SEDIMENT CHARACTERISATION

Stratigraphy and geochemistry. Well screens of the five monitoring wells are situated within a single, shallow aquifer. The sandy aquifer sediments are covered by olive-brown, thin layers that reach down to 3.85 m bls and comprise varying proportions of clay and silt (referred to as facies F4, Figure 6.1). The top 0.60 m are compacted and contain fragments of clinker. Between 1.90 and 3.85 m bls, mollusc shells as well as reddish and black mottles of Mn- and Fe-(oxyhydr)oxides occur, while Ca and TIC contents are here lower than in the surrounding sediments (Figure 6.2 and APPENDIX II, Table A 2.1). In this part, relatively high contents of As, Fe, Mn, Ni, Cu, Zn as well as TOC and TS appear. Between 3.20 to 3.85 m bls, a thin layer of silty fine sand is located, where the sediment colour changes into dark grey. Below, fine to medium grained sand dominates down to the target depth of 45.5 m bls. Due to the limited drilling depth, no information is available regarding the total thickness of the aquifer. From 3.85 to approximately 19 m bls (facies F3b), silt contents, TOC, major and trace elements (except for Zr and Ce) gradually decrease with depth in the silty medium and fine sands. The vertical distributions of sedimentary As, TOC and TS constitute an exception, they peak within a thin, brown coloured segment composed of fine sandy silt around 7.75 m bls. From about 19 to 29 m bls (facies F3a), silt is completely absent and previously decreasing elements stabilise at the lowest levels of the respective contents. Remaining sediments down to 45.5 m bls (facies F2) are composed of medium sandy

fine sands, which hold throughout low proportions of silt and an embedded gravel lens in about 31 to 33 m bls. In this part, most element contents strongly vary similar to the upper layers of F4. Sediments from facies F1 do not occur within this profile and are described in context of the high As site (chapter 7.2.1).

Figure 6.1: Grain size distribution of the sediment samples, including litho-facies boundaries.

Figure 6.2: Vertical distribution of major and trace elements, TOC, TIC, TS and silt and clay contents. The profile comprises 70 samples in intervals of 0.65 m.

Mineralogy. Mineralogical constituents are summarised in Table 6.1.

Detrital quartz grains represent the greater part of the aquifer sediments, followed in frequency by feldspar (according to XRD spectra presumably anorthite, see APPENDIX II, Figure A 2.1), carbonate (calcite and dolomite), mica (muscovite) and chlorite (clinochlore). All samples from the clayey surface sediments (facies F4) were analysed for the respective clay mineral inventory. Results reveal the presence of smectite and potentially traces of kaolinite and illite (see APPENDIX II, Figure A 2.2), which are considered common constituents of BDP sediments (MUKHERJEE et al. 2009).

Pre-concentration of the sample material by magnetic separation allowed identification of iron-rich minerals from concentrates. Traces of Fe-oxides (magnetite, hematite), garnet (almandine), biotite (phlogopite, which could be separated from muscovite), chloritoid, actinolite and presumably epidote could be additionally distinguished in respective concentrate fractions (APPENDIX II, Figures A 2.3 and A 2.4).

Table 6.1: Summary of identified minerals in representative sediment samples.

With “?” marked minerals could not be proved beyond any doubt due to weak and partly superimposed XRD peaks.

Main mineral phases

Arsenic content. Despite the comparatively high arsenic contents in samples of the clayey surface layers, bulk As contents in the aquifer sediments range between 1.8 and 12.6 mg kg^-1 (average: 3.2 ± 1.3 mg kg^-1; n: 64). The depth distribution of As correlates significantly (r >+0.75) with Fe, TOC, the fine grain fraction (<0.063 mm) and trace elements that are typically associated with iron bearing minerals (e.g., Ni, Cu and Zn) (Table 6.2). The comprehensive results are summarised in form of an agglomerative hierarchical cluster analysis, which groups the different variables (analytical parameters) in form of a dendrogram in dependence of their statistical similarities (Figure 6.3). This dendrogram indicates a close statistical distance of As to Fe and affiliated trace elements (Cu, Ni and Zn).

A SEP was performed to obtain detailed information about the sedimentary arsenic-bearing host phases (Figure 6.4 and APPENDIX II, Table A 2.2). Results indicate that a considerable percentage (42.4-84.7 %) of the total sedimentary As content is PO4

3--extractable (interpreted as strongly adsorbed fraction), although this fraction was found to be overestimated at the expense of fraction III (see chapter 4.2). Remaining As is associated with (a) acid volatile sulphides, carbonates, Mn-(oxyhydr)oxides and very amorphous Fe-Mn-(oxyhydr)oxides (fraction III, average: 14.5 %), (b) amorphous Fe-(oxyhydr)oxides (fraction IV, average:

6.1 %) and (c) crystalline Fe-(oxyhydr)oxides (fraction V, average: 8.6 %) as possible hosts. Only a minor fraction (average: 1.4 %) is SO4

2--extractable (fraction I, interpreted as weakly bound phase). Samples from depths below 13 m bls are characterised by similar distribution patterns and hold throughout low As contents of <3 mg kg^-1. In the two surface near clayey samples, As contents are higher, and As is additionally (d) associated with silicates (18.5 % in 2.60 m bls) and (e) As-sulphides, OM and/or refractory minerals (8.7 % in 3.20 m bls). These conclusions agree in general with findings from the well investigated Chakdah site (CHARLET et al. 2007, MÉTRAL et al. 2008). In contrast to other study sites in the BDP (AKAI et al.

2004, HARVEY et al. 2002, NICKSON et al. 2000), sulphides and OM as potential As sources are only in two samples detectable (Figure 6.4).

According to the generally very low TOC contents in the aquifer sediments (<0.1 %), OM is considered to play a minor role as As host in general.

Table 6.2: Statistical summary of relevant element contents in the aquifer sediments (ranging from 3.85 to 45.5 m bls, n: 64). Included are median values, lower and upper quartiles (25 % and 75 % Q.), minimum and maximum contents as well as average values (arithmetic mean) and correlation coefficients.

Range Silt &

clay K2O CaO TOC Fe2O3 MnO TS Ni Cu Zn As (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) Minimum 0.19 1.37 2.08 0.02 1.40 0.02 <26 21.2 8.73 19.0 1.8

25 % Q. 2.71 1.82 2.63 0.02 2.05 0.03 <26 23.2 9.75 27.1 2.8 Median 9.85 2.25 2.98 0.03 2.34 0.04 26.6 24.0 10.4 29.8 2.9 75 % Q. 15.7 2.60 3.80 0.04 2.95 0.05 54.6 27.1 11.2 38.3 3.2 Maximum 66.0 3.29 5.33 1.67 4.99 0.08 610 37.4 33.1 71.3 12.6

Average 13.0 2.25 3.18 0.07 2.58 0.04 51.5 25.4 11.6 33.6 3.2 r As- +0.76 +0.64 -0.11 +0.75 +0.86 +0.67 +0.53 +0.89 +0.92 +0.87 -

Figure 6.3: Dendrogram for 21 variables expressing the statistical distance between elements and other parameters in aquifer and aquitard sediments (0.0 to 45.5 m bls, n: 69; Ward’s method and Euclidean distances; one outlier removed). Arsenic in the sediments appears to be closely associated with Fe, Zn, Ni and Cu. This group is interpreted to be primarily composed of Fe-(oxyhydr)oxides (data summarised in APPENDIX II, Table A 2.1).

Figure 6.4: Sequential extraction procedure (according to EICHE et al. 2008) results for 9 samples that represent different parts of the aquifer. Sedimentary As is primary strongly adsorbed (extraction step II) and associated with fractions that comprise various forms of Fe-(oxyhydr)oxides (steps III-V).

Sedimentary organic matter. Since no peat layers were observed during drilling, sedimentary OM solely appears in dispersed form. In the organic-enriched clayey facies F4, TN contents as well as δ¹³C and δ¹⁵N values gradually decrease with depth, while the C/N ratios (weight ratio of TOC to TN) remain stable at values of about 8 (Figure 6.5). In sandy aquifer sediments of facies F3b beneath, C/N ratios shift to values above 10, while most δ¹³C values vary between -23 and -20 ‰, and δ¹⁵N values stabilise around + 2.5 ‰. Due to the throughout low TOC contents, no samples could be analysed from facies F3a (19–29 m bls), and only two from facies F2 (29-45 m bls).

Figure 6.5: Detailed characterisation of OM from sediment samples, including δ¹³C values, C/N-ratios, TOC fraction, TN contents and δ¹⁵N values (n: 22).

The comparison of C/N-ratios, δ¹³C and δ¹⁵N values is known to serve as a reliable and commonly used biomarker for source identification in fluvio-deltaic sedimentation environments (HOEFS 2009). In addition, isotopic characterisation of OM can be included in sequence stratigraphic considerations to interpret palaeo-environmental sequences in coastal sediment deposits (LAMB et al. 2006). In the Bengal Basin, different sediment facies occur that witness tectonic activities, eustatic sea level changes and climate changes, although erosion processes and subsequent

valley and channel fills have locally disrupted local facies successions (MIALL 1996). Thus, deposits can contain varying ratios of terrestrial and marine derived organic matter. The use of δ¹³C values allows differentiating between OM derived from C3 and C4 land plants. By combining δ¹³C with C/N-ratios, it is further possible to distinguish between C3 and C4 plant signatures and particulate organic matter (POM) derived from marine or freshwater algae and microbes (HOEFS 2009). Another common isotopic marker to distinguish marine from terrestric sources is the δ¹⁵N value.

Average C/N ratios indicate that algae and microbes (freshwater and marine) as well as C4 plants are the potential origin of the respective sedimentary OM (APPENDIX II, Table A 2.3). By comparing C/N ratios with δ¹³C values, potential OM sources can be further differentiated (Figure 6.6).

Samples from the sandy aquifer show C/N ratios of 6 to 12, and δ¹³C values of -24 to -20 ‰ that are characteristic for OM derived from marine POM (HOEFS 2009, LAMB et al. 2006, MEYERS 1994, SARKA et al. 2009, SHARP 2006). Despite this, δ¹⁵N values in this part of the aquifer are in a characteristic range of terrestric mangrove trees (C₃ plants), although most are very close the threshold value of marine POM (MUZUKA & SHUNULA 2006). A few signals even plot in range of δ¹⁵N values of marine POM samples from sediments of the Bay of Bengal (GAYE-HAKE et al. 2005).

Only few data is available regarding δ¹⁵N values for plants in general and for mangrove plants in particular. Hence, interpretation of the δ¹³C and C/N values that bases on a sound data base should be preferred to δ¹⁵N signature interpretation. Nevertheless, influences of terrestric C3 plant matter is typical for estuary environments and cannot be excluded in this case (MIALL 1996).

In the youngest silty and clayey deposit of facies F4, δ¹³C values occur (> -20 ‰) that represent mixed signals, indicating an increasing influence of terrestric C4 plant matter in the recent past. This is further supported by increasing δ¹⁵N signals. Due to the low TOC contents samples from below 19 m bls, nearly no information is available for facies F3a and F2.

Figure 6.6: Distribution of C/N ratios and δ¹³C and δ¹⁵N values in comparison to characteristic ranges for different OM sources (n: 22).

*δ¹⁵N range for mangrove (C3 type) tissues (MUZUKA & SHUNULA 2006)

**δ¹⁵N range for sedimentary POM from the Bay of Bengal (GAYE-HAAKE et al. 2005)

***δ¹⁵N range for C4 plant pollen (DESCOLAS-GROS & SCHÖLZEL 2007)

6.2.2 GROUNDWATER CHARACTERISATION