediments (g/ton) sediments (g/ton) 3 3 Suspended Suspended sediments (g/m ) sediments (g/m ) 3 3 Reported Sampling and Assay Data Reported Sampling and Assay Data (m /sec) (m /sec) River flow River flow Date of Date of sampling sampling (km) sea (km) Distance from Distance from Içá 31/oct/1995 5,354 41.4 3.5 0.15 0.15 0.14 0.29 0.14 River Jutaí 3/nov/1995 1,143 13.5 3.4 0.15 0.15 0.05 0.20 0.02 Beni 2,000 1/apr/1994 2,856 937.0 19.9 0.83 0.83 18.65 19.48 4.81 River Juruá 4/nov/1995 1,045 56.3 10.1 0.85 0.85 0.57 1.42 0.13 Purus 9/nov/1995 2,534 38.6 3.1 0.55 0.55 0.12 0.67 0.15 NegroNegroNegroNegroNegro 20/jun/1996 1,250 23/jun/1996 9,790 27/jun/1996 12/jul/1996 15,840 9/jul/1996 23,000 64,680 9.9 52,640 11.6 10.3 8.9 17.0 0.2 0.2 0.2 7.9 <0.05 1.7 <0.05 <0.05 <0.05 0.05 <0.05 <0.05 <0.05 0.00 <0.05 0.00 0.05 0.00 0.03 0.05 0.05 0.07 0.05 0.05 0.04 0.07 0.12 0.10 0.23 0.67 Javari 27/oct/1995 1,565 127.6 5.8 0.36 0.36 0.74 1.10 0.15 Japurá 4/nov/1995 10,264 28.5 13.6 0.33 0.33 0.39 0.72 0.64 Itaquaí 27/oct/1995 793 148.3 7.9 0.44 0.44 1.17 1.61 0.11 Branco 8/jul/1996 11,960 22.7 2.1 <0.05 <0.05 0.05 0.05 0.05 Tapajós 18/nov/1995 6,027 3.5 17.7 0.11 0.11 0.06 0.17 0.09 Mamoré 3/apr/1994 8,391 409.0 11.9 0.61 0.61 4.87 5.48 3.97 MadeiraMadeira 1,950 1,200 12/apr/1998 15/nov/1995 29,000 5,132 302.0 21.3 ( 2 ) 2.7 0.59 0.69 0.59 0.69 ( 0.60 ) 0.06 ( 1.19 ) 0.75 ( 2.98 ) 0.33 Solimões 2,500Solimões 26/oct/1995Solimões 2,200 20,115Solimões 28/oct/1995 166.5 24,251 1,900Solimões 29/oct/1995 14.5 3/nov/1995 74.5Solimões 1,380 32,539 34,333 1,200 1.53 7/nov/1995 15.7 46.0 60.9 46,847 10/nov/1995 1.53 1.38 52,477 9.7 10.0 63.7 127.1 2.41 1.38 0.49 1.01 3.0 2.8 3.94 1.17 0.49 1.01 0.88 6.85 0.77 2.55 0.45 0.61 0.88 5.34 0.77 0.94 1.62 0.19 0.36 2.63 4.80 1.07 1.13 4.34 5.10 Trombetas 16/nov/1995 1,258 14.8 17.1 0.12 0.12 0.25 0.37 0.04 Amazonas 1,000Amazonas 15/nov/1995 650 75,017 17/nov/1995 46.1 81,090 81.9 44.2 0.73 2.8 0.73 0.68 3.78 0.68 4.51 0.12 29.20 0.80 5.63 Madre Diós 2,050 2/apr/1994 5,092 424.0 10.0 0.61 0.61 4.24 4.85 2.13

23 W. Scarpelli TERRÆ 2(1-2):20-27, 2005 arsenic, and the tonnage of the S VENEZUELA E GUYANA

element transported per day. 5 D SURINAME Atlantic It is quite clear that the Ocean N COLOMBIA Fr. G. <0.1 Trombetas Andean waters have dramatically A <0.1

Branco higher concentrations of arsenic, <0.1 0 <0.1 Negro 0.7 <0.1 0.8 1.6 0.2 occasionally not too far from the 0.9 0.1 0.3 2.6 1.1 0.1 maximum limit of potability, 4.5 1.4 0.2 1.1 μ 1.1 3.9 0.7 0.8 which is 10 g/liter, or 10 grams -5 Solimões 1.6 Tapajós Purus adeira per cubic meter, as defined in PERU M year 2000 by the Brazilian Health BRAZIL Xingu 1.2

Ministry. When the concentra- -10 4.8 5.5 tions of arsenic in solution and ANDES Araguaia 19.5 in solids are added, the limit of

potability is occasionally sur- -15 passed. BOLIVIA Pacific The discrepancies observed 3.8 0.6 Ocean TOTAL GRADE OF 3 between the sites reflect differ- Andean Drainage Non-Andean Drainage ARSENIC (mg/m ) -20 ences in the flow regimens and (and sampled points) (and sampled points)

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70 65 60 55 should not be taken as represent- 50 45 ing errors in sampling or assay- S VENEZUELA GUYANA ing. Along the Amazon River it- E

5

D SURINAME Atlantic self, one of the reasons for the Ocean N COLOMBIA Fr. G. <0.1 Trombetas differences is the continuous A <0.1

Branco process of deposition and ero- <0.1 0 0.1 Negro 0.6 0.2 <0.1 5.6 sion of the transported solids on 2.6 4.8 0.7 0.1 5.3 4.3 0.1 the banks of the river (Martinelli 0.1 29.2 0.2 <0.1 5.1 6.8 0.2 0.3 et al. 1993). -5 Solimões 0.1 Tapajós The largest difference oc- Purus adeira PERU M curred with the samples taken at BRAZIL Xingu 3.0

the last two sampling stations in -10 2.1 4.0

ANDES Araguaia the Amazon River, near the 4.8 mouths of the Trombetas and the

Tapajós rivers. The values ob- -15 served at these stations revealed BOLIVIA Pacific that 29.2 and of 5.6 tons of ar- 3.8 0.6 Ocean ARSENIC TRANSPORT senic were being transported per Andean Drainage Non-Andean Drainage -20 (tons/day) day, respectively. While the true (and sampled points) (and sampled points)

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70 65 60 55 value could be between these 50 45 two figures, it is quite clear that Figure 5 – The total grades of arsenic in Amazonian rivers are shown above in soluble and suspended forms. Below, the tonnage of 5.6 or 29.2 tons per day of arsenic arsenic transported per day, considering the concentrations of represent a very large figure. arsenic and water flow volume

The source seems to be the sulfide-related Source of the arsenic transported mineralizations of gold and base metals that occurs by the Amazon Basin rivers in the eastern Andean ridges, many related to north- south faults and occurring at elevations on the order The information presented clearly indicates of 4,000 meters. Most of these steep sloped ridges that the arsenic is coming from the Andes, which receive high rainfall due to moisture rich winds is the source of 97% of the solids being transported from the area to the east, and are eroded at a by the Amazon River to the ocean. relatively fast rate (Figs. 6 and 7).

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ROSÁRIO CAMP

Figures 6 and 7 – Rosario de Araca, east of La Paz, Bolivia, one of the source areas for sediments containing arsenic. The Rosario de Araca camp is located over an arsenic-bearing auriferous deposit, at mid-slope of a 4,700-meter high cordillera. The photo at left shows the steep gulleys cutting through the mineralized area, and the photo at right, taken from the camp, shows extensive sedimentary terraces at the base of the mountain, submitted to intense erosion

Arsenic at the mouth of the Amazon River and from the foreland region of the Andes. Most of the grains are quartz, feldspars and micas, In the paper which drew attention to the issue followed by minor oxides and silicates. Quartz and of arsenic in Amazon waters, S.F.P. Pereira presented feldspars are round or angular, usually with iron- values of dissolved arsenic in waters of the mouth rich coatings. of the Amazon and the Guamá Rivers, that is, at McDaniel et al. (2002) detailed the relatively west, north, and south of the Marajó Island. As the unweathered conditions of the sediments sampled two rivers are connected by water channels west in the fan area and concluded, after comparative of the Marajó Island, Andean water from the studies of Pb and Nd isotopes, “that muds of the Amazon also flows through the Guamá River. Amazon Fan are derived dominantly from the Her paper (Fig. 8) revealed arsenic contents Andean highlands. Furthermore, during their similar to those observed in the upstream sections journey to the Atlantic Ocean, they were not of the Amazon River, but with an increase of values affected by the extreme weathering conditions such in direction of the sea. From concentrations of 3 as those that exist in the Amazon to 5 mg/m3 in front of Macapá and Belém, the con- today”. This conclusion was similar to that made centration reached 10 to 14 mg/m3 in the delta. earlier by Gibbs (1967), who identified angular These high concentrations are greater than the grains of fresh feldspars in the sediment load of maximum presently accepted by most countries for the Amazon River. waters for human consumption, which is 10 mg/m3. Burns (1997) describes the presence of a laterally persistent horizon rich in secondary High values of arsenic at the Amazon Fan diagenetic iron sulfides, phosphates and carbonates in the fan area, at about one meter of depth. He There are several reports describing the points out that these minerals form after dissolution sediments of the Amazon Fan area, among them and re-crystallization of iron oxides and hydroxides. those based on cores of the Ocean Drilling Program. The reactions occur under the reducing anoxic The observations support a proposed mechanism conditions created by the continuous burial of the to explain the greater values of dissolved arsenic in sediments. the coastal area of the state of Amapá. Sullivan and Aller (1996), examining the shal- Nanayama (1997) describes the constituent low holes drilled along the lines ‘ost’ and ‘rtm’ shown minerals of the sediments deposited in the Amazon at Figure 8, verified that during the diagenetic for- Fan, identifying assemblies derived from the mation of iron carbonates, sulfides and phosphates, Precambrian Brazilian and Guyana Shields, from the arsenic contained in the iron oxides and the Tertiary Sediments of the Amazon River Basin, hydroxides was liberated to the pore water of the from arc-volcanic rocks of the Andean Cordillera sediments, with the maximum grade of arsenic in

25 W. Scarpelli TERRÆ 2(1-2):20-27, 2005 pore water seen at depths of 0.25 to 1.5 meters (Fig. 9). A LAQUANAM SAMPLING AND SAMPLING HOLES OF THE sample with 300 ppb (300 mg/ MULTIDISCIPLINARY AMAZON SHELF SEDIMENTARY STUDY 3 m ) was reported at the hori- ost 3 zon rich in iron carbonates, Sampling sites ost 2 sulfates and phosphates. They where liberation of ost 1 AS from sediments express that “maximum pore- was observed rtm 3 water arsenic concentrations rtm 2 are one order of magnitude rtm 1 greater than levels reported in most other coastal marine en- Sampling of 14 LAQUANAM 9 vironments”, and “oxidized 3 currentcurrent (mg/m AS) 8 9 arsenic is associated with iron 8 direction 16 oxyhydroxides in surface sedi- 8 7 100 km ments, then subsequently re- Macapá 6 SantanaSantana 3 duced and released upon burial 6 <1 in the extensive suboxic zone”. 3 To aggravate the matters, “up- ward diffusion and intense physical reworking of sedi- 5 ments presumably releases the 5 Belém dissolved portion of arsenic 7 7 fraction to the water column”. 1 2

Summary and conclusions Figure 8 – Soluble arsenic determined by LAQUANAM in the Amazon River in the north, and the Guamá River in the south. There is an increase of soluble arsenic towards the ocean. The figure also shows the The high rate of erosion in location of the two lines of holes, “ost” and “rtm”, of the Amazon the Andean cordilleras is the Shelf Sedimentary Study, which revealed that the diagenesis of iron cause of the great volume of oxides and hydroxides liberates arsenic to the ocean waters, a sediments transported by the possible reason for the increase of arsenic in the coastal areas. Andean rivers, giving them their characteristic light brown color. These rivers to sulfides, carbonates and phosphates when are responsible for 97% of the sediments covered by about one meter of new sediment load, transported by the Amazon River to the Atlantic and during this reduction, their adsorbed arsenic Ocean. is liberated in soluble form to the pore water of the Together with their transported minerals, sediments. The top layers of sediments are mostly quartz, feldspars and micas, which are often disturbed during storms and the newly formed coated with iron oxides and hydroxides, the soluble arsenic is released into the sea water. Andean rivers also transport high concentrations As a consequence of this process, the con- of arsenic, in solution or adsorbed in the iron oxides centration of arsenic is higher in the Amazon Fan and hydroxides. The arsenic, which originates from than in the Amazon River, and this should be of the common occurrences of arsenic-bearing some concern since this area is a source of marine sulfides along the Andean cordilleras, appears with sea foods. concentrations which approach the maximum value of arsenic accepted for potable water. The References samples collected at the monitoring stations of ANEEL indicate that more than 5 tons of arsenic Brasil. Ministério da Saúde. 2000. Portaria nº 1469/ are transported annually to the Atlantic Ocean. GM. Brasília: Ministério da Saúde. 15p. After their deposition in the Amazon Fan area, (Regulation on water quality for human the iron oxides and hydroxides are slowly reduced consumption).

26 TERRÆ 2(1-2):20-27, 2005 W. Scarpelli

‘OST’ HOLES (NORTHWEST) ‘RTM’ HOLES (SOUTHEAST) 250 250

) )

3 200 3 200

150 150

100 100

50 50

As (ppb = mg/m

As (ppb = mg/m

0 0 5 15 25 35 55 75 95 115 135 155 185 215 245 5 15 25 35 55 75 95 115 135 155 185 215 245 Depth (cm) Depth (cm) 45 45 40 40 35 35 30 30 25 25 20 20

Fe (ppm) 15 Fe (ppm) 15 10 10 5 5 0 0 5 15 25 35 55 75 95 115 135 155 185 215 245 5 15 25 35 55 75 95 115 135 155 185 215 245 Depth (cm) Depth (cm) 25m 55m 75m

Figure 9 – Arsenic and iron in pore water of shallow sediments in the Amazon Fan area, as observed in holes OST-1,2,3 and RTM-1,2,3. The holes were drilled under sea water columns of 25 (blue lines), 55 (green lines) and 75 meters (red lines) of depth and were sampled every 10 centimeters of penetration to a depth of 2.5 meters. The marked increase of arsenic coincides with the horizon of reduction of iron oxides and hydroxides to iron sulfides, carbonates and phosphates

Burns S.J. 1997. Early diagenesis in Amazon Fan Piper, A. Klaus, L.C. Peterson, (eds.). Proceedings sediments: processes and rates of reaction. In: of the Ocean Drilling Program; Scientific Results, R.D. , D.J.W. Piper, A. Klaus, L.C. 155:169-176. Peterson, (eds.). Proceedings of the Ocean Drilling Nanayama F. 1997. An electron microprobe study of Program, Scientific Results, 155:497-504. the Amazon Fan. In: R.D. Flood, D.J.W. Piper, A. Gibbs R.J. 1967. The geochemistry of the Amazon Klaus, L.C. Peterson, (eds.). Proceedings of the Ocean River System: Part I. The factors that control the Drilling Program; Scientific Results, 155:147-168. salinity and the composition and concentration Pereira, S.F.P. [without a date]. Avaliação da contami- of the suspended solids. Geol. Soc. Am. Bull., nação por metais pesados no delta do rio Amazonas. 78(10):1203-1232. Belém: Universidade do Pará, LAQUANAM. Kasten S., Freudenthal T., Gingele F.X., Schulz H.D. (unpublished report). 1998. Simultaneous formation of iron-rich layers Seyler P.T., Boaventura G.R. 2001. Trace elements at different redox boundaries in sediments of the in the mainstream Amazon River. In: M.E. Amazon deep-sea fan. Geochim. Cosmochim. Acta, McClain, R.L. , J.E. Richey. (eds.). 2001. 62(13):2253-2264. The Biogeochemistry of the Amazon Basin. Oxford: Martinelli L.A., Victoria R.L., Dematte J.L.I., Richey Oxford University Press. J.E., Devol A.H. 1993. Chemical and mineral- Sullivan K.A., Aller R.C. 1996. Diagenetic cycling ogical composition of Amazon River floodplain of arsenic in Amazon shelf sediments. Geochim. sediments, Brazil. Appl. Geochem., 8(4):391-402. Cosmochim. Acta, 60(9):1465-1477. McDaniel D.K., McLennan S.M., Hanson G.N. 1997. Provenance of Amazon Fan muds: constraints Submitted in October, 2004 from Nd and Pb isotopes. In: R.D. Flood, D.J.W. Accepted in May, 2005

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