11 Mineralogy and sediment chemistry

Transcription

1 Mineralogy and sediment chemistry 11.1 SEDIMENT SAMPLES AVAILABLE Despite the large amount of drilling for tubewells that has been undertaken in Bangladesh, archives of well-catalogued and well-preserved sediment samples are relatively rare. This situation has been remedied recently to some extent by both DPHE and BWDB (with UNICEF assistance) undertaking a number of drilling programmes specifically aimed at retrieving core for investigations related to arsenic. Core recovery in early boreholes from these programmes was far from complete but as experience has accumulated, recovery has improved. However, in none of these programmes was core recovered without at least some oxidation of the pore water and sediment. This was also true during the drilling of the three boreholes carried out within this project. While this is clearly an important limitation for studies of sediment-water interactions in the reducing sediments of Bangladesh, it is not critical for many basic mineralogical studies and for those chemical analyses that are insensitive to the effects of oxidation, e.g. total arsenic contents. These and other archived sediments provide a valuable indication of the overall variability of sediment mineralogy. Undertaking sample collection with the necessary precautions to prevent oxidation must await more focused studies of the sediment chemistry. Sediment samples were therefore collected from various sources for whole-rock examination and analysis, and for selective extraction. A map showing the location of the various sediment samples used in this study is given in Figure Small amounts of archived sediments from three boreholes in and close to Nawabganj upazila were provided by the Barind Integrated Agricultural Development Project. Core material from these holes was obtained during drilling by the reverse-circulation method and samples had been stored in bags for several years. They will therefore have been at least partially oxidised. A limited amount of core material was also available from two other boreholes from the Dupi Tila aquifer in the Barind area. One borehole was located at Purba Fargilpur, some 25 km north of Chapai Nawabganj in Gomastpur upazila and the other at Khitta, 12 km east of Chapai Nawabganj. Core material was also available from a third borehole in the Recent alluvial sediments at West Bilat Haripur, in Shibganj upazila, around 15 km north-west of Chapai Nawabganj town. A number of additional sediment samples from northern and southern Bangladesh were also obtained from Dr. Aftab Alam Khan (DU) and Mizanur Rahman (BWDB). These had been obtained in separate studies of the sediments of Bangladesh as part of arsenic investigations by these organisations. In particular, sediments were analysed from two boreholes (DW1 and DW2) drilled by BWDB during January March The results of these were reported in detail in the Phase I report and their logs are 27 o 26 o 25 o 24 o 23 o 22 o 21 o 2 o 24 o o o 3 24 o 24 Thakurgaon Pirgacha Chapai Nawabganj INDIA Dhaka1 Sediment Locations Piezometer cluster Borehole INDIA Faridpur Bhimpur W. Latifpur Lakshmipur Groundwater Studies of Arsenic Contamination in Bangladesh BGS/DPHE/DFID (2) R. Ganges 88 o 89 o 9 o 91 o 92 o West Bilat Haripur R. Mahananda Khitta Chapai Nawabganj Chanlai Rajarampur Piezometer cluster Borehole Purba Fargilpur 1 km Chapai Nawabganj area 88 o 6 88 o o o o 3 Figure Location of the various sediment samples studied. presented in Chapter 3. These holes were drilled into the aquifer in the region of the Chapai Nawabganj hot spot: DW1 at Rajarampur and DW2 at Chanlai. Each borehole was drilled to a total depth of 15 m by reverse circulation and core material was recovered from a.6 m interval every 3 m using a split spoon. No drilling fluid was used.

2 188 Arsenic contamination of groundwater in Bangladesh Table Source of sediment samples Site Upazila, District Latitude Longitude Stratigraphy As-rich groundwaters Source Khitta Gomastapur, Nawabganj 24º36.7' 88º22.5' Dupi Tila No Barind Integrated Agricultural Development Project Purba Fargilpur, Gomastapur, Nawabganj 24º52.6' 88º26.6' Dupi Tila No same as above Rajarampur, DW1 Sadar, Nawabganj 24º34.67' 88º15.5' Holocene Yes DPHE/BWDB/DU (UNICEF assisted) Chanlai Primary Sadar, Nawabganj 24º35.37' 88º15.5' Holocene Yes same as above School, DW2 West Bilat Haripur, Kansat Shibganj, Nawabganj 24º41.5' 88º12.' Holocene No? Barind Integrated Agricultural Development Project Chanlai Primary Sadar, Nawabganj 24º35.37' 88º15.5' Holocene Yes Project piezometer, CPW5 School, Aliabad Union Sadar, Faridpur 23º35.22' 89º51.69' Holocene and Yes Project piezometer, FPW6 Parishad Compound older BWDB Compound Sadar, Lakshmipur 22º56.47' 9º5.6' Holocene and Yes Project piezometer, LPW6 older Bhimpur, Chatkhil, Noakhali 24º3.9' 9º58.7' Holocene Yes BWDB West Latifpur Sadar, Lakshmipur 22º57.5' 9º59.7' Holocene Yes BWDB Thakurgaon Sadar, Thakurgaon 26º4' 88º2' Holocene No BWDB alluvial fan Pirgacha Pirgacha, Rangpur 25º43' 88º22' Holocene Yes BWDB BWDB Compound Dhaka city, Dhaka 23º45.2' 9º23.' Dupi Tila No BWDB Details of the core material and drilling method are given in Khan et al. (1998). The main fraction of the core material was forwarded to Dhaka University for sedimentological investigation. A separate sub-sample was sealed on site with wax in PVC liners and freighted to the UK for analysis. Pore waters were recovered from the more sandy core samples from DW1 and DW2 by centrifugation using screw-topped centrifuge cups in a high-speed centrifuge but as reported in the Phase I report and Chapter 7, these showed evidence of oxidation and were all low in arsenic. They were therefore not thought to reflect the true in situ pore water chemistry. Nevertheless, the sediment provided samples for SEM analysis and chemical analysis. A set of piezometers for water quality and water-level monitoring was drilled in each of the Special Study Areas. The piezometers were installed by BWDB during Two drilling rigs were used: one for the Chanlai (Chapai Nawabganj) set and one for the Faridpur and Lakshmipur sets. The sediment was extracted in a PVC liner and both ends sealed with wax to minimise, but not totally exclude, oxidation of the sediment. Although visible signs of oxidation were not always visible, there was evidence for oxidation in some samples when the cores were opened. This was seen in the form of a brown rim on the outside of the core with grey sediment in the centre. Oxidation of the sediments was seen to progress slowly as the storage time increased. The borehole logs are described in detail in Chapter 3. As expected, clay was encountered below about 4 m at Chanlai, as in the earlier DW2 borehole (Figure 3.11), and so drilling was halted at 5 m. The deep hole at Faridpur was 155 m deep and at Lakshmipur it was 153 m deep. Sediments from these three purpose-drilled deep boreholes were used as the principal samples in the subsequent mineralogical and geochemical investigations. The samples used were derived from the bulking of sediment over.3 or.6 m (1 or 2 ft) intervals. Some sediment samples were also available from the BWDB. These included Thakurgaon in the extreme northwest of Bangladesh where the sediments are dominated by coarse sediments of the Tista Fan. It is also a region of low groundwater As concentrations. Pirgacha is also in the north-west but is a known As hot spot area. The Dhaka samples, from a borehole drilled on the main BWDB compound, are from the Dupi Tila aquifer. This aquifer has known low groundwater As concentrations. A summary of the properties of the sediments used in this study is given in Table Sediment samples from all boreholes were air-dried. Samples for determination of organic carbon and selective extraction using ammonium oxalate-oxalic acid and/or 6 M HNO 3 were gently disaggregated before extraction using a pestle and mortar. Organic carbon was determined using a Carlo Erba CNS analyser after initial dissolution in 1% HCl to remove inorganic carbon. Representative samples of sediments were analysed using an acid ammonium oxalate extraction. The most detailed profiles were for the DW1 (Rajarampur) and DW2 (Chanlai) boreholes and for the three project piezometer boreholes. Sediment samples from these profiles were taken at 3 m intervals for extraction, where available, and at 1.5 m intervals for the top 15 m of the three piezometer boreholes. These extracts provided upper limits on the amount of labile elements present, especially of arsenic and iron. They also provided a standard basis for comparing sediments from various locations.

3 Mineralogy and sediment chemistry 189 No. Table Description of the twenty one samples used for detailed mineralogical and geochemical studies Location Depth (ft) Depth (m) Description Colour 1 Chapai Nawabganj Very micaceous fine sand to silt Dark to olive brown 2 Chapai Nawabganj Micaceous fine sand Light grey 3 Chapai Nawabganj Micaceous medium to fine sand Brown grey 4 Chapai Nawabganj Silty clay Light yellow brown 5 Chapai Nawabganj Clayey silt Grey brown 6 Faridpur Micaceous silty fine sand Brown 7 Faridpur Fine sand and micaceous sandy silt Brown 8 Faridpur Clayey silt and micaceous fine sand Grey brown 9 Faridpur Fine to medium sand Brown grey 1 Faridpur Fine to very fine sand Grey 11 Faridpur Silty fine to medium sand Grey 12 Faridpur Micaceous fine sand Grey 13 Faridpur Coarse sand and gravel Grey 14 Lakshmipur Silty very fine sand Brown grey 15 Lakshmipur Micaceous fine sand Grey 16 Lakshmipur Medium to fine sand Grey 17 Lakshmipur Very fine to medium sand Grey 18 Lakshmipur Micaceous silt to fine sand Brown grey 19 Lakshmipur Micaceous fine to very fine sand Grey 2 Lakshmipur Coarse sand Grey 21 Lakshmipur Coarse to medium sand Grey Bold typeface in the Depth column signifies a significant aquifer horizon; bold typeface in the Description column indicates a fine-grained sediment identified from sieve analysis SAMPLES SELECTED FOR MINERALOGICAL ANALYSIS Twenty one samples of core material from the three Special Study Areas were selected for detailed sedimentological and mineralogical analyses. These were chosen to include the main aquifer as well as representative samples from the various lithologies found in the boreholes. Details are given in Table 11.2 for the Faridpur (FPW6), Chapai Nawabganj (CPW5) and Lakshmipur (LPW6) boreholes. The log descriptions were made immediately upon opening the plastic liner and so the sediments should not have been extensively oxidised. The aims of the analyses were twofold: (a) to determine the concentration of As and other elements in specific size and heavy mineral fractions, and (b) to determine the principal sources of As and other elements within the sediments. The geochemical and heavy-mineral analyses also enable conclusions to be drawn about the sediment provenance METHODS Sieve analysis, heavy mineral and magnetic separations Sample splitting A representative sub-split from each sample was taken either by riffle splitting (sand grade samples) or by cone and quartering (clay grade samples). This was ground by Tema mill for chemical analysis. The remainder was used as the active sub-sample (Figure 11.2). Particle size fractions The active sub-sample was split into several size fractions to obtain a suitably graded fraction for heavy liquid-separation (Figure 11.2). This was done using a combination of wet screening and hydrocycloning. The sample was wet screened on 5 and 63 µm stainless steel sieves. The <63 µm material was left in suspension and the <1 µm material was removed using a Pyrex bench-scale hydrocyclone. The 1 63 µm fraction was also dried and weighed and then combined with the 63 5 µm fraction to produce a 1 5 µm fraction for heavy-liquid separation. The data from the particle size fraction were used to produce particle-size frequency distributions (i.e. mass retained for each size fraction) and cumulative distributions (i.e. mass percentage less than a particular particle size). Heavy-liquid separation The 1 5 µm fraction was placed in a separating funnel containing bromoform (density 2.88 g cm 3 ) and was separated into a light (<2.88 g cm 3 ) and heavy fractions (>2.88 g cm 3 ). Each fraction was filtered and washed several times with acetone to remove any trace of the bromoform. The fractions were then air dried and weighed. Magnetic separation The heavy fraction was separated by magnetic separation to produce five fractions of different magnetic susceptibility as follows: (1) highly magnetic; (2) strongly magnetic; (3) moderately magnetic; (4) weakly magnetic, and (5) non-

4 19 Arsenic contamination of groundwater in Bangladesh Sample Wet screen on 63 and 5 µm sieves Dry & weigh >5 µm Representative sub-sampling Dry and weigh 63 5 µm magnetic. A permanent horseshoe magnet was used to separate the highly magnetic material, and an Eclipse variable hand magnet was used to split the remainder into the other fractions. Magnetic susceptibility measurements were also made on all 21 samples by R. Reynolds (USGS) Scanning electron microscopy (SEM) Subsamples of various horizons from the DW1 (Rajarampur) and FPW6 (Faridpur piezometer) boreholes were examined by SEM. These subsamples were mounted on aluminium pin-type stubs using a graphite carbon cement and coated with a thin layer of carbon, approximately 25 nm thick by evaporation in an Emitech evaporation coater. All specimens were examined in a Leo 435VP SEM fitted with a four element solid-state backscattered electron detector. An Oxford Instruments Isis 3 energy-dispersive X-ray microanalysis (EDXA) instrument allowed routine mineral identification X-ray diffraction Leave < 63 µm in suspension Hydrocylone to remove <1 µm ( slimes ) Dry and weigh 1 63 µm Combine 1 63 µm and 63 5 µm fractions and carry out heavy liquid separation using bromoform (2.88 g cm -3 ) Dry & weigh heavies (>2.88 g cm -3 ) Highly magnetic Active sub-sample ¾ split Magnetic separation Strongly magnetic Moderately magnetic Analysis ¼ split Dry & weigh lights (<2.88 g cm -3 ) Weakly magnetic XRF ICP AFS Sulphur Dry and weigh 1 µm Nonmagnetic Figure Scheme used for separation and analysis of sediment samples. In order to study the clay minerals present, a fine (<2 µm) fraction-oriented mount was prepared. A representative sub-sample of the <1 µm material was dispersed in 15 ml of deionised water by shaking overnight on a reciprocal shaker and subsequent treatment with ultrasound for approximately 3 minutes. The resultant suspensions were placed in a 25 ml measuring cylinder and allowed to stand. 1 ml of.1 M Calgon (sodium hexametaphosphate) was added to the suspensions to prevent flocculation. After a period dictated by Stokes Law, a nominal <2 µm fraction was removed and dried at 55 C. 1 mg of the <2 µm material were then re-suspended in a minimal amount of distilled water and pipetted onto a ceramic tile in a vacuum apparatus to produce an oriented mount. The clay mounts were then Ca-saturated using 2 ml 1M CaCl 2 solution and washed with distilled water twice to remove excess salt before being allowed to air-dry. XRD analysis was carried out using a Philips PW17 series diffractometer using Co-Kα radiation and operating at 45 kv and 4 ma. The oriented mounts were scanned over the range θ in both air-dried and ethylene glycol-solvated states, and after heating at 55 C for 2 hours, at a scanning speed of.5 2θ per minute. Diffraction data were analysed using the PC-based Philips X'Pert software coupled to an International Centre for Diffraction Data (ICDD) database Geochemistry Major element chemistry by XRF Major elements were determined by X-ray fluorescence (XRF) using a Philips PW 24 X-ray fluorescence spectrophotometer. Approximately 5 g of sample was dried for 24 hours at 15 C. Loss on ignition (LOI) was calculated from the weight loss of 1 g of sample heated at 15 C for one hour. Fused glass beads were prepared by fusing.9 g of sample with 9 g of dried lithium tetraborate (Li 2 B 4 O 7 ) flux at approximately 12 C in a muffle furnace. The melt obtained was poured into a platinum casting dish. Lithium iodide was then added to all samples before fusion to act as a releasing agent. Subsequently, 14 major elements were calculated as oxides using a standard Philips calibration algorithm and theoretically generated alpha coefficient corrections. The LOI values obtained represent the loss of volatiles from the samples due to reactions such as carbonate and organic matter decomposition, sulphide oxidation and the loss of moisture and structural water. Sulphur Total sulphur was determined on all 21 samples but only one (F154, Lakshmipur m) was found to contain an appreciable sulphur content (.22 %S). A sample of this material was incrementally ignited from 35 65ºC in air, and the sulphur content re-determined (.19 %S). Very little if any sulphur was lost, indicating that this sample probably contained sulphate and not sulphide. The small difference in the values is much less than the errors inherent in the method. Digested samples Two digestions were made prior to analysis by ICP-AES. The first was a mixed acid attack using a combination of HF/HClO 4 /HNO 3 acids and this was used to determine Sr, Ba, Fe, As Ti, Sc, Co, Rb, Cd, Pb and Bi. The second

5 Mineralogy and sediment chemistry 191 Table Fractionation of the Chapai Nawabganj sediments from test borehole CPW5 by sieve, heavy mineral and magnetic separation Description Chapai Nawabganj Depth (feet) Sample Sample code F145 F146 F147 F148 F149 Sieve >5 µm wt% µm wt% µm wt% <1 µm wt% Total wt% Heavy minerals >5 µm wt% µm lights wt% heavies Heavies wt% < <1 µm wt% Magnetic >5 µm wt% µm lights wt% ) highly magnetic wt% < <.1 2) strongly magnetic wt% < <.1 3) moderately magnetic wt% < <.1 4) weakly magnetic wt% < <.1 5) non-magnetic wt% < <.1 digestion involved the same mixed-acid attack followed by a fusion with NaOH and was used to determine REE, U, Th, Zr, Nb, Hf and Ta. Although the duplicate determinations for the fusions were not as reproducible as usual (i.e. differences sometimes greater than 1%) particularly for some of the more refractory elements, the data obtained for the CRMs was acceptable, suggesting that the cause was an inhomogeneous distribution of refractory minerals within the samples. Arsenic was determined by HG-AFS on the extracts from the mixed-acid attack without NaOH fusion Selective extraction using acid ammonium oxalate Acid-ammonium-oxalate extracts were prepared (following the method of McKeague and Day, 1966). Samples were weighed (1.25 g) into acid-washed Oak Ridge centrifuge tubes and 25 ml of.2 M acid oxalate solution (7 ml ammonium oxalate plus 535 ml.2 M oxalic acid, adjusted to ph 3; i.e. Tamm s Reagent) added. Solutions were then shaken in the dark (to prevent photochemical reactions) for 4 hr, centrifuged and supernatants decanted ready for chemical analysis. Acid ammonium oxalate extracts arsenic associated with iron and aluminium oxides, with carbonates including iron carbonate (siderite), and to some extent with clays, and is expected to give a closer estimate of the labile pool of arsenic than a total dissolution which will tend to overestimate the pool, or with a water-soluble extraction which will tend to underestimate it. In aerobic environments, this extract has been widely used for estimating the amount of amorphous iron oxides. With reducing sediments, the extraction is also likely to dissolve some crystalline iron oxides such as magnetite, akageneite, hematite and goethite (Heron et al., 1994; Kostka and Luther, 1994) and probably other mixed Fe(II)-Fe(III) oxides. It may also desorb As and other adsorbed solutes from a variety of minerals. The oxalate extracts were analysed for a wide range of elements by ICP-OES using matched standards and for As by HG-AFS as for the groundwaters MINERALOGY AND WHOLE ROCK GEOCHEMIS- TRY Particle size, sedimentology and mineralogy The results of the sieve and heavy-mineral separations are given in Tables Both coarse-grained and finegrained samples were analysed from the three Special Study Areas boreholes. The proportion of <63 µm (silt and clay) material provided a clear distinction between the two: the fine-grained sediments usually contained >9% of this fraction whereas the coarse-grained sediments usually contained <1%. In general, most samples in the Faridpur borehole (FPW6) were fine to medium sands. However, the fine-grained samples were distinguished by a higher clay and silt content. All samples contained conspicuous, coarse, sand-sized biotite. The mineralogy of the samples is dominated by quartz and feldspar, both plagioclase (albite and more Ca-rich compositions) and alkali feldspar. The proportion of <1 µm material varies in the Faridpur samples, from 38 46% in the fine-grained samples to typically less than 1% in the coarser-grained samples.

6 192 Arsenic contamination of groundwater in Bangladesh Table Fractionation of the sediments from the Faridpur borehole FPW6 by sieve, heavy mineral and magnetic separation Description Faridpur Depth (feet) Sample Sample code F137 F138 F139 F14 F141 F142 F143 F144 Sieve >5 µm wt% µm wt% µm wt% <1 µm wt% Total wt% Heavy minerals >5 µm wt% µm lights wt% Heavies wt% <1 µm wt% Magnetic >5 µm wt% µm lights wt% ) highly magnetic wt%.6.3 < ) strongly magnetic wt% < ) moderately magnetic wt% < < ) weakly magnetic wt% <.1.8 < ) non-magnetic wt% <.1.24 < Table Fractionation of the sediments from Lakshmipur borehole LPW6 by sieve, heavy mineral and magnetic separation Description Lakshmipur Depth (feet) Sample Sample code F15 F151 F152 F153 F154 F155 F156 F157 Sieve >5 µm wt% µm wt% µm wt% <1 µm wt% Total wt% Heavy mineral >5 µm wt% µm lights wt% heavies wt% < <1 µm wt% Magnetic >5 µm wt% µm lights wt% ) highly magnetic wt% ) strongly magnetic wt% ) moderately magnetic wt% ) weakly magnetic wt% < ) non-magnetic wt% < <1 µm wt% In the Chapai Nawabganj samples, the proportion of <1 µm fraction varied from 31 4% in the fine-grained samples to typically less than 4% in the coarse-grained samples. In the Lakshmipur samples, the proportion of this fine fraction varied from 13 33% in the fine-grained samples to typically less than 4% in the coarse-grained samples. Heavy mineral concentrations varied up to 8% in the coarser, more gravel-rich Faridpur samples, up to 6%

7 Mineralogy and sediment chemistry 193 in the Lakshmipur samples and up to 2% in the Chapai Nawabganj samples which were generally slightly better sorted. The magnetically-separated, heavy mineral fractions were very similar in all samples. The mineralogy changed gradually without distinct cut-offs between the different fractions. This is because the bulk of the heavy mineral populations were ferro-magnesian minerals including pyroxene (augite plus others), hornblende and other amphiboles and garnet. Other heavy minerals present in minor quantities included zircon in some of the less magnetic fractions and apatite in the least magnetic fractions. In the more magnetic fractions, a higher proportion of magnetite and possible hematite occurred. Pyrite was extremely rare and only observed in some of the less magnetic fractions Magnetic susceptibility The results of magnetic susceptibility measurements are given in Table They are based on bulk sediment samples of about 3 g. Values of the mass-normalised magnetic susceptibility ranged from emu g 1 to emu g 1. This is normally a measure of the magnetite content, with the higher values reflecting greater magnetite contents. The greatest values are found in the sandy aquifer horizons from Faridpur (18 55 m). Examination of grain mounts of 6 samples using reflected-light microscopy showed the presence of particles of detrital magnetite, the size of which corresponded Table Magnetic susceptibility (MS) measurements made on the subset of 21 samples from the three Special Study Areas Sample USGS sample no. Lithology Location Mass normalised MS 1 5 emu g 1 1 B99-1 mud CN B99-2 sand CN B99-3 sand CN.36 4 B99-4 mud CN B99-5 mud CN B99-6 mud F B99-7 silty sand F B99-8 mud F B99-9 sand F B99-1 sand F B99-11 sand F B99-12 sand F B99-13 sand F B99-14 mud L B99-15 sand L B99-16 sand L B99-17 sand L B99-18 mud L B99-19 sand L B99-2 sand L B99-21 sand L 2.17 Data kindly supplied by R. Reynolds, USGS. Sample numbers from Table Location key: CN=Chapai Nawabganj; F=Faridpur; L=Lakshmipur Figure Photomicrographs of Faridpur sediment. Top: m. The massive grain is composed mainly of an intermixed iron oxide mineral. The most reflective (white) parts are probably maghemite. The remainder of the grain is a mixture of finegrained hematite (reddish) and goethite (bluish grey). Bright flecks, mostly in the upper central region, appear to be α-iron. Bottom: 59 6 m. The large grain on the right is a rust fragment composed of iron oxide minerals. The particle on the upper left is detrital magnetite. The particle on the lower left consists of various Fe-Ti oxide phases, perhaps ilmenorutile. The bright bottom edge is composed of titanium dioxide (rutile or anatase). Photomicrographs kindly supplied by R. Reynolds (USGS). reasonably well with the overall grain size of the sediment. The magnetite appears to be of plutonic origin (optically homogeneous for the most part) with minor Ti content indicating low Ti-magnetites. Many grains contained some pleonastic spinel. Most grains exhibited post-depositional dissolution that has removed some Fe leaving relict TiO 2 at the margins of the grains. There were indications that the deeper samples showed a higher degree of dissolution. No unusual authigenic, magnetic Fe minerals were observed. Less common were titanomagnetite (magnetite subdivided by ilmenite lamellae) and ilmenite-magnetite composite grains, and grains of ilmenite-hematite intergrowths. Most samples from the Faridpur and Lakshmipur cores contained moderate to abundant particles of Fe oxide that could have been derived from contamination with rusty scale from the drilling equipment. Many such particles contained relicts of α-iron, indicative of steel fragments (Figure 11.3). The rust also contained some strongly magnetic phases, such as magnetite and

8 194 Arsenic contamination of groundwater in Bangladesh maghemite. The highest MS values came from the Faridpur samples that contained abundant rust (samples 9 12). Iron sulphides were uncommon. Some partly oxidised framboidal pyrite, a grain of pyrrhotite (probably detrital), pyrite in various associations, and one cluster of greigite were also observed Scanning electron microscopy (SEM) Subsamples of sediments from the DW1 (Rajarampur), West Latifpur and Faridpur piezometer boreholes were analysed by SEM (Figures 11.4 and 11.5). The sediments appear to be typical of young alluvial sediments with abundant quartz, mica and feldspars and minor amounts of heavy minerals such as pyroxenes, magnetite, chromite, TiO 2, Fe 2 O 3, and accessory minerals such as apatite (Figure 11.4a and b). The DW1 clay sample (Figure 11.4c) shows a clay matrix with various embedded lithic fragments. Authigenic pyrite was observed but was rare (Figure 11.4d f). It occurs both as a replacement for other minerals within exfoliating biotites and as very rare framboidal aggregates. In one sample (Faridpur FPW6 borehole m), extensive authigenic Fe(±Mn) phosphate developed as massive to blocky, subhedral rhombohedral laths. This formed coarse aggregates which filled in the pore space and locally cemented detrital grains. These aggregates may be derived from the ferricrete horizon observed in the Faridpur borehole at approximately 44.2 m depth (1.7 m below this sample). Three samples from the Faridpur borehole (samples m, m and m) were examined in detail to assess the nature of the clay minerals, iron oxides and heavy minerals present. The coarse, >5 µm fraction, comprised composite aggregates of smaller particles that reflected the mineralogy of the finer fractions, i.e. the finer fractions are probably derived from the disaggregation of coarser aggregates. However, it was noted in the coarser fractions that distinct lithic fragments were present including fine-grained, light brown sandstone, grey micritic limestone and possible granite. SEM analysis indicated that many of the coarse lithic clasts are made up of predominantly coarse to fine sand-sized, angular quartz, feldspar (albite, plagioclase and minor K-feldspar) and mica (biotite and minor muscovite) grains in a silty clay matrix. A minor but significant proportion of the grains from the Faridpur m sample were rounded suggesting possible reworking of a sandstone source. Generally, the grains appeared fresh although slight corrosion was seen in some feldspar grains. Biotites had been altered and opened slightly along their basal cleavage planes. The clay matrix (Figure 11.5a) comprised smectite, illite and chlorite as irregular platelets which had often coalesced to form coarser aggregates. Authigenic calcite was occasionally found as small, rhombohedral crystals up to 1 µm in diameter (Figure 11.5b). Iron oxide developed as patchy irregular, void linings (Figure 11.5c) and formed coalescing masses of subhedral to euhedral, hexagonal, sub-micron platelets (Figure 11.5d). These iron oxide fragments may be artefacts resulting from oxidation of the sediment or may even have been derived from rusty drilling equipment and entrained in the sediment during drilling. In the Faridpur m sample, iron oxide developed as microbotryoidal aggregates (Figure 11.5e) which were mostly found as isolated occurrences attached to sand grain surfaces. However, some examples (Figure 11.5f) developed as moderately extensive linings to relatively large voids in sandstone lithic clasts X-ray diffraction Examples of the X-ray diffactograms of the fine (clay) fractions are shown in Figure 11.6 and the overall results are summarised in Table The samples contain a clay mineral assemblage consisting of smectite, illite (an undifferentiated and hydrated mica species giving a 1 Å basal spacing), chlorite and kaolinite, in varying proportions. Of these, kaolinite was generally the least abundant clay mineral and was absent from, or below detection, in some samples. Smectite showed the most marked variation with two sub-groups showing a strong smectite-dominated assemblage (Table 11.7). In the Faridpur samples those from m, m, m, m and m, and in the Lakshmipur samples those from m and m (Figure 11.6) consist of a smectite-dominated clay mineral assemblage. In the Faridpur samples, there is an abrupt change in clay mineralogy from a chlorite-mica-dominated assemblage to a smectite-dominated assemblage between m and m. In the Lakshmipur borehole, a more gradual change occurs with depth, with the shallowest sample ( m) having a mica-dominated assemblage which changes to an assemblage containing major smectite between 85.3 and 13.6 m. The changes in clay mineral assemblage are not reflected in significant changes in whole rock geochemistry. However, when the Faridpur data are normalised to 1% SiO 2, significant increases occur in the Al 2 O 3 (36% to 45%), Na 2 O (3% to 9%) and K 2 O (8% to 11%) concentrations at and above 1.7 m, compared to the deeper, smectite-dominated samples. In the Faridpur borehole, the smectite-dominated assemblages coincide with the coarsegrained samples. The clay assemblages in the Chapai Nawabganj samples contain a much lower concentration of, but slightly more crystalline, smectite (Figure 11.6) relative to those of Lakshmipur or Faridpur. This may reflect the increased formation of smectite in the alluvial deposits downstream from Chapai Nawabganj or in the physical concentration of fine-grained particles in the lower part of the delta. Generally the coarser samples (i.e. aquifers) contained higher smectite concentrations Whole-rock geochemistry Whole-rock major and trace-element analyses are presented in Tables for the samples from Chapai Nawabganj CPW5, Faridpur FPW6 and Lakshmipur LPW6 respectively. The total As content of the 21 sediments varied from.4 to 1.3 mg kg 1 with averages of 5.9, 3.4 and 3.2 mg kg 1 for Chapai Nawabganj, Faridpur and Lakshmipur, respectively. These are within the typical range for soils and sediments even though the groundwaters from these areas are highly As-contaminated.

9 Mineralogy and sediment chemistry (a) Rajarampur fine sand, DW1: m (b) Rajarampur coarse sand, DW1: m (c) Rajarampur clay, DW1: m (d) Rajarampur clay, DW1: m (e) West Latifpur fine sand, m (f) Rajarampur clay, DW1: m 195 Figure SEM photomicrographs of polished thin sections from the DW1 (Rajarampur) and West Latifpur boreholes. (a) DW1 (12 13 m, 4 42 ft, bar=2 µm) fine sand showing grains of quartz, sodium feldspar, biotite, apatite, zircon; (b) DW1 (33 34 m, ft, bar=1 µm) coarse sand with abundant biotite and muscovite; (c) DW1 ( m, ft, bar=1 µm) grey clay showing aggregates; (d) close-up of (c) showing rare authigenic micron- to submicron-sized pyrite precipitating along the basal cleavage plane of an exfoliating mica (bar=3 µm); (e) West Latifpur (12 13 m, 4 42 ft, bar=1 µm) close-up of fine sand showing very rare authigenic framboidal pyrite, and (f) DW1 ( m, ft, bar=1 µm) rare authigenic pyrite replacing an earlier (ferro magnesian?) mineral.

10 196 Arsenic contamination of groundwater in Bangladesh (a) Faridpur: m (b) Faridpur: m (c) Faridpur: m (d) Faridpur: m (e) Faridpur: m (f) Faridpur: m Figure SEM photomicrographs of sediments from the Faridpur piezometer borehole (FPW6). (a) m (34 35 ft). Clay matrix including smectite, illite and chlorite within a coarse lithic clast. This matrix is typical of that in all samples examined (bar= 1 µm); (b) m (34 35 ft). Rare fine-grained authigenic calcite rhombohedra lining a void in a coarse lithic clast (bar=2 µm); (c) m (19 11 ft). Typical view of patchy, earthy iron oxide lining a void in a lithic clast (bar=1 µm); (d) m (19 11 ft). Detailed view of sub- to euhedral, sub-micron iron oxide platelets possibly hematite (bar=1 µm); (e) m ( ft). Botryoidal iron oxide aggregates attached to a feldspar substrate (bar=2 µm), and (f) m ( ft). Microbotryoidal iron oxide lining a void in a sandstone lithic clast (bar=1 µm).

11 Mineralogy and sediment chemistry 197 (a) CH. NAWABGANJ m (e) FARIDPUR m (b) CH. NAWABGANJ m (f) FARIDPUR m (c) LAKSHMIPUR m (g) FARIDPUR m (d) LAKSHMIPUR m Figure Examples of XRD traces of oriented <2 µm fractions from the test boreholes in the Special Study Areas highlighting variations in the proportions of smectite, mica and chlorite. Left column: Chapai Nawabganj CPW5 (a) m ( ft); (b) m ( ft); Lakshmipur LPW6 (c) m ( ft); (d) m ( ft); Right column: Faridpur FPW6 (e) m (4 5 ft); (f) m (14 15 ft) and (g) m (59 6 ft).

12 198 Arsenic contamination of groundwater in Bangladesh Table Summary of the clay minerals identified by X-ray diffraction Sample Sample code Location Depth (ft) Major Minor Trace 1 F145** Chapai Nawabganj Smectite, illite Chlorite?Kaolinite 2 F146 Chapai Nawabganj Illite Chlorite, smectite 3 F147 Chapai Nawabganj Illite Chlorite, smectite 4 F148 Chapai Nawabganj Smectite Illite Chlorite?Kaolinite 5 F149 Chapai Nawabganj Illite, chlorite, smectite?kaolinite 6 F137 Faridpur Illite Smectite, chlorite Kaolinite 7 F138 Faridpur Smectite Illite, kaolinite, chlorite 8 F139 Faridpur Smectite, illite Chlorite, kaolinite 9 F14 Faridpur Smectite* Illite, chlorite, kaolinite 1 F141** Faridpur Smectite* Illite, chlorite 11 F142 Faridpur Smectite* Illite, chlorite, kaolinite 12 F143 Faridpur Smectite* Illite, chlorite Kaolinite 13 F144 Faridpur Smectite* Illite, chlorite?kaolinite 14 F15 Lakshmipur Chlorite, smectite, illite Kaolinite 15 F151 Lakshmipur Smectite* Chlorite, illite 16 F152** Lakshmipur Illite, chlorite Smectite 17 F153 Lakshmipur Smectite Illite, chlorite Kaolinite 18 F154 Lakshmipur Illite, chlorite Smectite, kaolinite 19 F155 Lakshmipur Chlorite, illite, Smectite?Kaolinite 2 F156 Lakshmipur Smectite* Kaolinite, illite, chlorite 21 F157 Lakshmipur Smectite* Illite, chlorite, kaolinite * dominant phase in the clay assemblage ** poor quality traces due to low amounts of material available for analysis Bold text is used to highlight the marked changes in clay assemblage, from a smectite-mica-chlorite±kaolinite assemblages to a strongly smectite-dominated assemblage. Arsenic was strongly positively correlated with many major elements (Fe, Mg, Mn, Ca, K, P and Cr) and with LOI. These correlations, although all greater than r 2 =.64, are probably in large part due to the SiO 2 dilution effect. This results from the dominance of SiO 2 which is relatively pure, largely occurring as quartz in these sediments, and so effectively dilutes the concentration of all other elements. Similarly, As was strongly positively correlated with the amount of the <1 µm fraction (r 2 =.79) since this contained proportionately more feldspars, heavy minerals, clay minerals and authigenic minerals than the coarser fraction which was dominated by quartz. The As and Fe (total from XRF) depth profiles show very similar patterns for each of the three profiles (Figure 11.7) and the two elements are strongly correlated overall (Figure 11.8). The fitted regression equation is: As (mg kg 1 )= %Fe (r 2 =.79) (1) The high-fe and high-as sediments are generally the finergrained sediments and the Fe is closely associated with a number of other elements including Mg, Al, Mn, Ti, Co and Sc. The good correlation between Fe and a wide range of trace metals in GBM sediments has already been noted (Datta and Subramanian, 1998). The relatively high As content of the clay underlying the shallow sand aquifer in Chapai Nawabganj is notable. However, this probably has no direct relation to the high-as groundwaters found there and it is below the zone from where groundwater is generally derived. At Faridpur, there is quite a distinct drop in sediment As concentration with depth. The As concentration is strongly related to the texture of the sediment with a zone of relatively high-as sediments in the fine-grained overbank deposits at the top of the three profiles. There were a number of relatively strong correlations which tended to reflect the strong contrast between the Sirich and minor-element poor sands and minor elementrich silts and clays which are also rich in Mg, Al and K. Multiple regression analysis (not shown) indicated that the size of the <1 µm fraction, and the concentrations of MgO and Fe 2 O 3, and LOI together provided the best predictor of the sediment As concentration (R 2 =.9). This suggests that the As occurs principally in the fine (<1 µm) fraction and is associated with iron oxides and possibly smectite clay. In these samples, the LOI is likely to be associated with the clays. The generally low CaO contents indicate that while free carbonates are present in many of the sediments, they are only present as minor constituents. Calcite or dolomite are believed to be quite widely distributed in small amounts in Bangladesh sediments which is consistent with the finding that many Bangladesh groundwaters are saturated, or close to saturated, with these minerals. The Lakshmipur sediments have lower CaO concentrations than those from Faridpur and Chapai Nawabganj, indicating the importance of the carbonate-free Meghna Basin as a source of these sediments. Factor analysis, a commonly-used multivariate statistical technique, showed that two factors accounted for 86% of the variance in the data. The first factor can be called the heavy-mineral factor since high factor loadings were found for those trace and major elements known to be associated with heavy minerals (i.e. Y, Th, U, La, Zr, Ta, Ti,

13 Mineralogy and sediment chemistry 199 Table Whole-rock geochemical data for the Chapai Nawabganj CPW5 samples Description Chapai Nawabganj Depth (feet) Sample Sample code F145 F146 F147 F148 F149 Description brown fine sand grey fine sand brown grey yellow brown grey brown med/fine sand silty clay clayey silt SiO 2 % TiO 2 % Al 2 O 3 % Fe 2 O 3 t% Mn 3 O 4 % MgO% CaO% Na 2 O% K 2 O% P 2 O 5 % Cr 2 O 3 % SrO% ZrO 2 % BaO% LOI% Total% FeO% As Co Sr Ba Fe Ti Sc Rb Pb Bi Y Zr Nb La Ce Nd Sm Yb Hf Ta Th U Units are mg kg 1 unless otherwise indicated. Fe 2 O 3 t is the total Fe expressed as Fe 2 O 3 and FeO is the Fe(II) expressed as Fe, both from XRF. Fe is the total Fe determined by ICP-AES after dissolution. Nb, P). The second factor was the clay factor which contained a high factor loading for As in association with LOI, the <1 µm fraction and the MgO content. The As contents for the weight- and size-separated fractions of five selected samples (Table 11.11) indicated that the light sand fraction and <1 µm size fraction dominated both the weight fraction and also the As load of the sediment. The heavy mineral fraction does not contribute significantly to the total As concentration. This is in agreement with the multivariate analyses described above which emphasise the importance of the fine fraction. In the samples analysed (Table 11.11), As concentrations in the fine fraction (<1 µm) varied between 1 and 45 mg kg 1, significantly greater than the concentrations in the heavy-mineral fractions. The >5 µm fraction also tended to have quite high As concentrations.

14 2 Arsenic contamination of groundwater in Bangladesh Table Whole-rock geochemical data for the Faridpur FPW6 samples Description Faridpur Depth (feet) Sample MPG Code F137 F138 F139 F14 F141 F142 F143 F144 Description grey brown brown silty f brown f sand brown grey f/ grey silt f/vf Grey coarse clayey silt & f grey f/vf sand grey f sand sand & silt m sand sand sand & gravel sand SiO 2 % TiO 2 % Al 2 O 3 % Fe 2 O 3 t% Mn 3 O 4 % MgO% CaO% Na 2 O% K 2 O% P 2 O 5 % Cr 2 O 3 % SrO% ZrO 2 % BaO% LOI% Total% FeO% As Co Sr Ba Fe Ti Sc Rb Pb Bi Y Zr Nb La Ce Nd Sm Yb Hf Ta Th U Units are mg kg 1 unless otherwise indicated. Fe 2 O 3 is the total Fe expressed as Fe 2 O 3 and FeO is the Fe(II) expressed as Fe, both from XRF. Fe is the total Fe determined by ICP-AES after dissolution 11.5 NATURE AND ORIGIN OF THE SEDIMENTS Three lines of evidence can be used to make tentative suggestions about the provenance or origin of the sediments: the sediment chemistry, heavy-mineral assemblages and clay-mineral assemblages. However, the heavy-mineral assemblages from only three samples from the Faridpur borehole have been characterised and it is not known whether these findings can be extrapolated to the other samples within this borehole or to samples from Chapai Nawabganj and Lakshmipur. The heavy-mineral fractions from the Faridpur sediments that were analysed by SEM contained varying amounts of magnetite (largely retained in the magnetic and strongly magnetic fractions), hematite, titanite, rutile and ilmenite in the strongly to moderately magnetic fractions as well as pyroxene, epidote, tourmaline(?), amphiboles including hornblende and garnet. Minor minerals included

15 Mineralogy and sediment chemistry 21 Table Whole rock geochemical data for the Lakshmipur LPW6 samples Description Lakshmipur Depth (feet) Description brown grey grey med/f grey vf/med brown grey grey coarse grey coarse/ Grey f sand grey f/vf sand silty vf sand sand sand silt/f sand sand med sand Sample MPG Code F15 F151 F152 F153 F154 F155 F156 F157 SiO 2 % TiO 2 % Al 2 O 3 % Fe 2 O 3 t% Mn 3 O 4 % MgO% CaO% Na 2 O% K 2 O% P 2 O 5 % Cr 2 O 3 % SrO% ZrO 2 % BaO% LOI% Total% FeO% As Co Sr Ba Fe Ti Sc Rb Pb Bi Y Zr Nb La Ce Nd Sm Yb Hf Ta Th U Units are mg kg 1 unless otherwise indicated. Fe 2 O 3 is the total Fe expressed as Fe 2 O 3 and FeO is the Fe(II) expressed as Fe, both from XRF. Fe is the total Fe determined by ICP-AES after dissolution zircons, apatite, sillimanite/kyanite/andalusite and xenotime. Pyrite was only observed very rarely. This range of heavy minerals is comparable to that from the Miocene and younger sediments reported for the Bengal Basin (Uddin and Lundberg, 1998). The young alluvial sediments studied here sometimes contained significantly more heavy minerals as a proportion of the total rock than older sediments, up to 8.1% in the Faridpur m ( ft) sample (Table 11.4) but the proportion was highly variable. The heavy-mineral assemblage from the Plio-Pleistocene Dupi Tila sands are dominated by non-opaque minerals, mainly tourmaline, kyanite, epidote, garnet, pyroxene, hornblende, mica, chlorite and apatite. The opaque minerals made up to 11% of the total heavy-mineral fraction. Uddin and Lundberg (1998) suggested that the varied nature and types of heavy minerals in the younger sediments of the Bengal Basin indicate an orogenic source and reflect input from varied sources ranging from high-grade contact metamorphic rocks to acidic to ultramafic igneous suites. In the Bengal Basin, this reflects