Holocene Aquifer Research Articles

Solubility of Symplesite (Ferrous Arsenate): Implications for Reduced Groundwaters and Other Geochemical Environments

Arsenic in drinking water represents a major public health concern globally, with tens of millions of people thought to be consuming unsafe amounts of As daily. Groundwater is particularly vulnerable to As contamination, and one geochemical environment conducive to natural mobilization of As is found in shallow alluvial Holocene aquifers. Bacterial consumption of organic matter in these aquifers creates anoxic conditions, leading to dissimulatory reduction of Fe oxides. In these environments, dissolved As and Fe often co‐occur. While As(III) is thermodynamically favored under these conditions, As(V) often is found as well. Concentrations of dissolved As(V) and Fe(II) can be constrained by the precipitation of ferrous arsenate solid phases such as symplesite [Fe(II)3(As(V)O4)2·8H2O]. We present a new solubility product for symplesite (pKso = 33.25) based on controlled laboratory precipitation experiments. Using this solubility product, we conducted equilibrium geochemical modeling using one of the most extensive groundwater data sets from Bangladesh to show that a number of water samples are oversaturated with a variety of solid phases, including siderite, calcite, rhodochrosite, vivianite, and symplesite. Symplesite or other ferrous arsenate solid phases could be a significant sink for As(V) and Fe(II) in Bangladesh and other countries with similar geochemical environments. Symplesite could also control As(V) solubility in extreme environments such as alkaline lakes or in brines used for regeneration of anion exchange resins.

Study of stable isotopes for highly deformed aquifers in the Hsinchu-Miaoli area, Taiwan

This study was based on the analysis of isotopic compositions of hydrogen and oxygen in samples from precipitation, groundwater and stream water. In addition, parts of groundwater samples were dated by carbon-14 and tritium. These data are integrated to provide other views of the hydrologic cycle in the Hsinchu-Miaoli groundwater district. The groundwater district is principally composed of Pleistocene and Holocene aquifers. The Pleistocene aquifers are highly deformed by folding and faults into small sub-districts with areas of only tens of square kilometers. These aquifers are exclusively recharged by local precipitation. The Holocene aquifers cover narrow creek valleys, only tens of meters in thickness. The local meteoric water line (LMWL), constructed from rainfall samples in the Hsinchu Science Park, is described by the equation δD=8.02δ18O+10.16, which agrees with the global meteoric water line. In addition, the precipitation isotopic compositions can be categorized into two distinct end members: typhoon type and monsoon type. The groundwater isotopic compositions are perfectly located on an LMWL and can be considered a mixture of precipitations. Based on the mass balance of isotopic compositions of oxygen and hydrogen, infiltration is more active in the rainy season with depleted isotopic compositions. The amount of infiltration during May–September is roughly estimated to comprise at least 55% of the whole year’s recharge. The isotopic compositions of stream water are expressed by a regression equation: δD=7.61δ18O+9.62, which is similar to the LMWL. Although precipitation isotopic compositions are depleted during summer time, the isotopic compositions contrarily show an enriched trend in the upstream area. This is explained by the opposite altitude effect on isotopic compositions for typhoon-related precipitations.

Solid-phases and desorption processes of arsenic within Bangladesh sediments

Arsenic is a contaminant in the groundwater of Bangladesh and West Bengal, India, where an estimated 57 million people may be drinking water with unsafe arsenic levels. The source of arsenic appears to be natural, solid-phase arsenic in the sediments, and various theories have been put forth regarding the modes of arsenic release, ranging from the oxidative or reductive degradation of arsenic-bearing solids to competitive ligand displacement by phosphate. Currently, however, reductive dissolution of Fe(III) (hydr)oxides and concomitant arsenic release is the most widely accepted explanation of high arsenic groundwater concentrations. Using micro-X-ray fluorescence elemental mapping and micro-X-ray absorption near-edge structure spectroscopy, we detect arsenic-bearing sulfides in the aquifer sediments from our field site in Munshiganj, Bangladesh. Reduction of Fe and As in surface soil layers is apparent, but Fe(III) (hydr)oxides are not detected in the Holocene aquifer materials. Rapid abiotic desorption of arsenic from sediments is observed in batch experiments, and positive controls with ferrihydrite dismiss the role of ferric (hydr)oxides in arsenic retention within the aquifer sediments. Based on our laboratory results, we do not see evidence for ferric (hydr)oxide reductive dissolution at well-depths in the Holocene aquifer. In contrast, our data suggest that arsenic is only released via redox cycling in surface soils/sediments and thus must then be transported to well-depth through the sandy aquifer.

Processes conducive to the release and transport of arsenic into aquifers of Bangladesh

Arsenic is a contaminant in the groundwater of Holocene aquifers in Bangladesh, where approximately 57 million people drink water with arsenic levels exceeding the limits set by the World Health Organization. Although arsenic is native to the sediments, the means by which it is released to groundwater remains unresolved. Contrary to the current paradigm, ferric (hydr)oxides appear to dominate the partitioning of arsenic in the near surface but have a limited impact at aquifer depths where wells extract groundwater with high arsenic concentrations. We present a sequence of evidence that, taken together, suggest that arsenic may be released in the near surface and then transported to depth. We establish that (i) the only portion of the sediment profile with conditions destabilizing to arsenic in our analysis is in the surface or near-surface environment; (ii) a consistent input of arsenic via sediment deposition exists; (iii) retardation of arsenic transport is limited in the aquifers; and (iv) groundwater recharge occurs at a rate sufficient to necessitate continued input of arsenic to maintain observed concentrations. Our analyses thus lead to the premise that arsenic is liberated in surface and near-surface sediments through cyclic redox conditions and is subsequently transported to well depth. Influx of sediment and redox cycling provide a long-term source of arsenic that when liberated in the near surface is only weakly partitioned onto sediments deeper in the profile and is transported through aquifers by groundwater recharge.

Spatial pattern of arsenic contamination in shallow wells of Bangladesh: regional geology and nonlinear dynamics

Since the discovery of large-scale arsenic contamination of groundwater in Bangladesh more than a decade ago, studies related to its spatial characterization have relied on geostatistical approaches and the classical notion of linear stochastic dynamics. This study explores an alternative nonlinear approach, with a motivation to possibly achieve more cost-effective solutions for Bangladesh. It investigates the existence of nonlinear deterministic and chaotic dynamic behavior in the spatial pattern of arsenic contamination in the shallow wells (depth<150 m). The database comprises the nationwide arsenic survey completed in 1999 by the British Geological Survey (BGS) in collaboration with the Department of Public Health Engineering (DPHE) of Bangladesh. Distinction is made in terms of regional geology (Pleistocene vs. Holocene deposits/Northwest vs. Southwest) to understand the geologic dependency. Identification of possible presence of nonlinear deterministic and chaotic patterns is made via the Grassberger-Procaccia correlation dimension algorithm. The analysis yields correlation dimension values ranging anywhere from 8 to 11 depending on the region, suggesting that the arsenic contamination in space, from a chaotic dynamic perspective, is a medium- to high-dimensional problem. The dimension results also indicate that the spatial dynamics of arsenic may be moderately sensitive to geology, with Pleistocene aquifers appearing to require a minimum of about two less dominant processes/variables for its description when compared to that required by the Holocene aquifers. Based on these results, a qualitative discussion is also cast on the potential opportunities offered by a nonlinear deterministic and chaotic dynamic approach towards improving cost-effectiveness in siting new safe wells.

Natural arsenic contamination of Holocene alluvial aquifers by linked tectonic, weathering, and microbial processes

Linked tectonic, geochemical, and biologic processes lead to natural arsenic contamination of groundwater in Holocene alluvial aquifers, which are the main threat to human health around the world. These groundwaters are commonly found a long distance from their ultimate source of arsenic, where chemical weathering of As‐bearing minerals occurs. We propose a “GBH‐As” model that ties together all of the important tectonic, biologic, and hydrologic processes that cause natural As release in groundwater in Holocene terrestrial deposits. Processes highlighted by the “GBH‐As” model can explain the movement of arsenic from lithosphere to hydrosphere. However, we propose a factor that has increased dissolved As concentrations in Holocene aquifers to levels where human health is threatened: mechanical weathering associated with Pleistocene glaciation. Our model invokes erosion of mountain belts aided by glaciers, transport of arsenic by surface waters, adsorption of As by stream sediments, and deposition of stream sediments and organic matter in alluvial deposits. Subsequently, Fe(III)‐reducing bacteria present in alluvial aquifers cause the release of sorbed As to groundwater under moderately reducing conditions.

Characterising scale-dependence of hydraulic conductivity in carbonates: evidence from the Bahamas

Aquifer and core test data for the calcitic Pleistocene Lucayan Limestone aquifer of the Bahamas exhibit a strong dependence of hydraulic conductivity on test scale, to length scales of 102–103m. In contrast, the scale-dependence of hydraulic conductivity in the poorly consolidated Holocene aquifer is much less marked, being independent of scale above 100m. Core scale conductivities of the Pleistocene aquifer are much less than those in the Holocene because of pore occlusion by cementation, but the development of linked vug porosity, fissure and cavernous porosity along paleo-exposure horizons gives higher hydraulic conductivities at larger scales. We suggest that the observed scale dependence is related to the existence of a range of void types (termed characteristic conductive elements) with aperture and connectivity increasing, and abundance decreasing with sample scale.

SEA LEVEL CHANGE AND THE DEPOSITIONAL HISTORY OF A SHALLOW, COASTAL AQUIFER SYSTEM, NORTHERN MONTEREY, CA, BASED ON INDICATOR GEOSTATISTICS

We conducted a multidisciplinary investigation of shallow aquifers and confining layers within a coastal groundwater basin in northern Monterey County, CA. Shallow sediments within this basin comprise a Holocene, unconfined, aeolian and valley-fill aquifer, and an underlying, Pleistocene aquifer of fluvial and alluvial silt, sand, and gravel. These two aquifers are separated by a confining unit of clay, silt, and sand, previously interpreted to be a lagoonal and shallow marine deposit of latest Pleistocene to Holocene age. The 40 km2 study area contains 231 wells and borings, from which cuttings and core description data were digitized and compiled at 0.6-m depth intervals. Additional geophysical logging (natural gamma-ray and induction resistivity) data was collected in 55 wells. This information was used for proxy lithological identification based on calibration from core descriptions, and combined with the core and cuttings data to generate a database of lithological interpretations at 22,000 X-Y-Z locations. These data were used to generate geostatistical realizations, using a binary (clay versus not clay) indicator scheme for the confining unit and shallow, valley-fill aquifers. The realizations indicate a complicated, three-dimensional geometry that is not apparent in earlier cross-sections of the confining unit, and appears to record multiple sealevel changes, most likely associated with isotope Stage 5. This interpretation has important implications for the hydrogeology of this site and the Salinas Valley in general, as well as other coastal California basins.

Hydrogeology of an Atoll Island: A Conceptual Model from Detailed Study of a Micronesian Example

ABSTRACTFindings from a detailed hydrogeologic study of a Micronesian atoll island are combined with published descriptions of the hydrogeology of other atoll islands. The purpose is to propose a conceptual model which describes how the occurrence and flow of ground water in atoll islands are related to the underlying geologic framework. The study island is Deke, a small, uninhabited island on Pingelap Atoll in the Eastern Caroline Islands.Integrated studies on Deke of surface geology and physiography, water levels and their variation, surface geophysics (seismic refraction and resistivity), and subsurface core samples indicate a hydrogeologic system that is more complex than would ordinarily be expected for a small and, at first glance, uncomplicated island. Of utmost importance to the occurrence and flow of ground water is the fact that the island straddles the lagoonward edge of the very firmly indurated reef‐flat plate. This plate is a hard, impermeable substrate beneath the northern portion (ocean side) of the island, occurs at sea level, and is underlain by unconsolidated or loosely cemented sediments. Study results indicate that this reef‐flat plate acts as a confining bed along the ocean side of the lens; elsewhere the lens is unconfined, receives recharge directly, and forms a thicker fresh‐water nucleus.The conceptual model of atoll‐island hydrogeology involves a dual aquifer system: (1) an aquifer of mostly unconsolidated Holocene sediments resting on (2) a once emergent and now very permeable Pleistocene limestone platform. The Holocene aquifer, which is where the island lens occurs, is heterogeneous with respect to its hydraulic properties and is confined in part of the island and unconfined elsewhere, with the location of the two regions depending on where the island sits relative to the pinchout of the reef‐flat plate. In the unconfined region, there is a hydrologically‐important central depression, where low lying sediments are enclosed by ocean‐derived washover fans and a lagoon‐bordering dune. This area is a likely ground‐water sink due to direct evapotranspiration during the dry season. Accordingly, the model includes two flow patterns: a wet‐season pattern radiating outward from the unconfined lagoon‐side of the island, and a dry‐season pattern that includes a superimposed area of centripetal flow at the central depression.