skip to primary navigation skip to content
 

SESSION 3 - Hydrochemistry and management of groundwater

SESSION 3 - Hydrochemistry and management of groundwater

(1) Arsenic in groundwater: simplicity and complexity.

John M. McArthur
Professor of Geochemistry, Earth Sciences, University College London
Gower Street, London WC1E 6BT. j.mcarthur@ucl.ac.uk

The widespread problems posed by pollution of groundwater by arsenic require us to understand how arsenic gets into groundwater, moves through aquifers, and in some instances is immobilized within them. When concentrations of arsenic are high (>200 g/L) identifying the source is simple - it is either reductive dissolution of iron oxides, geothermal sources, or weathering of sulphide ores. Geothermal sources are eaasy to identify. Weathering of ores to give identifiable pollution is rare in the subsurface, but mining activity on the surface can generate acidic, arsenic-rich, waters that can invade aquifers locally to poison groundwater.

When arsenic concentrations in groundwater are 100 /L, matters of source, transport and fate are much harder to determine. Reductive dissolution of iron oxides, competitive exchange with other anions such as hydroxide, diffuse geothermal sources, and weathering of silicates and trace sulphides, may all play a part. The balance of influences will change along a flowpath to further complicate matters.

Reduction of iron oxide is a microbial process and the source of organic matter to drive the reaction is arguably the most important, and least known, part of the process. Competitive exchange is not biologically mediated, and has been studied much in the laboratory, but at arsenic concentrations that are far higher than those found in nature, so there is a problem with interpretation. Such problems will be addressed in this presentation.

(2) Mobilisation of arsenic in the groundwater of the Blackfoot Disease area in Chia-Nan Plain, southwestern Taiwan

Jiin-Shuh Jean Professor Department of Earth Sciences, National Cheng Kung University
Bibhash Nath Postdoctoral researcher Department of Earth Sciences, National Cheng Kung University
Li-O Weng Undergraduate student Department of Earth Sciences, National Cheng Kung University
Chia-Chuan Liu PhD student Department of Earth Sciences, National Cheng Kung University
Ying-Wen Yang PhD student Department of Earth Sciences, National Cheng Kung University
jiinshuh@mail.ncku.edu.tw

Presentation

The adsorption/desorption of arsenic and its mineral species were investigated in the Blackfoot Disease (BFD) area of Chia-Nan Plain, southwestern Taiwan. In this study, groundwater samples from twenty-five wells were collected and analyzed in May 2005. The result shows that all the groundwater samples contained considerable amounts of arsenic, iron, manganese, and strontium (0.015 to 0.796 mg/L, 0.036 to 4.43 mg/L, 0.025 to 0.901 mg/L, and 0.062 to 6.22 mg/L, respectively). The arsenic speciation studies show that the reduced arsenic species (As-III) is widely distributed, with reduction ratio ranged from 57 to 99%, suggesting reducing nature of the BFD groundwater. The saturation indices computed by PHREEQC were positive for iron assemblages (i.e., iron hydroxides, goethite, hematite, jarosite, maghemite, and magnetite), while negative for arsenic assemblages (i.e., arsenolite, native arsenic, and scorodite). This demonstrates that the iron species had been precipitated, while arsenic species was dissolved in groundwater. Some bacterial strains were also isolated from groundwater, which includes Acinetobacter radioresistens,Bacillus benzoevorans, Bacillus circulans, Brevundimonas sp., Exiguobacterium aestuarii, Glacial ice bacterium, etc. The adsorption/desorption experiment revealed that the arsenic could be adsorbed upto 44% onto iron hydroxides at lower concentration level for initial arsenic (5 mg/L), while the adsorption is relatively low (29%) for 10 mg/L of initial arsenic.

(3) Arsenic enrichment of ground water at two regions of the Chacopampean Plain, northwest Argentina

Ondra Sracek1, María Gabriela García2
1 OPV s.r.o., Bĕlohorská 31, 169 00 Praha 6, Czech Republic
2 Centro de Investigaciones Geoquímicas y de Procesos de la Superficie, FCEFyN, Universidad Nacional de Córdoba, Av. Vélez Sarsfield 1611, X5016CGA Córdoba, Argentina

Presentation

High concentrations of arsenic have been encountered in ground water of the Chacopampean Plain (CPP). Two regions have been studied: Santiago del Estero in semiarid central CPP and more humid Tucumán at western limits of CPP. In both regions concentrations of dissolved As may reach values above 1,000 μg/l and high As concentrations are linked to shallow loessic sediments (upper 30 m of sedimentary sequence). There is a positive correlation between dissolved As and Na and HCO3 concentrations. Also, there is positive correlation between As and several oxyanion-forming elements including V, F, B, and Mo present in volcanic ash in loessic sequence. Primary source of arsenic in ground water cannot be determined unequivocally, but highly weathered glass present in volcanic ash is a principal candidate. Contents of ferric iron extracted by the oxalic extraction step are low because there is a limited amount of oxidizable ferrous iron minerals in volcanic ash of acid composition. Thus, adsorption capacity of solid phase for As is quite limited and may be further decreased by competition with other oxyanions for adsorption sites. Conditions in shallow aquifer are generally oxidizing and arsenic is present mostly as As(V). However, in the proximity of the Salí River in Tucumán As can be released as a consequence of the penetration of surface water contaminated by organic wastes and causing reductive dissolution of ferric mineral adsorbents in the neighboring aquifer. Similarly, high concentration of DOC with a potential impact on As mobilization were observed in the proximity of unlined irrigation canals at Santiago del Estero.

In Santiago del Estero high As concentrations seem to be linked to slow ground water flow zones with long ground water residence times and high pH (up to 9.0) and TDS values. In Tucumán, As concentrations seem to increase from west towards east along with direction of flow, decreasing precipitation and longer residence time of ground water that recharges at the mountain front of Sierra del Aconquija at western limits of the region. Many of these findings are also applicable to other sites in Argentina with high dissolved As concentrations like La Pampa south of both studied sites.

(4) Towards a regional characterisation of the 'deep aquifer' in southern Bangladesh

Mohammad Hoque PhD researcher UCL
William Burgess Lecturer UCL
Matin Ahmed Professor Dhaka University
william.burgess@ucl.ac.uk

Presentation

In southern Bangladesh and West Bengal it has long been known that a deeper aquifer, separated from the shallow, arsenic-bearing groundwater system by an effective aquitard at ca. 150 m depth, occurs in places. This deep aquifer has been exploited sustainably to provide water supplies for individual towns - eg at Khulna where for over 20 years the aquitard has protected the deep aquifer from incursions of salinity and arsenic occuring in the shallow aquifer. Nevertheless, for a variety of alternative hydrogeological scenarios, exploratory models demonstrate the vulnerability of the deep aquifer to vertical leakage from the shallow system, limiting its potential to act as an 'arsenic safe' source of water. The lateral variability and regional extent of the aquitard is unknown, and many questions concerning the deep aquifer remain to be answered. We present empirical descriptions and preliminary conceptualisations of the deep aquifer environment from current research at individual sites across southern Bangladesh, incorporating: lithostratigraphy, sedimentology and hydrostratigraphy from geological and geophysical logs and head measurements, and profiles of groundwater chemistry, groundwater age and groundwater isotopic character for indications of groundwater flow. Uncertainties, and their implications for the viability of the deep aquifer as a source of arsenic-safe water, will be emphasised.

(5) Arsenic contaminated aquifers: a study of the Ganga Levee zones in Bihar, India

Dr. Ashok Ghosh, Prof.-in-Charge, Department of Environment and Water Management
A.N.College [Magadh University], Patna, India ghosh51@hotmail.com
Prof. Shatrunjay K. Singh, Coordinator, Dept. of Environment and Water Management
A.N.College [Magadh University], Patna, India
Dr. Nupur Bose, Lecturer, Dept. of Geography, A.N.College [Magadh University], Patna, India
Dr. Sunil Choudhary, Reader, Dept. of Botany, T.M.Bhagalpur University, Bhagalpur, India

Presentation

'In Bihar Plains, ground water is the most important source of drinking and irrigation water. The purpose of this interdisciplinary study, undertaken along the levee of river Ganga in the Mid Ganga Plain, was to determine the existence and intensity of arsenic contamination in aquifers being tapped for direct and indirect ingestion of the properties of the region's ground water, in the four districts of Bihar [India], i.e., Patna, Bhojpur, Vaishali and Bhagalpur. The methodology involved formulation of a protocol for arsenic detection in ground water, use of Field Test Kits for initial detection, obtaining GPS coordinates of arsenic hotspots for spatial analysis of the problem, and confirmatory testing of arsenic hot samples by U.V., and Atomic Absorption Spectrophotometry. Water samples of 28000 private and government owned hand pumps were tested. Many arsenic hotspots were detected in all the four districts, the coordinates of which were recorded by GPS. Arsenic contamination up to 1861 ppb. was found in the western district of Bhojpur, against the W.H.O. permissible limit of 10 ppb. The greatest frequency of contaminated hand pumps was noted in the eastern district of Bhagalpur. Sharp spatio-temporal variations of contamination levels were detected in this densely populated study belt.