Arsenic contamination of groundwater

In a collaboration with Instituto de Geoquímica in Buenos Aires, the BGS has been involved with investigations of arsenic occurrence in groundwater in Argentina over the last decade.

Studies have focused on La Pampa Province, part of the vast Chaco-Pampean Plain, which covers a large part of the country.

The region is a semi-arid plain where shallow groundwater resources occur largely in Quaternary loess deposits.

Arsenic in Pampean groundwater

The shallow Pampean aquifer of La Pampa is typically oxic and analysed samples of groundwater show a very large range of arsenic concentrations (<4 – 5300 µg/L observed in our studies).

The arsenic is dominated by As(V) and correlates positively with a number of other anions and oxyanions (HCO₃, V, B, F and to a lesser extent, Mo and U). Concentrations of V, B, F, Mo and U range up to 5.4 mg/L, 14 mg/L, 29 mg/l, 990 µg/L and 250 µg/L respectively and many exceed WHO guideline values or national limits for drinking water. Concentrations of arsenic and the associated trace elements in the groundwaters are extremely variable on a local scale.

Release of arsenic(V) from the Pampean sediments under high-pH (alkaline) conditions is seen as a critical part of the mobilisation mechanism.

In urban areas, response to the groundwater-quality problems is to treat by reverse osmosis to remove solutes from drinking water. Such options are less viable in rural areas and poor drinking-water quality remains an important issue for the rural populations.

Publications

Nicolli, H B, García, J W, Falcón, C M, and Smedley, P L.  2012.  Mobilization of arsenic and other trace elements of health concern in groundwater from the Salí River Basin, Tucumán Province, Argentina.   Environmental Geochemistry & Health, 34, 251–262.

Smedley, P L, Nicolli, H B, Macdonald, D M J, and Kinniburgh, D G.   2008.   Arsenic in groundwater and sediments from La Pampa Province, Argentina. In: Bundschuh, J Armienta, M A, Birkle, P, Bhattacharya, P, Matschullat, J, Mukherjee, A B. (editors).   Natural Arsenic in Groundwaters of Latin America, Taylor & Francis, 35–45.

Smedley, P L, Kinniburgh, D G, Macdonald, D M J, Nicolli, H B, Barros, A J, Tullio, J O, Pearce, J M, and Alonso, M S.  2005.   Arsenic associations in sediments from the loess aquifer of La Pampa, Argentina.   Applied Geochemistry, 20, 989-1016.

Smedley, P L, Nicolli, H B, Macdonald, D M J, Barros A J, and Tullio, J O.   2002.   Hydrogeochemistry of arsenic and other inorganic constituents in groundwaters from La Pampa, Argentina.   Applied Geochemistry, 17, 259–284.

The project ‘Groundwater Studies for Arsenic Contamination in Bangladesh’ was a reconnaissance investigation of the arsenic problem, carried out over the period 1998 to 2001. Its remit was to collate available data and conduct new groundwater surveys.

The project was funded by the UK Department for International Development (DFID). One of the main aims of the investigation was to assess the scale of the groundwater arsenic problem in order to aid the rapidly developing arsenic mitigation programme.

A second aim was to increase our understanding of the origins and behaviour of arsenic in Bangladesh aquifers. These aims were subsequently expanded to include a broader range of hydrochemical parameters.

The project was carried out in a collaboration between a number of organisations. On behalf of the Government of Bangladesh, DFID appointed the British Geological Survey (BGS) as lead consultants for the study. The Department of Public Health Engineering (DPHE), which is responsible for water supply throughout the country other than in the cities of Dhaka and Chittagong, was the executing agency. The Bangladesh Water Development Board (BWDB) and Geological Survey of Bangladesh (GSB) also provided counterparts.

Phase 1: Rapid investigation phase

The BGS appointed Mott MacDonald Ltd (MML) to carry out much of the Phase 1 work and a team of national experts was recruited to assist. A report from the Phase 1 investigations was completed in 1999.

Phase 2: Groundwater surveys and more detailed investigations

Hydrochemical surveys were undertaken at various scales — national, upazila and village.

Two national-scale surveys were undertaken:

• a systematic survey of 61 of the 64 districts of Bangladesh involving the collection of 3534 tubewell samples
• a survey of 113 tubewells from the national water quality monitoring network maintained by the Bangladesh Water Development Board (BWDB)

Three upazilas (the sadar upazilas of Nawabganj, Faridpur and Lakshmipur districts) were also selected as Special Study Areas for a broader range of investigations.

In one of these upazilas, a single mouza or village (Mandari, Lakshmipur) was selected for a detailed survey using on-site arsenic analysis with an Arsenator. A limited amount of monitoring (time series) data were also collected from tubewells and piezometers in the three Special Study Areas.

Full details of the outputs from the Phase 2 project are available:

• Bangladesh reports
• Bangladesh maps
• Bangladesh data

Use of material from the BGS and DPHE project ‘Arsenic contamination of groundwater in Bangladesh’

Permission for reproduction of materials contained in these web pages is granted subject to the inclusion of the following acknowledgement:

‘This material was produced by the British Geological Survey and the Department of Public Health Engineering (Bangladesh) undertaking a project funded by the UK Department for International Development (DFID). Any views expressed are not necessarily those of DFID’. In cases where only a map or diagram is reproduced or where data from the report are used, the above acknowledgement may be substituted by a full citation to the report as follows:BGS and DPHE.  2001.  Arsenic contamination of groundwater in Bangladesh. Kinniburgh, D G and Smedley, P L (Editors). British Geological Survey Technical Report WC/00/19. British Geological Survey: Keyworth

In collaboration with COWI and DAEPA (Direction de L’Approvisionnement en Eau Potable et de L’Assainissement), the BGS has carried out an investigation of arsenic in groundwaters from the Ouahigouya area of rural northern Burkina Faso.

Previous surveys had shown that some sources of drinking water in the area had concentrations of arsenic above the WHO guideline value for arsenic (10 µg/L) and some villagers were presenting with skin problems consistent with chronic arsenic exposure.

Groundwater in northern Burkina Faso

Northern Burkina Faso is an arid area and traditional sources of water have been from shallow hand-dug wells. These are still used by many villagers in the region, although boreholes were drilled in some villages in the 1990s to 2000s to provide drinking water from deeper groundwater sources.

Reconnaissance findings

BGS collected 45 samples of groundwater from hand-pumped boreholes and dug wells in villages from the Ouahigouya area. Results of the survey showed:

• concentrations of arsenic had a large range (<0.5–1630 µg/L) although most analysed samples contained less than 10 µg/L
• highest concentrations were found in borehole waters (all dug-well waters had <10 µg/L As)
• arsenic present in the waters was dominantly As(V)
• spatial variability in arsenic concentrations was large

Sources of the arsenic

The high-arsenic groundwaters, that were observed, derive from zones of gold mineralisation in ancient (Lower Proterozoic) volcano-sedimentary rocks. The gold occurs in vein structures along with quartz and altered sulphide minerals (pyrite, chalcopyrite, arsenopyrite).

The source is likely to be the oxidised sulphide minerals and secondary iron oxides in the mineralised zones. Links were also found between the concentrations of dissolved arsenic and the concentrations of dissolved molybdenum and tungsten. These also probably derive from ore minerals and oxides in the mineral veins.

Mitigation of the problem

Since discovery of the groundwater quality problems in the region, the Burkina Faso government has taken steps to close many of the problematic wells and install alternative supplies. The government has carried out subsequent sampling and analysis, conducted awareness campaigns and carried out health surveillance.

The discovery of high arsenic concentrations in groundwaters from crystalline basement rocks in northern Burkina Faso reiterates the need for reconnaissance surveys of trace elements (including arsenic) in groundwater from basement aquifers.

Publications

Smedley, P L, Knudsen, J, and Maiga, D.  2007.   Arsenic in groundwater from mineralised Proterozoic basement rocks of Burkina Faso.   Applied Geochemistry, 22, 1074–1092.

The BGS, in collaboration with the Huhhot Anti-Epidemic and Sanitation Station (Huhhot, Inner Mongolia), carried out an investigation of the occurrence, distribution and causes of arsenic in groundwater from Quaternary aquifers in rural parts of the Huhhot Basin (around 4800 km²), Inner Mongolia, China.

The investigation followed the discovery, by our collaborators, of health problems consistent with chronic arsenic exposure in village populations. Observed health problems include skin lesions (keratosis, melanosis, skin cancer) and internal cancers (lung and bladder cancer).

The BGS hydrogeochemical investigation was funded by the UK government’s Department for International Development (DFID ).

Huhhot Basin

The Huhhot Basin lies on the southern edge of the Gobi Desert and has an arid climate. The rural population of the area relies solely on groundwater for drinking and domestic use.

Groundwater from a shallow aquifer is accessed using traditional open dug wells and more recent hand-pumped boreholes, usually <30 m deep. Boreholes >100 m deep tap into a deeper aquifer, which is often artesian.

The aquifers in the basin are young (Quaternary) sediments of fluvial and lacustrine origin.

Arsenic-affected aquifers

We sampled a representative set of groundwaters from both aquifers. Analysis showed:

• groundwaters have a large observed range of arsenic concentrations (<1–1480 µg/L)
• concentrations greater than 10 µg/L occur in groundwaters from both the shallow and deep aquifers
• some open dug wells have arsenic concentrations >10 µg/L
• high arsenic concentrations are frequently associated with high concentrations of dissolved Fe, Mn, NH₄, DOC, HCO₃ and P, and low SO₄ concentrations, consistent with occurrence under strongly reducing conditions
• dissolved arsenic is dominated by inorganic As(III)
• deep groundwaters have particularly high dissolved organic carbon (DOC) concentrations (up to 30 mg/L), reflecting enrichment in humic acid

Our results for limited sediment analysis from the area revealed:

• total arsenic concentrations in the range 3–29 mg/kg, 30% being oxalate-extractable and suggesting an origin largely from amorphous and poorly-structured iron oxides

Groundwater chemistry and arsenic release

Groundwater in the Huhhot aquifers flows from the basin margins towards the low-lying central part (see map).

The groundwaters display a strong change in redox conditions with flow, being oxic along the basin margins but strongly reducing downgradient in the low-lying part of the basin.

High groundwater arsenic concentrations appear to occur in villages taking water from the strongly reducing parts of the aquifers.

Release of arsenic from iron oxides by desorption and reductive dissolution is proposed as the principal mechanism.

Organic carbon is a strong driver for the microbially-mediated redox reactions, including that of arsenic release.

Alternative supplies

Alternative options for water supply are somewhat limited in this arid region. Use of piped groundwater supplies from the resources available on the margins of the basin is an option, though careful monitoring of groundwater for other trace elements is important.

Concentrations above the respective WHO guideline values for trace elements such as fluoride, uranium, manganese, boron and molybdenum have been found in some groundwaters from the area.

Unlike in Bangladesh, accessible deep groundwaters are not a viable option for low arsenic drinking water in the worst affected areas of the Huhhot Basin.

Publications

Smedley, P L, Zhang, M, Zhang, G, and Luo, Z.  2003.  Mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the Huhhot Basin, Inner Mongolia.   Applied Geochemistry, 18, 1453–1477.

Ashanti Region in Ghana has been a centre of major gold-mining activity since the late 19th century. One of the main mining towns is Obuasi.

The principal gold-bearing ore is arsenopyrite (FeAsS) and the mining activity is known to have given rise to substantial airborne arsenic pollution from the ore-roasting chimney in the town as well as riverborne arsenic pollution derived from nearby tailings dams.

A project funded by the UK government’s Department for International Development (DFID) was undertaken between 1992 and 1995(DFID Project R5552) in collaboration between BGS and the Water Resources Research Institite, (WRRI), Accra, Ghana.

The DFID project involved sampling of streams, shallow dug wells and tubewells used for drinking water in a 40 by 40 km area around Obuasi town. Samples of deep groundwaters (70 – 100 m depth) from mine exploration boreholes as well as mining effluent were also collected.

Results

Arsenic concentrations in water from streams, shallow wells and boreholes were found to range between <2 and 175 μg/L. The main sources are mine pollution and natural oxidation of sulphide minerals, predominantly arsenopyrite.

Streamwaters have apparently been most affected by the mining activity and contain some of the highest arsenic concentrations observed. They are also of poor bacteriological quality. Some of the streams have relatively high As(III) concentrations(As(III)/As_T>0.5), probably as a result of methylation and reduction reactions mediated by bacteria and algae.

Concentrations of arsenic in groundwaters reach up to 64 μg/L, being highest in deeper (40 – 70 m depth) and more reducing (Eh 220 – 250 mV) waters. The arsenic is thought to build up as a result of the longer residence times undergone by groundwaters and the increasingly reducing conditions in the deeper parts of the aquifer.

The proportion of arsenic (As) present as As(III) is also higher in the deeper groundwaters. Deep mine exploration boreholes (70 – 100 m) have relatively low arsenic concentrations of 5 – 17 μg/L, possibly as a result of arsenic adsorption onto precipitating hydrous ferric oxides or to localised low concentrations of arsenic-rich sulphide minerals.

Publications

Smedley, P L, Edmunds, W M and Pelig-Ba, K B.  1996.  Mobility of arsenic in groundwater in the Obuasi gold-mining area of Ghana: some implications for human health. In: Appleton, J D, Fuge, R and McCall, G J H (editors), Environmental geochemistry and health: with special reference to developing countries.   Geological Society Special Publication 113, 163–181.

Smedley, P L.  1996.   Arsenic in rural groundwater in Ghana. Journal of African Earth Sciences, 22, 459–470.