COMUNICACIÓN DE RETORNO Y PARTICIPACIÓN POPULAR

Contradictory arsenic concentrations. Arsenic concentrations in samples of the five monitoring wells were found to be surprisingly much higher than in the adjacent tube well no. 125, which was the reason to choose this location as low As study site (see chapter 5.3.2). Despite comparable depths of the well screens, some important parameters (Cl^-, SO4

2-, NO3

-, As-, Fe-, and PO4

3-) strongly differed in values observed in groundwater from well 125. Consequently, well 125 was re-sampled during the field trip in 2009. In this new sample, As, Fe and PO4

3- concentrations were more alike the monitoring wells than to the sample taken during the post-monsoon season in 2007 (Table 6.5). These concentration changes are attributed to on-going redox processes in the aquifer as discussed later on in chapter 6.3.6. Since As concentrations in all five monitoring wells turned out to be throughout high, the designation “low As site” appears inappropriate and the site should be considered rather as a “lower As site”

compared to the second study site, where As concentrations are twice as high (see Table 8.1 in chapter 8).

Arsenic release and groundwater evolution. Compared to the field survey (chapter 5.2), numerous additional parameters were determined during the monitoring, allowing a much more precise identification of processes related to groundwater evolution. Solute concentrations, calculated SI and results of the sediment analyses were used to identify hydrogeochemical reactions that have influenced the groundwater composition in the study area. In addition, conservative tracers (Cl^-, K, stable isotope compositions of O and H) were used to identify influences of physical processes like mixing or evaporation (SENGUPTA & SARKA 2006). Nevertheless, subsequent reactions caused by microbial activity or ion exchange reactions may have re-changed distribution patterns of non-conservative solutes like As in groundwater as discussed in the following.

The following processes and mechanisms were identified:

(a) Carbonate and silicate weathering

As previously assumed from results of the field survey, throughout elevated concentrations of Ca, Mg and HCO3

in groundwater indicate pronounced influences of carbonate weathering during evolution of the groundwater (Tables 6.4 and 6.7). This is attributed, on the one hand, to the presence of considerable amounts of carbonates in the aquifer sediments (calcite and dolomite), and on the other hand, to the slow groundwater flow supporting the carbonic acid–bicarbonate–carbonate–equilibrium to balance. Dissolved Si concentrations, as well as the mineral inventory, both point at silicate weathering, which is most likely related to the dissolution of instable feldspars (e.g., anorthite).

Due to the carbonate-rich sediments, the groundwater pH is circum-neutral.

This controls the protonation of As species and influences in turn their adsorption behaviour. The predominating As species is here uncharged H3AsO3 (see EH-pH-diagram in Figure 6.9). This strongly reduces the importance of ionic surface adsorption / desorption processes, but increases the sorption potential of As(III) to Fe-(oxyhydr)oxides by ligand exchange (DIXIT & HERING 2003). Results of the SEP support these assumptions. Nearly no weakly bound As was observed (fraction I), but strongly adsorbed As (fraction II) was identified as a principal form (even if this fraction was overestimated, see chapter 4.2).

(b) Ion exchange reactions

The presence of surface reactive minerals like Fe-(oxyhydr)oxides and clay minerals generally foster ion exchange reactions, especially Na-Ca-exchange. The Na content of the sediments originates from past transgression events (GOODBRED et al. 2003), when intrusions of marine water caused the reverse exchange reactions (APPELO & POSTMA 1996).

Thus, Na/Cl ratios in solution are a good proxy to estimate the groundwater maturity and further support identification of mixing influences, which is

3-ratios in solution seem to be not high enough to induce As-PO4

3--exchange and release of adsorbed As by PO4

as proposed by ARCHARYA et al.

(1999). This assumption is further supported by results of the in-situ experiments that were carried out the high As and the low As study sites.

Here, strong increases in local PO4

3- concentrations did not provoke an additional As release. The long-term monitoring additionally revealed that As to PO4

ratios and concentrations in almost all monitoring wells remained constant during the time of observation (see chapter 6.3.6 and chapter 7.3.5.2).

(c) Hydrochemical stratification

Results reveal the presence of two distinctive hydrochemical layers with partially sharp concentrations gradients in many different solutes between the well screens of wells A and B. This set of solutes comprises a wide range of hydro- and geochemically deviant acting compounds including As (Table 6.3). For example, Cl^- is considered a conservative parameter that does not tend to react with solid phase compounds (SENGUPTA & SARKA 2006). In contrast, Na and Ca can be influenced by ion exchange, while As, NH4

+, SO4

2-, and Co are sensitive to redox changes. The observed stratification effects can be hardly explained by a single process and are attributed to multiple processes. First, groundwaters with different compositions (especially concerning major solutes, like the conservative Cl^-) must have been mixed around the depth range of well A. After mixing, consecutive reactions additionally superimposed the initial distribution of non-conservative solutes including As, what is discussed in the following.

(d) Microbially mediated reactions

Temperatures of around 27°C as observed in the investigation area provide optimum conditions for microbiological growth and activity. Depth decreasing DOC and SOM, as well as the presence of considerable amounts of cultivatable germs and an increased alkalinity in groundwater all indicate pronounced microbial degradation processes in the recent past.

Anaerobic decomposition of OM is known to cause formation of fatty acids

as intermediate products and HCO3

as end product (LOVLEY 1993).

Microbial released organic acids further trigger carbonate dissolution through acid neutralisation reactions. Hence, microbial decomposition of OM in reducing aquifer parts entails an increase in groundwater alkalinity, which was demonstrated by the sucrose injection experiment (chapter 6.3.5). In addition, high concentrations of Mn, Fe(II), As(III) coupled with low O2, NO3

-, and SO4

concentrations reflected a decisive influence of microbial mediated redox reactions on the local groundwater composition.

According to the redox classification criteria of JURGENS et al. (2009), methanogenesis is currently the predominating redox process (Table 6.9).

Although not being the dominating TEA consuming process any more, Fe(III) reduction can still be a relevant process (JURGENS et al. 2009), which could be demonstrated by the biostimulation experiment (described in chapter 6.3.5).

Table 6.9: Biogeochemical fingerprints of groundwater samples (03/12/09) following the classification scheme of JURGENS et al. (2009). Groundwater from the monitoring wells reflects strongly reducing conditions allowing formation of methane (methanogenesis).

2- reduction <2.22 >0.05 >0.10 >0.90 Methanogenesis <2.22 >0.05 >0.10 <0.90

Concentration in groundwater (depth m bls):

A (12-21) <0.88 0.77 4.34 <0.85 B (24-27) <0.88 0.47 3.76 <0.85 C (30-33) <0.88 0.41 5.57 <0.85 D (36-39) <0.88 0.42 2.86 <0.85 E (42-45) <0.88 0.60 2.15 <0.85

In addition to the problematic enrichment of As, Mn concentrations exceeded the Indian threshold value of 0.10 mg L^-1 in drinking water (IS 10500; standard value is extendable to 0.30 mg L^-1 if no other sources of drinking water are available), raising the risk of chronic intoxications for the local population (MC ARTHUR et al. 2012).

(e) Arsenic mobilisation

Similar to arsenic-rich tube wells, hydrochemical properties of groundwater at the low As site appear to be closely related to the microbial reduction of both Fe(III) and As(V). Poorly ordered Fe-(oxyhydr)oxides like ferrihydrite are highly instable under the prevailing hydrochemical conditions, while ordered Fe-minerals (e.g., hematite, magnetite and siderite) are super-saturated and therefore considered as stable (STUMM & MORGAN 1996).

From a kinetic point of view, a strong negative or positive SI does not necessarily mean that the respective minerals dissolve or precipitate in a certain period of time (MERKEL & PLANER-FRIEDRICH 2008). However, the occurrence of arsenic-bearing, poorly crystalline Fe-(oxyhydr)oxides in the aquifer sediments (see chapter 6.3.3) can support on-going microbial Fe(III) reduction with concomitant release of As if degradable OM is available. In fact, Fe(III) reduction and As release can be still active processes, which was demonstrated by both, the biostimulation experiment and the long-term monitoring (chapter 6.3.5).

Increased concentrations of Mn reflect that reductive dissolution of Mn-oxides took also place, which often accompanies Fe(III) reduction (LOVLEY 1993). Although aqueous to solid phase ratios of Mn were comparable to those of Fe, concentrations in groundwater and sedimentary contents underline that As mobilisation via Mn(IV) reduction is secondary compared to Fe(III) reduction.

Another potential As release mechanism is biogenic weathering of apatite (MAILLOUX et al. 2009). The presence of potentially arsenic-hosting apatite could neither be proved nor ruled out by the characterisation of the sediments. It is difficult to detect traces of apatite in carbonate-rich sandy sediments by means of XRD, since the main peaks are superimposed by

those of quartz. Occurrence of arsenic-bearing apatite was not reported from other study sites in the Bengal Basin until now. According to MAILLOUX et al. (2009), this kind of release would be decoupled from redox processes. Hence, increased As would also appear under less reducing or even oxic conditions, what stands in contrast to common observations in the Bengal Basin.

(f) Competitive adsorption

Groundwater from the investigation area reflects a close connection between As and PO4

3-, which was reported from other locations in the BDP as well (e.g., BGS & DPHE 2001). In the present thesis, the hypothesis was developed that dissolved As and PO4

competitively adsorb to residual and newly formed Fe-minerals. This process is considered as an important process in the development of reducing groundwaters in the Bengal Basin as well as other Asian delta areas.

Reductive dissolution of Mn- and Fe-(oxyhydr)oxides should be accompanied by a fast re-adsorption of previously released As until the sorption capacity is reached or available potential sorbents are completely dissolved (DIXIT & HERING 2006, WELCH et al. 2000). Geochemical data support the former, since amorphous and weakly ordered Fe-(oxyhydr)oxides with considerable amounts of As are still available within the aquifer sediments, despite the strong negative SI (see chapter 6.3.3). In contrast to the prevailing reducing conditions, some Fe-mineral phases that are capable of arsenic retention are supersaturated in groundwater (see SI of hematite, magnetite and siderite in Table 6.4). While precipitation of Fe-(oxyhydr)oxides like hematite or magnetite is considered unlikely due to the nearly absence of dissolved O2, previous studies revealed that high concentrations of dissolved Fe(II) can induce transformation of weekly ordered Fe-(oxyhydr)oxides into more stable forms like magnetite. These transformed minerals are again capable of As(III) and As(V) adsorption (HANSEL et al. 2003, PEDERSEN et al. 2005) as well as incorporation of previously adsorbed As(V) into the magnetite crystal structure (COKER et al. 2006). In addition, precipitation of siderite is also possible (positive SI

values according to the low redox potential and the throughout high HCO3

-and Fe concentrations), which has a pH-EH-stability field close to magnetite and is able to remove dissolved As(III) as well as As(V) by adsorption (GUO et al. 2007, JÖNSSON & SHERMAN 2008). Detection of siderite in the sediment samples by XRD was not possible (principal siderite peaks are superimposed by mica and feldspar peaks), but magnetite was proved by bar magnet. However, magnetite and concretes of siderite have been previously identified in sediments of West Bengal under reducing conditions (PAL et al. 2002).

In fact, outcomes of the field survey and the biostimulation experiment strongly support the assumption of As (re-)adsorption within aquifers of the investigation area (see chapters 5.3.1 and 6.3.5). The predominating As species in local groundwater is always As(III), which can strongly adsorb to free binding sites of Fe-mineral surfaces via inner-sphere surface complexes (ONA-NGUEMA et al. 2005). At neutral pH values, PO43-

has higher binding affinities for Fe-minerals than both, As(III) and As(V) (GOH &

LIM 2004). Hence, it can be expected that adsorption to newly formed Fe-minerals is competitive and that PO4

adsorption is preferred. All available As and PO4

3- binding sites in the aquifer sediments are considered to be currently occupied because of the constantly increased concentrations in groundwater. This assumption is further supported by the high percentages of strongly adsorbed As in local sediments, based on the SEP results. The aqueous to solid phase ratios (Table 6.3) show that As is relatively more enriched in groundwater than PO4

3-, suggesting that PO4

3- is favoured over As(III) and/or As(V) adsorption. Depending on available binding sites and prevailing solute concentrations, distinctive As-PO4

ratios developed in groundwater (Table 6.10). This alludes to the conclusion that As concentrations in groundwater would be distinctively lower in absence of dissolved PO4

3-. The concept of competitive adsorption of As and PO4

is further developed in the following chapters.

Table 6.10: Comparison of As to PO4

3- mol ratios in groundwater (samples from 03/12/09) and in sediments from respective depths (determined from micro-wave acid digestions).

Since the study site is located in a rural area of intense agriculture, the question arises whether there are any traceable anthropogenic influences that influence the mobilisation and distribution of As in the groundwater.

Three possible effects were identified:

 Inflow of anthropogenic derived OM

An intensively discussed issue is the infiltration of OM either by sewage from pit latrines, by artificial ponds, or by irrigation water from fertilised fields. Since OM is considered as a limiting factor for microbial mediated reactions in aquifer sediments of the BDP (RADLOFF et al. 2008), an inflow of OM into the aquifer would most likely induce additional As release (NEUMANN et al. 2009).

However, DOC concentrations in the monitoring wells range below 10 mg L^-1, which is considered as common for pristine groundwater (COZZARELLI & WEISS 2007). Total microbial abundance as indicated by TPC reflects a moderate burden of the groundwater, which is fostered by the comparatively high groundwater temperatures of about 27°C.

The absence of NO3

and coliform bacteria (especially E. coli) further supports the assumption that groundwater at the study site is not affected by sewage or organic fertiliser.

 Influences of excessive pumping

Potential influences on the groundwater composition can arise from an adjacent irrigation pump. Since irrigation is restricted to the dry pre-monsoon season, this issue is discussed in context of the monitoring results (see chapter 6.3.5). Nevertheless, the pumping stations of Chakdah described by NATH et al. (2008) are assumed to have no or minor effects on the local hydrology, since they are located more than 10 km east of the study site.

 Influences of wet rice cultivation

Cultivation of wet rice is widespread in the BDP (NORRA et al.

2005). The typically clayey paddy field soils constitute an effective barrier that prevents infiltration of O2 into the aquifer parts below, supporting development of reducing conditions and by association As release (MÉTRAL et al. 2008).

6.3.5 INFLUENCE OF THE AVAILABILITY OF ORGANIC