Contamination of groundwater

Although aspects of groundwater chemistry have been discussed elsewhere in this chapter, this section highlights issues relating to the contamination of ground-water. Groundwater is critically important to humans since it is a major source of drinking water. For example, in the USA over 50% of the population rely on groundwater as a source of drinking water. Groundwater quality is therefore very important and, in most developed countries, water must conform to certain standards for human consumption. Groundwater may fail to meet water quality standards because it contains dissolved constituents arising from either natural or anthropogenic sources. Typical anthropogenic mechanisms of groundwater contamination are shown in Fig. 5.16. In the USA, major threats to groundwater include spillage from underground storage tanks, effluent from septic tanks and leachate from agricultural activities, municipal landfills and abandoned hazardous waste sites. The most frequently reported contaminants from these sources include nitrates, pesticides, volatile organic compounds, petroleum products, metals and synthetic organic chemicals.

The chemistry of contaminated groundwater is little different from that of surface waters, except that most groundwaters are anaerobic. This has posed a major problem for remediation of benzene (C6H6), one of the most prevalent organic contaminants in groundwater and of concern because of its toxicity. Although much is known regarding aerobic degradation of benzene (see Section 4.10), until recently no pure culture of an organism capable of anaerobic degradation of benzene existed, which made remediation almost impossible. It is now known, however, that the bacterium Dechloromonas aromatica strain RCB oxidizes benzene anaerobically, using nitrate as the electron acceptor.

Municipal landfill

Leachate

Leaking septic tank

Agricultural leachate (pesticides, fertilizers)

De-icing chemicals, oils

Leachate

Leaking septic tank

Agricultural leachate (pesticides, fertilizers)

De-icing chemicals, oils

Fig. 5.16 Typical anthropogenic mechanisms of groundwater contamination.

Accidental spillage

Leaking petroleum tank

Abandoned oil well

Leaking petroleum tank

Abandoned oil well

Industrial landfill leachate

(benzene)

In addition to benzene this bacterium also oxidizes toluene, ethylbenzene and xylene and thus offers great potential for the treatment of petroleum-contaminated aquifers (Section 5.7.1).

Natural degradation processes that occur in days or weeks in surface waters may take decades in groundwater, where flow rates are slow and microbiological activity is low. This limits the potential for natural purification through flushing or biological consumption. Once contaminated, groundwater is difficult and expensive—in many cases impossible—to rehabilitate.

The location of older sites of contamination may be imprecisely known, or even unknown, and hydrogeological conditions may dictate that contaminated groundwater discharges at natural springs into rivers or lakes, spreading contamination to surface waters.

The following sections highlight different styles of groundwater contamination where chemical considerations have proved important. Nutrient element contamination of groundwater was discussed in Section 5.5.1.

5.7.1 Anthropogenic contamination of groundwater

Landfill leakage — Babylon, Long Island, New York, USA

At Babylon landfill site, New York, shallow groundwater contamination of a surface sand aquifer has resulted from leakage of leachate rich in Cl-, nitrogen compounds, trace metals and a complex mixture of organic compounds. Land-filling began in the 1940s with urban and industrial refuse and cesspool waste. The refuse layer is now about 20 m thick, some of it lying below the water table. Chloride behaves conservatively (see Section 6.2.2) and is thus an excellent tracer of the contaminant plume, which is now about 3 km long (Fig. 5.17).

Close to the landfill, most nitrogen species are present as NH4+, indicating reducing conditions resulting from microbial decomposition of organic wastes. With increasing distance from the landfill, NO- becomes quantitatively important due to the oxidation of NH+ (Fig. 5.12), brought about by mixing of the leachate plume with oxygenated groundwater. This demonstrates how nitrogen speciation can be used to assess redox conditions in a contaminant plume.

Reducing conditions within the leachate plume also cause metal mobility, particularly of manganese and iron. The plume near the landfill has a pH of 6.0-6.5 and is reducing (-50 mV), making Fe2+ stable (Box 5.4). The transition to oxidizing conditions down gradient in the aquifer allows solid iron oxides (e.g. FeOOH) to precipitate, dramatically, reducing the mobility of metals which co-precipitate with iron.

This relatively inoffensive example illustrates the importance of redox conditions in contaminated groundwater. Worse scenarios are known where toxic chlorophenolic compounds in very alkaline groundwaters (pH 10) ionize to neg-

Line of equal chloride concentration, 1974—dashed where approximately located. Interval 50 and 100 mg l-1

Line of equal chloride concentration, 1974—dashed where approximately located. Interval 50 and 100 mg l-1

Landfill deposits

Fig. 5.17 Map of Cl plume at 9-12m depth below the water table, Babylon landfill site, 1974, showing the extent of groundwater contamination. After Kimmel and Braids (1980).

atively charged species and become much more mobile than in the neutral conditions generally typical of groundwater.

Petroleum contamination — Bowling Green, Kentucky, USA

Although groundwater flow rates are generally low when compared to surface waters, large cracks and conduits in some contaminated aquifers cause specific problems. The US city of Bowling Green, Kentucky, is built on limestone bedrock (Ste Geneviève limestone), with underground drainage through the Lost River Cave (Fig. 5.18). Limestone bedrock is often heavily fissured and joints in the rock are enlarged by dissolution, resulting in interconnected caves. Sinkholes may divert surface streams into these fissures and caves, resulting in a subsurface drainage system. Accidental spillage of toxic chemicals or any other contaminant is rapidly dispersed in these conduits, making remediation particularly difficult.

In the 1970s and 1980s up to 22 000 litres of petroleum leaked from storage tanks at auto service stations in Bowling Green into the subsurface water. Petroleum, being a non-aqueous phase liquid (NAPL), floats on the surface of groundwater and its volatile components (see Box 4.14), for example benzene, rapidly fill air spaces with explosive fumes, particularly at sumps or traps in the cave system (Fig. 5.18). The trapped fumes then escape into the basements of buildings, water wells and storm drains.

In addition, leaking tanks at a chemical company are believed to have delivered benzene, methylene chloride (CH2Cl2), toluene (C6H5CH3), xylene (C6H4(CH3)2) and aliphatic hydrocarbons (see Section 2.7) to the subsurface water. These toxic (some carcinogenic) chemicals vaporize in the cave atmosphere, collect at traps and then rise into homes in a similar way to petroleum fumes.

The potential explosive/toxicity risk in Bowling Green has resulted in a number of evacuations of homes in the last 20 years. Remediation measures have

Toxic and/or explosive fumes from trapped chemicals may rise into

Toxic and/or explosive fumes from trapped chemicals may rise into

Lost River

Cave entrance Solution enlarged fractures

Distance (km)

Fig. 5.18 Cross-section through the Lost River Cave drainage system underlying the city of Bowling Green, Kentucky, showing potential trap for floating or gaseous contaminants. After Crawford (1984). With permission from Swets & Zeitlinger Publishers.

Lost River

Cave entrance Solution enlarged fractures

Distance (km)

Fig. 5.18 Cross-section through the Lost River Cave drainage system underlying the city of Bowling Green, Kentucky, showing potential trap for floating or gaseous contaminants. After Crawford (1984). With permission from Swets & Zeitlinger Publishers.

included better storage tank containment, regular monitoring of cave conduit outlets and ventilation of basements in homes at risk. It is hoped that these will prevent a disaster such as occurred in nearby Louisville, Kentucky, where an underground sewer explosion travelled 11 blocks, causing damage estimated at over 43 million dollars.

5.7.2 Natural arsenic contamination of groundwater

Those who have seen Victorian melodramas will know that arsenic is a common and effective poison; murderers have used the oxide (As2O3) successfully for the last two millennia. Ingestion of just 20 mg of the oxide is said to be lethal (but see Box 5.5), caused through damage to the stomach and intestines; lower exposures cause cancers. Given this macabre background it is perhaps obvious that exposure to arsenic in foodstuffs and drinking water should be low. Although tiny amounts of arsenic occur in some foods, it is typically a water-soluble form of organic arsenic that is easily excreted. Moreover, most drinking water contains much less than 5 mgl-1 of inorganic arsenic, such that typical daily intakes are about 4 mg.

Arsenic contamination of drinking waters by industrial and commercial activities is not particularly commonplace, although arsenic is still used in pesticides in some underdeveloped countries. Surprisingly, however, natural arsenic contamination of groundwater is now well known, and today affects areas of Argentina, Taiwan, Vietnam, Cambodia, China, Hungary, Bangaladesh, India and the USA. The symptoms of low-level arsenic poisoning are dependent on dose received, but can take up to 10 years to develop, while cancers may take 20 years. It is therefore important to identify arsenic contamination of water supplies quickly.

Nearly all cases of large-scale arsenic contamination in groundwater are caused by reduction of iron oxides in aquifer sediments.

Microbial reduction of arsenic-bearing iron oxides

Reduction of iron oxide (FeOOH) is a potential source of arsenic to groundwa-ter because it releases arsenic adsorbed to the oxide surfaces. A representation of this process might be:

4FeOOH(s) + CH2O(s) + 7H2CO3Uq)-> 4Fe?+,) + 8HCO-Uq) + 6H2O(„

The reaction requires anoxic conditions, since iron is barely soluble in oxic water (Box 5.4), and is fuelled by the microbial metabolism of organic matter (depicted as CH2O in eqn. 5.26).

This mechanism of arsenic release to groundwater has recently been proposed as the cause of high arsenic concentrations in millions of deep drinking water wells across Asia, the example in the Ganges-Meghna-Bramaputra delta plain of Bangladesh and West Bengal being much publicized. The original source of the arsenic in these aquifer sediments is not known with much certainty, but probably comes from weathering of arsenic-rich coals and sulphide ores in the upstream drainage basin. The arsenic was then transported downstream in solution, adsorbing to clay-rich and organic-rich sediments that accumulated in the delta over the last 2 million years or so.

Many of these drinking water wells, which provide 20 million people with almost all their drinking water, exceed both the World Health Organization (WHO) guideline value of 10 |igl-1As and the Bangladesh drinking water maximum of 50 |igl-1 As. The world's press have recently picked up on this problem, first discovered in the mid 1990s, with headlines claiming 'the largest mass poisoning of a population in history'. It is feared that up to 20000 people could soon die each year as a delayed result of accumulating arsenic in their bodies from wells sunk up to 25 years ago.

The highest levels of contamination (>250 |igl-1As) typically occur between 25 and 45 m depth—too deep to implicate sulphide oxidation (Section 5.4.2) — and concentrations in excess of 50 |igl-1As occur down to 150 m. The requirement for microbial metabolism of organic matter has recently led scientists to link the presence of highly organic peat beds in the deltaic aquifer sediments as the driver for equation 5.26. Clearly other sedimentary aquifers that host peats or organic-rich muds might be vulnerable to arsenic contamination, including other large deltas such as the Mekong and Irrawaddy.

As the release of arsenic requires reducing conditions, a logical application of environmental chemical principles suggests that aeration of the water might reverse the effects of equation 5.26, for example:

This would force iron oxide (Fe(OH)3) to precipitate, but also remove some of the dissolved arsenic as it adsorbs to the precipitating oxide surface. Trials at water-treatment plants in the area have shown that water with 220 |igl-1As, can be reduced to around 40 |igl-1As (below the Bangladesh drinking water maximum) by this method. Longer-term mitigation strategies might include the construction of shallower wells in the oxidized upper zone of the aquifer. Whatever the long-term mitigation strategies are, it is clear that these wells are currently critical for the water supply of millions of people. As abandoning the wells is not an option in many cases, short-term mitigation methods, even imperfect ones such as aeration with a stick in a bucket, seem preferable to continued exposure to high levels of arsenic.

It is worth noting that an 'early warning' of potential arsenic contamination by reduction of iron oxide is given by high values of dissolved iron in reducing drinking water wells. This element is simple to analyse, and routinely measured during water quality analysis. It could therefore be used as a sign that arsenic contamination is possible, prompting further analysis for dissolved arsenic.

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