ORIGINAL PAPER Increasing Iron Concentrations in UK Upland Waters C. Neal S. Lofts C. D. Evans B. Reynolds E. Tipping M. Neal Received: 12 December 2007 / Accepted: 17 June 2008 / Published online: 15 July 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Iron distributions in rainfall, streams, soils and groundwaters are described for the Upper River Severn catchment of mid-Wales. Iron is mainly supplied from within- catchment sources with highest concentrations occurring under reducing conditions. Iron concentrations have doubled over the past 20 years (*5.0 lg yr -1 for the forest and *3.7 lg yr -1 for the moorland). For the forested sites, the gradients are particularly high post- 1993. UK rivers/lakes monitored by the UK Acid Waters Monitoring Network show similar increases. Generally, Fe correlates with dissolved organic carbon (DOC). The greatest rates of Fe increase coincide with those for DOC. Thermodynamic modelling using WHAM/Model VI indicates that Fe(III) is mainly in microparticulate form (probably oxyhydroxides) apart from under reducing conditions. It is proposed that Fe increases for surface waters are associated with increased microparticulate Fe(III) due to stabilisation against aggregation by binding of DOM to its surface. The results relate to acidification declines and deforestation leading to land disturbance and wetter conditions within the soil. There will be greater acidification reversal following tree harvesting due to lowering of atmospheric SO x scavenging and this may have resulted in the greater increase in Fe in the later years of the study. Keywords Iron River Acidification Colloids/nanoparticles DOC WHAM C. Neal (&) M. Neal Centre for Ecology and Hydrology Wallingford, Crowmarsh Gifford, Wallingford, Oxon OX10 8BB, UK e-mail: [email protected]S. Lofts E. Tipping Centre for Ecology and Hydrology Lancaster, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster LA1 4AP, UK C. D. Evans B. Reynolds Centre for Ecology and Hydrology Bangor, Orton Building, Deiniol Road, Bangor, Gwynedd LL57 2UP, UK 123 Aquat Geochem (2008) 14:263–288 DOI 10.1007/s10498-008-9036-1
Transcript
ORI GIN AL PA PER
Increasing Iron Concentrations in UK Upland Waters
C. Neal Æ S. Lofts Æ C. D. Evans Æ B. Reynolds Æ E. Tipping Æ M. Neal
Received: 12 December 2007 / Accepted: 17 June 2008 / Published online: 15 July 2008� Springer Science+Business Media B.V. 2008
Abstract Iron distributions in rainfall, streams, soils and groundwaters are described for
the Upper River Severn catchment of mid-Wales. Iron is mainly supplied from within-
catchment sources with highest concentrations occurring under reducing conditions. Iron
concentrations have doubled over the past 20 years (*5.0 lg yr-1 for the forest and *3.7
lg yr-1 for the moorland). For the forested sites, the gradients are particularly high post-
1993. UK rivers/lakes monitored by the UK Acid Waters Monitoring Network show
similar increases. Generally, Fe correlates with dissolved organic carbon (DOC). The
greatest rates of Fe increase coincide with those for DOC. Thermodynamic modelling
using WHAM/Model VI indicates that Fe(III) is mainly in microparticulate form (probably
oxyhydroxides) apart from under reducing conditions. It is proposed that Fe increases for
surface waters are associated with increased microparticulate Fe(III) due to stabilisation
against aggregation by binding of DOM to its surface. The results relate to acidification
declines and deforestation leading to land disturbance and wetter conditions within the soil.
There will be greater acidification reversal following tree harvesting due to lowering of
atmospheric SOx scavenging and this may have resulted in the greater increase in Fe in the
later years of the study.
Keywords Iron � River � Acidification � Colloids/nanoparticles � DOC � WHAM
C. Neal (&) � M. NealCentre for Ecology and HydrologyWallingford, Crowmarsh Gifford, Wallingford, Oxon OX10 8BB, UKe-mail: [email protected]
S. Lofts � E. TippingCentre for Ecology and Hydrology Lancaster, Lancaster Environment Centre,Library Avenue, Bailrigg, Lancaster LA1 4AP, UK
C. D. Evans � B. ReynoldsCentre for Ecology and Hydrology Bangor, Orton Building,Deiniol Road, Bangor, Gwynedd LL57 2UP, UK
The UK uplands provide headwaters of many major British rivers and they are a major
source of potable, industrial and agricultural water supplies (Hudson et al. 1997). These
uplands often constitute areas of outstanding natural beauty and they are of high amenity/
ecological status. However, they are ecologically threatened due to pollutant inputs
especially from ‘acid rain’, including sulphur and nitrogen compounds (CLAG 1995;
Hornung et al. 1995). In addition to acidification, there are issues of land use change that
has affected water quality. In particular, in the early to mid parts of the twentieth century
conifer plantations were extensively introduced onto acidic and acid sensitive upland
moorland. Now many of the plantations have reached maturity, widespread felling is
affecting water quality (Hudson et al. 1997; Neal et al. 2004). The main focus on upland
water quality has been on those components directly linked to acidification and forestry
rotation cycles such as pH, inorganic aluminium and strong acid anions (chloride, sulphate
and nitrate). There has been less emphasis on the behaviour of other elements such as iron
(Fe) that have not been directly associated with previously known upland water quality
issues. Iron is a key element, being essential for all organisms and yet limiting in certain
environments such as the open ocean (Turner et al. 2001). Understanding the biogeo-
chemical cycling of Fe is therefore a key research topic. There is evidence that surface
water iron concentrations may be significant on an international scale. For example, in the
FOREGS Geochemical Atlas of Europe (Salminen et al. 2005) 31% of measurements of
filterable (\0.45 lm) Fe in surface waters exceed the mandatory EU drinking water limit
of 200 lg l-1. In the UK, studies have shown that iron can be high in upland waters,
especially those that are acidic and organic rich (e.g. Abesser et al. 2006).
Iron is the second most abundant metal in the crust after aluminium (Greenwood and
Earnshaw, 2005). Despite this, Fe concentrations are relatively low in surface waters
(typically lg l-1 to mg l-1 levels) with the ratio of its concentration in river water and
seawater to that in the crust being around three orders of magnitude lower than ions such as
sodium and chloride and two orders of magnitude lower than transition metals such as
copper, nickel and zinc (Neal 2000). Iron is predominant in solution and in mineral forms
in oxidation states of Fe(II) (Fe2+) and Fe(III) (Fe3+). Fe(II) occurs under reducing con-
ditions encountered in some groundwaters and soils, while Fe(III) predominates in more
oxygenated conditions such as most surface waters. The low solubility of Fe is associated
with the ease of hydrolysis and the formation of insoluble (weathering) components such as
oxides/hydroxides and clay minerals, and under reducing conditions with sulphate
reduction, Fe sulphides can also precipitate (Appelo and Postma, 1999; Drever 1997). In
general, Fe solubility increases as pH decreases as well as when the waters become more
reducing and Fe2+ predominates. For surface waters, Fe is primarily as Fe(III) and in this
oxidation state Fe readily binds to ligands, particularly natural organic matter, and also
hydrolyses to form solid oxyhydroxides that may be encountered at sizes below the pore
sizes of conventional filters (0.1–1 lm). Additionally, Fe(III) oxyhydroxides may them-
selves bind natural organic matter at their surfaces (e.g. Tipping 1981, Filius et al. 2000), a
phenomenon that contributes to their colloidal stability in waters.
Recently, Lofts et al. (2008) have shown that the amount of Fe(III) bound to inorganic
and organic ligands in waters (the ‘‘truly dissolved’’ Fe(III)) can be estimated by assuming
oxyhydroxide control of the Fe3+ ion activity. This allows the distribution of Fe(III)
between ‘‘truly dissolved’’ and oxyhydroxide forms to be estimated. Application of such
knowledge to the field can help develop understanding of the processes controlling Fe
concentrations and transport from soil to waters. Currently, widespread and significant
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increases in dissolved (filterable) organic carbon concentrations are being observed in
temperate surface waters across Europe and North America probably in relation to the
reversal of anthropogenic acidification (Evans et al. 2006; Monteith et al. 2007). In the
context of such widespread change and our improving understanding of Fe(III) speciation,
it is timely to examine concentrations of iron in surface waters, and their temporal trends.
Here, ‘‘dissolved’’ (\0.45 micron filtered fraction) Fe concentrations are described for
rainfall, cloud water, soil water, stream water and groundwater at the Plynlimon experi-
mental catchments in the upper river Severn, mid-Wales, where there have been extensive
hydrological and water quality studies over the past 30 years (Neal 1997). The catchments
cover acidic and acid sensitive moorland and forest typologies characteristic of much of the
British uplands, and the work is of relevance to environmental science and environmental
management issues for the uplands in general. The issues include: Do changes in pH affect
Fe mobilisation? How important is organic complexation given that the dissolved organic
carbon (DOC) levels have been increasing over the last two decades or more (Freeman
et al. 2001; Worrall et al. 2004; Neal et al. 2005)? Are there any systematic trends in Fe
concentrations over time in line with changing acidity, pollutant deposition and DOC
levels (Evans and Monteith 2001; Neal et al. 2001; Evans et al. 2006)? Does conifer
plantation rotation cycles affect iron mobilisation? Are Fe levels such that they may affect
potability of the important upland water resource (Buckley and Keil 1990)? These issues
are addressed here and are put in the context of UK rivers by comparisons between the
Plynlimon and other long-term studies in the acidic and acid sensitive areas of the UK (The
UK Acid Waters Monitoring Network, UKAWMN, Monteith and Evans 2000; Evans and
Monteith 2001). Given the issues of increasing DOC levels in many areas where acidifi-
cation is in decline and relatively high Fe concentrations across many parts of Europe, the
work is of relevance to much broader regions than just the UK. It is also of relevance to
contemporary issues of colloidal and nanoparticle transport, function and form.
2 Study Area and Monitoring Data
2.1 The Plynlimon Study
The headwaters of the River Severn have an upper half of hill-top acid moorland plateau,
with hag-peat and podzolic soils, and a lower half of conifer plantation, the Hafren Forest,
with podzolic and gley soils (Brandt et al. 2004). The upper Severn has two main tribu-
taries, the Afon Hafren and the Afon Hore, and a minor one, the Nant Tanllwyth, which
joins the Afon Hafren near its confluence with the Afon Hore. The bedrock consists of
Lower Palaeozoic mudstone, shale and grit. The Hafren Forest is mainly of Sitka spruce
with some Norway spruce, larch and lodgepole pine that were planted in various phases
from the mid-1940s through to the late 1960s. Felling occurred in various phases across the
catchment (Table 1).
Here, information is drawn from 1 to 25 years of rainfall, cloud water, throughfall,
stemflow and stream and ground water quality information (Table 1). The total length of
data record covered is March 1983 to the end of December 2006. Two rainfall sites cover
the altitude range for the catchment, and rainfall samples were combined for an integrated
value. There are five main stream sites: the upper and lower Hafren (117 ha per 100%-
moorland and 347 ha per 50%-moorland, respectively), the upper and lower Hore (178 ha
per 50%-moorland and 334 ha per 25%-moorland, respectively) and the lower Tanllwyth
(51 ha per 100%-forest). Several small streams, ditch waters and ground waters were also
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Table 1 Catchment summary information
Site Area(ha)
Soiltype
Vegetationtype
Felling Samplingper year
Startdate
Enddate
Atmospheric inputs
Rain – – – – 52 10/5/83 Cont
Cloud water – – – – 52 25/9/90 Cont
Throughfall – – – – 26 01/02/84 02/09/91
Stemflow – – – – 26 01/02/84 02/09/91
Main streams
Upper Hafren 117 M/Pe M M 52 17/07/90 Cont
Lower Hafren 347 M/P/G SS Y100a 52 10/05/83 Cont
Upper Hore 178 M/P/G SS Y100%b 52 28/08/84 Cont
Lower Hore 335 M/P/G SS Y100%c 52 10/05/83 Cont
Intermediate size stream
Tanllwyth 51 G SS Y50%d 52 17/09/91 Cont
Small streams
South2Hore 3–6 P SS Y100%e 52 19/04/88 20/02/01
SE1f 2–4 P SS Y100%f 26 20/09/94 14/02/01
SE3c 2–4 P SS N 26 11/10/94 27/04/99
Tanllwyth Nc \2 G SS N 26 28/04/94 27/04/99
Tanllwyth Sf \2 G SS Y100%d 26 28/04/94 14/02/01
Boreholes
HA4b P SS Y100%f 12/52 24/04/94 14/02/01
Quarry P SS N 12/52 24/04/94 14/02/01
SE1bf P SS Y100%f 26 10/05/95 27/04/99
SE3bc P SS N 26 10/05/95 27/04/99
TanNbc G SS N 26 05/07/94 27/04/99
TanSbf G SS Y100%d 26 09/08/94 27/04/99
US1 Pe M M 12 24/04/94 12/07/95
US2 P SS N 12 24/04/94 12/07/95
US3 P SS N 12 24/04/94 14/06/95
US4 P SS N 12 24/04/94 12/07/95
LS1 P SS N 12 24/04/94 12/07/95
LS2 P SS N 12 24/04/94 12/07/95
LS3 P SS N 12 24/04/94 12/07/95
LS4 P SS N 12 24/04/94 12/07/95
IS1 P SS N 12 24/04/94 12/07/95
IS2 P SS N 12 24/04/94 12/07/95
VB1 P/Gr M M 12 24/04/94 12/07/95
The paired sites have control and felled catchments, the site names have suffix ‘‘c’’ for the control and suffix‘‘f’’ for felled sites. SS = Sitka Spruce; M = Acid Moorland. Soil type (Soil) G = Gley; P = Podzol; Pe =Peat, Gr = Gravel. Under felling activity Y = Yes, 100% fell unless indicated otherwise; N = No fell.The superscript denote felling date: a ongoing thinning; b March 2000; c March 1985 to October 1988;d February 1996; e August–October 1989; f September–October 1995
266 Aquat Geochem (2008) 14:263–288
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monitored across the catchment and were collected on a regular weekly to monthly basis
(Table 1). The groundwater samples represent fracture flow in the bedrock (Neal et al.
1997b).
The waters were filtered on return to the Plynlimon laboratories for rainfall, cloud water,
throughfall and stemflow. The stream and groundwater samples were filtered in the field.
Filtration was undertaken under vacuum using 0.45 lM cellulose acetate circles. The
filtered waters were stored at 4�C in the dark in acid washed polypropylene bottles and
were acidified to 1% (v/v) with high purity concentrated nitric acid to avoid sample
deterioration for the trace metals. Iron concentrations were subsequently determined using
inductively coupled plasma emission spectroscopy. The methodologies used for all the
determinands monitored are provided by Neal et al. (1997a).
There is a large background of information on soil water chemistry to be drawn from,
and it would be remiss not to deal with this information within this publication. Since some
of the findings have been published previously (Reynolds et al. 1988; Hughes et al. 1990),
here only the salient features are provided with information on the findings of the earlier
research being reported later in the paper.
Soil water was sampled from five horizons within a peaty podzol and peaty gley soil
under mature Sitka spruce in the Hafren forest and data for grassland is also available for
the adjacent River Wye catchment. Soil water was collected using tensionless lysimeters
from beneath Lf- and Oh-horizons and porous ceramic cup samplers were used in the
mineral soil horizons. Samples were collected every two weeks and filtered through 0.45
lm membrane filters prior to analysis for a range of analytes at CEH Bangor and CEH
Merlewood research stations. pH was determined on an unfiltered sample, Fe by atomic
absorption spectrophotometry and DOC by auto analyser using a UV oxidation procedure
on filtered samples. Part way through the study, felling took place for the forested site and
hence comparisons could be made pre- and post-fell.
2.2 The UK Acid Waters Monitoring Network
The UKAWMN sites cover a wide area of the UK (from the north of Scotland to the south
of England as well as Northern Ireland, Monteith and Evans 2000) and they comprise 11
rivers and 11 lakes. 17 of the catchments are mainly moorland, with five having significant
conifer cover (up to 70%). Bedrock geology varies, but is predominantly low weathering
(sandstone, grits slate, schist, extrusive volcanics and granite). The soil type and % forest
cover are shown in Table 2. Streams are sampled monthly, and lakes quarterly. Data for the
period 1988 to 2006 have been analysed for long-term trends in both Fe and DOC using the
modified Seasonal Kendall Test (Hirsch and Slack 1985). At six of the sites, Fe concen-
trations frequently fell below detection limits, which decreased in 2000 from 0.015 to
0.002 mg l-1; trends at these sites have therefore been treated with caution.
3 Results
3.1 Atmospheric Inputs, Throughfall and Stemflow
Iron concentrations in rainfall, cloud-water, stemflow and throughfall vary considerably,
especially in cloud-water (Table 3), and the highest concentrations occur when the volume
of catch is low. The lowest average concentration occurs for rainfall (14 lg l-1) with
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throughfall intermediate (35 lg l-1) and cloud water and stemflow highest (85 and 73 lg
l-1, respectively). There is no statistically significant trend in Fe concentration over time
for any of these waters (Table 4).
3.2 Streams
The average Fe concentration varies between 12 and 911 lg l-1 (Table 3). For the main
streams, Fe concentrations increase over time and the effects of felling activity are
indistinct against this overall pattern (Fig. 1). In the case of the forest streams, there is a
marked steeper increase in trend around 1993, and to illustrate the magnitude of the
changes occurring, the data is split between the earlier (pre-1994) and the later (post-1993)
periods and the data is then assessed using average values and linear regressions. In terms
of average, for the forested catchments, the Fe concentration increased between the two
periods typically by around 60%, with concentrations almost doubling over the full 22
years of monitoring. For example, the average Fe concentrations in 1983/1984 and 2005/
2006 for the lower Hafren are 71 and 139 lg l-1, respectively. With regard to the linear
regression (Table 4):
• For the period prior to 1994, the gradients are low, mainly statistically insignificantly
different from zero, and the correlations are weak. However, for the period post 1993,
there are statistically significant positive gradients with time for the forested
catchments averaging around 5.5 lg l-1 yr-1.
• Comparison cannot be undertaken for the earlier part of the record in the case of the
Tanllwyth and upper Hafren due to an insufficient record (only 3 years of data pre-
1993). They can however be examined for the later period as there is sufficient overlap.
The gradients occur in the sequence upper Hafren (3.73 ± 1.23 lg l-1 yr-1), lower
Table 2 Catchment types within the UKAWMN and regression of Fe with time and DOC across the sitesUKAWMN
Site name Type Forest % Soil Fe regression against DOC DOC
Time 2*std const 2*std r2 N Avg Min Maxlg mg-C-1 mg-C l-1
Allt na Coir n Con R 42 pdz 22 1 0 37 0.68 199 4.6 0.1 11.8
River Etherow R 0 p, pdz 28 2 59 129 0.584 196 6.7 0.3 34.0
Old Lodge R 30 pdz 66 6 11 307 0.423 195 6.1 0.2 45.0
Afon Gwy R 0 pdz 36 4 48 64 0.271 195 2.3 0.1 15.6
Beaghs Burn R 0 p 57 6 142 515 0.33 197 12.5 3.1 37.0
Bencrom River R 0 p 11 1 11 31 0.417 197 4.4 1.2 16.0
Coneyglen Burn R 0 p 26 8 648 581 0.054 176 8.3 1.4 26.9
Loch Coire n Arr L 0 p 19 3 -14 27 0.482 67 2.6 0.1 5.9
Hafren (4.41 ± 1.06 lg l-1 yr-1), upper Hore (5.59 ± 1.60 lg l-1 yr-1), lower Hore
(6.48 ± 1.25 lg l-1 yr-1), South2Hore (9.18 ± 2.16 lg l-1 yr-1) and the Tanllwyth
(15.1 ± 2.3 lg l-1 yr-1). This represents a sequence of increasing forest/harvested-
forest cover and increasing gley soils.
• There are also four small stream sites draining podzol (SE3c, SE1c) and gley (TanNc,
TanSf). For these sites (c = control, f = felled and the monitoring periods were from
1994 to 1999 and 2001, respectively), the felled sites showed no statistically significant
trend, but there were some clear increases over time for the controls with the gley site
showing the greatest increase, although near the start of the record there were some
high Fe concentrations encountered. The gradients for the podzol (SE3c) and gley
(TanN) sites were 2.81 ± 1.31 and 151 ± 56 lg l-1 yr-1, respectively. In the case of
TanN, this had the highest average Fe concentration for all the sites monitored and it
was also the most reducing with low levels of nitrate, often below the detection limit.
In terms of average, there is a separation between catchments dominated by peat and
podzolic soils and those dominated by gley. For peat and podzolic soil cases, Fe con-
centrations average around 70 lg l-1 (range of averages 12 to 138 lg l-1) and the highest
averages occur for the larger catchments. In contrast, for catchments that are dominated by
gley soils, Fe concentrations are higher. For example, for the two small gley drainage areas
monitored (Tanllwyth Nc and Sf), Fe concentrations average around 900 lg l-1 while for
the main stem of the Nant Tanllwyth, concentrations are lower averaging 277 lg l-1.
The increases in Fe concentrations over time occur generally for both across the flow
range and for baseflow and stormflow, but the greatest proportionate change seems to occur
under baseflow conditions (Table 3). For example for the pre- and post-1993 periods Fe
concentration increases for the lower Hafren by 50% on average, by 77% for the low-flow
average and 32% for the high-flow average: the corresponding averages for the upper
Hafren is 18, 97 and 6% while for the lower Hore it is 63, 74 and 20%.
In the case of the Hore, Fe concentrations peak around 2002 and it is at this time when
there was relatively high forestry activity within the upper part of the catchment and some
land/forest-road disturbance with heavy vehicles used for felling. Nonetheless, there was
no clear nitrate felling signal, and the spikes in Fe concentration at this time probably
reflect some aspect other than the ‘normal’ felling response such as land disturbance. In
addition, there is an increase in Fe concentration during the latter part of 2006 when there
was further felling activity—as earlier in the record, there is no clear nitrate increase with
felling, but at this later time concentrations of other ions such as potassium increase and
this seems tied to weathering components.
Table 3 continued
Period Average Minimum Maximum Low flow High flow
IS1 1994–1995 1 0 20 NA NA
IS2 1994–1995 131 0 1945 NA NA
LS1 1994–1995 126 0 760 NA NA
LS2 1994–1995 1207 0 2715 NA NA
LS3 1994–1995 7 0 60 NA NA
LS4 1994–1995 117 0 885 NA NA
VB1 1994–1995 745 0 3735 NA NA
The low flow and high flow values represent the average for the bottom and top 10% of flows, respectively.An outlier point is excluded for the Upper Hore and the Tanllwyth and 2 outlier points for South2Hore
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3.3 Groundwater
Groundwater Fe concentrations vary considerably across the sites, with a range in average
of 1 to 7181 lg l-1 (Table 3). The highest Fe concentrations occur for one borehole
associated with gley soils (TanNBc) and an artesian borehole with podzolic soil cover
(LS2). In both cases, green flocs can occur in the water especially under artesian conditions
Table 4 Linear regression of Fe with time and with DOC for Plynlimon streams. Excluded is 1 outlier eachfor the Upper Hore and Tanllwyth and 2 for South2Hore
Period Fe regression against time
Time 2*std Const 2*std r2 Nlg Fe yr-1
Rainfall 1983–2006 0.22 0.19 –434 38 0.006 922
Cloud water 1990–2006 -6.4 3.1 12,983 374 0.026 681
For the Upper Hafren and the Tanllwyth, the regression just for the period prior to 1994 is not included dueto the shortness of record
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Fig. 1 Time series plots of Fe concentration. The upper Hafren represents a moorland case; the lowerHafren represents gradual/phased felling over the past 20 years while the Afon Hore represents major clear-fell between 1985 and 1989. For the Tanllwyth and South2Hore felling occurred in 1996 and in 1989,respectively
272 Aquat Geochem (2008) 14:263–288
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within the borehole when water turbulence is at its highest. These flocs turn brown on
exposure to the air. This seems to represent the classic case of Fe(II) mobilisation within
the anoxic groundwater conditions and oxidation to Fe(III) on exposure to the air. This type
of feature is also observed within the streams where there are clear groundwater seepages
especially in gley areas.
3.4 Soils
The data are summarised in Table 5 together with analogous data from four horizons
within a peaty podzol under acid grassland and moorland vegetation in the adjacent Wye
catchment.
All the soils are highly acidic and both Fe and DOC concentrations can vary consid-
erably and the highest Fe concentrations occur for the podzol within the Oh-horizon. Prior
to felling, Fe concentrations average 410 lg l-1 compared with 40 and 20 lg l-1 in the
Lf- and C-horizons, respectively, while post-felling Fe concentrations are double to treble
Table 5 Concentrations of Fe (mg l-1) in soil waters at Plynlimon
Sampling periods: grassland podzol: 14/8/1984 to 27/9/1984; forest podzol: 20/11/1984 to 2/1/1990; forestgley: 18/12/1984 to 2/1/1990. Clear-felling: forest podzol: between 10/2/1987 and 7/4/1987; forest gley:between 21/4/1987 and 19/5/1987
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these values in the Lf- and Oh-horizons with little difference in the lower horizons. The
highest Fe concentrations occur for the gley within the Bs-horizon and there are elevated
concentrations in the Oh-horizon as well. Prior to felling, the average concentrations are
40, 430, 250, 1390 and 300 lg l-1 in the Lf-, Oh-, Eag-, Bs- and C-horizons, respectively,
while post-felling Fe concentrations are higher with factors of 3.0, 2.0, 1.8, 1.2 and 1.7,
respectively. For both podzol and gley soils, the Fe concentrations are correlated with DOC
for the Lf-, Oh- and Eag-horizons, but much more poorly correlated in the Bs- and
C-horizons as well as for the Eag-horizon in the podzol (Table 6). With felling, the
increases in Fe concentration are mirrored with increases in DOC.
Concentrations of Fe in soil solutions from beneath mature conifer forest were higher
than those observed in grassland and moorland soils, whilst clear-felling of the forest
resulted in a further increase in Fe concentrations (Table 5). The Lf-horizon comprises
mainly forest litter plus decomposing vegetation from the pre-existing moorland. As a
consequence of this, Fe concentrations are very low, and Fe in the soil water will be
primarily derived from the organic matter itself plus any mineral material incorporated as a
result of site preparation and planting. The site was ploughed before planting but the
lysimeters were installed beneath mature forest in the undisturbed ‘bench’ at the side of the
plough furrows. The mobility of Fe is clearly linked to DOC in this horizon although Fe
availability is probably limited. In the Oh-horizon, the relationship between Fe and DOC is
much stronger with relatively little scatter. The Oh-horizon contains highly weathered
mineral material and abundant organic matter. The relationship weakens in the E horizon
as other controls, probably redox, become more important during periods of waterlogging
and anaerobicity. Redox processes probably dominate Fe mobility in the mainly anaerobic
B- and C-horizons where Fe concentrations are large but unrelated to DOC. In the Bs- and
C-horizons of the gley, conditions are highly reducing (in the field, the waters/soils smelt
strongly of hydrogen sulphide). In the Eag-horizon, mottling indicated seasonal water
logging which resulted in short-lived increases in Fe concentrations where a sharp peak in
Fe and a more muted DOC response coincided with heavy rainfall in the late summer of
1985. Thus, there are very different processes are operating in the two soil types.
Table 6 Coefficients and r2 values for the least squares regression between Fe and DOC in podzol and gleyforest soil waters at Plynlimon
Gradient 2*std Intercept 2*std r2 N Fe/DOC
Forest Podzol
Lf 1.93 0.38 6 155 0.480 113 2.1
Oh 28.1 3.3 84 418 0.731 110 32.1
Eag 5.96 2.66 244 223 0.147 119 25.2
Bs 3.17 2.38 30 127 0.057 120 7.5
C 0.87 0.50 11 36 0.094 120 3.2
Forest Gley
Lf 2.05 0.29 5 91 0.642 2.2
Oh 26.8 2.6 80 345 0.815 30.5
Eag 62.7 23.8 -183 335 0.464 42.6
Bs -76.2 137.1 1804 1988 0.014 295
C 102 50 -90 650 0.166 96.7
Italicised data are for regressions of low significance. The data cover pre and post-felling, but there is nosignificant change in gradient pre- and post-fell
274 Aquat Geochem (2008) 14:263–288
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3.5 Fe Relationships with Alkalinity and DOC
In order to assess Fe mobilisation in relation to acidity and DOC changes, linear regression
was used. Gran alkalinity was used as a marker of acidity rather than pH as it is much more
suitable for examining soil and groundwater inputs: while pH is non-linear, Gran alkalinity
changes are approximately linear on mixing of soil and groundwater.
Iron concentrations show no statistically significant correlation with Gran Alkalinity
based on an analysis of the full dataset and subsets for baseflow and stormflow where there
is a clear contrast in pH. Hence, only DOC will be considered further in the results section.
For the atmospheric inputs, there is a positive correlation between Fe and DOC and the
gradients are variable (Table 4). The highest gradient is for the cloud water (33.8 ± 2.0 lg
Fe mg-C-1) with rainfall intermediate (9.8 ± 1.7 lg Fe mg-C-1) and throughfall and
stemflow the lowest (0.6 ± 0.2 and 3.6 ± 0.4 lg Fe mg-C-1, respectively).
For the streams, Fe concentrations are strongly and linearly correlated with DOC
(Table 4). For the main streams the gradient is similar across the sites at around 36 lg-Fe
mg-C-1 with a range of 28.9 to 49.3 lg-Fe mg-C-1 and the highest gradients occur for the
upper Hafren and the Tanllwyth. The gradients are similar for the earlier and later parts of
the record and there is little difference between pre- and post-fell periods. For the small
streams there is a similar Fe:DOC gradient range (21.5 to 39.1 lg-Fe mg-C-1).
For the groundwater, there is no statistically significant relationship between Fe and
DOC concentrations.
For the soils, the highest ratios of Fe:DOC occur within the Oh- and Eag-horizons of the
podzol (32.1 and 25.2 lg mg-1, respectively, compared with 2.06–7.51 lg mg-1 within
the other horizons). Correspondingly, the Fe/DOC ratio increases with depth for the gley
soils with highest average concentrations in the Bs-horizon (295 lg mg-1 in the
Bs-horizon compared with 2.19 and 96.7 lg mg-1 in the Lf- and C-horizons, respectively).
3.6 The UK Acid Waters Monitoring Network
Seasonal Kendall analysis indicates that there are significant (p\0.05) Fe trends at 14 of
the 22 sites, of which 12 were positive. Of the 10 sites with non-significant, zero or
negative trends, 6 had very low Fe concentrations (\0.04 mg l-1), and trend estimates are
believed to have been influenced by frequent below-detection samples: only one site with
mean Fe \0.04 mg l-1, Dargall Lane, had a significant rising trend.
Time series for all sites with mean Fe C0.04 mg l-1 (Fig. 2) illustrate the general
pattern of increasing Fe concentrations. In some cases these trends appear linear and
monotonic, in others there is some indication of a step change and/or levelling off of
concentrations from around 1998. One site, the Bencrom River, had a significant
decreasing trend, which again appears to be associated with a change from 1998 onwards.
Several sites (e.g. Afon Hafren, Old Lodge, Llyn Cwm Mynach, and Bencrom River) are
affected by one or more high-Fe outliers. These could be associated with analytical errors,
although some appear to be sustained over more than one consecutive sample, or to
coincide with peaks in other determinands, suggesting that they may be associated with
real but infrequent high-Fe events. In particular, the very large September 2002 peak at Old
Lodge coincided with unprecedented high DOC concentrations at this stream, and very low
discharge.
There is a close association between Fe and DOC within the UKAWMN, both spatially
and temporally. Site mean concentrations of Fe and DOC are strongly correlated (Fig. 3a;
Aquat Geochem (2008) 14:263–288 275
123
r2 = 0.83, p \ 0.001), although within this general relationship there is considerable
variability in the site mean Fe:DOC ratio, from 6 to 93 lg Fe mg-C-1. The amount of Fe
per unit DOC increases as mean DOC increases (Fig. 3b, r2 = 0.49, p\0.001), indicating
that the relationship shown in Fig. 3a is non-linear.
There is also a strong temporal correlation between Fe and DOC concentrations of
individual samples at most sites (Table 2), but again the gradient of this relationship varies
between sites (9 to 66 lg Fe mg-C-1). In part, this correlation may reflect similar short-
term discharge responses (i.e. both Fe and DOC increase at high flow). However, analysis
of Seasonal Kendall Trend estimates show that rates of long-term change in Fe and DOC
are also correlated. Significant Fe and DOC trends coincide at 13 sites, and for these sites
there is a positive correlation between Fe and DOC trend slope (Fig. 4, r2 = 0.56, p =
0.003). Incorporating the remaining sites (seven with non-significant Fe trends and two
with non-significant DOC trends) still produces a significant correlation (r2 = 0.40, p =
0.002). Therefore, both the spatial and temporal factors that control Fe concentration are
similar (but not identical) to those that determine DOC concentration.
There is one moorland site within the UKAWMN, the Gwy, which lies within the
Plynlimon catchments: the Gwy is the upper part of the River Wye and it is adjacent to
the upper River Severn and both have sources in the Plynlimon plateau area. The Gwy and
Fig. 2 Time series of Fe concentration for the UKAWMN sites: time series for 7 sites with mean Feconcentrations below 0.04 mg l-1 are not shown. Also, in a few cases the scale has been expanded toexclude anomalously high values as including them would obscure the underlying pattern. Numbers onfigures correspond to UKAWMN site numbers
276 Aquat Geochem (2008) 14:263–288
123
the upper Hafren moorland sites show similar trends: the gradients for the Gwy (excluding
4 outlier values) and the upper Hafren are 3.66 ± 2.15 and 3.73 ± 1.23 lg l-1 yr-1,
respectively (both p \ 0.001).
3.7 Modelling Iron Speciation
3.7.1 ‘‘Truly Dissolved’’ and Microparticulate Fe(III)
The Fe determined in this study is operationally defined (Kennedy et al. 1974) as the
fraction that passes a 0.45 lm filter. This fraction will comprise in part ‘‘truly dissolved
Fe’’, that is Fe2+ and Fe3+ ions together with their complexes with inorganic ligands and
organic matter. It is also clear from past research that this fraction contains Fe-rich colloids
(e.g. Stolpe et al. 2005), probably in the form of amorphous Fe oxyhydroxides. Knowledge
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 5 10 15
DOC (mg l-1)
Fe
(mg
l-1)
a)
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15
DOC (mg l-1)F
e/D
OC
(m
g m
g-1
)
b)
Fig. 3 The relationship between Fe and DOC mean concentrations is examined for the UKAWMN riversand lakes. The graph on the left represents a plot of mean Fe versus mean DOC, while the graph on the rightrepresents the mean of the ratio of Fe to DOC against the mean DOC for each site
-0.004
-0.002
0
0.002
0.004
0.006
0.008
0.01
0 0.1 0.2 0.3 0.4 0.5
DOC trend slope (mg l-1 yr-1)
Fe
tren
d sl
ope
(mg
l-1 y
r-1)
Fig. 4 Estimated trend slopesfor Fe versus DOC at theUKAWMN sites. Filled circlesindicate sites with significant (p\ 0.05) trends in both Fe andDOC. Open circles indicate siteswith non-significant Fe trends,open diamonds sites with non-significant DOC trends.Regression best-fit line is basedon sites with significant trends inboth Fe and DOC
Aquat Geochem (2008) 14:263–288 277
123
of the distribution of measured Fe between microparticulate and ‘‘truly dissolved’’ forms is
important in interpreting the temporal trends, particularly to establish whether the corre-
lation observed between Fe and DOC in the streams can be attributed to changes in Fe
bound to natural organic matter. Lofts et al. (2008) recently proposed a method for the
estimation of the ‘‘truly dissolved’’ Fe(III) in a water, based on the assumption that (i) Fe3+
activity is controlled by equilibrium with a solid oxyhydroxide phase and (ii) that the
concentrations of Fe(III) bound to ligands may be predicted given knowledge of the
relevant binding strengths. Conventionally, the oxyhydroxide precipitate is defined as
having the formula Fe(OH)3(s) and the solubility equilibrium is:
Fe OHð Þ3ðsÞ# þ3Hþ $ Fe3þ þ 3H2O; Kso ¼aFe3þ
a3Hþ
ð1Þ
where aFe3þ and aHþ are the solution activities of Fe3+ and H+, respectively, and Kso is the
solubility product. Research on the solubility of ferric hydroxide in artificial solutions (e.g.
Byrne and Luo 2000) indicates that the apparent Fe3+:H+ stoichiometry in the solubility
relationship is less than 3, either due to incorporation of anionic elements into the solid, or
due to particle size effects on the activity of Fe(OH)3 (s). By dialysing natural waters to
remove Fe oxyhydroxide and speciation modelling of the dialysates, using WHAM/Model
VI (Tipping 1994, 1998), Lofts et al. (2008) developed the following solubility relationship:
K 0so ¼aFe3þ
a2:70Hþ¼ 102:93 at 283 K ð2Þ
Using this relationship and the chemical composition of the water in question (after
filtration) it is possible to calculate the concentration of ‘‘truly dissolved’’ Fe(III) without
a priori knowledge of the filterable Fe. The concentrations of non ‘‘truly dissolved’’ Fe, e.g.
Fe(III) oxyhydroxides, microparticulate Fe-containing clays and Fe(II) species, may then
be calculated by difference. In all the calculations done here, dissolved organic matter
(DOM) was assumed to be 65% chemically ‘‘active’’. The chemically active fraction of
DOM was represented in WHAM/Model VI by fulvic acid, with an Fe(III) binding con-
stant of 2.6.
Speciation of filterable Fe in the Lower Hafren was calculated for mean stormflow,
baseflow and overall conditions. Under mean stormflow conditions (pH 4.5 and DOC 3.0
mg l-1) the model predicted 6% of Fe to be Fe(III) bound to organic matter (Fe(III)-DOM),
18% to be inorganically bound Fe(III) and 76% to be microparticulate or Fe(II). Under
mean baseflow conditions (pH 6.5 and DOC 0.9 mg l-1), the model predicted 5% of Fe to
be Fe(III)-DOM, 2% to be inorganically bound Fe(III) and 93% to be microparticulate or
Fe(II). Under overall mean conditions (pH 5.4 and DOC 1.5 mg l-1) the model predicted
3% Fe(III)-OM, 6% inorganically bound Fe(III) and 91% microparticulate or Fe(II). Thus
the predicted speciation indicates that microparticulate forms of Fe, probably mostly
Fe(III) oxyhydroxides, predominate across all flow conditions.
Speciation of Fe may also be considered by calculating the predicted ratio of Fe(III) to
dissolved organic matter (DOM) due to Fe(III):DOM complexation, for a range of pH and
a typical water composition. This ratio includes free ionic and inorganically complexed
Fe(III) and Fe(III) bound to DOM. This ratio (termed the apparent m, with units of moles
Fe(III) per g DOM, to distinguish it from the true m, which is the amount of Fe(III) bound to
DOM per g DOM) takes a characteristic set of values given a particular Fe(III) oxyhy-
droxide solubility, under the assumption of solubility control of the Fe3+ activity. If the
measured ratio of filterable Fe to filterable organic matter exceeds the predicted ratio, this
can be taken as an indication of the presence of microparticulate Fe and/or Fe(II). This
278 Aquat Geochem (2008) 14:263–288
123
relationship is examined in Fig. 5 for the lower Hafren, the Nant Tanllwyth, and Plynlimon
groundwater separated into catchments with podzolic soils (lower Afon Hafren) and gley
soils (Nant Tanllwyth).
The apparent m values for both systems are characteristic of the presence of micropar-
ticulate Fe or Fe(II) across the entire range of pH encountered. In the surface waters this Fe is
likely to be mostly oxyhydroxides rather than Fe(II). However, in the case of groundwater,
Fig. 5 Apparent m and our estimate of true m versus pH for the lower Afon Hafren, the Nant Tanllwyth,Plynlimon groundwater with podzol and gley cover and the UKAWMN rivers and lakes. N.b. for theUKAWMN data, no value for apparent m is used where Fe or DOC concentrations are below the detection limit
Aquat Geochem (2008) 14:263–288 279
123
there is a clear separation between podzolic and gley soil. For both the podzolic and gley soil
cases the data mainly fall above the solubility line. The results are similar to that for the
streams except that the apparent m values are an order of magnitude greater with the gley
groundwater than the streams while for the podzols the values are intermediate. For the
groundwater podzol case the greatest apparent m values occurs for the HA4 borehole with the
early part of the record. For this early part of the record and for the gley groundwater cases in
general, microparticulate Fe is probably of importance as with the streams, but there is also
an additional Fe(II) component that reflects the reducing conditions.
With respect to Fe speciation for the UKAWMN rivers the thermodynamic plots
(Fig. 5) indicate a similar feature to that for the Plynlimon streams with m values above or
approaching the solubility line. Further, the observation for the UKAWMN of the amount
of Fe per unit DOC increases as the mean DOC concentration is at odds with the theory
that complexation to DOM controls the variability in Fe concentrations since the amount of
Fe bound per unit DOC should be independent of DOC concentration. WHAM/Model VI
predicts that the Fe:DOC ratio resulting from Fe(III) complexation to DOM (the true m) to
vary between 32.4 and 1.8 lg Fe mg-C-1 between pH 4 and 8. The Fe:DOC ratios
observed in the higher DOC systems clearly exceed these predicted ratios. Thus, the
variability in Fe does not appear to result directly from complexation to DOM in the
streams and lakes.
For the soils, notwithstanding the possible occurrence of solid phase organic complexes
(Van Schaik et al. 2008), analysis of the data using WHAM/Model VI (Fig. 6) indicates for
the podzols that apparent m values are close to the solubility line for soils within the lower soil
horizons, but lie under the line for the near surface horizons that are depleted in inorganic
sources of Fe. The same situation applies for the gley except that in the lower soil horizons
the data lie just above the solubility line and this is consistent with Fe being present in part in
reduced form. However, it must be noted that the use of ceramic cups for extracting soil
waters may filter out micro-particulate material (Reynolds et al. 2004) and that the micro-
particulate form may be underestimated in the lower parts of the soil profile when iron
solubility controls might come into play. Indeed, microparticulate transport of Fe is often
taken as an important mechanism for the formation of these soils (Russell 1966).
3.7.2 Binding to DOM to Fe(III) Oxyhydroxides
The binding of DOM to the surfaces of Fe(III) oxyhydroxides is a well-attested phe-
nomenon under laboratory conditions (e.g. Tipping 1981). Under typical surface water
conditions of circumneutral pH, the process is believed to stabilise oxyhydroxides against
aggregation by giving the particle surface a net negative charge. However, the system is
complex to model mechanistically due to the heterogeneous nature of DOM, and although
some progress has been made in well-defined conditions (e.g. Filius et al. 2000), prediction
of binding in the field using full mechanistic models is not currently feasible. To obtain
some broad estimates of the importance of this process in the absence of a mechanistic
model, we used data obtained by Tipping (1981) to produce an indicate empirical model.
The data in question were for the binding of humic substances (HS), isolated from
Esthwaite Water (a lake in the English Lake District), to a sample of the crystalline iron
oxide goethite, in a water of composition mimicking that of Esthwaite Water. These data
were considered most appropriate for modelling since Tipping (1981) found that binding
was enhanced in solutions mimicking natural waters as opposed to simple electrolyte
solutions. Esthwaite Water is a soft water lake and thus the composition can be regarded as
broadly representative of UK waters draining non-calcareous landscapes.
280 Aquat Geochem (2008) 14:263–288
123
Tipping (1981) measured isotherms for HS binding to the goethite at pH 6, 7 and 8, and
measured the proportion of HS bound for a number of HS:goethite ratios in the pH range
5 to 9. We used the isotherm data to parameterise the model. The model used was a
standard Freundlich isotherm:
HS½ �ads mg m�2 FeOx� �
¼ K � HS½ �diss mg l�1� �1=n
h ið3Þ
We assumed that the concentration of adsorbed HS per unit surface area of the oxide
would be independent of the specific surface area. This would allow the results for the
goethite (specific surface area 15 m2 g-1) to be used to predict binding to amorphous
oxyhydroxides of substantially greater specific surface area. We also assumed that the
Freundlich binding constant K could be described as a simple function of pH,
log K ¼ m � pHþ c; ð4Þ
thus allowing the isotherm to be applied to data at multiple pH values:
log HS½ �ads mg m�2 FeOx� �
¼ m � pHþ cð Þ þ 1=nð Þ � log HS½ �diss mg l�1� �
ð5Þ
Fitting this model gave parameter values m = -0.12, c = 0.96 and n = 2.85 and the
model provides an excellent fit to the observations (Fig. 7). The pH-dependent adsorption
data were used to check the performance of the model. The predictions are largely
Fig. 6 The variations in apparent m and our estimates of true m with pH for podzolic and gley forest soilwaters for the LF-, O-, E-, B- and C-horizons. Data source: Reynolds et al. (1988) and Hughes et al. (1990)
Aquat Geochem (2008) 14:263–288 281
123
excellent, including below the lower calibration limit of pH 6 (Fig. 7). The binding of the
lowest concentration of HS is overestimated by the model in comparison with the mea-
surements: this may be due to errors in the measurement of the low residual concentrations
of unadsorbed HS.
For model application it was necessary to estimate the concentration of microparticulate
Fe(III) oxyhydroxide. This was done by taking the difference between the total filterable Fe
and the calculated ‘‘truly dissolved’’ Fe(III) and Fe(II). The truly dissolved Fe(II) was
estimated to be 24% of the total filterable Fe, based on measurements in UK waters
reported by Lofts et al. (2008). The amount of Fe calculated was assumed to be entirely in
oxyhydroxide form and its concentration (g dm-3) was calculated assuming a stoichi-
ometry of Fe(OH)3. A surface area of 600 m2 g-1 was used for scaling adsorption data.
This value was recommended by Dzombak and Morel (1990) for the modelling of the
binding of small ions to Fe(III) oxyhydroxide. It may be an overestimate with respect to
DOM binding, since a large proportion of the surfaces of iron(III) oxyhydroxide are
believed to comprise internal pore space which might not be readily accessible to DOM
molecules. However, since the purpose of the modelling is to ascertain the general sig-
nificance of the DOM binding process rather than to make precise predictions, we believe
that the assumption is justified. We have also assumed that microparticulate Fe(III) is
entirely in the form of oxyhydroxides. In principle some Fe(III) may well be present in clay
minerals. However we have no information allowing us to reasonably estimate how much
Fe(III) might be in such a form. As before, we justify our assumption on the basis that the
dissolved HS (mg l-1)
0 5 10 15 20 25ad
sorb
ed H
S (m
g m
2 )
0
1
2
3
4
5
pH
5 6 7 8 9
% H
S ad
sorb
ed
0
20
40
60
80
100
Fig. 7 Top pane: Results offitting binding model toadsorption isotherms for HSbinding to goethite after Tipping(1981). Symbols denote bindingat different pHs: circles, pH 6;triangles, pH 7; squares, pH 8.Total oxide surface area *1.5 m2
g-1. Lines are model fits. Bottompane: predictions of HS bindingto goethite made using theparameterised binding model.Symbols different totalconcentrations of HS: circles,1.08 mg l-1; triangles, 2.71 mgl-1; squares, 5.42 mg l-1;diamonds, 10.83 mg l-1; invertedtriangles, 27.08 mg l-1. Totaloxide surface area *1.5 m2 g-1.Lines are model fits
282 Aquat Geochem (2008) 14:263–288
123
model is intended to give a broad rather than a precise picture of the importance of surface
DOM binding.
For baseflow, stormflow and average conditions at Plynlimon, the model predicted that
4%, 6% and 5% of DOM would be bound to iron(III) oxyhydroxides. Thus, at least under
the conditions prevailing at Plynlimon, the association of DOM and Fe by the binding of
the former to oxides of the latter appears to be a minor process. By corollary the con-
centrations of unbound DOM in the streams are in excess relative to the concentrations
bound to the oxyhydroxide surfaces.
Predictions of DOM binding to Fe oxyhydroxides for the UKAWMN sites generally
indicate similar results to that for Plynlimon. Of the 22 UKAWMN sites, only at five
(River Etherow, Old Lodge, Afon Hafren, Beagh’s Burn and Coneyglen Burn) was more
than 5% of DOM predicted to be bound on average. Considerable temporal variability in
the proportion of DOM bound was indicated, for example, between 2% and 81% for Old
Lodge and between 0.3% and 34% for Beagh’s Burn. In Beagh’s Burn and Coneyglen
Burn, high bound proportions of DOM were associated with high pH (typically above 6.5),
high Fe and low flow conditions.
4 Discussion
Iron concentrations in rainfall are relatively low, and the highest levels occur at low
volumes of catch due to lack of dilution of particles scavenged from the atmosphere. There
can be a significant deposition flux related to the cloud water input (Wilkinson et al. 1997).
Iron concentrations in throughfall and stemflow are generally low and this reflects limited
cycling through the vegetation. As with rainfall, the highest Fe concentrations occur with
low volumes of catch/flow in cloud water, stemflow and throughflow. Iron in the streams is
essentially derived from the catchment (typically around 90 to 95%; Durand et al. 1994;
Neal et al. 1997a). The highest Fe concentrations occur within the soil and groundwater
and the highest values are probably associated with redox controls where Fe is mobilised
under reducing conditions (Drever 1997; Appelo and Postma 1999). Under the well-
oxygenated surface waters, Fe2+ oxidises and there is the potential for Fe3+ precipitation/
colloid formation. Indeed, the trends in Fe within the streams are consistent with increased
anoxia within the organic rich soils as the catchments wet up—this is a particular feature
for the Plynlimon forested catchments where there has been a relatively large and long-
lasting increase in the low flows (Q95) and a corresponding but limited change in the
baseflow index (Robinson and Dupeyrat 2005) as felling has occurred. Nonetheless, the
increase in the rate of increase in Fe concentration around 1993–1994 does not correspond
with forest harvesting, and the changes observed cannot simply be linked to the timing and
extent of the fell.
One of the strongest features of the Fe stream data for Plynlimon and the rivers and lakes
within the UKAWMN is that there is a positive trend over time with a corresponding
increase in extremes concentration. A similar pattern has been observed for DOC at Ply-
nlimon (Neal et al. 2005) and in the UKAWMN streams (Evans et al. 2006). This feature is
probably associated with a concentration increase in the Fe- and DOC-rich soils. While lakes
clearly function in different ways to the rivers, with longer residence times, and greater
mixing of water from different sources within the lake, there nonetheless seems to be some
similarity. A particular feature of the analysis, for both the Plynlimon and UKAWMN
datasets, is a general correlation between Fe and DOC concentrations in both space and time.
The spatial relationship exhibits higher Fe:DOC ratios in waters having higher DOC
Aquat Geochem (2008) 14:263–288 283
123
concentrations. DOC concentrations are generally highest in waters draining peat (Hope
et al. 1997), but substantial DOC export can also occur from other soils, particularly soils
with a deep organic horizon, such as peaty podzol and peaty gley (Evans et al. 2007). In the
case of peaty podzols, which cover much of the UK upland area including large areas of
Plynlimon and the UKAWMN catchments, these organic-rich horizons are normally aero-
bic, and do not therefore favour Fe export. On the other hand, waterlogged peat and peaty
gley are normally anaerobic, favouring Fe (as well as DOC) export. In other words, the
spatial controls on Fe and DOC release are similar, but not identical: runoff DOC concen-
trations appear to be a function of the presence and extent of organic soils, whereas Fe
concentrations also depend on the extent of waterlogging within this soil.
As the Fe and DOC trends are correlated, and since it is believed that a major reason
DOC is increasing is that acid deposition is falling (Evans et al. 2006; Monteith et al.
2007), the implication is that Fe must be responding to the same driver. However, our
mechanistic modelling clearly indicates that increases in Fe(III)–organic matter com-
plexation in surface waters cannot account for the increases observed in Fe concentrations.
A more likely mechanism is the increased leaching into surface waters of microparticulate
Fe(III) oxyhydroxides formed during oxidation of groundwaters, due to increased binding
of DOM to their surfaces. Alternatively, there may be increased transport of Fe(III) within
soil and groundwaters as organic complexes followed by transformation into micropar-
ticulate form in surface waters following loss of DOM, e.g. by mineralization. Further,
only in the case of the forested catchments at Plynlimon do there seem to be higher rates of
Fe concentration increase post-1993. For the Plynlimon area there seems to be an increase
in the gradient of the Fe time series from moorland to forest and to gley, but this pattern
does not seem to be observed for the UKAWMN sites and general statements cannot be
made as to what particular typology the differences in gradients relate. Thus, it is plausible
that the recent increases in DOM concentrations in surface waters are causing the observed
increases in Fe concentration by binding to oxyhydroxides, thus promoting their solubility
and transfer from soil and groundwater to surface water. One feature to note in this regard
is that for the Plynlimon forested areas, there have been extensive clear-fell or tree-
thinning, and this has led to reduced SOx and non-marine SO4 scavenging from the
atmosphere, thus intensifying the rate of acidification recovery (Neal et al. 2001) relative to
sites with no forest cover or intact forest.
With regard to the soils, the process of podzolisation and gleying are major mechanisms
of soil formation and podzols and gleys are major soil categories for the British uplands
(Curtis et al. 1976). Some of the highest Fe concentrations are observed in the Oh-horizon
relative to other soil units as well as the atmospheric inputs, stream water and groundwater.
Indeed, the Fe:DOC gradient is similar for the Oh-horizon and the Plynlimon streams
draining podzol. The situation is more complex in the case of the gley soils and the Bs-
horizon has particularly high Fe concentrations. An earlier work reported on the controls
on the mobility of Fe (and Al) in the podzols which are spatially dominant in the Plynlimon
catchments (Hughes et al. 1990). The work by Hughes et al. (1990) on the peaty podzols
points to the classic processes of mobilisation of Fe and soluble organic matter in the
surface horizons with downward leaching and deposition of Fe-organic matter complexes
in the Bs- and C-horizons. DOC probably has an important controlling influence on Fe
mobility in the surface horizons of the gley, but redox processes appear to be more
important with depth in the mineral soil. Hughes et al. (1990) conclude that dissolved
organic matter determined the chemistry, solubility and transport of Fe in the Oh-horizon
of the podzols. The distribution of Fe through the podzol profile was consistent with
traditional podzolisation theory, with a marked decrease in Fe and DOC concentrations in
284 Aquat Geochem (2008) 14:263–288
123
the Bs- and C-horizons compared to those observed in the Oh- and Eag-horizons. The
strong positive relationship between DOC and soil temperature and the further increase in
DOC in response to clear-felling provided evidence of a strong biological control on DOC
in these soils. Consequently biological processes also indirectly influence Fe mobility.
In terms of the questions raised in the introduction, the following can be surmised.
Do changes in pH affect Fe mobilisation? Some increased mobilisation of ‘‘truly dis-
solved’’ Fe(III) might be expected with increasing acidification although in practice this
might be countered by decreases in DOM concentrations. As acidification decreases, the
amount of ‘‘truly dissolved’’ Fe(III) mobilised per gram of DOM should decrease, so any
increase in ‘‘truly dissolved’’ Fe(III) must be due to a more than compensatory increase in
DOM concentration. In the present case, Fe concentrations are increasing as acidity is
reducing (i.e. ANC is increasing). The observed increases in Fe are then consistent with our
hypothesis that increasing stream DOM concentration increases microparticulate Fe(III),
since it is now believed that increasing DOM is a response to decreases in acidity. Thus,
changes in pH do affect Fe mobilisation, albeit the mechanisms appear to be indirect and
linked to the response of DOM concentrations to changes in acid inputs.
How important is organic complexation given that the dissolved organic carbon (DOC)levels have been increasing over the last 20 years or more in such systems? WHAM/Model
VI modelling results indicate that despite the clear spatial and temporal relationships
between filterable Fe and DOC, this is not directly related to the degree to which Fe(III) is
complexed by DOM in the streams. Rather, while there is often a correlation between Fe
and DOC, much of the Fe is associated with microparticulates rather than organically
complexed. Association of DOM with the surfaces of microparticulate Fe does not appear
to be an important vector for the transport of DOM itself; increased stabilisation and
solubilisation of microparticulate Fe due to increasing DOM concentrations is a more
likely hypothesis.
Does conifer plantation rotation cycles affect iron mobilisation? There is no clearly
discernable direct forestry harvesting effect within the streams (other than perhaps associ-
ated with extensive harvesting/land-disturbance for the Upper Hore late on in the study), but
there is evidence for Fe (and DOC) concentration increases within the soils following felling.
Are Fe levels such that they may affect water potability given that the uplands provide amajor water resource in this area? The water quality standard for Fe in drinking water is 200
lg l-1 (total, unfiltered maximum allowable concentration) and for the protection of sal-
monids and less sensitive fish is 2000 lg l-1 (dissolved annual average) (Gardiner and Zabel
1991). There are issues for drinking waters and for some of the UK acidic waters as the
extremes in Fe concentrations can exceed the dissolved annual average for fish. In addition,
there are issues of potential impacts on spawning grounds and smothering of benthos from Fe
microparticulates as they flocculate out of solution (Marsden and Mackay 2001; Younger
2001). As previously noted, the FOREGS monitoring data clearly indicate that the issue of
exceedance of the drinking water quality standard is not confined to the UK.
5 Conclusion
Long-term monitoring of UK upland water quality is revealing changes in water quality
that link not only to acidification but also to climate change and climate instability and
presently acidity seems to be reducing (Evans et al. 2001; Jenkins et al. 2001) while DOC
is generally increasing (Worrall et al. 2004), possibly as a consequence of recovery from
acidification (Evans et al. 2006; Monteith et al. 2007). The present study indicates that Fe
Aquat Geochem (2008) 14:263–288 285
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also is increasing and that the increase is generally associated with DOC and other organic-
related determinands such as bromine and iodine (Neal et al. 2005). For the forested sites at
Plynlimon, where felling will have reduced the atmospheric deposition of acidifying SOx,
the greatest increase in Fe concentrations occur, and the two features may well be related.
Mechanistic insights suggest that this phenomenon of increasing Fe concentrations is not
driven by formation of dissolved Fe(III)–organic matter complexes; a more likely mech-
anism is the stabilisation of increased concentrations of microparticulate Fe (probably
largely Fe(III) oxyhydroxides but possibly some Fe-containing clays) by increasing con-
centrations of DOC. Nevertheless, this remains a tentative conclusion at the moment, since
there remains a need to examine in more detail redox controls, the rate of aeration and
Fe-oxidation of the surface water, and the Fe speciation (in particular Fe2+, Fe3+, inorganic
versus organic forms and micro-particulate components). These aspects warrant further
research. Extension of the work to other temperate regions is also a clear priority in the
context of drinking water quality standards. Correspondingly on the fundamental research
front for Fe in the aquatic environment, there are many issues with regards to colloid/
nanoparticle stabilisation, changes in the pH of zero-point charge for the Fe-DOM couplets
and novel applications of techniques such as small-angle neutron scattering that can
examine controls on natural aquatic nano-colloid interactions in aqueous dispersion (Lead
and Wilkinson 2006; Jarvie and King 2007), all of which merit further study.
Acknowledgements We are indebted to the UKAWMN for allowing us to use their data, and we thankthem for their enthusiasm and openness. Their data show how the UK upland environment is changing ratherthan opinion and long may the programme continue. We are also grateful to David Cooper for running trendanalyses on the UKAWMN dataset.
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