In rural Bangladesh, the drinking water supply is mostly dependent upon manually operated hand pumped tubewells, installed by the local community. The presence of natural arsenic (As) in groundwater and its wide scale occurrence has drastically reduced the safe water access across the country and put tens of millions of people under health risk. Despite significant progress in understanding the source and distribution of As and its mobilization through sediment-water interactions, there has been limited success in mitigation since the problem was discovered in the country’s water supply in 1993. This study evaluated the viability of other kinds of alternative safe drinking water options and found tubewells are the most suitable due to simplicity and technical suitability, a wide acceptance by society and above all low cost for installation, operation and maintenance. During planning and decision making in the process of tubewell installation, depth of the tubewell is a key parameter as it is related to groundwater quality and cost of installation. The shallow wells (usually &lt; 80m) are mostly at risk of As contamination. One mitigation option are deep wells drilled countrywide to depths of around 250 m. Compared to safe water demand, the number of deep wells is still very low, as the installation cost is beyond affordability of the local community, especially for the poor and disadvantaged section of the society. Using depth-specific piezometers (n=82) installed in 15 locations spread over the 410 km2 area of Matlab (an As-hot spot) in southeastern Bangladesh, groundwater monitoring was done over a 3 year period (pre- and post-monsoon for 2009-2011 period). Measurements were performed for hydrogeological characterization of shallow, intermediate deep and deep aquifer systems to determine the possibility of targeting safe aquifers at different depths as the source of a sustainable drinking water supply. In all monitoring piezometers, As was found consistently within a narrow band of oscillation probably due to seasonal effects. Hydrogeochemically, high-As shallow groundwaters derived from black sands are associated with elevated DOC, HCO3, Fe, NH4-N and PO4-P and with a relatively low concentration of Mn and SO4. Opposite to this, shallow aquifers composed of red and off-white sediments providing As-safe groundwater are associated with low DOC, HCO3, Fe, NH4-N and PO4-P and relatively higher Mn and SO4. Groundwaters sampled from intermediate deep and deep piezometers which were found to be low in As, are characterized by much lower DOC, HCO3, NH4-N and PO4-P compared to the shallow aquifers. Shallow groundwaters are mostly Ca-Mg-HCO3 type and intermediate deep and deep aquifers’ groundwaters are mostly Na-Ca-Mg-Cl-HCO3 to Na-Cl-HCO3 type. A sediment color tool was also developed on the basis of local driller’s color perception of sediments (Black, White, Off-white and Red), As concentration of tubewell waters and respective color of aquifer sediments. A total of 2240 sediment samples were collected at intervals of 1.5 m up to a depth of 100 m from all 15 nest locations. All samples were assigned with a Munsell color and code, which eventually led to identify 60 color varieties. The process continued in order to narrow the color choices to four as perceived and used by the local drillers for identification of the targeted As-safe aquifers. Munsell color codes assigned to these sediments render them distinctive from each other which reduces the risk for misinterpretation of the sediment colors. During the process of color grouping, a participatory approach was considered taking the opinions of local drillers, technicians, and geologists into account. In addition to the monitoring wells installed in the piezometer nests, results from 87 other existing drinking water supply tubewells were also considered for this study. A total of 39 wells installed in red sands at shallow depths producing As-safe water providing strong evidence that red sediments are associated with As-safe water. Average and median values were found to be less than the WHO guideline value of 10 μg/L. Observations for off-white sediments were also quite similar. Targeting off-white sands could be limited due to uncertainty of proper identification of color, specifically when day-light is a factor. Elevated Mn in red and off-white sands is a concern in the safe water issue and emphasizes the necessity of a better understanding of the health impact of Mn. White sediments in shallow aquifers are relatively uncommon and seemed to be less important for well installations. Arsenic concentrations in more than 90% of the shallow wells installed in black sands are high with an average of 239 μg/L from 66 wells installed in black sediments. It is thereby recommended that black sands in shallow aquifers must be avoided. This sediment color tool shows the potential for enhancing the ability of local tubewell drillers for the installation of As-safe shallow drinking water tubewells. Considering the long-term goal of the drinking water safety plan to provide As-safe and low-Mn drinking water supply, this study also pioneered hydrogeological exploration of the intermediate deep aquifer (IDA) through drilling up to a depth of 120 m. Clusters of tubewells installed through site optimization around the monitoring piezometer showed a similar hydrochemical buffer and proved IDA as a potential source for As-safe and low-Mn groundwater. Bangladesh drinking water standard for As (50 µg/L) was exceeded in only 3 wells (1%) and 240 wells (99%) were found to be safe. More than 91% (n=222) of the wells were found to comply with the WHO guideline value of 10 µg/L. For Mn, 89% (n=217) of the wells show the concentration within or below the previous WHO guideline value of 0.4 mg/L, with a mean and median value of 0.18 and 0.07 mg/L respectively. The aquifer explored in the Matlab area shows a clear pattern of low As and low Mn. The availability of similar sand aquifers elsewhere at this depth range could be a new horizon for tapping safe drinking water at about half the cost of deep tubewell installation. All findings made this study a comprehensive approach and strategy for replication towards As mitigation and scaling-up safe water access in other areas of Bangladesh and elsewhere having a similar hydrogeological environment. / <p>QC 20151211</p> / Sida-SASMIT project (Sida Contribution 75000854).

Dans le cadre de la recherche de nouvelles sources d'énergie propres et durables, nous étudions les mécanismes de formation du dihydrogène (H2) dans les environnements intra-cratoniques. Des émanations naturelles de H2 ont précédemment été décrites à proximité des dorsales médio-océaniques et des ceintures ophiolitiques. Cette production naturelle de gaz, telle que documentée dans la littérature, est étroitement liée au métasomatisme de roches ultrabasiques d'origine mantellique, riches en minéraux ferromagnésiens, à travers les réactions de serpentinisation. Au Kansas (USA), des émanations de H2 intra-cratonique ont été révélées dès les années 80 par l'étude de puits riches en H2. Nos travaux s'appuient sur un nouveau forage, D#2 (Kansas, USA), ainsi que deux forages existants depuis les années 80, Heins#1 et Scott#1 (Kansas, USA). Le puits D#2 permet d'accéder à un aquifère modérément profond (~300 m) chargé en H2. Le gaz est composé également de N2 et de CH4 tout comme le gaz issu des contextes ophiolitiques. Du He est présent en quantité substantielle -comparé aux contextes précédents- dans ces forages. Afin de comprendre les processus engendrant la production de H2 dans ce contexte géologique, de quantifier le gaz ainsi généré, et de déterminer la relation du H2 avec les autres espèces gazeuses une étude multidisciplinaire gaz/eau/roche a été réalisée. Les résultats obtenus lors de l'étude de ces fluides mis en parallèle avec le contexte géologique régional et les lithologies observées ont permis (1) de proposer différents scénarios pour expliquer les associations de gaz observées et (2) de discuter de l'origine et des processus de production du H2, de l'He, et du N2. / As part of the search for new sources of clean and sustainable energy, the mechanisms for the formation of dihydrogen (H2) in intracratonic environments were studied. Natural emissions of H2 have been described in the vicinity of mid-ocean ridges and ophiolite belts. This natural gas production, as documented in the literature, is closely related to the metasomatism of mantle rocks which are rich in mafic minerals, through the serpentinization reaction. In Kansas (USA), intracratonic H2 seepages were revealed in the 80’s by studies of H2-rich wells. Our work is based on a new borehole D#2 (Kansas, USA), and two boreholes previously studied in the 80s, Heins#1 and Scott#1 (Kansas, USA). The D#2 well provides access to a moderately deep aquifer (~ 300 m) loaded with H2. The gas is also composed of N2 and CH4 similarly to the gases issued from ophiolitic contexts. Helium is present in substantial quantities -compared to preceding contexts- in these boreholes. A multidisciplinary gas/water/rock study was carried out to understand the processes generating the production of H2 in this geological setting, to quantify the gas so generated, and to determine the relationship of H2 with other gaseous species. The results of these studies in parallel with the regional geological setting and observed lithology allowed (1) to propose several scenarios to explain the observed associations of gas and (2) to discuss the origin and production process of H2, He and N2.

L’aquifère molassique du Bas-Dauphiné est situé le long de la vallée du Rhône, dans le Sud-Est de la France. Cet aquifère d’une superficie proche de3000 km², et d’une épaisseur moyenne de 400m renferme une eau d’excellente qualité, utilisée par de nombreuses collectivités pour l’alimentation en eau potable. Cependant certains secteurs montrent une forte vulnérabilité de la nappe aux activités agricoles. L’utilisation d’outils géochimiques etisotopiques a permis de préciser le fonctionnement hydrodynamique de l’aquifère. Dans un premier temps, nous avons montré que la stratification des écoulements décrite par De La Vaissière (2006) sur la partie drômoise de l’aquifère s’étend au secteur isérois. Les eaux les plus profondes ont des vitesses de circulation de l’ordre du mètre par an alors que les flux superficiels ont des vitesses de circulations d’une centaine de mètres par an. D’autre part, le marquage des nappes superficielles et des rivières par des teneurs faibles en tritium (de 3 à 4 UT) et forte en magnésium (jusqu’à 18 mg/L)indique un apport d’eaux anciennes, issues de l’aquifère molassique vers ces eaux superficielles. La définition de deux pôles d’eaux et l’application d’une équation de mélange couplés à la réalisation de bilans hydrogéologique a permis d’appréhender les volumes échangés. Il apparaît finalement que les réservoirs d’eaux superficiels constituent l’exutoire principal de l’aquifère molassique. L’utilisation des éléments traces a mis en avant le rôleessentiel du temps de séjour des eaux dans l’aquifère ainsi que des conditions d’oxydo-réduction dans l’acquisition de la minéralisation. L’évaluation de la qualité naturelle des eaux de la nappe a montré l’impact des activités agricoles sur l’aquifère, avec prés de 80% des échantillons ayant une teneur en nitrates supérieure à la concentration naturelle supposée. L’étude des teneurs en pesticides conforte ce constat. De plus l’étude de l’évolution des concentrations en polluants montre une dégradation de la ressource. / The molassic basin of Bas-Dauphiné is located in south-eastern France, in the Rhône valley. With an averagethickness of about 400 m, and a surface area of about 2900 km², this aquifer is an important groundwaterresource for freshwater supply and agriculture. However, this resource is also vulnerable and is impacted byhuman activities. The use of geochemical and isotopic analyses made it possible to understand the hydrodynamicsurvey of the aquifer. Firstly, we showed the stratification of the groundwater previously described in thesouthern part of the molassic aquifer could be extended to the north. The groundwater flow velocity is about onemeter / year for the deepest flow and around 100-200 meter /year for the shallowest flow. Secondly, superficialaquifers and rivers are marked by low tritium activities (3 to 4 UT) and high magnesium concentrations (until 18mg/L), which indicate ancient molassic water contribution. The definition of two water types and the applicationof a mixing equation combined with hydrogeological balances lead to an estimation of the contribution of thedeep aquifer to the shallow aquifer. It is finally established that surface water (aquifers and rivers) constitute themain outlet of the molassic aquifer. The use of trace elements shows the importance of the groundwaterresidence time and of redox conditions in the water mineralization. The assessment of baseline quality shows theimpact of agricultural activities upon the aquifer, with nearly 80 % of samples showing higher nitrateconcentration than the maximal natural concentration. This is confirmed by the study of pesticidesconcentrations. Furthermore, the evolution of pollutants concentrations points out a deterioration of the resource.