ANNEX 1 THEMATIC INTERPRETATION OF STREAM WATER …weppi.gtk.fi/publ/foregsatlas/articles/Annex 1 old.pdf · the interpretation of the hydrochemistry, but also to illustrate the results

ANNEX 1

THEMATIC INTERPRETATION OF STREAM WATER CHEMISTRY

E. L. Ander, B. Smith and S. Reeder

British Geological Survey, Keyworth, Nottingham, UK

Introduction

The natural composition of stream waters

across the FOREGS region has been shown to

vary considerably in the single analyte images,

which provide a great deal of information on the

changing baseline for each analyte. However,

natural waters are a complex mixture of species,

arising from initial recharge (rainfall) components

and subsequent water – mineral interactions.

Thus, the understanding of individual analyte data

can be enhanced by undertaking the simultaneous

assessment of more than one, related, parameter in

relation to a controlling geochemical process.

Such an interpretation relies upon the

comprehensive analytical suite collected by the

FOREGS-EuroGeoSurveys project, and an

understanding of geographical and geological

variations across this region of Europe.

The purpose of the calculations presented in

this section is not only to provide greater depth to

the interpretation of the hydrochemistry, but also

to illustrate the results in such a way that the

output information can be conveyed to a wider

audience than those already familiar with these

geochemical methods. The benefits of a

systematic survey, with a wide range of analytical

parameters, are found in the ability to undertake

these types of interpretations. This is because

many ions which are not in themselves of especial

concern, arising from naturally elevated

concentrations or anthropogenic inputs, can be

crucial in controlling the behaviour of trace

elements, or understanding reaction pathways.

The interpretation of data at this regional scale

has focused on the systematic variations observed

over wider areas, rather than concentrating on

isolated anomalous samples, which may have an

over-riding influence from small scale local

sources (natural or anthropogenic). Whilst large

datasets, such as this, are frequently used to make

assessments of hydrochemical concentration

variations due to anthropogenic inputs

(contamination), much of the work in this section

has been undertaken with a focus on

hydrochemical variations arising from natural

sources and reaction paths. This is because it is

important to understand the controls on major-ion

chemistry, and parameters such as pH, in natural

waters, in order to then understand how these

conditions control natural concentrations

(baseline) of other analytes, and how any

anthropogenic inputs may be attenuated by

precipitation or sorption, or rapidly flushed

through the aqueous sytem as highly soluble

species. Detailed description of these and other

geochemical reaction processes, and associated

thermodynamic data, can be found in key

references such as Appelo and Postma (1994),

Stumm and Morgan (1996) and Langmuir (1997).

The thematic studies have been undertaken by

looking at 10 different hydrochemical and

mineralogical controls, which can be summarised

as either reflecting major ion chemistry, mineral

saturation controls or aqueous speciation.

Data manipulation and presentation

The interpretative calculations and methods

used in this chapter are described in detail in the

appropriate sections. These methods vary from the

study of solubility using just two or three

parameters, to more complex modelling using

programmes designed to allow the simultaneous

calculation of all key species of interest.

It is important to note the detailed post-

455

analysis checking which was undertaken on the

data, in order to ensure that there is validity to the

raw data and, thus, any data derived from it. The

rigorous quality control procedures (already

described) applied to the comprehensive

analytical suite for each stream water sampling

site in the FOREGS project have enabled the data

to be used as the input parameters for these more

detailed analyses with some confidence.

The results of applying these methods to the

stream water data have been summarised as either

ternary or x-y plots, with a thematic shading

applied to the data according to the relative

position of each data point on the graph, which

has previously been applied in other systematic

mapping areas (e.g., Smith et al. 1998). The

ternary plots are based on the Piper Plot, widely

used in the assessment of groundwater chemistry

(Appelo and Postma 1994) and previously used

for the study of regional stream water chemistry

data (e.g., Smith et al., 1998). This thematic

shading has then been mapped for the FOREGS

region, using the colour derived for each point,

plotted in its geographical location. Thus, the

graph acts as the key for the map in each case,

which are otherwise presented in the common

FOREGS format. These thematically coloured

images use a red-green-blue scale for the ternary

plots, and a simple shading between two

differently coloured end-points for the binary

plots. Further details are provided in the

appropriate sections below.

Figure 1. Trilinear predominance diagram for the cations in stream water.

Major ion chemistry

Cation predominance

An understanding of the relative abundance of

the major cations (Ca, Mg, Na and K) and the

major anions (HCO3-, SO4

2-, Cl- and NO3-) in

natural waters provides useful indicative

information on the key geochemical processes in

streams. This section describes the analysis of the

cation data using these methods, and there is a

separate discussion of the anion dominance. These

derived data promote a greater understanding of

the mineral-water interactions which control the

regional stream water chemical variations, and

provide an indication of the controls on the

speciation (and thus potential toxicity) of trace

elements, irrespective of their total concentration.

The cation predominance map has been

constructed to illustrate the relative abundance of

the major cations in water across the FOREGS

sampling region. The concentration of these ions

has been compared to the total sum of the major

cations in each sample. The results have been

plotted thematically according to the percentage

dominance of the different ions.

456

Construction of the trilinear and mapped cation

predominance diagrams

The concentrations of the major cations in

water, Ca, Mg, Na and K, have been converted

from the measurement units of mg l-1, into

equivalence units (meq l-1), which takes into

account the charge on the molar concentrations

These cationic species are expected to account

for >90% of the cations in most natural waters

irrespective of their total concentration, and are

thus summed as if they represent the whole water

composition. The proportion of that total cation

equivalence is calculated for Ca, Mg and, by

convention, Na plus K (Hem 1992). The result of

this process is then plotted as the trilinear diagram

shown in Figure 1, where each axis varies from 0

– 100% abundance.

In order to represent the variation in relative

abundance of the cations across the study area, the

position of the samples on the predominance

trilinear diagram is further represented by a

gradational colour scheme; 100% Ca is red, 100%

Mg is green and 100% Na-K is blue. This is based

on the methods of Smith et al. (1998), enabling

data to be projected onto a regional map. The

trilinear diagram can then be used not only to

show the range of compositions encountered

during the FOREGS sampling (on the triangular

diagram), but also the location of the different

water types, by projecting the appropriate colour

from the trilinear diagrams onto each FOREGS

sample location, as demonstrated in Figure 2.

Interpretation of the cation predominance

trilinear diagram and map

Maps also considered: Ca, Mg, Na, K,

electrical conductivity. The geological

information is taken from Ager (1980).

Figure 1 shows that the majority of the 808

FOREGS samples for which there is a full suite of

major ion analyses, are dominated by Ca, and are

thus ‘calcium-type’ waters. A smaller proportion

of samples have a predominantly Na+K cationic

composition, and only two samples are >50% Mg.

These relative abundance relationships are

independent of concentration.

Figure 2 shows the map of the trilinear

diagram projected onto location of the FOREGS

samples. It can be seen that the areas of relative

Ca dominance closely follow the areas of higher

Ca concentration across south-east Britain, the

Aquitaine Basin and the north-east of France, and

the North European Plain (north-east Germany

and north Poland), the Baltic states and Croatia,

which are dominated by Mesozoic and Cenozoic

carbonate and carbonate-cemented sediments.

These areas are characterised by stream waters

which are frequently >75% Ca, and almost

ubiquitously >50% Ca. The Carboniferous

Limestones of central Ireland can clearly be seen

as Ca dominated waters. The predominance of Ca

in the area of Greece may arise from the

weathering of ophiolites as well as from

sedimentary strata. Very low Ca proportions can

be seen over the Palaeozoic strata of southern

Portugal and Spain, which contrasts with the

generally high Ca concentrations that can be seen

on the Ca stream water map (Part 1, p. 129).

Despite the low concentrations of Ca in the stream

waters sampled in Sweden, Norway and Finland,

it can be seen that in some instances Ca is the

predominant ion in those very low total dissolved

solid (TDS) samples.

The release of Ca and Mg will take place

rapidly from calcite (or aragonite) in soils and

aquifers, as the kinetics of the reaction (illustrated

below for low-Mg calcite) are rapid (Appelo &

Postma 1994).

Cax(Mg1-x)CO3 + H2O + CO2

xCa2+ + (1-x)Mg2+ + 2HCO3

This reaction is important in pH buffering, and

where calcite is abundant will maintain pH values

from circum-neutral to typically slightly alkaline.

This, however, is not the only source of Ca to the

stream environment, with the weathering of Ca

bearing silicates assuming a greater significance

in aquifers with a very low abundance of calcite

(primary or secondary mineralisation). Minerals

such as plagioclase feldspars and pyroxenes

dissolve incongruently to release Ca (and other

cationic species) into solution, accompanied by

the precipitation of a secondary silicate phase, as

exemplified below by anorthite (Appelo and

Postma, 1994).

CaAl2Si2O8 + 2H+ Al2SiO5(OH)4 + Ca2+

In some environments, sufficient Ca

concentrations can be reached to exceed calcite

saturation and result in the precipitation of

secondary calcite. It is thus expected that the

significant proportions of Ca in the low Ca

concentration waters of Scandinavia and Finland

457

are likely to arise from the weathering of Ca

containing silicates.

The relative dominance of Mg is less than that

of Ca or Na+K, with a minority of samples shown

on Figure 1 and 2, as having Mg as the major

cation. The sample draining basalt in Northern

Ireland can be seen as having more predominant

Mg. The ophiolites of Greece may be responsible

for the more elevated proportions of Mg that can

be seen there, due to the weathering of Mg-

bearing minerals, such as olivine and pyroxene.

The areas with very low Mg proportions are

generally associated with the Mesozoic and

Cenozoic carbonates, which are frequently very

low-Mg calcites, which is in contrast to the

generally moderate absolute concentrations which

can be seen in the Mg interpolated image.

The areas which are dominated by Na+K can

be seen as blue data points on Figure 2. These can

be seen over the crystalline basement lithologies

on the coasts of northern Scotland, Norway, the

north of Ireland, south-west England, central

Wales, Brittany, the Iberian Massif (Spain and

Portugal) and south-west Portugal. These acid

crystalline lithologies are expected to contain a

greater abundance of Na and K bearing silicates

(e.g., alkali feldspars), which when undergoing

incongruent dissolution will give rise to Na and K

ions in solution (cf. the example for anorthite

above) (Appelo and Postma 1994). However, the

geographical location, and sometimes elevation of

these areas, means that they are greatly affected

by the higher rainfall on the Atlantic seaboard and

the higher wet and dry deposition of Na in

precipitation in these areas. The influence of a

marine source for these regions may be gauged

from the available annual average rainfall and

wind-roses for these areas (which show a

predominantly south-westerly source) (Figure 3

and 4) (IIASA 2005, Troen and Petersen 1989).

These areas with proportionally elevated Na+K

abundances, can be seen to be almost wholly

independent of total concentrations, if Figure 2 is

compared to the Na stream water map (Part 1, p.

341), and generally reflect areas of low total

dissolved solids (as shown by the electrical

conductivity image, Part 1, p. 219).

Anion predominance

The relative abundance of the major anions

(HCO3-, SO4

2-, Cl- and NO3-) is shown here, to

provide additional interpretation of the controls on

stream water chemistry than can be obtained from

the ion concentration maps alone. This

information is complemented by that calculated

for the major cations (Ca2+, Mg2+, Na+ and K+),

discussed separately (above). These derived data

promote a greater understanding of the mineral-

water interactions which control the regional

stream water chemical variations, and provide an

indication of the controls on the speciation (and

thus potential toxicity) of trace elements,

irrespective of their total concentration.

The map has been constructed to illustrate the

relative abundance of the major anions in water

(HCO3-, SO4

2-, Cl- and NO3-) across the FOREGS

sampling region. The concentration of these ions

has been compared to the total sum of the major

anions in each sample. The results have been

plotted thematically according to the

predominance of the different ions.

Construction of the trilinear and mapped anion

predominance diagrams

The concentrations of the major anions in

water, HCO3-, SO4

2-, Cl- and NO3-, have been

converted from the measurement units of mg l-1,

into equivalence units (meq l-1), which takes into

account the charge on the molar concentrations

(Hem 1992).

These anionic species are expected to account

for >90% of the total anionic species in most

natural waters irrespective of their total

concentration, and are thus summed as if they

represent the whole water composition. The

proportion of that total anionic equivalence is

calculated for HCO3-, SO4

2- and, by convention,

Cl- plus NO3- (Hem 1992). The result of this

process is then plotted as the trilinear diagram

shown in Figure 5, where each axis varies from 0

– 100% abundance.

In order to represent the variation in relative

abundance of the anions across the study area, the

position of the samples on the predominance

diagram is further represented by a gradational

colour scheme; 100% HCO3 is red, 100% SO4 is

green and 100% Cl-NO3 is blue. This is based on

the methods of Smith et al. (1998), enabling data

to be projected onto a regional map. The trilinear

diagram can then be used not only to show the

range of compositions encountered during the

458

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Figure 3. Annual precipitation in Europe (IIASA

2005).

Figure 4. Simplified wind roses for two stations in France (a)

Brest and (b) Caen (Modified after Troen and Petersen 1989).

FOREGS sampling, but also the location of the

different water types, by projecting the colours

from the trilinear diagrams onto the FOREGS

sample locations, as demonstrated in Figure 6.

460

Interpretation of the anion predominance

trilinear diagram and map

Maps considered: HCO3-, SO4

2-, Cl- and NO3-,

electrical conductivity. The geological


Figure 5 shows the anion predominance of the

808 samples for which there were complete major

ion analyses. It can be seen that the great majority

(67%) lie in the range of HCO3- 50%, and can

thus be considered of the ‘bicarbonate type’.

Smaller subsets are of sulphate or chloride type. A

large number of samples are also represented by

intermediate brown-red colouration – these show

no predominance of one ion over another. It

should be remembered that this diagram is

independent of total concentration of any of these

ions. The relative position of these samples is

projected onto a map of the FOREGS sampling

area to allow the spatial variation to be

understood, as well as the range of compositions

shown in Figure 6.

The mapped data (Figure 6) shows systematic

variations across Europe, which can be

understood in terms of regional geological and

geographical terrains. Isolated occurrences of

some water types may also indicate more locally

significant processes, rather than those occurring

across wider regions of Europe. A selection of

these will be described below.

Despite the very low TDS nature of the waters in

Fennoscandia (as shown on the electrical

conductivity map) and the low concentration of

HCO3- in solution, it can be seen from Figure 6

that HCO3- predominates in north-central Sweden

and Finland. The HCO3- is likely to be derived

largely from atmospheric CO2(g) equilibrium and

biological respiration, rather than any dissolution

of carbonate minerals (also indicated by the large

undersaturation with respect to calcite – see

Figure 10). This implies that there is not a large

reservoir of HCO3- as would be expected in a

carbonate aquifer. The same process may be

responsible for the moderate proportions (~50%)

over the granites of the Massif Central. As

described for the cation predominance, it is

possible for very localised calcite (CaCO3)

precipitation to occur where concentrations of

both Ca and HCO3- are such that the waters

become super-saturated with respect to calcite.

The HCO3- dominated outcrop areas of

Aquitaine, north-east France, Estonia, Latvia and

Lithuania, the Pannonian Basin (Hungary), the

North European Plain and south-east England are

related to the occurrence of Mesozoic and

Caenozoic carbonate and carbonate cemented

sediments, and are generally coincident with high

or moderate absolute concentrations of HCO3- in

stream waters. The area of the Alps in Austria,

Figure 5. Trilinear predominance diagram for the anions in

stream water.

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northern Italy and Slovenia and the Dinaric Alps

of Croatia can be seen to be dominated by HCO3,

despite the low to moderate total concentrations of

HCO3- in stream waters measured there. Older

limestone, such as the Carboniferous of central

Ireland, can be seen to dominate the aqueous

chemistry as is found with more recent calcareous

strata. It is expected that these concentrations of

HCO3- are primarily controlled by the dissolution

of calcite (or other carbonate minerals). The

reaction kinetics are rapid for the dissolution of

calcite, and equilibrium is soon attained.

Concentrations may also be elevated by biological

respiration processes in soils and the unsaturated

zone, which raise the CO2(g) partial pressure and

increase the solubility of calcite (Appelo and

Postma 1994).

Mafic igneous rocks are responsible for the

high proportion of HCO3 found in solution in

north-east Ireland (Tertiary basalt), central Italy

(mafic volcanics) and ophiolites may be included

in the predominance of HCO3- in Greece (as well

as the widely occurring limestones). The high

HCO3- concentrations are a result of reactions

between Ca2+ derived from the (congruent or

incongruent) dissolution of mafic minerals within

these strata and CO2(g), which may be increased

in the soil zone by biological respiration.

There are very few waters which have a

significant proportion of SO42-, and those that

have are generally isolated in occurrence. In the

case of central Spain this is likely to be due to the

weathering of evaporites (especially gypsum)

within the Cenozoic sediments. The other major

source of SO42- as a weathering product is likely

to be drainage arising from pyrite (FeS2)

oxidation, which may elevate concentrations in

individual river systems, both as a natural

occurrence and enhanced as a result of mining

activities and weathering of mine waste debris.

The locations where Cl dominated waters are

found are characteristically associated with the

Atlantic seaboard of Europe, such as south-west

Britain, north-west France, Donegal (Ireland), and

south-west Norway. In these areas the geology is

largely composed of low carbonate lithologies

(e.g., granite and greywacke) and the maritime

climate from the prevailing south-westerly storm

direction from the Atlantic makes for increased

deposition (wet and dry) of Na and Cl- (IIASA

2005, Troen and Petersen 1989), and as shown by

wind-rose and annual precipitation data (Figure 3

and 4). These samples are often associated with

low total dissolved solid (TDS) concentrations

(indicatively illustrated by electrical

conductivity), where the composition of rainfall is

far more significant than it is at more distal

locations where water-rock interaction is greater,

or mineral dissolution reaction rates are higher

(e.g., calcite). The very high TDS samples from

southern Portugal and south-west Spain can be

seen to be clearly dominated by Cl despite their

anomalously high concentrations of all the

individual anions on the interpolated images. In

low-lying coastal areas of the North Sea fringe,

where recent sediments often overlay older

Caenozoic sedimentation, saline incursion and

mixing to produce brackish waters is likely to be

responsible for the dominance of Cl, typically

around eastern England, western Netherlands and

Denmark.

Sodium and Chloride ratios

The molar ratio of Na to Cl has been plotted

for the FOREGS stream water chemistry. This

ratio is expected to be largely controlled by sea

water composition, and thus close to 0.85, in

coastal regions. Chloride is a conservative

element in solution, very rarely subject to

precipitation as secondary phases in active stream

environments, and rarely released in significant

quantities by mineral dissolution. In some areas

natural brine springs with very high

concentrations of Cl may occur, but these are rare

on a (FOREGS) regional scale. In coastal areas

Na is expected to be largely controlled by the

contribution of proximal wet and dry deposition.

This relative contribution may be altered with

distance from marine sources of rainfall, as

sorption processes, mineral weathering and dry

deposition play a greater role.

Thus the presentation of the Na:Cl map allows

some indicative conclusions to be drawn about

these major ions, and whether their concentrations

are greatly affected by water-rock interaction

processes in different European environments.

Comparison with the sea water ratio

The average concentration in seawater for

these two elements is as follows (Hem 1992):

Cl – 19,000 mg l-1, which is equal to 536

mmol l-1.

463

Na – 10,500 mg l-1, which is equal to 457

mmol l-1.

Thus the seawater molar Na:Cl ratio is 0.85,

and this is the ratio that would be expected in

diluted, but unreacted, marine derived recharge to

streams and aquifers (irrespective of the actual

concentrations and degree of dilution), and is the

ratio found in coastal Europe (Appelo and Postma

1994). Increasing values of the ratio suggest an

additional source of Na or depletion of Cl- (which

is unlikely given the generally conservative

behaviour of Cl-). A decrease in the value of the

ratio suggests the removal of Na or addition of

extraneous Cl-. The ions may be deposited via wet

and dry deposition from sea water (aerosols). The

effect of these aerosols can be enhanced by

changes in the dominant vegetation cover, with

coniferous forests increasing concentrations in

relation to deciduous and low scrub vegetation

(Appelo and Postma 1994). When rainfall

becomes more distal from marine areas, the

increase in ions such as Ca and Na is due to the

increasing proportion of continental dust

incorporated in precipitation, which can be

significant in relation to the marine derived

proportions of these elements (Appelo and Postma

1994).

Water-rock interaction processes may affect

the Na:Cl, by increasing the relative proportion of

Na by the dissolution of Na bearing silicates and

evaporites, and decrease it by the loss of aqueous

Na due to ion-exchange reactions. An example of

this is the plagioclase feldspar, albite

(NaAlSiO3O8), a common component of igneous

rocks, and readily weathered. The incongruent

reaction of albite to kaolinite releases Na into

solution (Appelo and Postma 1994):

2NaAlSi3O8 + 2H+ 9H2O

2Na+ + Al2Si2O5(OH)4 + 4H4SiO4

Ion exchange reactions may occur between the

recharge of precipitation to the soil and aquifer

zones and its discharge to a stream network.

Where recent marine sediments are subject to

subaerial weathering, it is possible that the cation

exchange sites are dominated by Na. The

dissolution of calcite will release Ca2+ into

solution, which freshens the aquifer by release of

Na+ into solution (Appelo & Postma, 1994):

½Ca2+ + Na-X ½Ca-X + Na+

The dissolution of evaporites with Na and Cl

occurring as halite (NaCl), would result in an

aqueous molar ratio of 1. However, this is not the

case when either is released from the dissolution

of other evaporite minerals such as Na2CO3 or

KCl. Brines may also occur in sedimentary

formations, such as Coal Measures sequences,

which may have a Na:Cl value different to that of

sea water.

Interpretation of the Na:Cl graph and mapped

image

Additional maps considered: Na and Cl-.

Geological information is taken from Ager

(1980).

Figure 7 shows the Na and Cl data plotted on a

logarithmic scale, with a gradational colour

scheme employed according to whether the stream

water ratio is greater than that of sea water (red),

the same or less (blue). It can be seen that whilst

most data points plot relatively close to the line,

the logarithmic scale indicates that differences

become very large for some samples. The colour

scheme used in Figure 7 has been projected onto

the FOREGS sample locations in Figure 8, and

Figure 7 should be used as a key for Figure 8.

The mapped data (Figure 8) shows that the

Na:Cl is most similar to that of sea water in many

of the coastal regions of north-west Europe. The

high rainfall (Figure 3) in low-lying coastal areas

such as the Amorician Massif (France), the west

of Ireland, the North Sea and Baltic coasts appear

to have a Na:Cl little altered by water-rock

interaction.

In contrast Scandinavia is characterised by

excess Na, apart from southernmost Sweden (it

should be noted that this area has very low total

concentrations of Na and Cl- and the ratio

calculations may be affected by increased

uncertainty in the concentration values). There is

also a systematic variation within Fennoscandia,

where the north of Sweden and Finland have

greater Na excess than does the rest of the area.

Northern and western Scotland is also of a similar

ratio distribution to Norway. These areas of high

relief and high rainfall may represent mineral

weathering sources of Na to the stream waters,

which are more important than those of the lower-

lying areas. In the drier areas of northern Sweden

and Finland it is also possible that some dry

deposition of Na bearing particulates is increasing

the ratio value, as noted by Appelo and Postma

(1994) for Kiruna in the north of Sweden.

464

The elevated, granite and base-poor sediment

dominated areas of the north-west Iberian

Peninsula and Massif Central appear to represent

areas where Na is increased by release from

mineral phases, to increase the Na:Cl ratio above

that of sea water. The Italian Alps show the

greatest proportional increase of Na (with

northern Sweden and Finland) and these must

reflect a considerable amount of remobilisation of

Na particulates and/or Na-bearing silicate

weathering.

Equally of interest, although difficult to

explain, are the large areas of inland France,

Germany and Poland which show either an excess

of Cl or a depletion of Na. Extraneous sources of

Cl may arise from industrial processes such as

coal burning or the leaching of pore fluids or

evaporite salts that are enhanced in Cl with

respect to Na. Depletion of Na suggests that a

significant amount of precipitation or ion-

exchange must be taking place. Concentrations

are sufficiently elevated that these ratios will not

be an artefact of analytical error. It is not clear

what the major mechanism controlling these

concentrations are, and whether it is due to natural

geochemical processes, or affected by

anthropogenic activity.

Figure 7. Comparison of Na and Cl concentrations for

FOREGS stream water sites.

Solute saturation and mineral stability

Calcite solubility

This map takes into consideration theconcentrations of Ca and HCO3 and the pH value for all FOREGS stream water sites which have all three measurements. These values are comparedto those which would theoretically exist, if thestream water were to be in thermodynamicequilibrium with calcite, thus showing streamswhich are undersaturated, in equilibrium with, or supersaturated with respect to calcite. The resultshave been plotted thematically according thesaturation state.

These calculations enable an assessment of the regions where calcite is at, or above, the equilibrium value and thus considered to have a

carbonate controlled pH buffering environment, or undersaturated which indicates that silicate weathering is likely to be an important pH buffering mechanism. Calcite buffering of pH is likely to protect against regional, diffuse acidification sources and has implications fortrace element solubility and mobility. Those which are undersaturated are likely to representalumino-silicate pH buffering environments, andthey may be more susceptible to larger fluctuations in stream water pH. These variations can be related to the geological variations seen on the European scale, with base-poor lithologies such as granites contrasting strongly with the carbonate rich sediments seen in many Mesozoic basins.

465

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Calculation of calcite solubilityThe solubility of calcite, at a fixed temperature

and pressure, is a function of Ca concentration

and that of HCO3-, as affected by the partial

pressure of CO2(g) with which it is in equilibrium.

These interactions are also controlled by, and

affect, the system pH. These procedures

previously have been used to map stream water

data obtained in Wales and part of England in the

UK (British Geological Survey 1999, Smith et al.

1998).

Thus, at 15°C, the following equations

describe the equilibrium reactions between

dissolved inorganic carbon and a calcite phase:

[HCO3-] [CO3

2-] + [H+] log K = -10.43

CaCO3 [Ca2+] + [CO32-] log Kcalcite = -8.43

thus:

2

-8.42

Ca

10 =

H

HCO10 3

33.10

Which using the prefix ‘p’ to denote –log10,

8.42 - pCa = 10.33 + pHCO3-

or

2pH – pCa = pHCO3- + pH + 2.00

Thermodynamic data from Busenburg and

Plummer (1982). The stream water data have been

plotted according to this equation onto Figure 9.

These data are thematically coloured according to

whether they are above or below the equilibrium

concentrations, in relation to the equations above.

They are represented on a blue (oversaturated) to

red (undersaturated) colour scale. The

thematically coloured data points in Figure 9, are

then projected onto the geographical location for

each site in Figure 10. Thus Figure 9 should be

used as the key for Figure 10.

The calculations also give a relative

assessment of pCO2 changes, as the solubility of

calcite is intimately related to the concentration of

CO2(g). An indication of this is shown in Figure 9.

It should be noted that temperature has not been

varied for these calculations (i.e., calculated at

15oC). Calcite solubility has a temperature

dependence, which can be generalised as

decreasing solubility with increasing temperature,

this is illustrated on Figure 9 by plotting the

location of calcite equilibrium at 5oC and 25oC in

addition to 15 C. This trend of solubility reduction

is likely to be a result of the decreasing solubility

of CO2(g) in warmer waters (Krauskopf and Bird

1995). The geographical scale of the FOREGS

sampling may reasonably be expected to have

encountered systematic temperature variations

from the north of Scandinavia to the

Mediterranean region in the south. Thus, it may be

estimated that samples in the north of the mapped

region will be more undersaturated than indicated

by the calculated figures, and those in the south

will be more oversaturated than indicated by the

calculated figures (which used a temperature of

15oC, because temperature is a parameter that was

not recorded at the time of sampling).

Interpretation of the calcite solubility graph and

mapped image

Maps also considered : pH, Ca and HCO3. The

geological details have been taken from Ager

(1980).

Figure 9 shows that a wide range of calcite

saturation states occur throughout the FOREGS

sampling area. The samples are evenly distributed

between those which are undersaturated and those

which are supersaturated. The undersaturated

condition can be exp lained by insufficient Ca to

reach equilibrium with calcite. This is usually as a

result of a low abundance of highly soluble Ca-

bearing salts (sulphates or carbonates primarily),

although the weathering of some Ca containing

silicates may result in equilibrium with calcite

being reached (Appelo and Postma 1994). The

geographical distribution of the calcite saturation

state can be seen in Figure 10.

Streams flowing over the crystalline basement

lithologies of Scandinavia and the north of

Scotland can be seen to be systematically

undersaturated with respect to calcite. These strata

are generally devoid of carbonate minerals and

have low concentrations of Ca in the major rock

forming alumino-silicate minerals of which they

are composed, thus restricting the importance of

carbonate buffering of the pH system.

Undersaturation is also observed over granites,

such as those of south-west England, Brittany, the

Massif Central and the north-west Iberian

Peninsula. The base-poor sediments of central

Wales, the north-east of Ireland and the Black

Forest (Germany) are also clearly distinguishable

as areas of general undersaturation with respect to

calcite. When compared to the pH image, it can be

seen that these areas are characterised by low

467

values, confirming that pH buffering by carbonate

equilibrium is not an important process, and

emphasising the vulnerability of such waters to

decreases in rainwater pH (naturally 5.6) or other

sources of H+(aq).

Areas where calcite saturation is an important

control in the aqueous chemistry can be seen

where limestones and carbonate-rich sediments

occur, such as the Baetics (Spain), the Aquitaine

and Paris Basins (France), south-east Britain, the

Pannonian Basin (Hungary) and Lithuania, Latvia

and Estonia. In the North European Plain

(northern Germany and Poland) large-scale liming

of the siliceous soils for agriculture causes calcite

saturation in water. Mafic igneous rocks can also

show equilibrium with calcite, due to the

weathering of Ca bearing silicates (e.g., anorthite)

and this can be observed over the mafic volcanics

of the Naples-Rome region (Italy) and the basalts

of Antrim (Ireland), and the widespread ophiolites

of Greece.

Gypsum solubility

Gypsum is a soluble salt of Ca and SO4, which

occ

(equilibrium) or supersaturation

(ex

urs as a primary mineral in evaporite deposits,

and as a secondary mineral in other sedimentary

sequences. A simple calculation, based on the

concentrations of Ca and SO42- measured in the

FOREGS stream water samples, has been used to

predict whether the systems are in equilibrium

with this mineral. The results have been plotted

graphically, and the relationship between the

actual data and the theoretical limits on solubility

have been projected onto the map of the FOREGS

atlas region.

Saturation

cess Ca and SO42-) would indicate that gypsum

may be an important control on the solubility of

these elements, and may limit concentrations of

these ions in solution.

pH + pHCO3

6 8 10 12 14 16

4

6

8

10

12

14

16Supersaturated

Undersaturated

calcite (CaCO

3)

equilibruim

Increasing pCO2

Temperature = 5 ºC

Temperature = 15 ºC

Temperature = 30 ºC

Figure 9. Calcite saturation index for FOREGS stream water data

468

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Calculation of gypsum solubility

The calculation of the relationship of the actual

data in comparison to the theoretical limits on

gypsum solubility has been made using the

following equations and data (Appelo and Postma

1994):

CaSO4 Ca2+ + SO42-; Kgypsum = 10-4.60 @ 25ºC

The solubility product (Kgypsum) has been

compared to the measured data by conversion of

the concentrations into molar units to calculate the

IAP (ion activity product):

IAPgypsum = [Ca2+] x [SO42-]

The IAP is then compared to the solubility

product to produce a ratio (the saturation index,

SI) which describes whether the sample is

supersaturated (SI > 0), in equilibrium (SI = 0) or

undersaturated (SI < 0).

SI = log [IAPgypsum / Kgypsum]

The data have been plotted in Figure 11,

thematically coloured to show the proximity of

the data to the theoretical equilibrium line. Thus

which are red are supersaturated. These colours

have been transferred onto the mapped image

(Figure 12) (as illustrated in British Geological

Survey 1999) to illustrate the geographical

distribution of these values across the FOREGS

sampling area.

blue data points are undersaturated, and those

nterpretation of the gypsum solubility

grO4 and pH. The

geo

aqu

I

aph and mapped imageMaps considered: Ca, S 2-

logical information is taken from Ager (1980).

Figure 11 shows the FOREGS stream water

eous Ca and SO4 plotted, and compared to the

equilibrium saturation condition of gypsum. These

data demonstrate that the majority of stream water

samples measured were undersaturated with

respect to gypsum. The undersaturated condition

can be explained by other controls on the

concentration of Ca and SO42- in solution, which

includes insufficient sources of these elements

from the parent lithologies, or lower solubility

minerals limiting the concentration of either ion in

solution (such as calcite). The supersaturated

condition can be explained by the complexation of

2+

-6 -5 -4 -3 -2 -1

4

-6

-5

-4

-3

-2

-1Supersaturated

Undersaturated

gypsum(CaSO

4.2H2O)

equilibruim

Figure 11. Comparison of the FOREGS stream water data with the gypsumsaturation index

470

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ions in solution increasing the apparent solubility

of ostma 1994). The

ge

Streams flowing over much of the European

area sampled by FOREGS can be seen to be

undersaturated with respect to gypsum and thus

this mineral phase is not expected to be an

important control on concentrations of Ca and

SO42- in those areas. The samples which are very

supersaturated are located in Spain and Sicily

(Italy), and these are most likely due to the

dissolution of evaporite minerals, which will be

expected to include gypsum, anhydrite and other

Ca and SO4 phases. There are more samples

which are very close to the equilibrium condition

(-0.5 < SI < 0.5), and these generally appear to be

associated with the dissolution of Mesozoic

evaporite sequences. An alternative cause of

equilibrium with respect to gypsum is where the

oxidation of sulphide minerals such as pyrite is

acid generating, and leads to the dissolution of

increased Ca concentrations as a result of

buffering reactions. Thus the source of the SO42- is

the pyrite, and the source of the Ca can be

carbonate or silicate phases.

Fluorite solubility

Fluorite is a low solubility calcium mineral

(CaF2), which can prevent high concentrations of

fluoride occurring in natural waters. Where Ca

concentrations are insufficient to limit the

solubility of fluoride, it can occur in sufficiently

elevated concentrations to potentially cause

human and animal health problems. A simple

calculation, based on the concentrations of Ca and

F occurring in FOREGS streamwater samples has

been used to predict whether the systems are in

equilibrium with this mineral phase. The results

have been plotted graphically and the relationship

between the actual data and the theoretical limits

on solubility have been projected onto the map of

the FOREGS region.

Saturation or supersaturation would indicate

that fluorite is an important control on fluoride

concentrations in streamwaters. High fluoride

concentrations are of potential concern to human

and animal health, as a result of the detrimental

effect it can have on the skeletal system.

Calculation of fluorite solubility

The calculation of the relationship of the actual

data in comparison to the theoretical limits on

fluorite solubility has been made using the

following equations and data (Appelo and Postma,

1994):

CaF2 Ca2+ + 2F- Kfluorite @ 25ºC = 10-10.57

The solubility product (Kfluorite) has been




IAPfluorite = [Ca2+] x [F-]2






SI = log [IAPfluorite / Kfluorite]



the data to the theoretical equilibrium line. Thus

blue data points are undersaturated, and those

which are red are supersaturated. These colours


(Figure 14) to illustrate the geographical


sampling area.

Interpretation of the fluorite solubility graph and

mapped image

Maps considered: F, Ca and pH. Geological


Figure 13 shows the FOREGS stream water Ca

and F concentrations plotted in relation to the

fluorite equilibrium line. The diagram illustrates

that all the data are below the saturation for

fluorite, suggesting that this mineral phase is not

an important limitation on F concentrations across

the FOREGS atlas region. It can also be seen that

only one sample collected had a F concentration at

the WHO drinking water limit of 1.5 mg l-1. The

geographical distribution of the data is shown in

Figure 14.

Figure 14 demonstrates that the only samples

which are close to the equilibrium condition with

respect to fluorite are found in Spain (1 sample)

and Italy (2 samples). Those in Italy are possibly

associated with F-rich volcanic lithologies.

Comparison of the stream water map for F and

gypsum (Appelo and P

ographical distribution of these data is shown in

Figure 12.

472

Figure 13 confirms that the higher F

concentrations in stream water occur over a much

wider area, so fluorite equilibrium is generally not

a limiting process on F concentrations.

B

ment not measured in

eochemistry of

wh

uir 1997). As is the case with Ba, the

e low

sol

y SO4 , the same may

be expected for Ra.

Ca

portant limiting factor on

the aqueous transport of Ba. The calculation of the

relationship of the actual data in co

the

arite solubility

Barite is a low solubility salt of Ba and SO4,

which is expected to limit the Ba concentration in

most natural waters. A simple calculation, based

on the concentrations of Ba and SO4 measured in

the FOREGS stream water samples, has been used

to predict whether the systems are in equilibrium

with this mineral. The results have been plotted

graphically, and the relationship between the

actual data and the theoretical limits on solubility

have been projected onto the map of the FOREGS

atlas region.

Saturation (equilibrium) or supersaturation

(excess Ba and SO42-) would indicate that barite

may be an important control on the solubility of

Ba in the stream systems where this occurs. This

information can also be used to illustrate how

knowledge of the chemistry of elements with

similar geochemical behaviour can be used to

predict the mobility of an ele

solution. For example, Ra, the g

ich is analogous to that of Ba, is of interest as a

result of its presence in the 238U and 235U decay

series, with additional interest due to the release

of Rn as its progeny in these decay series

(Langm

solubility of Ra is very low in most natural SO4

bearing wat rs, due to precipitation of the

ubility phase RaSO4 (Ksp = 10-9.99), frequently

as an accessory within the BaSO4 phase (Zhu

2004). Thus, in environments where the aqueous

transport of Ba is limited b 2-

lculation of barite solubility

Barium solubility is substantially reduced in

the presence of SO4 due to the very low solubility

of barite in comparison to other salts (e.g. BaCl2),

and thus SO4 is a very im

mparison to the

oretical limits on barite solubility has been

made using the following equations and data

(Appelo & Postma 1994):

BaSO4 Ba2+ + SO42- Kbarite @ 25ºC = 10-9.97

2+

-5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

Supersaturated

ta plotteWHO

WHO drinkingwater limit

Undersaturated

fluorite (CaF2) equilibruim

Figure 13. FOREGS Ca and F daequilibrium. Guideline value from

d in relation to the fluorite (2004).

473

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2+

-9.0 -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0

4

-6

-5

-4

-3

-2

-1

Supersaturated

Undersaturated

Barite (BaSO4 ) equilibruim

Figure 15. Plot of Ba and SO aqueous data from the FOREGS

sampling

2+ 2-

4

The solubility product (Kbarite) has been




IAPbarite = [Ba2+] x [SO42-]






SI = log [IAPbarite / Kbarite]



the data to the theoretical equilibrium line. The

vertical lines of data seen at lower Ba

concentrations are due to the numerical truncation

of the data at the lower end of the detectable range

by the analytical laboratory. Blue data points are

undersaturated, and those which are red are

supersaturated with respect to barite. These

colours have been transferred onto the mapped

image (Figure 16) to illustrate the geographical


sampling area.

Interpretation of the barite solubility graph and

mapped image

Maps considered : Ba, SO42-

, Cl- and pH. The

geological information is taken from Ager (1980).Streams flowing over lithologies with low Ba

concentrations were generally undersaturated withrespect to barite. These include Scandinavia,western UK, western Ireland, the north-westIberian peninsula, the Massif Central (France), the Alps and Dinaric Alps. Isolated samples withinthese areas are close to equilibrium (-0.5 < SI<0.5), and these are due to local conditions varying from those in the larger areas. Figure 15 shows that a wide range of barite saturation states occur in the FOREGS sample data. The samplesare reasonably evenly distributed between thosewhich are supersaturated and undersaturated. The undersaturated condition is more likely to be explained by a low abundance of Ba than of themajor ion, SO4. Supersaturation can be explained by aqueous complexation increasing the apparent solubility of barite in solution (Appelo and Postma 1994). The geographical distribution ofthe barite saturation state can be seen in Figure 16. Whilst it is not suggested the water samplescollected are used for drinking water, it is worthnoting for comparison that all measuredconcentrations are below the WHO drinking water guideline value of 0.7 mg l-1 Ba (WHO 2004).

475

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Large areas of the FOREGS region are

supersaturated with respect to barite. The highest

supersaturation conditions are associated with the

dissolution of evaporite sequences, and are

coincident with the samples for which gypsum

was (super)saturated, such as in western France,

southern Spain and central Italy. Other areas may

show supersaturation for barite in the presence of

the Cl- ion (see Cl Figure), complexation with

which may increase the solubility of Ba in SO4

rich waters. In eastern UK, Benelux, northern

Germany, Denmark, Poland and the Baltic states

the area of barite supersaturation is coincident

with high SO42- and Ba concentrations, with

respect to both datasets (see Ba and SO42-

Figures). There are no areas with a relatively high

abundance of Ba and SO4 in solution which are

undersaturated with respect to barite. These data

suggest that SO4 concentrations were a limiting

factor on Ba solubility in stream waters over much

of the FOREGS atlas area, but that complexation

of ions in solution increases the Ba2+ beyond that

which would be expected by purely equilibrium

conditions occurring.

Aluminium and silicon speciation and

solubility

The map has been constructed to illustrate the

variation in likely controls on Al concentrations in

streamwaters sampled during the FOREGS

project. Information on the concentration of Al, Si

and the pH have been used to calculate the

abundance of these ions in relation to those

predicted by thermodynamic principles.

The aluminium-silicate bearing minerals are

important in the natural cycling of these ions

through the near surface environment, and may

act as, and be affected by, variations in pH and the

pH buffering environment. High concentrations of

Al in solution are detrimental to aquatic life,

particularly fish.

Calculation of silicon and aluminium solubility

The solubility of Si in a simple system may be

considered to relate only the SiO2 phase.

However, secondary quartz (or other crystalline

SiO2 phase) is not generally recorded in nature

and the dissolution of quartz is recorded to be

immeasurably slow in surficial environments

(Langmuir 1997). Thus it is considered that

precipitation of a silica phase is likely to be as

an amorphous mineral, which may slowly

recrystallise to a more crystalline state, and

4 5 6 7 8 9 10

44

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

Quartz

SiO2(am)

Soil silica

Figure 17. Comparison of FOREGS data with crystalline quartz andamorphous soil SiO solubility limits.2

477

that this is what controls the Si concentration in

soil, sediment and stream-waters (log Kquartz = -

3.7; KamorphSiO2 = -2.7) (British Geological Survey

1999). Figure 17 shows the FOREGS stream

water data plotted with the stability constants

cited above used for comparison. It can be seen

that most systems appear to be controlled by

crystalline quartz with ‘average soil silica’

controlling the other samples. It is considered that

it is unlikely that actual precipitation of quartz is

taking place, given the kinetic controls already

discussed, and the relationship between Si and Al

will be discussed below. Further controls that

could limit Si concentrations, particularly in

spring and summer months, include the

consumption of large amounts of Si by terrestrial

and aquatic vegetation and biota, and co-

precipitation with iron oxyhydroxides (Langmuir

1997).

The solubility of Al in a simple system is

considered to be controlled by hydroxide phases

and the relationship of Al concentrations to those

phases (amorphous Al(OH)3 and gibbsite

(Al(OH)3) has been investigated using the

following equilibrium relationships (Langmuir

1997):

Al(OH)3 Al3+ + 3OH-

log K = -31.4 for amorphous Al(OH)3

log K = -33.9 for gibbsite

Whilst it can be seen that gibbsite should

control concentrations of Al in solution, kinetic

constraints make it more plausible that amorphous

Al(OH)3 is the first phase to precipitate in soil and

aqueous sediment systems (Langmuir 1997).

Frequently in natural waters, sufficient

dissolved silica occurs to favour the precipitation

of Al-Si phases, such as kaolinite (Al2Si2O5(OH)4)

rather than pure Al or Si minerals. The

equilibrium concentration for the reaction

between gibbsite and kaolinite (below) is

2.4 mg l-1 Si, and thus higher concentrations of Si

would be expected to favour the precipitation of

kaolinite over that of gibbsite (Langmuir 1997).

½Al2Si2O5(OH)4+5/2H2O -Al(OH)3+H4SiO4º

kaolinite gibbsite

The procedures of Langmuir (1997, p. 248)

have been followed to calculate the equilibrium

conditions expected for each of the FOREGS

stream waters, with the reactions shown above.

These equations relate in detail the relationship

between pH, Al, Si and the solubility constants for

each reaction. The calculation of equilibrium

conditions with respect to kaolinite has used a

H4SiO4 concentration of 3.7 mg l-1 (6.19x10-5 mol

l-1), which is the median of the FOREGS dataset,

rather than 7.9 mg l -1 (1.3x10-4 mol l-1) used by

Langmuir (1997). Despite the lower

concentration, it can be seen that this median

concentration is still higher than that of theoretical

equilibrium between gibbsite and kaolinite. These

reaction constants assume a temperature of 25oC.



the data to the theoretical equilibrium line. The

blue data points are undersaturated with respect to

the lowest solubility phases studied, kaolinite.

Those which are green, are in equilibrium, or

supersaturated, with kaolinite, those which are

yellow are supersaturated (or in equilibrium) with

gibbsite, and those which are red are

supersaturated with respect to Al(OH3)amorphous, the

most soluble Al phase considered. These colours


(Figure 19) to illustrate the geographical


sampling area.

Interpretation of the aluminium solubility graph

and mapped image

Maps considered : Al, Si, pH. The geological


Figure 18 shows that considerable variation

exists in the saturation state of FOREGS waters

with respect to the three Al phases considered in

this section. The undersaturation with respect to

the most stable phase, kaolinite, is shown by the

blue data points. These areas are predominantly in

Spain, eastern France, Italy and Greece, with

isolated occurrences in other countries. These

samples were characterised by low Al

concentrations (see Al map), and in the case of the

Alps and eastern France, low Si as well.

Most of the waters sampled in the FOREGS

atlas area are supersaturated with respect to

kaolinite, with all the green, yellow and red

samples giving this result. However, the

oversaturated samples have been investigated for

other phases and will be discussed below. The few

samples which are indicated to be controlled by

kaolinite equilibrium occur in isolated areas

across the FOREGS sampling region.

However, the narrowness of the area in which this

478

pH

2 4 6 8 10 12

1e-8

1e-7

1e-6

1e-5

1e-4

1e-3

1e-2

1e-1

1e+0

1e+1

Al(OH)3(a)

Gibbsite

Kaolinite ([H4SiO4°] = 3.7 mg/l)

Figure 18: Comparison of FOREGS data with solubility limits forAl mineralphases and Kaolinite

equilibrium occurs on Figure 18 (and the frequent

supersaturation observed for other mineral phases,

e.g. calcite) suggests that some of the yellow data

points may in reality be controlled by kaolinite

rather than gibbsite.

The data points which appear to be controlled

by gibbsite supersaturation occur over a wide area

of the FOREGS atlas region (Figure 19). These

streams are characterised by a wide range of

absolute abundances of Al and Si and a wide

range of underlying lithologies. Those areas

supersaturated with respect to all the phases

considered (of which amorphous Al(OH)3 is the

most soluble) occur largely in southern Sweden

and Finland and Denmark. These areas are

particularly high in Al in relation to the rest of the

FOREGS dataset (see Al Figure), and have

moderate to low Si abundance (see Si Figure),

suggesting that an Al phase is more likely to

control Al concentrations than a Al-Si phase.

These waters have high relative concentrations of

DOC (see DOC Figure), and it may be that

organo-Al complexation increases the Al

solubility to apparent supersaturation with respect

to the most soluble phase studied.

Aqueous speciation

Iron speciation

The mapping of iron speciation gives some

indication of the potential changes in the

predominant species across the FOREGS

sampling region. These changes are caused by

predicted changes in the ratio of reduced (FeII)

and oxidised (FeIII) iron, and by variations in the

pH (a parameter which was measured). The

results have been plotted graphically and

projected onto the FOREGS atlas region for

selected Fe species.

479

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These calculations and the presentation of the data

enables some interpretation to be added to the

distribution of Fe in solution across the FOREGS

sampling region. Iron mobility controls may give

important indicators to limits on the mobility of

those trace elements which are readily sorbed onto

FeIII phases.

Calculation of Fe speciation

The likely speciation of Fe in the stream waters

of the FOREGS atlas region has been studied

using the geochemical modelling programme

PHREEQ-C and the wateq4f database (Parkhurst

and Appelo 2001). This computer programme has

been used to extend the study of speciation from

the limited suite of ions considered when looking

at saturation states (e.g., calcite, kaolinite) to the

full range available as a result of the analytical

suite and field observations (e.g., pH, alkalinity)

of the FOREGS project.

Two of the key parameters determining Fe

speciation in natural waters are Eh and pH (Figure

20). The variations in these physico-chemical

parameters have a strong influence on the

solubility and transport of Fe in catchments.

Solubility is generally enhanced in low pH and

low Eh conditions when the FeII valency is

dominant. This is particularly the case when the

Eh is still sufficiently elevated to favour SO42-

over dissolved sulphide, which would precipitate

Fe as iron(II)-sulphide phases. The solubility of

Fe in moderate pH, oxidising waters should be

low and controlled by the precipitation of FeIII as

iron oxyhydroxide species. These phases have a

high surface area, and may be the site of co-

precipitation and sorption for many minor and

trace ions in solution, thus restricting their

mobility. The availability and transport of many

trace elements of potential concern in deficiency

and excess environmental health studies are

greatly affected by the cycling of iron between

these valency conditions (Appelo and Postma

1994, Langmuir 1997, Stumm and Morgan 1996).

The data entered into PHREEQC was as

measured during the FOREGS project, however

the parameters of temperature and pe (redox

potential) were also required as input parameters.

These were set to equal 10ºC and 4 respectively.

The temperature is reasonable for an indicative

analysis of the streamwater data. The assignation

of pe to a sample is the most complex, and is the

value to which the result is most sensitive. There

is no direct measurement method of assigning this

value, thus a moderate initial value was assigned,

which is then recalculated by PHREEQC during

the iteration towards a solution for the measured

chemistry. It should thus be reiterated that the

results of this study are very much indicative,

rather than definitive. However, the results have

been studied in conjunction with redox sensitive

element maps (in addition to Fe) to ensure that the

predictions are feasible for the environmental

conditions encountered.

The key reactions are the Eh controlled

equilibrium of FeII and FeIII:

Fe2+ Fe3+ + e- Eº = 0.770 V

and the precipitation of FeIII oxyhydroxides

(where FeOOH represents poorly crystalline or

amorphous ferrihydrite) (Appelo and Postma

1994):

Fe3+ + 3H2O FeOOH + 3H+

log K = 3.0 – 5.0

The precipitation of poorly crystalline Fe

phases is favoured over the more crystalline

goethite in most stream waters. This phase may

initiate as a colloidal phase suspended in solution.

Interpretation of the Fe speciation graph and

mapped image

Maps considered: Fe, Mn, pH, DOC, SO42-,

HCO3-. The geological information is taken from

Ager (1980).

The predominance of FeII species (specifically

Fe2+) is shown in Figure 21 to be strongly

controlled by pH, and predominant at the lower

pH values measured in the FOREGS survey.

Consideration of the mapped image of FeII

abundance (on a percentage scale of the total

Fe(aq)) is shown in Figure 22, and the regions

where FeII is dominant are generally Scandinavia,

the west of Ireland, the Massif Central (France),

the north-west Iberian peninsula and parts of

central Germany. Consideration of the images for

Fe, Mn, DOC and pH all show these results to be

reasonable as these regions are characterised by

conditions that would be expected to favour FeII

over FeIII. The Scandinavian region and north-

west Ireland all have high DOC, low pH and high

Fe (in the context of the dataset) concentrations.

Some areas also have high Mn concentrations.

These parameters are all indicative of more poorly

oxidising stream waters, where Fe would be

481

expected to be more mobile in solution. There

may be areas where FeII is dominant, but not

highlighted on this indicative image.

The dominance of the aqueous species

Fe(OH)3 is predicted to become important at pH

values around 7.5 – 9 in the FOREGS dataset

(Figure 21). Figure 23 shows that this is also

reflected in the expected saturation state for

amorphous Fe(OH)3 phases in streams. These data

show that many streams are expected to be

supersaturated, and the colour range from blue to

red is proportional to the SI value. The mapped

0 2 4 6 8 10 12 14

–.5

0

.5

1

pH

Eh(volts)

Fe++

Fe(OH)2+

Fe(OH)4

-

FeOH++

FeSO4+

Fe(OH)3(ppd)

FeO(c)

Pyrite Siderite

15°C

land Wed Feb 15 2006

DiagramFe++,T=15°C

,P=1.013bars,a[main]=10–6,a[H2O]=1,a[HCO3-]=10–2.7,a[SO4-- ]=10–3.8;Suppressed:Hematite

,Goethite,Magnetite

Figure 20. Eh-pH diagram for Fe, using Geochemist's Workbench(Bethke, 2002).

pH

4 5 6 7 8 9 10

0

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

100

Fe(OH)3°(aq)

Fe2+(aq)

Figure 21. Predicted dominance of two Fe species in solution in FOREGSstream waters.

482

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data in Figure 24 demonstrates the geographical

distribution of the predicted Fe(OH)3º values. It

can be seen that the oxidised aqueous species is

dominant across much of the FOREGS region

outside the areas described above. These areas

were also frequently supersaturated with respect

to Fe(OH)3 amorphous, which suggests that they either

occur as colloids which pass through the <0.45

µm filter, or aqueous complexation is increasing

the apparent solubility of these precipitates, or the

solubility constant for such a variable phase is not

appropriate across the full range of hydrochemical

conditions encountered in the FOREGS region.

Copper speciation in solution

The speciation of Cu in solution has been

predicted from the FOREGS stream water data

using a geochemical modelling programme,

PHREEQC. This procedure allows many more

ions in solution to be considered simultaneously

than would otherwise be possible. The speciation

of ions in solution can be important in

understanding their mobility in catchments and

potential toxicity to receptor organisms.

The data have been plotted as graphs, and the

predicted occurrence of organically bound Cu has

been mapped for the FOREGS atlas region to

show the geographical distribution of the data.

Calculation of copper speciation

The likely speciation of Cu in the stream

waters of the FOREGS atlas region has been

studied using the geochemical modelling

programme PHREEQC using the phreeq database

(Parkhurst and Appelo 2001). This computer

programme has been used to extend the study of

speciation from the limited suite of ions

considered when looking at saturation states (e.g.

calcite, kaolinite) to the full range available as a

result of the analytical suite and field observations

(e.g., pH, alkalinity) of the FOREGS project (see

also the section on Fe).

Copper speciation and solubility, like that of

Fe is strongly dependent on pH and Eh variations,

with concentrations being limited in reducing

environments by the precipitation of copper-

sulphide phases. The solubility of Cu is thus

expected to be greatest in low pH, oxic

environments. The solubility of Cu is more

complex than indicated by a simple Eh-pH

diagram though, with organic complexation and

sorption to mineral phases, such as iron

oxyhydroxides, likely to be important in the

transport of Cu in catchments (Hem 1992, Rozan

and Benoit 1999). Copper solubility may be

enhanced by organic complexation, whilst

sorption to mineral surfaces is likely to inhibit

transport in the aqueous phase (although sorption

may also be to highly mobile colloidal phases).

4 5 6 7 8 9 10

-10

-8

-6

-4

-2

0

2

4

Supersaturated

Undersaturated

Figure 23. Predicted amorphous iron hydroxide saturation state in relation to stream pH across for the FOREGS data.

484

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Figure 25 shows the relative dominance

predicted for two Cu species from the PHREEQC

output, across the measured pH range. The data

for the Cu-fulvate complex are mapped across the

FOREGS atlas area in Figure 26, with a colour

range of blue (low) to red (high) on the percentage

abundance of the total Cu in solution. These data

are indicative of the speciation, as they are based

on prediction rather than actual measurements of

the individual species.

Interpretation of the copper speciation graph

and mapped image

Maps considered: Cu, DOC, HCO3 and pH.

Geological information is taken from Ager

(1980).

Figure 25 shows that the hydroxy species

Cu(OH)2º is predominant in most of the waters

predicted speciation above a pH of ~7.5. In lower

pH waters is can be seen that an understanding of

the dissolved organic carbon (DOC) concentration

is essential to predict the likely speciation of Cu,

as (using the equilibrium data for fulvate in

PHREEQC) Cu-fulvate is predicted to be

dominant in the majority of the waters with a pH

of <7.5. Figure 26 shows these data mapped

across the FOREGS atlas region.

Figure 26 shows a clear geographical

association of organically-bound aqueous Cu with

pH and DOC distributions (see DOC and pH

Figures). These areas of predominance are in fact

independent in many instances of the total

concentration of Cu (see Cu Figure) as can be

observed particularly well in Finland. Organic

complexation is expected to be an important part

of the aqueous-Cu cycle in many of the stream

waters of Scandinavia, Ireland, north-west Iberian

Peninsula, the Amorican Massif, Massif Central

(France) and along the North Sea coast from the

Netherlands to Denmark.

pH

4 5 6 7 8 9 10

0

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

100

Cu(OH)2

Cu-Fulvate

Figure 25. Proportion of selected Cu species predicted in the FOREGSstream waters.

486

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References

Ager, D.V., 1980. The geology of Europe. McGraw-

Hill Book Company Ltd., Maidenhead, U.K., 554

pp.

Appelo, C.A.J. & Postma, D., 1994. Geochemistry,

groundwater and pollution. A.A. Balkema,

Rotterdam, 535 pp.

Bethke, C.M. 2002. The Geochemist's Workbench -

version 4.0. University of Illinois, Illinois, U.S.A.

British Geological Survey, 1999. Regional

geochemistry of Wales and part of west-central

England: stream water. British Geological Survey,

Keyworth, Nottingham.

Hem, J D., 1992. Study and interpretation of the

chemical characteristics of natural water. United

States Geological Survey Water Supply Paper 2254,

Third edition, 263 pp.

IIASA, 2005. Europe - Annual precipitation.

International Institute for Applied Systems Analysis,

Laxenburg, Austria. Downloaded from

http://www.iiasa.ac.at/Research/LUC/GIS/clim_eur.

htm. IIASA data have been constructed based on

CRU TS 2.1, Mitchell and Jones, 2005.

Krauskopf, K.B. & Bird, D.K., 1995. Introduction to

geochemistry. McGraw Hill Inc., New York, Third

edition, 637 pp.

Langmuir, D., 1997. Aqueous environmental

geochemistry. Prentice Hall, New Jersey, U.S.A,

436 pp.

Parkhurst, D.L. & Appelo, C.A.J., 2001. PHREEQC-2

- version 2.4.2. United States Geological Survey,

Denver, Colorado

Plummer, L.N. & Busenberg, E., 1982. The

solubilities of calcite, aragonite and vaterite in CO2-

H2O solutions between 0-Degrees-C and 90-

Degrees-C, and an evaluation of the aqueous model

for the system CaCO3-CO2-H2O. Geochimica et

Cosmochimica Acta, 46, 1011-1040.

Rozan, T.F. & Benoit, G., 1999. Geochemical factors

controlling free Cu ion concentrations in river water.

Geochimica et Cosmochimica Acta, 63, 3311-3319.

Smith, B., Hutchins, M.G., Rawlins, B.G., Lister, T.R.

& Shand, P., 1998. Methods for the integration,

modelling and presentation of high-resolution

regional hydrochemical baseline survey data.

Journal of Geochemical Exploration, 64, 67-82.

Stumm, W. & Morgan, J.J., 1996. Aquatic chemistry.

Chemical equilibria and rates in natural waters. John

Wiley & Sons, N.Y., Third edition, 1022 pp.

Troen, I. & Petersen, E.L., 1989. European Wind

Atlas. Risø National Laboratory, Roskilde, 656 pp.

WHO, 2004. Guidelines for drinking water quality.

World Health Organisation, Geneva, Third edition,

515 pp.

Zhu, C., 2004. Coprecipitation in the barite

isostructural family: 2. Numerical simulations of

reactions and mass transport. Geochimica et

Cosmochimica Acta, 68, 3339-3349.

488

Documents

ANNEX 1 THEMATIC INTERPRETATION OF STREAM WATER …weppi.gtk.fi/publ/foregsatlas/articles/Annex 1 old.pdf · the interpretation of the hydrochemistry, but also to illustrate the results