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Redox processes. In the following, presented results of the hydro-chemical survey are used to answer the questions raised in the introduction (chapter 1.2). The flat topography of the Bengal Basin and constantly slow horizontal groundwater flow velocities entail intensive water-rock interactions (MICHAEL & VOSS 2009b). Results of groundwater dating conducted in nearby areas revealed that shallow groundwater in Holocene aquifer sediments is not older than 100 years (HARVEY et al. 2005, MC ARTHUR et al. 2010, STUTE et al. 2007). Increase and/or decrease of redox sensitive parameters reflect decisive influences of anaerobic microbial metabolic processes, of which different TEA consuming redox processes were identified, ranging from NO3

- reduction over Mn(IV), Fe(III) and SO4

reduction to methanogenesis.

The applied redox classification concept after JURGENS et al. (2009) is generally a simplification and does not necessarily match with situations met in complex natural systems, where groundwater is often not in redox equilibrium. Here, multiple redox reactions may partly overlap at a certain point of time, although usually one process dominates (JURGENS et al.

2009, STUMM & MORGAN 1996). For example, NO3

- and Mn(IV) reduction often occur parallel to each other, as well as Fe(III) and SO4

2- reduction.

2-methanogenesis, or in a mixed redox state (Figure 5.3). It was not possible to further differentiate between Fe(III)/SO4

reduction since no sulphide determination was done (MC MAHON & CHAPELLE 2008). Spatial variations of these redox indicators reflect the presence of inhomogeneous redox zones in the aquifer. This aspect is discussed later in detail for the two study sites (chapters 6.3.4 and 7.3.4).

Indications for arsenic release. The hydrochemistry of groundwater from the investigation area supports the assumption that microbially mediated processes are the key mechanism for As mobilisation. The presence of As concentrations exceeding 50 µg L^-1 is limited to samples in state of Fe(III)/SO4

reduction, methanogenesis or a mixed redox state (see Figure 5.3). The more reducing the redox conditions, the higher the concentrations of dissolved As. This trend manifests in a moderate positive correlation among As and Fe concentrations (correlation coefficient: +0.46), pointing at release according to the Fe(III) reduction hypothesis (chapter 2.2.3). It is noteworthy that samples in state of Fe(III)/SO4

reduction were found to have not yet reached As concentrations found in samples that are in state of methanogenesis. This can be explained by the assumption that these samples originate from more mature aquifer parts, where Fe(III) reduction and associated As release is completed or at least non-dominant.

Samples indicating mixed redox states. A considerable number (50) of samples did not fit into one of the four principal redox classes and were therefore assigned as “mixed redox state”. Two important types can be distinguished within these samples. The first group includes 34 samples with elevated As concentrations, where the state of Fe(III)/SO4

2- reduction or methanogenesis should be reached, but NO3

concentrations clearly exceed the threshold value of 2.25 mg L^-1. This is attributed to mixing processes, which are caused by infiltration of nitrate-rich surface water into more reducing zones. For example, extensive pumping has been proven to cause such effects in case of the Chakdah area (CHARLET et al. 2007), as well as in Hanoi, Vietnam (NORRMAN et al. 2008).

The second group contains 16 samples that appear to be in a transition between Mn(IV) and Fe(III)/SO4

reduction. These samples hold low NO3

-and Fe concentrations that remain closely below the threshold values of the classification scheme, while Mn concentrations are increased. Here, measured As concentrations are comparatively low, except for three samples. Results point at precipitation and adsorption processes, which are discussed in the following.

Arsenic and phosphate. The strongest correlation between As concentrations and any other determined parameter in all samples is related to PO4

3- (rAs-P: +0.53). Phosphate in groundwater generally originates from mineralisation of organic matter and from reductive dissolution of phosphate-hosting Mn- and Fe-(oxyhydr)oxides, which is known from oxbow lake sediments (LEWANDOWSKI & NÜTZMANN 2010, O’DAY 2006). Phosphate is further considered a strong competitive anion that can induce As release from host surfaces. Since As(III/V) and PO4

3- have similar geochemical properties and preferentially adsorb to surfaces of Fe-(oxyhydr)oxides (GOH & LIM 2004), it is here more likely that both are controlled by the same mobilisation mechanism, which would be the activity of FeRB. Additionally, P could be fertiliser derived and enter the aquifer via infiltrating irrigation water (ACHARYYA et al. 2000).

Highest concentrations of dissolved As, PO4

and Fe occur in wells that are 20 to 40 m deep. Only a few samples (n: 36) were taken from wells exceeding 40 m in depth, but this distribution is in accordance to results from other surveys that have been previously conducted in the Bengal Basin (BGS & DPHE 2001, HARVEY et al. 2002). Here, dissolved As, PO4

3-and Fe concentrations never were that high, or enrichment processes have stopped and solutes were slowly, but constantly flushed-out into the Bay of Bengal. In the BDP, increasing depth is equivalent to increasing groundwa-ter age (HARVEY et al. 2005, MC ARTHUR et al. 2010). This increases the influence of kinetic based solid-water equilibrium reactions like the precipitation of supersaturated mineral phases. Hence, precipitation of supersaturated minerals, especially Fe-minerals, is another potential

concentrations in groundwater. Precipitation of Fe-minerals enables an immobilisation of dissolved As and PO4

3-. This aspect is discussed later on in the discussion of the low As site (chapter 6.3.4) and in the final summary (chapter 8.2).

Manganese. In addition to the problematic As concentrations, manganese exceeds in 79 samples the Indian drinking water threshold value of 0.30 mg L^-1 (if no other drinking water source is available, other-wise 0.10 mg L^-1; IS 10500, reaffirmed 1993). This is known from other areas of the BDP as well and is considered to originate from the reductive dissolution of Mn-minerals under mildly reducing conditions (MC ARTHUR et al. 2012, VAN GEEN et al. 2007). The occurrence of neurobiological effects related to chronic Mn uptake is still under debate, but the WHO decided to remove the provisional guideline recommendation in the latest edition of the guideline values for drinking water (WHO 2011). This was decided because the former guideline value of 0.40 mg L^-1 was considered well above concentrations normally found in drinking water, which is definitively not true in case of the BDP (MC ARTHUR et al. 2012).

Arsenic in public wells. Most people meanwhile rely on official governmental wells, which deliver water from deep aquifer parts and are partly equipped with As filters (Figure 5.4). Pipelines were recently installed to provide a central water supply with treated surface water. Nevertheless, some small communities still rely on private tube wells. All in all, 15 wells turned out to be critically burdened with As in the investigation area, reaching high concentrations of 159 to 333 µg As L^-1. Unfortunately, providing technical solutions exceeded the competence of the project. The Indian project partner intends to contact the corresponding local responsibilities in order to find a solution for critically burdened wells with As concentrations well above the national Indian threshold value, including one public school well (GSFC school, 83.0 µg As L^-1).

Figure 5.4: Picture of a public well installed in 2003 in the village Chakudanga, which is equipped with an As filter cartridge (well depth: 163 m). Surprisingly, the As concentration in filtered water was higher as compared to the raw water (77.4 compared to 101 µg L^-1), strongly suggesting that the filter material needs to be replaced. This example demonstrates that current mitigation strategies (use of deep wells and filter systems) can be unreliable.