Earth and Environmental Sciences

In arid and semi-arid regions of the world, such as Mongolia, the future of water resources under a warming climate is of particular concern. The influence of increasing temperatures on precipitation is difficult to predict because precipitation trends in coming decades could have a high degree of spatial variability. In this study, we applied a rotated principal component analysis (RPCA) to a network of 20 treering chronologies across central Mongolia from 1790 to 1994 to evaluate spatial hydroclimatic variability and to place recent variability in the context of the past several centuries. The RPCA results indicate that the network consists of four tree-growth anomaly regions, which were found to be relatively stable through time and space. Correlation analyses reveal spatial linkages between the tree-growth anomalies and instrumental data, where annual streamflow variability was strongly associated with tree-growth anomalies from their respective regions from 1959 to 1994 (r = 0.52-0.64, p < 0.05). This study highlights the extent of spatial variability in hydroclimate across central Mongolia and emphasizes the value of using tree-ring networks in locations with limited instrumental records.

The depth of the 2006–9 drought in the humid, southeastern US left several metropolitan areas with only a 60–120 day water supply. To put the region’s recent drought variability in a long-term perspective, a dense and diverse tree-ring network—including the first records throughout the Apalachicola–Chattahoochee–Flint river basin—is used to reconstruct drought from 1665 to 2010 CE. The network accounts for

Extreme spatial heterogeneity has emerged as a salient characteristic of groundwater arsenic in many complex fl uviodeltaic environments. Here we examine patterns of arsenic heterogeneity in the shallow (<23 m) groundwaters of a well-studied fl oodplain setting in Araihazar, Bangladesh. Ninetyfi ve augers and 200 shallow wells sampled at spacings of 10 1 -10 3 m in the 25 km 2 area indicate that the concentration of arsenic in shallow groundwater largely varies with the grain size, thickness, and distribution of fi ne-grained (<63 µm) sediments that overlie buried aquifer sands. The overall pattern shows that lower arsenic concentrations are typically found where aquifer sands outcrop at or near the surface, whereas higher arsenic levels typically underlie, or are adjacent to thicker, fi ne-grained deposits. Furthermore, chronostratigraphic reconstructions of aquifer sediments indicate that sediment distribution, and consequently the patterning of dissolved arsenic, is readily explained in the context of local river history and fl oodplain development within the past 1000 yr. An important implication is that complex patterns of groundwater arsenic in affl icted fl uviodeltaic settings can be better understood through reconstructions of local aquifer history. This fi nding is especially relevant because the village and tube-well locations are closely linked with surface landforms such as former levees and bars. An additional and worrisome fi nding is that the artifi cial fi lling of villages to protect from fl ooding can mimic the natural fi ne-grained stratigraphy commonly associated with high concentrations of arsenic.

Groundwater drawn daily from shallow alluvial sands by millions of wells over large areas of South and Southeast Asia exposes an estimated population of over 100 million to toxic levels of arsenic (1). Holocene aquifers are the source of widespread arsenic poisoning across the region (2, 3). In contrast, Pleistocene sands deposited in this region more than ~12,000 years ago mostly do not host groundwater with high levels of arsenic. Pleistocene aquifers are increasingly used as a safe source of drinking water (4) and it is therefore important to understand under what conditions low levels of arsenic can be maintained. Here we reconstruct the initial phase of contamination of a Pleistocene aquifer near Hanoi, Vietnam. We demonstrate that changes in groundwater flow conditions and the redox state of the aquifer sands induced by groundwater pumping caused the lateral intrusion of arsenic contamination over 120 m from Holocene aquifer into a previously uncontaminated Pleistocene aquifer. We also find that arsenic adsorbs onto the aquifer sands and that there is a 16-20 fold retardation in the extent of the contamination relative to the reconstructed lateral movement of groundwater over the same period. Our findings suggest that arsenic contamination of Pleistocene aquifers in South and Southeast Asia as a consequence of increasing levels of groundwater pumping have been delayed by the retardation of arsenic transport.

1 Earth & Environmental Sciences, Vanderbilt University, Nashville, TN 37235 2 Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964 3 Queens College, City University of New York, Flushing, NY 11367 4 Physical Research Laboratory, Navrangpura, ...

One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1-1000 μg/L), Fe (0.01-40 mg/L), Mn (0.2-4 mg/L) and S (0.04-14 mg/L) are compared. The P-extractable (0.01-36 mg/kg) and HCl-extractable As (0.04-36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2-1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored.

A new sampling device was used to obtain 8 detailed profiles of groundwater and associated sediment properties to ∼30 m depth in a 4 km 2 area of Bangladesh that is characterized by high spatial variability in groundwater As. Concentrations of dissolved As, Fe, and Mn ranged from b 0.1 to 600 μg/L, b 0.1 to 18 mg/L, and b 0.1 to 4 mg/L, respectively. Voltammetric measurements conducted in the field indicate that N 95% of the dissolved arsenic was As(III). The proportion of Fe(II) in the hot-leachable iron fraction of the sediment ranged from 0.2 to 0.9. The concentration of phosphate-extractable As in aquifer sands ranged from b 0.1 to 1.1 mg/kg. Although the highest dissolved As concentration was measured at the site where the highest groundwater Fe and Mn concentrations were also observed, the dissolved As maximum was located several meters below the dissolved Fe and Mn maxima. Fe(II) / Fe ratios and P-extractable As concentrations in the solid phase were particularly elevated at the same site, though again the profiles of these properties did not consistently match that of groundwater As concentrations. A 500-m section composed of 5 vertical profiles confirms that whereas low As concentrations are broadly associated with low dissolved Fe and Mn concentrations, as well as low Fe (II) / Fe ratios in the solid phase, the spatial distributions of these properties in the subsurface do not follow a simple relationship. In combination with other observations from the area, the results suggest that elevated local recharge in areas where the permeability of surface soils is high prevents As from accumulating in groundwater. Conversely, dissolved As concentrations tend to be high in areas where local recharge is restricted by a surface cover of low permeability. The data demonstrate that further investigation of the mechanisms underlying As mobilization is likely to require sampling at lateral scales b 1 km.

This study reexamines the notion that extensive As mobilization in anoxic groundwater of Bangladesh is intimately linked to the dissolution of Fe oxyhydroxides on the basis of analyses performed on a suite of freshly collected samples of aquifer material. Detailed sediment profiles extending to 40 to 70 m depth below the surface were obtained at six sites where local groundwater As concentrations were known to span a wide range. The sediment properties that were measured include (1) the proportion of Fe(II) in the Fe fraction leached in hot 1.2 N HCl, (2) diffuse spectral reflectance, and (3) magnetic susceptibility. In parallel with local concentrations of dissolved As ranging from Ͻ5 to 600 g/L, Fe(II)/Fe ratios in shallow (gray) Holocene sands tended to gradually increase with depth from values of 0.3 to 0.5 to up to 0.9. In deeper (orange) aquifers of presumed Pleistocene age that were separated from shallow sands by a clay layer and contained Ͻ5 g/L dissolved As, leachable Fe(II)/Fe ratios averaged ϳ0.2. There was no consistent relation between sediment Fe(II)/Fe and dissolved Fe concentrations in groundwater in nearby wells. The reflectance measurements indicate a systematic linear relation (R 2 of 0.66; n ϭ 151) between the first derivative transform of the reflectance at 520 nm and Fe(II)/Fe. The magnetic susceptibility of the shallow aquifer sands ranged from 200 to 3600 (x 10-9 m 3 /kg SI) and was linearly related (R 2 of 0.75; n ϭ 29) to the concentrations of minerals that could be magnetically separated (0.03 to 0.79% dry weight). No systematic depth trends in magnetic susceptibility were observed within the shallow sands, although the susceptibility of deeper low-As aquifers was low (up to ϳ200 ϫ 10-9 m 3 /kg SI). This set of observations, complemented by incubation results described in a companion paper by van Geen et al. (this volume), suggests that the release of As is linked to the transformation of predominantly Fe (III) oxyhydroxide coatings on sand particles to Fe(II) or mixed Fe(II/III) solid phases with a flatter reflectance spectrum such as siderite, vivianite, or magnetite, without necessarily resulting in the release of Fe to groundwater. The very low As/Fe ratio of magnetically separated minerals compared to the As/Fe of bulk acid leachate (2 vs. 40 10-6 , respectively) suggests that such a transformation could be accompanied by a significant redistribution of As to a mobilizable phase on the surface of aquifer particles.

The extremely heterogeneous distribution of As in Bangladesh groundwater has hampered efforts to identify with certainty the mechanisms that lead to extensive mobilization of this metalloid in reducing aquifers. We show here on the basis of a highresolution transect of soil and aquifer properties collected in Araihazar, Bangladesh, that revealing tractable associations between As concentrations in shallow (b 20 m) groundwater with other geological, hydrological, and geochemical features requires a lateral sampling resolution of 10-100 m. Variations in the electromagnetic conductivity of surface soils (5-40 mS/m) within a 500 m  200 m area are documented with 560 EM31 measurements. The results are compared with a detailed section of groundwater As concentrations (5-150 Ag/L) and other aquifer properties obtained with a simple sampling device, bthe needle-samplerQ, that builds on the local drilling technology. By invoking complementary observations obtained in the same area and in other regions of Bangladesh, we postulate that local groundwater recharge throughout permeable sandy soils plays a major role in regulating the As content of shallow aquifers by diluting the flux of As released from reducing sediments.

The high-degree of spatial variability of dissolved As levels in shallow aquifers of the Bengal Basin has been well documented but the underlying mechanisms remain poorly understood. We compare here As concentrations measured in groundwater pumped from 4700 wells <22 m (75 ft) deep across a 25 km 2 area of Bangladesh with variations in the nature of surface soils inferred from 18,500 measurements of frequency domain electromagnetic induction. A set of 14 hand auger cores recovered from the same area indicate that a combination of grain size and the conductivity of soil water dominate the electromagnetic signal. The relationship between pairs of individual EM conductivity and dissolved As measurements within a distance of 50 m is significant but highly scattered (r 2 = 0.12; n = 614). Concentrations of As tend to be lower in shallow aquifers underlying sandy soils and higher below finer-grained and high conductivity soils. Variations in EM conductivity account for nearly half the variance of the rate of increase of As concentration with depth, however, when the data are averaged over a distance of 50 m (r 2 = 0.50; n = 145). The association is interpreted as an indication that groundwater recharge through permeable sandy soils prevents As concentrations from rising in shallow reducing groundwater.