5. Inorganic substances

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5. Inorganic substances

authors: Pieter-Jan van Helvoort, Mike Edmunds (Oxford University), Jasper Griffioen (TNO)

Contents

5.1 Introduction 5.2 Aluminium

5.2.1 General geochemistry 5.2.2 Aqueous chemistry

5.2.3 Natural background levels of aluminium in groundwater 5.2.4 Anthropogenic sources of aluminium

5.2.5 Aluminium in European aquifers – the Groundwater Quality Data Base 5.2.6 Summary of aluminium sources, sinks and mobility controls in aquifers 5.3 Arsenic

5.3.3 Natural background levels of arsenic in natural waters 5.3.4 Anthropogenic sources of arsenic

5.3.5 Arsenic in European aquifers – the Groundwater Quality Data Base 5.3.6 Summary of arsenic sources, sinks and mobility controls in aquifers 5.4 Chloride

5.4.3 Natural background levels of chloride in natural waters 5.4.4 Anthropogenic sources of chloride

5.4.5 Chloride in European aquifers – the Groundwater Quality Data Base 5.4.6 Summary of chloride sources and mobility controls

5.5 Mercury

5.5.3 Natural background levels of mercury in natural waters 5.5.4 High natural background levels

5.5.5 Anthropogenic sources of mercury

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5.5.6 Mercury in European aquifers – the Groundwater Quality Data Base 5.5.7 Summary of mercury sources and mobility controls

5.6 Nitrogen (Nitrate and Ammonium) 5.6.1 General geochemistry 5.6.2 Aqueous chemistry

5.6.3 Natural background levels of nitrogen in natural waters 5.6.4 Anthropogenic nitrogen sources

5.6.5 Nitrogen in European aquifers – the Groundwater Quality Data Base 5.6.6 Summary of nitrogen sources, sinks and mobility controls in aquifers 5.7 Sulphur (sulphate and sulphide)

5.7.3 Natural background levels of sulphate in natural waters 5.7.4 Anthropogenic sources of sulphate

5.7.5 Sulphate in European aquifers – the Groundwater Quality Data Base 5.7.6 Summary of sulphate sources, sinks and mobility controls in aquifers 5.8 Trace metals: Zinc, Cadmium, Copper, Lead, and Nickel

5.8.3 The mobility of Zn, Cd, Cu, Ni, and Pb in groundwater 5.8.4 Natural background levels of trace metals in natural waters 5.8.5 Anthropogenic sources of trace metals

5.8.6 Trace metals in European aquifers – the Groundwater Quality Data Base 5.8.7 Summary of trace elements sources, sinks, and mobility controls

5.9 Concluding remarks 5.10 References

5.1 Introduction

Chapter 5 aims at a state-of-the-art survey of the hydrogeochemistry of inorganic groundwater

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aqueous geochemistry and speciation

natural background concentrations and natural extremes anthropogenic sources and pollution

the occurrence in EU aquifers - the Groundwater Quality Data Base a summary table of geochemical controls and environments

All references used in this survey are filed in the Endnote Reference Base (see Chapter 2, this file is available through the CIRCA website), and are also listed in the final section of this chapter. Wherever possible, the survey is based on European groundwater studies, but in some cases others were used as well.

The data in the Groundwater Quality Data Base was used as illustration of element occurrence in groundwater across Europe, which is indicative of concentration ranges and occasionally of geochemical controls. For a quick survey and presentation, the data were organised into four main lithologies (hard rock, sandstone, carbonate and unconsolidated aquifers) and plotted as cumulative frequency diagrams. A group-wise statistical summary (selected percentiles) is reported in Appendix 2.5 and a list of medians of 27 substances for each individual aquifer is included with Appendix 2.6. Median values are therefore given for major ions and some other trace elements, for which adequate data existed, which are not classed as priority substances.

For each substance a summary diagram is given for each or the main lithologies (hard rock, sandstone, carbonate and unconsolidated aquifers). Then, for each of the individual aquifers within each group, the individual cumulative frequency curves are shown. Although these are difficult to follow, the reference to the table of medians (Appendix 2.6) can assist

interpretation.

The main goal of including groundwater data in the substance sheets was illustration of abundance and in some cases overall processes, but not derivation of NBL, which is done in other Work Packages. It is pointed out that the Groundwater Quality Data Base includes aquifers from a wide variety of hydrological and climate settings, both polluted and pristine, and its illustrative power is merely on a generic level than for the individual aquifer.

Nevertheless, the concentration ranges obtained from these plots are the first interesting results of a standardised multi-aquifer data set of Europe.

The substance sheets are found in sections 5.2 to 5.8. Section 5.9 lists some conclusive remarks on both the literature survey and the data presentation. The references are in section 5.10.

5.2 Aluminium

5.2.1 General geochemistry

Aluminium (Al) is the most abundant metal in the Earth's crust (8.4% by weight, Table 5.1), and the third most abundant element after oxygen and silica. It is lithophile, and a major

constituent of many rock-forming minerals, such as feldspars and micas, where it substitutes for Si in tetrahedral coordination with oxygen. Clay minerals, as one of the weathering products of primary rocks, contain the highest amounts of aluminium, up to 25% by weight. Weathering of alumino-silicates results in the formation of a range of secondary minerals, such as Al-goethite and gibbsite (Table 5.1).

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Table 5.1. Aluminium contents in rocks and sediments, and phosphate minerals, adapted from Hitchon et al. (1999).

Material Al content (formula)

crust 8.4 %

Igneous rocks

granite (Ca rich – Ca poor) basalt

7.2 - 8.2 % 8.3 % Sediments

shale sandstones limestones

8.0 % 2.5 % 0.4 % Aluminium minerals (other than rock-

forming minerals and clay minerals) Alunite

Al-Goethite Gibbsite Kaolinite

KAl3(SO4)(OH)6

AlOOH Al(OH)3

Al2Si2O5(OH)4

5.2.2 Aqueous chemistry

Aquatic speciation of aluminium

The Eh-pH diagram shows that at low pH, Al occurs as free Al³⁺, and at pH > 6, it forms the very insoluble Al(OH)3 solid (gibbsite, see Figure 5.1 and Figure 5.2). Several Al-OH

complexes add up to the total aqueous speciation of Al; in order of dominance with increasing pH: AlHO²⁺, Al(OH)2

+, Al(OH)4

-. Other inorganic complexes are formed with F^- and SO4 2-, but complexes with DOC or soil organic matter also exist (see e.g. Evans, 1986; Wesseling, 1996;

Weng et al., 2002), where Al competes with other trace elements. Aluminium does not take part in redox reactions at all.

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Figure 5.1. Eh-pH diagram for aluminium in the Al-O-H system, after Hitchon et al. (1999).

Figure 5.2. Al-OH complexes and the solubility of gibbsite in the Al-O-H system, after Appelo

& Postma (1993).

The mobility of aluminium in natural waters

In the pH range of most natural waters, aluminium is not mobile. However, in acidic waters (pH < 4) Al can be mobilised by dissolution of gibbsite and the accelerated weathering of both clay minerals (e.g. kaolinite) and rock-forming minerals. Aluminium concentrations in acidic waters containing considerable amounts of sulphate could also be limited by the formation of

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Al-SO4 minerals like alunogen or alunite and hydroxysulphates such as basaluminite and jarosite (Nordstrom, 1982). In some cases, Al solubility could be limited by variscite precipitation (AlPO3).

5.2.3 Natural background levels of aluminium in groundwater

Due to its ability to readily form complexes, the concentrations of Al in true solution are often overestimated. It is necessary to report concentrations of water filtered through sub micron filters (preferably 0.1µm). Compared to its high abundance in the crust, monomeric Al concentrations are very low in natural waters due to the extreme low solubility of Al-bearing minerals. Concentrations in groundwater are strongly pH dependent (Nordstrom, 1982) and it is most unlikely that high aluminium will be found in well-buffered carbonate-containing

lithologies. Hitchon et al. (1999) report a background concentration in the ppb range.

Publications from the UK (Edmunds et al., 2003), Norway (Frengstad et al., 2000), The Netherlands (Meinardi et al., 2003) and Germany (Kunkel et al., 2004) report median values in the range of 5 - 43 µg/l for pristine aquifers with quite diverse lithology. Concentrations up to the mg/l level occur in acidic, unconfined aquifers, especially consisting of unconsolidated materials and at shallow depths (e.g., Kjöller et al., 2004). Fast dissolution of alumino-silicates, clay minerals, and or gibbsite in unbuffered soils or aquifers is then the responsible process. In case of Al mobilisation, Al³⁺ could also take part in cation exchange reactions. Groundwater acidification leading to enhanced Al mobility could have several causes, including the release of acidity coincident with the oxidation of sulphides or ammonia in the subsurface, or

infiltration of acid rain.

5.2.4 Anthropogenic sources of aluminium

High groundwater aluminium concentrations are almost without exception linked to acidic conditions which may have either natural or anthropogenic causes. Industrial emissions, especially long range pollution from fossil fuel combustion has caused acidification of shallow groundwaters (Edmunds and Kinniburgh 1986) with concomitant Al mobilisation, although rainfall acidity is now becoming less of a problem. Apart from atmospheric deposition, groundwaters may be affected by acid mine drainage or leachates from colliery wastes.

Aluminium concentrations in mine drainage can reach 100's of mg/l in combination with other metals and extreme low pH's (<2).

5.2.5 Aluminium in European aquifers – the Groundwater Quality Data Base

Observations

The aluminium data of the European aquifers are summarised in Figure 5.3 and 5.4, and Table 5.2. The following observations were made from these data and plots:

Figure 5.3 shows that the aluminium concentration varies 7 orders of magnitude (0.01 - 14000 µg/l), however, this is affected by just a few outliers

the carbonate group has the lowest median concentration (0.1 µg/l), the unconsolidated

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Figure 5.3. Cumulative frequency plot for aluminium for each of the four main lithological groups. The vertical line indicates Maximum Admissible Concentration (200 µg/l) for drinking water.

Table 5.2. Selected percentiles of aluminium concentrations in European groundwaters, divided into four main aquifer groups. Percentiles include observations below detection (DL).

Aluminium (µ g/l) Cases Percentiles

Group Valid Missing 10.0 50.0 68.3 90.0 95.4 97.7

Carbonate group 260 42 DL 0.1 1.0 8.0 26.0 94.9

Hard rock group 465 61 DL 20.0 94.2 390.0 620.2 722.8

Sandstone group 453 109 DL 1.0 3.2 11.0 27.5 58.5

Unconsolidated group 1134 883 DL 0.7 7.0 41.7 97.8 289.7

Ungrouped 2324 1095 DL 1.2 7.1 80.0 260.3 501.6

Discussion

From Figures 5.3 and 5.4, it is clear that a strong relation exists between aluminium concentrations and lithology. The effect is pronounced in both the hard rock and

unconsolidated aquifer group, leading to extreme aluminium concentrations of 1000's µg/l.

However, the dominance of low pH observations in the hard rock group is significantly influenced by two UK aquifers (UK07 and UK10) with relatively acidic groundwaters, which distort the overall picture for this lithological group. On the other hand, elevated aluminium concentrations in unconsolidated aquifers are based on a large suite of observations from different aquifers (Figure 5.4), and the median reported in Table 5.2 (0.7 µg/l) may even be underestimated by a factor five due to a number of data sets with rather high detection limits (0.5 -1.0 µg/l). Generally, clay contents in unconsolidated aquifers are high if compared to other aquifer groups leading to elevated aluminium concentrations when clay minerals dissolve in response to acidification (Figure 5.5). It is anticipated that many data sets may contain overestimates of dissolved (monomeric) Al since there is no check on the filtration conditions.

Probability

Hard rock group Unconsolidated group Carbonate group Sandstone group

concentration (µg/l)

10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴

0 20 40 60 80 100

Al

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10 10 10 10 10 10 10

Probability

0 20 40 60 80 100

A05 B03 B04 (NO DATA) B05 BUG02 D03 DK01 PL05 UK01 UK05 UK08

Al

Probability

0 20 40 60 80 100

CZ01 CZ02 (NO DATA) D01

PL07 PL08 UK07 UK10 Hard rock group

Probability

0 20 40 60 80 100

D02 EE01 EE02 F02 F03 P01 PL01 UK02 UK03 UK04 UK09 Sandstone group

Concentration (µg/l)

10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴

Probability

0 20 40 60 80 100

A01 A03 A04 A06 B01 B02 B06

BUG01 (NO DATA) D04

D05 DK02 DK04 DK05 ES01 F01

GR01 (NO DATA) GR02 (NO DATA) GR03 (NO DATA) GR04 GR05 (NO DATA) GR06 (NO DATA) GR07 (NO DATA) NL01 NL02 NL03 P02

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Figure 5.5. Aluminium concentration data vs. pH. Colours indicate main aquifer group. Arrow indicates overall trend of increasing Al concentration with decreasing pH.

5.2.6 Summary of aluminium sources, sinks and mobility controls in aquifers

Table 5.3A. Sources and mobilisation processes of aluminium.

Source/ process Natural environment Anthropogenic / polluted environment

weathering of Al -silicates no specific environment dissolution of Al-oxides,

kaolinite, clay minerals

possibly acidic thermal waters (pH<4)

acidified aquifers, especially unconfined, non-calcareous aquifers only to shallow depth

aquifers affected by acid mine drainage or leachates from colliery wastes

10 9 8 7

6 5

4

pH

0.01 0.1 1 10 100 1000 10000

Al (µg/l)

Carbonate group Hard rock group Sandstone group Unconsolidated group acidification

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Table 5.3B. Sinks and immobilisation processes of aluminium.

Sink/ process Natural waters Antropog. / polluted

precipitation of secondary Al- silicates, Al-oxides, or Al- sulphates

virtually all groundwaters with pH>4

adsorption to DOC DOC rich waters DOC rich waters

5.3 Arsenic

Arsenic is a chalcophile element that occurs in minerals, mostly with a formal oxidation state of (V) (arsenates) or (III) (arsenites). Arsenic-containing minerals are dominantly (60%)

arsenates, 20% sulphides and sulpho-salts, with the remainder including arsenides, arsenites, oxides and alloys. In rock-forming minerals As(III) and As(V) can substitute for Si(IV), Al(III), Fe(III), and Ti(IV). Arsenic is adsorbed by hydrous iron oxide in (sub)oxic environments, and is co-precipitated with sulphide minerals in reducing environments.

The arsenic content in some important rock-forming minerals is given in Table 5.4. As the chemistry of arsenic closely follows that of S, the greatest concentrations occur in sulphide minerals. One of the most important As-bearing mineral is pyrite, which occurs both in ore bodies and in low temperature sedimentary environments under reducing conditions. It is present in the sediments of rivers, lakes, oceans and in many aquifers. High arsenic also occurs in metal oxides and (oxy)hydroxides, and adsorption to these minerals is very strong at lower pH values. Arsenic concentrations in carbonates, gypsum, and halite are low, and variable concentrations occur in phosphates and jarosite.

Table 5.4. Arsenic concentrations (modified from Smedley and Kinniburgh, 2002).

Mineral Range concentration (mg/kg)

Sulphide minerals (pyrite, marcasite, galena, sphalerite, chalcopyrite)

5-126000

Oxide minerals

Fe-oxide up to 2000

Fe(III) oxyhydroxide up to 76000

Carbonates 1-8

Other minerals

gypsum/ anhydrite 1-6

jarosite 34-1000

apatite 1-1000

halite 3-30

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especially in those of marine origin. Unconsolidated material is not notably different from those in their indurated equivalents, but higher concentrations are found in fine sediments (e.g. clays and till). Relative As enrichments have been recorded in reducing sediments with high pyrite contents or Fe oxides, especially in young unconsolidated sediments, such as Quaternary deltaic environments.

Table 5.5. Abundance of arsenic in rocks and sediments (modified from Smedley and Kinniburgh, 2002).

Rock type Average content or range (mg/kg)

Igneous rocks

basic (basalt) 1.5-2.3

acidic (granite) 1.3-4.3

Metamorphic rocks (schist/ gneiss) 1.1 Sedimentary rocks

shale 48-361

limestone 2.6

sandstone 4.1

Evaporites (gypsum) 0.1-10

Unconsolidated sediments

alluvial sand 1.0-6.2

alluvial clay 2.7-14.7

glacial till 1.9-170

Aquatic speciation of arsenic

Arsenic is sensitive to mobilisation at the pH values typically found in groundwaters (6.5-8.5), under both oxidising and reducing conditions. In natural oxic waters, arsenic occurs as ortho- arsenic acid (H3AsO4) and its complex anions, formed on dissociation – the monovalent arsenate anion, (H2AsO4

-), in acidic waters (pH 3-7) and the divalent arsenate anion, HAsO4 2-, in alkaline waters (pH>7). Reducing conditions favour the uncharged arsenite anion, H3AsO3, and its anions, depending on pH (see Figure 5.6).

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Figure 5.6. Eh-pH diagram for As speciation in the As-H-O-S system (Hitchen et al., 1999).

The mobility of arsenic in groundwater

The fate of arsenic in groundwater is a complex matter regarding the large variety of arsenic species found under different redox and pH conditions (Smedley & Kinniburgh, 2002).

Whereas most toxic elements occur as cations, arsenic occurs mainly as oxyanions, which tend to desorb as the pH increases. At lower pH, As(V), and to a lesser extent As(III) can adsorb to a variety of aquifer minerals, like Al-(hydr)oxides, Fe/Mn-hydroxides and clay minerals. The most important sorbents are iron-(hydr)oxides (Fe-oxyhydroxide and goethite), which have strong sorption sites and are abundant as grain coatings in many aquifers. The adsorption of As is enhanced in the presence of freshly precipitated metal hydroxides, and decreases with ageing of mineral surfaces. Weaker adsorption of arsenic is expected when competing anions such as phosphate, bicarbonate and silicate, are present in the groundwater.

The mobility of arsenic is also controlled by precipitation/ dissolution reactions. Arsenic occurs as impurities in several minerals (Table 5.4), but iron (hydr)oxides and metal sulphides (pyrite) are the most important. Arsenic forms arsenopyrite (FeAsS) under strong reducing conditions (sulphate reduction) either in aquifers associated with large amounts of fresh organic matter, or ore mineralisation in hydrothermal systems (where the temperature is typically >100°C). Under oxidising conditions and low pH, arsenate is likely to be incorporated in iron hydroxides.

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5.3.3 Natural background levels of arsenic in natural waters

Natural background levels and controls

Natural background levels of As in river waters are low (0.1-0.8 µg/l), but can range up to 2 µg/l. Variations occur due to contributions from base flow and bedrock lithology, but highest concentrations are found as a result of inputs from geothermal sources, up to 10-70 µg/l.

Background levels of As in groundwater are mostly less than 10 µg/l, and sometimes substantially lower. However, values quoted in the literature show a very large range (<0.5- 5000 µg/l) for natural conditions (see Table 5.3), depending on the presence of As-bearing minerals such as pyrite or Fe-oxides. High-As groundwater areas have been found in many regions of the world (e.g. Argentina, Chile, Mexico, China, West Bengal, Vietnam) as well as in Europe (Hungary, Spain, The Netherlands).

The concentrations of As may vary within a groundwater body as a result of time-dependent hydrochemical processes, as shown (Figure 5.7) in the UK East Midlands aquifer (Smedley and Edmunds 2001). The cross section shows a profile of As concentrations relative to Eh along a flow line of 30km from outcrop where the groundwater temperature is shown as a proxy for distance. In this aquifer, anthropogenic impacts are limited to the near outcrop and for As are negligible. The timescale of flow is around 30 kyr. Arsenic concentrations build up with time within the oxic section of the aquifer and then are less mobile in the anoxic groundwaters due to Eh and pH controlled surface reactions with iron oxides.

E h (m V )

-100 0 100 200 300 400

Temperature (

^o

C) (as proxy for distance)

10 15 20

A s ( µ g l

-1

)

0 2 4 6 8 10 12 14

Redox Boundary

groundwater flow

Figure 5.7. Downgradient changes in arsenic concentration across redox boundary in the East Midlands Triassic Sandstone aquifer (UK). N.B.: temperature used as proxy for distance.

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High natural background levels

High natural arsenic concentration of groundwater tends to be found in closed basins in arid or semi-arid areas, or in strongly reducing aquifers of alluvial and deltaic sediments with rapid burial of large amounts of sediment together with high fresh organic matter contents (Smedley and Kinniburgh, 2002). Both environments tend to contain geologically young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. Historically, these are poorly flushed aquifers and any As released from the sediments following burial has been able to accumulate in the groundwater, even when the As content of the aquifer materials is not exceptionally high (in the range of 1-20 mg/kg).

There appear to be two distinct triggers that can lead to the release of As on a large scale (Smedley and Kinniburgh, 2002). The first one is the development of high pH (>8.5) and oxidising conditions in semi-arid or arid environments as a result of the combined effects of mineral weathering and high evaporation (salinity). In such environments, As(V) predominates and arsenic concentrations are positively correlated with those of other anion-forming species such as HCO3

-, F^-, H3BO3, and H2VO4

-. The high pH leads either to desorption of adsorbed As from metal oxides, or it prevents them from being adsorbed.

Under strongly reducing conditions at near neutral pH typically found in young (Quaternary) alluvial aquifers, arsenic is mobilised by reductive dissolution of Fe and Mn oxides. Iron (II) and As(III) are relatively abundant in these groundwaters and SO4 concentrations are small (typically 1 µg/l or less). Large concentrations of phosphate, bicarbonate, silicate and possibly organic matter can enhance the desorption of As because of competition for adsorption sites. A characteristic feature of areas with a groundwater arsenic anomaly is the large degree of spatial variability in As concentrations in groundwater. This means that it may be difficult, or

impossible, to predict reliably the likely concentration of As in a particular well from the results of neighbouring wells and means that there is little alternative but to conduct intensive

investigations of As occurence. Arsenic-affected aquifers are restricted to certain environments and appear to be the exception rather than the rule.

Arsenic-rich groundwaters are also found in geothermal areas and hot springs, and are associated with the decomposition and weathering of arsenopyrite. Arsenic concentrations in thermal waters of Iceland have been found to be in the range of 50-120µg/l (White et al., 1963).

Examples of high natural background values

Hernandez-Garcia et al. (2004) report high arsenic concentrations (up to 91 µg/l) in the Madrid Tertiary aquifer, one of the largest and most important aquifers of Spain for drinking water production. The aquifer is situated in the sedimentary basin of Madrid, consisting of cemented sands, silts, and clays from Teritiary age. The deeper regions of the aquifer are considered pristine and the groundwater chemistry has been derived from water-rock interaction processes.

However, the water quality is affected by high arsenic concentrations, rising from <10 µg/l in the recharge areas to as high as 91 µg/l at discharge areas, thus showing an evolutionary trend from recharge to discharge areas. The natural arsenic contamination has its origin in pH-

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arsenic concentrations were highly variable, even in the same well at different depths. They suggested that arsenic mobilisation may be controlled by complexation by organic molecules (humic substances).

High natural arsenic concentrations (up to hundreds of µg/l) occur in groundwaters throughout The Netherlands, where large tracts of the aquifer system consist of Holocene and Pleistocene alluvium. Naturally high arsenic concentrations appear in reduced groundwaters under natural flow conditions at greater depth (typically 30-40m below surface), but also near surface in upward seepage zones along fractures or the foot of ice-pushed ridges. The main natural source of arsenic is the desorption from iron oxides, which have been formed during intensive upward seepage in the Late Pleistocene and Early Holocene. Under suboxic and oxic conditions, often a result of dewatering or the influx of contaminants, decomposition of arsenopyrite may be the most important source of arsenic (personal communication).

5.3.4 Anthropogenic sources of arsenic

Anthropogenic arsenic contamination occurs associated with acid mine drainage (AMD) and where arsenic-based pesticides have been applied. In mining areas, extreme arsenic

concentrations have been reported in highly acidic mine-effluent due to sulphide oxidation (up to 12 mg/l). Extreme arsenic concentrations are often found with heavy metals, but arsenic is readily precipitated when AMD is neutralised to pH>3. Arsenic mobilisation is also induced where dewatering of aquifers has resulted in a lowering of the groundwater table promoting pyrite oxidation. High arsenic concentrations have been found in the zone of fluctuation where water table oscillation occurs (Schreiber et al., 2000).

The application of arsenic-based pesticides may be an accessory source for arsenic. Although the mobility of arsenic in top soils is low, leaching over long timescales may increase arsenic concentrations in groundwater under arable lands.

5.3.5 Arsenic in European aquifers – the Groundwater Quality Data Base

Observations

The arsenic data for the European aquifers are summarised in Figures 5.8 and 5.9, and Table 5.6. The following observations were made from these data and plots:

Figure 5.8 shows 30 to 50% of the observations lie below the detection limits, ranging from 0.01 - 0.5 µg/l

Figure 5.8 shows that the sandstone and unconsolidated groups tend to have higher As concentrations than carbonate and hard rock aquifers and this is supported by the percentiles reported in Table 5.6

Figure 5.9 shows (see also Appendix 2.6) that the median values vary by almost 2 orders of magnitude, between 0.33 and 6.0 µg/l

Figure 5.9 shows that the lowest median concentration was in D01 (0.33 µg/l, hard rock group), and the highest in F02 (6.0 µg/l, sandstone group)

Figure 5.9 shows that in the unconsolidated group a number of aquifers (A01, B02, DK02, DK04, NL03) have As > 5.0 µg/l at the 70th percentile, which is also the case for some sandstone aquifers (D02, F02, UK03).

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Figure 5.8. Cumulative frequency plot for arsenic for each of the four main lithological groups. The vertical line indicates the maximum admissible concentration (10 µg/l) for drinking water.

Table 5.6. Selected percentiles of arsenic concentrations in European groundwaters, divided into four main aquifer groups. Percentiles include observations below detection (DL).

Arsenic (µg/l) Cases Percentiles

Group Valid Missing 10.00 50.00 68.30 90.00 95.40 97.70

Carbonate group 259 43 DL 0.20 0.40 1.00 1.81 4.16

Hard rock group 291 235 DL 0.02 0.20 1.84 3.06 4.83

Sandstone group 509 53 DL 1.04 2.63 11.14 19.17 26.86

Unconsolidated group 1060 957 DL 0.45 1.18 8.98 19.19 34.01

Ungrouped 2131 1288 DL 0.38 1.00 6.95 15.00 26.73

Discussion

The arsenic concentrations in the European aquifers are generally low, as the large amount of observations below detection limit suggest. The higher median values in sandstone and unconsolidated aquifers are indicative of an elevated natural background concentration compared to the other lithology groups. The As data in the groundwater quality base seem to support the fact that a higher natural background is more common in young sediments, e.g.

Neogene unconsolidated aquifers, but also suggest that this may be the case in sandstone aquifers of older age (Cretaceous, Triassic). The cumulative frequency plots in Figure 5.9 show that high As aquifers are best separated from others using the 70th percentile, which is close to 2σ (68.3%). The EU limit for drinking water, shown on the plots, is 10 µg/l and about 10% of

concentration (µg/l)

10^-2 10^-1 10⁰ 10¹ 10²

Probability

20 40 60 80 100

As Hard rock group

Unconsolidated group Carbonate group Sandstone group

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Figure 5.9. Cumulative frequency plots for arsenic for the individual aquifers. Aquifer details are in Appendix 2.3, median values in Appendix 2.6.

10^-2 10^-1 10⁰ 10¹ 10²

Probability

0 20 40 60 80 100

A05 B03 B04 B05 BUG02 D03 DK01 PL05 UK01 UK05 UK08 Carbonate group

As

Probability

0 20 40 60 80 100

CZ01 CZ02 (NO DATA) D01

PL07 PL08 (NO DATA) UK07 UK10 Hard rock group

Probability

0 20 40 60 80 100

D02 EE01 EE02 F02 F03 P01 PL01 UK02 UK03 UK04 UK09 Sandstone group

Concentration (µg/l)

10^-2 10^-1 10⁰ 10¹ 10²

Probability

0 20 40 60 80 100

A01 A03 A04 A06 B01 B02 B06 BUG01 D04 D05 DK02 DK04 DK05 ES01 F01

GR01 (NO DATA) GR02 (NO DATA) GR03 (NO DATA) GR04 GR05 (NO DATA) GR06 (NO DATA) GR07 (NO DATA) NL01 NL02 NL03 P02 PL02 PL03 (NO DATA) PL04 (NO DATA) PL06 UK06

Unconsolidated group

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5.3.6 Summary of arsenic sources, sinks and mobility controls in aquifers

Table 5.7A. Sources and mobilisation processes of arsenic.

Source/ process Subsurface conditions Natural environment Anthropogenic / polluted.

environment anion competition and

mineral weathering

high to near neutral pH under aerobic conditions

arid regions with high evaporation rates

desorption due to reduction of As(V) to As(III)

change to reducing conditions

river valleys and deltas with high burial rates and organic C desorption due to reduction

of surface area of oxide minerals by “ageing”

near-neutral pH under aerobic conditions

?

desorption due to reduction binding strength by reductive dissolution of oxides

strong reducing conditions river valleys and deltas with high burial rates and organic C mixing/ dilution alkaline, high temperature

and

geothermally influenced groundwater dissolution of pyrite by

oxidation

a change to aerobic conditions

Mining (Acid Mine Drainage), lowering of groundwater table

- contaminant source application of

arsenic-based pesticides

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Table 5.7B. Sinks and immobilisation processes of arsenic in groundwater.

Sink/ process Subsurface conditions Natural environment Anthropogenic / polluted.

environment adsorption acidic to near neutral pH

under aerobic conditions

fluvial aquifers Mining areas

co-precipitation with metal sulphides

sulphate reducing conditions

river valleys and deltas with high burial rates and organic C;

hydrothermal systems

5.4 Chloride

Chloride is lithophile and the most abundant halogen in the continental crust. In nature, it accumulates in terminal water reservoirs (oceans, and closed-basin lakes), because it behaves as a conservative substance. As a result, three quarters of crustal chloride occurs in the oceans. In igneous rocks, it probably replaces OH^- in apatites, micas, and hornblendes. Evaporites are the main source of chloride (see Table 5.8), where it forms soluble salts such as halite (NaCl) and potassium chloride (KCl).

Table 5.8. Chloride contents in rocks and unconsolidated materials (Hitchon et al., 1999).

Material Cl content

crust 130 ppm

Igneous rocks

granite (Ca rich – Ca poor) basalt

130-200 ppm 55 ppm Sediments

shale sandstones limestones

180 ppm 10 ppm 150 ppm Evaprorites (salts)

KCl NaCl

46 % 60 %

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Aquatic speciation of chloride

Chloride occurs predominantly as the free Cl^- ion, and due to its strong electronegativity, it is not affected by pH and redox conditions. In addition, chloride does not form insoluble salts, except at extreme salinities. At higher ionic strengths, chloride can form complexes with many metals including, Na, K, Ba, Ca, Cd, Cu, Ag, Zn, Hg, Mn, Fe, Pb, and Sr.

The mobility of chloride in groundwater

Chloride is extremely mobile and its mobility is not dependent on pH or redox conditions, neither does it form insoluble salts under environmental conditions.

5.4.3 Natural background levels of chloride in natural waters

Natural background levels

Chloride is an inert constituent and concentrations are derived either from rainfall with a degree of evaporative concentration during recharge. The background Cl concentrations are also likely to be strongly linked to distance from coastlines in relation to the decreasing influence of the deposition of marine aerosols. Background levels of chloride concentrations in shallow unpolluted aquifers will be limited to a few tens of ppm. Baseline concentrations higher than this will be influenced either by non-marine formation evaporites, marine or non-marine formation waters or modern sea water. These may be readily distinguished using the Br/Cl ratios as indicators (Edmunds 1996; Herczeg and Edmunds 1999).

The UK East Midlands Triassic sandstone aquifer (Figure 1) illustrates well how the natural baseline may vary with time/distance along flow lines and also the extent of anthropogenic influence can be easily recognised (Edmunds et al.1982). The Cl concentrations are dominated by the rainfall input signature over the timescale of 30kyr as confirmed by use of ³⁶Cl isotope studies (Andrews et al 1994). These studies illustrate that spatial/temporal information must be taken into account as well as purely statistical information.

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Temperature ( ^oC) as proxy for distance

10 15 20

Cl (mg l-1 )

0 50

100 Redox

Boundary a)

b)

c)

Direction of flow

Figure 5.10. Chloride concentrations in groundwater along a flow line in the East Midlands Triassic sandstone aquifer (temperature as proxy for distance from outcrop and the timescale is around 30000 years). Concentrations near outcrop (a) show the influence of recent pollutants;

the horizontal line indicates maximum concentrations from modern local rainfall after allowing for evapotranspiration. The pre-industrial baseline concentrations (b) show the rainfall derived Cl with little or no geogenic additions. The same is true also for Cl downgradient (c) where little Cl is added from internal sources.

Kunkel et al. (2004) estimate chloride baseline concentrations for a wide variety of aquifer lithologies in Germany, ranging from 1.0 mg/l in Alpine Limestones to 106 mg/l in

unconsolidated Rhine Valley deposits. Unpolluted wells in a coastal alluvial aquifer in Mersin (Turkey) revealed Cl concentrations of about 40 mg/l (Demirel, 2004). Edmunds et al. (2002) report a median background concentration of 21 mg/l for a Chalk aquifer in Dorset (UK) with some seasonal variation for shallow groundwaters (Schürch et al., 2004). Unpolluted sand aquifers in the Netherlands have 26 mg/l Cl for groundwaters older than 50 years (Meinardi et al., 2003 RIVM rapport)

Table 5.9. Chloride concentrations in water (adapted from Hitchon, 1999).

Water type chloride (mg/l)

rain water 3.8 stream water 7.8

groundwater 1 – 250

groundwater (polluted) up to 15000 groundwater (maximum) up to 180000 formation water (maximum) up to 403000

thermal water up to 30000

High natural background levels

High chloride concentrations in groundwater (see above) are due to additions from internal sources – either marine formation waters, recent sea water of dissolution of marine evaporates;

locally thermal waters may be significant. Elevated chloride concentrations (e.g. ~200-250 mg/l; Banaszuk et al., 2005) may be found in shallow groundwaters directly under the plant

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root zone due to intensive evapotranspiration; this is likely in forested ecosystems and in semi- arid regions. Some other examples of brines, thermal circulation, and water-rock interactions (or combinations) are given below.

Conti et al. (2000) give an overview on the geochemistry of formation waters in the Po plain (northern Italy). The presence of brackish groundwaters results from mixing of meteoric water with deep-seated brines and subsequent upwelling to the surface due to a weak geothermal circulation system. Extreme chloride concentrations in the deep groundwaters influenced by brines reach up to 118600 mg/l, whereas shallow groundwaters with a larger share of meteoric water may have up to ~8000 mg/l.

Bein & Arad (1992) discuss saline groundwaters occurring in the Baltic region through freezing of seawater during glacial periods. The brines typically occur in crystalline and metamorphic rocks below fresh groundwater in various localities in Sweden and Finland. High total

dissolved solids (up to 20 g/l in Sweden, and 120 g/l in Finland) are indicators of high chloride concentrations up to 16800 mg/l. The brines are subsequently gradually diluted by fresh post- glacially infiltrated groundwater. Another example of very high salinity groundwaters was reported by Grobe & Machel (2002). In the Münsterland Cretaceous Basin (Germany) where chlorine concentrations up to 62000 mg/l resulted from evaporite (halite) dissolution.

In granitic rocks there is the possibility that some Cl addition results from water-rock interaction with rock-forming minerals rich in volatiles. Such cases are infrequent, but

described for example from the Carnmenellis Granite (Cornwall, UK) by Edmunds et al (1984).

Cl concentrations up to 11500 mg/l are found in thermal waters (around 54°C) and the saline groundwaters are encountered in tin mines, and attributed to hydrolysis of Cl-rich biotites in the thermal aureole.

Hot thermal waters as a source of high Cl concentrations have been discussed by many workers in several countries. A good example comes from Valentino & Stanzione (2003), who describe some important Cl sources for groundwater associated with thermal activity: 1. inflow of magmatic HCl gas into deeply circulation groundwaters; (2) intensified rock leaching due to high temperatures; (3) uptake of seawater or marine components in powerful deep circulation systems. The combined effects of these processes led to very high Cl concentrations (up to 18150 mg/l) in thermal groundwaters beneath the Phlegraean Fields (Naples, italy).

5.4.4 Anthropogenic sources of chloride

There are many anthropogenic sources of chloride in groundwater, but in most cases Cl

concentrations do not exceed 1000 mg/l (Hitchon, 1999). Ford and Tellam (1994) list industrial and domestic waste, sewage, road de-icing salts,and processing chemicals as the main sources of Cl in shallow groundwater (up to 235 mg/l) of the Birmingham urban aquifer, UK. Edmunds et al. (1982) attributed chloride concentrations of 50 – 174 mg/l to either induced recharge of nearby rivers or local pollution of the East Midlands Triassic sandstone aquifer. Bank filtrated Rhine water in the Netherlands has led to Cl concentrations up to 160 mg/l (Stuyfzand, 1989).

Stigter et al. (1998) suggest that high chloride concentrations (~350 mg/l) under irrigated fields in Campina de Faro (southern Portugal) are caused by the return flow of irrigation water into

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groundwaters have increased a factor 2 – 4 under arable land (~40-70 mg/l) with respect to older uncontaminated groundwaters (10–30mg/l) (Meinardi et al., 2003).

There are numerous studies on man-induced seawater intrusion due to over-exploitation of coastal aquifers causing major problems in most, if not all Mediterranean countries (Italy, Spain, Turkey, Greece, France, Cyprus), but to some extent in the coastal provinces in countries with temperate climate as well. The problem of over-exploitation increases rapidly and is most profound in semi-arid regions due to ever-increasing urban developments and expanding resorts as built in the coastal plains. Chlorine concentrations easily reach 1000 mg/l and this type of pollution can be easily recognised using major ion chemistry and stable isotopes of water.

5.4.5 Chloride in European aquifers – the Groundwater Quality Data Base

Observations

The chloride data from the present survey are summarised in Figures 5.11 and 5.12, and Table 5.10. The following observations were made from these data and plots:

Figure 5.11 shows that chloride concentrations cover 5 orders of magnitude (0.3-10000 mg/l);

Figure 5.11 and Table 5.10 show that hard rock and carbonate aquifers have the lowest median concentrations (15.9 and 18.3 mg/l respectively), unconsolidated the highest median (32.3 mg/l), and sandstone intermediate (23.0 mg/l);

Figure 5.12 shows (see also Appendix 2.6) that the medians of individual aquifers vary over 3 orders of magnitude (lowest: A05 - carbonate group, 0.59 mg/l; highest: GR07 - unconsolidated group, 3347.7 mg/l), but the majority of medians are well within 1 order of magnitude (10 - 50 mg/l);

Figure 5.12 shows bi modality or multi modality within all groups, but is profound in the hard rock and unconsolidated groups;

Figure 5.12 shows that extreme high chloride concentrations tend to occur from the 90th percentile;

Figure 5.12 shows some individual aquifers with very low overall chloride

concentrations and strong bimodality (CZ01, CZ02, PL07, PL08, EE02, GR06), aquifer A05 has extreme low chloride contents.

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Figure 5.11. Cumulative frequency plot for chlorine for each of the four main lithological groups. The vertical line indicates maximum admissible concentration for drinking water.

Table 5.10. Selected percentiles of chlorine concentrations in European groundwaters, divided into four main aquifer groups. Percentiles include observations below detection (DL).

Chloride (mg/l) Cases Percentiles

Group Valid Missing 10.0 50.0 68.3 90.0 95.4 97.7 Carbonate group 302 0 6.2 18.3 26.0 51.6 58.9 77.7 Hard rock group 525 1 1.5 15.9 25.0 39.0 53.0 64.6 Sandstone group 543 19 3.6 23.0 35.2 105.7 209.5 352.8 Unconsolidated group 2006 11 2.0 32.3 62.0 415.1 2062.5 5567.8 Ungrouped 3388 31 2.1 24.7 42.0 213.0 655.4 2773.2

concentration (mg/l)

10^-1 10⁰ 10¹ 10² 10³ 10⁴

0 20 40 60 80 100

Cl Hard rock group

Unconsolidated group Carbonate group Sandstone group

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Figure 5.12. Cumulative frequency plots for chlorine for the individual aquifers. Aquifer details are in Appendix 2.3, median values in Appendix 2.6.

10^-1 10⁰ 10¹ 10² 10³ 10⁴ 10⁵

Probability

0 20 40 60 80 100

A05 B03 B04 B05 BUG02 D03 DK01 PL05 UK01 UK05 UK08 Carbonate group

Cl

Probability

0 20 40 60 80 100

CZ01 CZ02 D01 PL07 PL08 UK07 UK10 Hard rock group

Probability

0 20 40 60 80 100

D02 EE01 EE02 F02 (NO DATA) F03 P01 PL01 UK02 UK03 UK04 UK09 Sandstone group

Concentration (mg/l)

10^-1 10⁰ 10¹ 10² 10³ 10⁴ 10⁵

Probability

0 20 40 60 80 100

A01 A03 A04 A06 B01 B02 B06 BUG01 D04 D05 DK02 DK04 DK05 ES01 F01 GR01 GR02 GR03 GR04 GR05 GR06 GR07 NL01 NL02 NL03 P02 PL02 PL03 PL04 PL06 UK06

Unconsolidated group

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Discussion

Figure 5.13 shows that the chloride concentration goes from rain water composition (0.5 - 4 mg/l) to dilute seawater (>10000 mg/l). Although chloride is a conservative element and is not involved in chemical reactions in the subsurface, there is a large diversity of statistical

distribution within and between groups (Figures 5.11 and 5.12). As indicated earlier chloride input to aquifers depends strongly on regional factors such as climate (evaporation factor and precipitation rate), geographic location (distance from the sea), the degree and type of pollution (high or low anthropogenic input), and whether an aquifer is confined or non-confined

(sensitivity to external input).

The regional factors will complicate the identification of natural background values and

possible anthropogenic components. For instance, the range of minimum chloride concentration in the UK aquifers (4.4 - 17.4 mg/l) is higher than found in the aquifers of central Europe - Czech Republic, Bulgaria, Germany, south-east Poland (0.6 - 7.8 mg/l). This suggests that the geographical distance from the sea and regional climate translates into different natural background concentrations, even for pristine aquifers with comparable lithology.

The aquifers affected by mixture with saline formation waters or brines can be clearly identified from Figure 5.13, all having very high Cl concentrations (> 500 mg/l) close to the Cl/Na ratio of 1.8 in seawater. The affected aquifers are predominantly young unconsolidated coastal aquifers containing a fraction of residual marine water, showing increased Cl

concentrations from the 90th percentile. These aquifers are intensively exploited for abstraction (eg. GR04 - Athens alluvial aquifer; GR01 - Argolida coastal aquifer) or host (connate)

seawater (NL03 - Holland sandy aquifer group; DK02 - Ribe sands aquifer).

0.1 1 10 100 1000 10000

Cl (mg/l) 0.1

1 10 100 1000 10000

Na (mg/l)

Carbonate group Hard rock group

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5.4.6 Summary of chloride sources and mobility controls

Table 5.11A. Sources and mobilisation processes of chloride.

Source/ process Natural environment Anthropogenic / polluted environment

dissolution of evaporites evaporites -

degassing of magma deep thermal waters seawater intrusion sea level changes coastal

aquifers

over exploited coastal aquifers

thermal circulation thermal waters leaching, rainwater

infiltration, atmospheric deposition

uinconfined aquifers unconfined aquifers in urban and agricultural regions

evapotranspiration

Table 5.11B. Sinks and immobilisation processes of chloride.

Sink/ process Natural waters Antropog. / polluted

there are no sinks for chlorine in groundwater

5.5 Mercury

Mercury is chalcophile and occurs in three valencies: Hg⁰, Hg⁺, and Hg²⁺. The average abundance in the upper crust is 0.08 ppm, and it is enriched in intrusive magmatic rocks and locations of subaerial and submarine volcanism. Therefore, high mercury abundance in surface and near surface rocks mirrors zones of current and past tectonic activity mainly found along tectonic plate boundaries (so called “global mercury belt”). High Hg contents (0.4 ppm) are also found in shales, resembling the affinity for clay minerals and organic matter. Cinnabar (HgS) is the most important mercury mineral and is very insoluble (logK = -36.8). Zero valency Hg is volatile and is emitted to the atmosphere by volcanic degassing, coal combustion and natural emission from biochemical processes.

Aqueous speciation of mercury

The divalent state of Hg would dominate in most natural waters, however, Hg⁰ is the most stable form in a broad Eh/pH range (Figure 5.14). The mercurous ion (Hg⁺) is not stable under environmental conditions, since it decomposes into Hg²⁺ and Hg⁰. Divalent mercury forms strong complexes with hydroxy ligands, (Hg(OH)2), sulphur ligands (with increasing pH:

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Hg(SH)2, HgS2H^-, HgS2

2-), as well as chloride (with increasing Cl concentration: HgCl⁺, HgCl2, HgCl3

-, HgCl4

2-) - see Figure 5.15. Common species in natural waters, but not mentioned in the Eh-pH diagram are methyl mercury species (CH3Hg⁺, CH3HgOH, CH3HgCl, CH3HgS^-,), all of biogenic origin. Solubility-limiting species under oxic conditions could be the hydroxides, although saturation would be never reached. The sulphide (HgS) limits the solubility to the ng/l level in reduced sulphur environments. In absence of sulphide, Hg⁰ (gaseous species) limits solubility, and many surface waters are close to saturation leading to Hg⁰ degassing to the atmosphere (Fitzgerald & Lamborg, 2004).

Figure 5.14. Eh – pH diagram for Hg in the Hg-O-H-S system (from Hitchon, 1999).

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The mobility of mercury in groundwater

The mobility of Hg is, like other heavy metals, mainly controlled by adsorption to solid particles and organic substances. The association of the Hg²⁺ ion with organic matter is very strong, and would be dominant when compared to adsorption to mineral surfaces (Schuster, 1991). Thus, the mobility of Hg in most soils is expected to be rather low. However, Hg²⁺

mobility is increased by ligands in solution (Cl^-, OH^-), and organic anions. The organic anions form strong complexes with mercury and have a weak positive or even negative net charge, thus diminishing the adsorptive tendency of Hg in water. Under reducing conditions and where sulphide is present, the mobility of Hg is likely to be determined by the formation of highly insoluble HgS, since the activity of free Hg²⁺ remains too low to exceed the solubility product of any other Hg solid. In thermal groundwaters, however, Hg would be more mobile due to increased solubility of HgS at high temperatures and salinity.

The mobility of Hg is also enhanced by methylation, a biochemical process leading to the formation of the very volatile monomethyl mercury (HgCH3

+) and dimethyl mercury

(Hg(CH3)2) compounds (MacLeod et al., 1996). Methylation of Hg is enhanced in waters and sediments with low oxygen levels, low pH, and in the presence of sulphur-reducing bacteria.

The methylated Hg species are entirely produced by microbial conversions and highly toxic for most life, including human.

Compared to other metals, Hg has an exceptionally low boiling point of 357°C, and therefore it readily vaporises at low temperature. The vaporisation of Hg to the atmosphere is an important link in the global mercury cycle.

5.5.3 Natural background levels of mercury in natural waters

Natural background levels

The average concentration of mercury in natural surface waters is very low (0.07 µg/l in surface water, 0.001 µg/l in seawater). For crystalline bedrock groundwaters in Norway, Frengstad et al. (2000) reported a median Hg concentration of 0.018 µg/l and a maximum concentration of 0.13 µg/l, which was associated with Permian intrusive igneous rocks. In an extensive survey of German groundwaters, Kunkel et al. (2004) report background concentrations ranging from 0.03 µg/l in sandstone to 0.56 µg/l in unconsolidated sediments. Thus mercury has a very low natural aqueous abundance and anomalies in normal groundwaters are not to be expected.

5.5.4 High natural background levels

Murphy et al. (1994) measured mercury species in 78 potable wells in southern New Jersey, and found that Hg⁰ and HgCl2

0 were the main species. Total mercury concentrations reported here as background values were in the range of 0.001 - 0.042 µg/l. A compilation of literature values by Allard (1995) revealed a wide range of background concentrations from a variety of geohydrological settings (0.0001 – 2.8 µg/l), but it is not clear whether or not these represent solely natural values.

Grassi & Netti (2000) found Hg concentrations up to 11.2 µg/l (drinking water is 1.0 µg/l) in some coastal alluvial aquifers in Tuscany (Italy), where increased groundwater abstraction has led to seawater intrusion. The elevated chloride concentrations enhance dissolution of mercury minerals originally present in the aquifer material by the formation of stable Hg-Cl complexes such as HgCl2

-, HgCl3

-, HgCl4

2-, and HgBrCl^-. In a comparable study on Tuscany aquifers