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Nitrate leaching as a confounding factor in chemical recovery from acidification in UK upland waters
Environmental Pollution 137 (2005) 73e82 www.elsevier.com/locate/envpol
Nitrate leaching as a confounding factor in chemical recovery from acidification in UK upland waters C.J. Curtisa,*, C.D. Evansb, R.C. Helliwellc, D.T. Monteitha a
ECRC, University College London, 26 Bedford Way, London WC1H 0AP, UK CEH Bangor, Orton Building, Deiniol Road, Bangor, Gwynedd LL57 2UP, UK c Macaulay Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK
b
Received 31 October 2004; accepted 17 December 2004
With declining excess sulphate, nitrate will become the dominant agent of continued anthropogenic acidification in many UK upland waters within a decade. Abstract Over the period 1988e2002, data from 18 of the 22 lakes and streams in the UK Acid Waters Monitoring Network (AWMN) show clear trends of declining excess sulphate concentrations in response to reductions in sulphur deposition, but fewer trends in increasing pH or alkalinity. There has been no significant decline in the deposition of total nitrogen over the same period, and no sites show a trend in nitrate concentration. Peak nitrate concentrations have already surpassed excess sulphate on occasion in half of the AWMN sites. Furthermore, current understanding of terrestrial N saturation processes suggests that nitrate leaching from soils may increase, even under a constant N deposition load. Best-case projections indicate that nitrate will overtake sulphate as the major excess acid anion in many sites within 10 years, while worst-case predictions with steady-state models suggest that in the longer-term, nitrate could become the dominant excess acid anion in most of the UK. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Nitrogen; N saturation; Acid deposition; Sulphate; Critical load
1. Introduction The UK AWMN has provided direct evidence of rapidly declining surface water non-marine (or ‘‘excess’’) sulphate (xSO4) concentrations in 18 of 22 monitored lakes and streams in response to reductions in the deposition of anthropogenic sulphur (S) compounds since the mid-1990s (Cooper and Jenkins, 2003). However, declining trends in xSO4 correspond with increases in alkalinity or pH at only ten sites. This is in part due to a decline in base cation leaching in response to a reduced acid load. In addition, significant positive * Corresponding author. Fax: C44 207 679 7565. E-mail address: [email protected] (C.J. Curtis). 0269-7491/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2004.12.032
trends in dissolved organic carbon (DOC) confound the detection of alkalinity trends and are at least partly responsible for increasing trends in acid neutralizing capacity (ANC) (Davies et al., 2005, this issue). Other possible reasons for the lack of significant trends in alkalinity and pH at some sites are the confounding effects of hydrological influences, seasalt inputs and nitrate (NO3) leaching (Evans and Monteith, 2002). While xSO4 is generally still the dominant excess acid anion in anthropogenically acidified surface waters, NO3 contributes significantly to the total excess acid anion load in several regions and its relative role will increase as xSO4 concentrations decline. However, temporal variability in NO3 leaching into surface waters complicates assessment of its importance, and
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C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
potentially hinders detection of longer-term trends in alkalinity and pH. NO3 leaching into surface waters follows a seasonal pattern partly because of hydrological controls and partly because of the biological demand for nutrient N in the terrestrial ecosystem (INDITE, 1994; Reynolds and Edwards, 1995). Furthermore, mean concentrations on a year-to-year basis may be affected by climatic factors (Monteith et al., 2000; Davies et al., 2005, this issue). More importantly for recovery prospects in upland regions where S deposition is declining, not only is N deposition expected to decline to a smaller degree than S, but for a given level of N deposition, NO3 leaching into surface waters may increase in the longer term as N saturation of terrestrial ecosystems occurs (A˚gren and Bosatta, 1988; Skeffington and Wilson, 1988; Aber, 1992; Stoddard, 1994). The timescale over which this process may occur is largely unknown. Only small trends confined to central and south-eastern England have been observed for N deposition (Fowler et al., 2005, this issue), while no relationship was found between N deposition and leaching of inorganic N species by Cooper (2005, this issue). Hence, while future changes in both deposition and leaching of inorganic N species are rather uncertain, major reductions in either are unlikely in the short term. This paper provides an assessment of the increasing importance of NO3 as an acidifying anion in both relative (to xSO4) and absolute terms using best-case/ worst-case models of NO3 leaching.
2. Methods This work is focussed on the impacts of acid anions derived from anthropogenic atmospheric deposition, which are assumed to be primarily xSO4 and NO3. It is assumed that anthropogenic chloride deposition is generally negligible. Furthermore, the effects of organic acid anions are beyond the scope of this work. Hereafter, the term ‘‘excess acid anions’’ is therefore used to describe the sum of xSO4 and NO3 as the main anthropogenic agents of acidification. Protocols for the collection and analysis of water chemistry data for the AWMN sites are described in Monteith and Evans (2000), while trend analysis is reported in Davies et al. (2005, this issue). Methods for the derivation and trend analysis of deposition data are provided in Fowler et al. (2005, this issue). To place the AWMN sites into a spatial context, several regional datasets have been drawn from the national critical loads mapping programme (Curtis and Simpson, 2004), which provides good regional coverage of surface water samples (commonly standing waters) from the key areas impacted by acidification. The correspondence between AWMN sites and regional
datasets is given in Table 1. AWMN site numbers are linked to site names in Table 2. Note that sampling frequency and periods represented vary between regional datasets, while seasonal variations in NO3 mean that one-off samples are very sensitive to sampling season. However, the problem of obtaining representative data with a one-off sample has generally been minimised by sampling in spring or autumn, when chemistry is assumed to most closely represent annual, flow-weighted mean values (Forsius et al., 1992). Site-specific xSO4 trends are unavailable for regional datasets other than the AWMN sites, so the scaling and FAB modelling approaches described below were applied. 2.1. Scaling from deposition trends: ‘‘best-case’’ NO3 leaching Given the strong links between xSO4 concentrations and S deposition on an annual mean basis at AWMN sites (Cooper and Jenkins, 2003; Cooper, 2005, this issue) it is possible to predict surface water xSO4 concentrations for projected future deposition levels by rescaling measured values to changes in deposition. Links between NO3 leaching and total N deposition on a mean annual basis are poor (Cooper, 2005, this issue) but a ‘‘best-case’’ NO3 leaching scenario assumes that NO3 is leached at a fixed proportion of inputs according to hydrological controls (e.g. see Evans et al., 2004), i.e. the current proportion of total N deposition leached may be determined by estimated inputeoutput budgets and applied to different deposition levels in future. It is assumed that catchment N saturation does not occur over the period under consideration so that the remaining proportion of inputs is biologically retained in catchment plants and soils over the long term. Mean surface water xSO4 and NO3 concentrations were scaled to projected changes in excess S and total N deposition by 2010 according to the FRAME model (Singles et al., 1998). Since the most recent modelled deposition data were mean values for the period 1998e 2000, mean AWMN water chemistry data from the same period were used as the starting point for scaling. For regional datasets which pre-date the period 1998e 2000, best available contemporary deposition data were used as the starting point for scaling to 2010 concentrations. Methods for the modelling of deposition data from measurements made at a national monitoring network are given in CLAG Deposition Fluxes (1997). 2.2. FAB model predictions of S and N leaching: ‘‘worst-case’’ NO3 leaching Mean water chemistry data for 2002 from the 22 AWMN sites feed into the UK national critical loads mapping programme. Critical loads and exceedances
75
C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82 Table 1 Regional datasets used in spatial comparison with AWMN sites Dataset
ntot
nex
Sampling period
Site selection criteria
Reference
AWMN site no.
AWMN, UK
22
15
1988e2002a,b
11
1
Davies et al., 2005, this issue Allott et al., 1995
All
NW Scotland
Cairngorms, NE Scotland Trossachs, central Scotland Galloway, SW Scotland
37 32 61
13 0 31
1999c 2002c 1996e8d/2002a
Undisturbed, acid-sensitive lakesa and streamsb Sea-salt input gradient (altitude, distance to sea); lochs Comprehensive e lochs Comprehensive e lochs Comprehensive e lochs
2, 4 5, 6 7, 8, 9
Lake District, NW England
53
28
2000c
Helliwell et al., 2002 Curtis and Simpson, 2004 Ferrier et al., 2001d, Helliwell et al. (unpublished)a Evans et al., 2001
S Pennines, NW England
64
56
1998c/2002a
Evans et al., 2000
12
S England Dartmoor, SW England
25 20
7 12
2002c 1991e1993c
Curtis and Simpson, 2004 Curtis et al., 2000
13 14
Snowdonia, NW Wales Welsh streams
74 102
31 34
1996c 1995b
Kernan and Allott, 1999 Stevens et al., 1997
15, 16 17, 18
Mournes, N Ireland N Ireland
8 105
8 9
2002a 2000c
Helliwell et al., unpublished Curtis et al., 2001
20, 21 19, 22
1992e1993a
Lakes>0.5 ha without improved land in catchment Reservoirs between Kinder and Ilkley with mostly unimproved catchment Acid sensitive; lakes Critical loads mapping sites !50 km from Narrator Brook; lakes and streams All in 40 km square - lakes Extensive spatial coverage; streams Comprehensive - lakes 10 km grid survey e lakes and streams
1, 3
10, 11
ntot, total number of sites; nex, number exceeding critical load in 2010. a Annual mean based on quarterly samples. b Annual mean based on monthly samples. c One-off sample in spring or autumn. d Mean of annual one-off samples in spring or autumn.
under various deposition scenarios have therefore been calculated for all AWMN sites using the most recent version of the first-order acidity balance (FAB) model (Henriksen and Posch, 2001), as well as for another 1700 surface water sites around the UK (Curtis and Simpson, 2004). The FAB model employs a simple steady-state mass balance for S and N to predict leaching under given deposition loads over the long term, i.e. assuming that N saturation is occurring and that currently elevated rates of catchment N retention cannot be maintained. Low default values for N immobilization in soils are based on chronosequence studies of N content in soil profiles. The N mass balance predicts the leaching of a large proportion of N deposition at long-term steady-state (Curtis et al., 1998; Kaste et al., 2002). The FAB mass balance was used here to provide theoretical steady-state concentrations of xSO4 and NO3 according to mean modelled deposition levels for 2010 following implementation of the EU National Emissions Ceiling Directive (NECD). It may be argued that the future dominance of NO3 as an excess acid anion is only important if surface waters have not achieved their recovery targets. The critical load provides the threshold for deposition above which
significant harmful effects will occur to a specified sensitive element of the ecosystem (Nilsson and Grennfelt, 1988). For surface waters, critical loads are set with respect to a selected critical value of ANC (Henriksen and Posch, 2001), generally taken as 20 meq lÿ1 in the UK unless site-specific evidence suggests that pre-industrial ANC may have been lower than this threshold, in which case 0 meq lÿ1 is used (Curtis and Simpson, 2004). Within the AWMN, the only site with a reconstructed preindustrial ANC of !20 meq lÿ1 for 1850, using both a palaeolimnological transfer function (Curtis and Simpson, 2004) and the dynamic model MAGIC (e.g. Evans et al., 2001) is Blue Lough. Exceedance of the critical load implies that at long-term steady-state, ANC will decline below the pre-selected threshold for a site. Critical load exceedance was calculated here for modelled deposition in 2010 (NECD). A positive value of exceedance indicates that the critical ANC threshold will be crossed at long-term steady-state. Note that for the regional data, only sites exceeding their critical loads in 2010 are included in comparisons with AWMN sites, as these are the sites at which long-term recovery cannot occur. Lowland sites with high NO3 from nonatmospheric sources are also screened out.
76 Table 2 Measured, scaled and modelled excess acid anion concentrations (meq lÿ1), deposition (keq haÿ1 yearÿ1; NECD, EU National Emissions Ceiling Directive) and critical load exceedance (keq haÿ1 yearÿ1) at AWMN sites Site name
2002 Long-term xSO4 declinea Significance Year of Mean deposition mean mean (meq lÿ1 level ( p) convergenceb 1998e2000 xSO4 NO3a yearÿ1)
a
0.42 0.42 0.42 1.04 1.46 1.25 1.88 2.71 1.25 0.42 1.25 9.38 9.17 n/a 1.67 0.83 1.04 1.67 1.88 1.88 3.33 1.04
NS !0.01 NS !0.01 !0.05 !0.05 !0.05 !0.01 !0.01 NS !0.01 !0.01 !0.01 n/a !0.01 NS !0.01 !0.01 !0.01 !0.01 !0.01 !0.05
2013 2064 e 2021 2015 2016 2010 2007 2030 e 2030 2013 2011 n/a 2008 e 2014 2011 2004 2013 2005 2011
Mean chemistry 1998e2000
Scaled 2010 concentrations
FAB predicted concentrations and exceedance (Exc.)
S
N
S
N
xSO4
NO3
xSO4
NO3
xSO4
NO3
Exc.
0.35 0.25 0.50 0.66 0.91 0.92 0.84 0.67 0.74 1.00 0.82 1.54 0.50 0.54 0.71 0.48 0.78 0.78 0.67 0.67 0.67 0.50
0.37 0.42 0.54 1.06 1.43 1.44 1.42 1.16 1.24 1.61 1.28 2.21 1.14 1.14 1.03 0.76 1.25 1.25 1.18 1.38 1.37 1.02
0.12 0.12 0.19 0.19 0.34 0.34 0.46 0.38 0.41 0.52 0.48 0.69 0.19 0.22 0.30 0.23 0.34 0.34 0.23 0.34 0.34 0.21
0.35 0.42 0.52 0.64 0.99 0.96 1.42 1.32 1.30 1.77 1.57 1.77 0.75 1.35 1.15 0.90 1.39 1.40 0.91 1.29 1.30 0.93
8.1 27.9 21.7 39.5 33.0 21.0 27.2 43.8 45.9 36.5 44.0 170.7 122.3 44.0 27.6 43.2 51.0 35.9 10.3 44.6 43.5 14.6
1.6 1.3 3.6 15.7 12.8 2.5 7.0 23.7 9.6 16.8 6.7 48.2 7.3 8.2 6.9 9.9 21.2 8.6 2.3 19.1 22.9 2.3
2.7 12.7 8.3 11.5 12.6 7.9 14.8 25.3 25.5 18.8 25.8 76.7 47.7 17.7 11.6 20.2 22.1 15.6 3.5 22.7 22.1 6.0
1.5 1.3 3.5 9.4 8.9 1.6 7.0 26.9 10.0 18.5 8.2 38.6 4.8 9.7 7.7 11.7 23.7 9.6 1.8 18.0 21.7 2.1
4.5 10.9 9.2 13.6 18.2 18.1 24.4 20.5 23.8 19.8 23.6 60.6 48.1 17.7 10.4 12.7 17.5 16.6 18.7 25.5 27.2 18.9
2.6 13.7 5.9 20.9 29.0 27.8 50.9 40.3 65.4 57.8 63.1 128.0 108.0 81.3 29.0 33.8 53.3 54.3 50.9 75.0 76.4 56.2
ÿ0.58 ÿ0.40 ÿ0.60 0.22 ÿ0.11 ÿ0.31 1.24 1.03 0.85 1.81 0.32 0.89 0.23 0.64 0.54 0.23 0.30 0.48 ÿ0.29 0.52 0.88 ÿ1.32
Calculated over the period of AWMN monitoring from 1988 to 2002. Hypothetical year of convergence based on the continuation of current xSO4 trends and the maintenance of NO3 at long-term mean concentrations. Sites with non-significant trends excluded. n/a, not applicable (no trend). b
C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
1 Loch Coire nan Arr 7.5 2.5 2 Allt a’Mharcaidh 27.7 1.5 3 Allt na Coire nan Con 13.6 4.0 4 Lochnagar 36.4 16.4 5 Loch Chon 31.0 12.3 6 Loch Tinker 20.6 2.8 7 Round Loch of Glenhead 22.3 6.9 8 Loch Grannoch 32.1 18.3 9 Dargall Lane 46.0 10.6 10 Scoat Tarn 34.0 19.6 11 Burnmoor Tarn 41.4 5.6 12 R. Etherow 149.3 46.5 13 Old Lodge 89.0 7.1 14 Narrator Brook 45.9 7.0 15 Llyn Llagi 18.2 8.2 16 Llyn cwm Mynach 34.0 9.8 17 Afon Hafren 31.2 20.8 18 Afon Gwy 22.6 8.9 19 Beaghs Burn 7.4 3.0 20 Bencrom River 48.2 27.0 21 Blue Lough 37.0 26.6 22 Coneyglen Burn 12.5 2.5
2010 deposition (NECD)
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C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
3. Results 3.1. Current situation The ratio of NO3:(xSO4CNO3) provides an index of the influence of NO3 on chronic acidification status, assuming that both anions are derived from anthropogenic acid deposition (i.e. even if NO3 was deposited initially as reduced forms of N and was subsequently converted to NO3). A value of 0.5 indicates an equal influence on surface water acidification for both NO3 and xSO4, while larger values would indicate a greater influence of NO3 (i.e. NO3>xSO4). Based on annual means for 2002, NO3 provides the greatest proportion of total excess acid anions in Scoat Tarn, the Afon Hafren and Blue Lough (Table 2). Mean NO3 concentrations also exceed half of the xSO4 concentration in Loch Grannoch and the Bencrom River. Time-series plots of NO3:(xSO4CNO3) are shown for the Afon Hafren and Scoat Tarn in Fig. 1, with the value of 0.5 indicated by a solid line. Seasonal variations in this ratio, peaking in late winter or early spring, are clearly seen in both sites. At the Afon Hafren, the ratio has exceeded 0.4 every year since 1997 but only twice in the preceding 9 years. In the winter of 2001e2002 the concentration of NO3 exceeded xSO4 for the first time over the whole period of monitoring. At Scoat Tarn, NO3 has exceeded xSO4 on numerous occasions during the period of monitoring, but there is no indication of an increase in the maximum value of this ratio. In total, 11 of the 22 AWMN sites have at some point experienced
peak NO3 concentrations that exceed xSO4: Loch Coire nan Arr, Allt na Coire nan Con, Loch Grannoch, Scoat Tarn, River Etherow, Old Lodge, Afon Hafren, Beaghs Burn, Bencrom River, Blue Lough and Coneyglen Burn (data not shown). The same ratio (NO3 as a proportion of excess acid anions) is shown both for AWMN sites using annual mean chemistry data and the distribution in regional datasets in Fig. 2. At present, mean xSO4 is greater than NO3 in all AWMN sites (numbered circles in Fig. 2). The same is generally true for the regional datasets; box plots of NO3 as a proportion of excess acid anions show only a few outlying sites at which NO3 is greater than xSO4 (Fig. 2). Northwest Scotland contains only one exceeded site, while none are exceeded in the Trossachs. The two Northern Ireland datasets are the only ones in which NO3 is approaching xSO4 in importance in a high proportion of exceeded surface waters.
3.2. Current trends The response of NO3 as a proportion of excess acid anions for AWMN lake sites with significant trends in xSO4 is shown in Fig. 3. A rising pattern is clearly seen, as expected given the declining trend in xSO4 and lack of trend in annual mean NO3 concentrations. For annual means, the increase appears to start between 1992 and 1995 (Fig. 3a), while for annual maxima, the ratio has been increasing in most of these sites throughout the period of monitoring (Fig. 3b). A similar pattern is also observed for stream sites (data not shown). An apparent step change in NO3 leaching since 1995 at Lochnagar has 1.0
0.5
0.9
0.4
0.8
NO3 / (NO3 + xSO4)
NO3 / (xSO4 + NO3).
a: Afon Hafren 0.6
0.3 0.2 0.1 0
0.6
Scotland
Wales
NI
0.6 0.5 0.4 4
0.3 0.2 0.1
0.4
0.0
5
8 7 9
17 10
16 12
1 3
18
20
21
19
14 15
6 2
0.3
England
0.7
0.5
11
22
13
r s s d s d s s y ct d N ia M tlan orm ach wa stri ine lan moo on am rne lan i o n AW Sco irng oss all e D en Eng art owd stre Mou Ire N D a Tr G ak S P S Sn elsh L NW C W
0.2 0.1 Jul-02
Jul-01
Jul-00
Jul-99
Jul-98
Jul-97
Jul-96
Jul-95
Jul-94
Jul-93
Jul-92
Jul-91
Jul-90
Jul-89
0 Jul-88
NO3 / (xSO4 + NO3).
b: Scoat Tarn
UK
Sampling date Fig. 1. NO3 as a proportion of total excess acid anions (eq eqÿ1) in (a) Afon Hafren and (b) Scoat Tarn. Bold horizontal line indicates equivalence of NO3 and xSO4.
Fig. 2. Surface water NO3 as a proportion of excess acid anions (eq eqÿ1) in AWMN sites and exceeded sites in regional datasets; measured data (B numbered AWMN sites, using long-term mean NO3 from 1988 to 2002 and annual mean xSO4 for 2002). Boxes show 25th and 75th percentiles (inter-quartile range). Whiskers show extent of data excluding outliers. *Represents outliers that are located 1.5e3! the inter-quartile range from the median (horizontal bar).
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C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
a. Annual mean ratio
NO3:(NO3+xSO4) ratio
0.6 0.5 0.4 0.3 0.2 0.1 0 1988
1990
1992
1994
1996
1998
2000
2002
2000
2002
b. Annual maximum ratio
NO3:(NO3+xSO4) ratio
0.6 0.5 0.4 0.3 0.2 0.1 0 1988
1990
1992
Lochnagar Round Loch of Glenhead Llyn Llagi
1994
1996
1998
Loch Chon Loch Grannoch Blue Lough
Loch Tinker Loch Tink Burnmoor Burnmoor Tarn
Fig. 3. Response of NO3 as a proportion of total excess acid anions to trend in xSO4 for the eight AWMN lakes with significant xSO4 trends in Table 2. (a) Annual mean ratio; (b) annual maximum ratio.
six sites experience a decrease in total N deposition by 2010 of more than 10%. The greatest predicted decline in deposition is at Lochnagar, with a 2010 level only 60% of that in 1998e2000. As a result of the minor changes in total N deposition predicted for 2010, the best case for NO3 leaching does not differ appreciably from mean 1998e2000 concentrations when scaled to deposition. Only in Loch Grannoch, Scoat Tarn, Afon Hafren and Blue Lough will NO3 concentration be almost equal (G10%) to xSO4 when scaled to deposition in 2010, although it will be approaching xSO4 in importance in Lochnagar and Bencrom River (Table 2). These are all sites that exceed their critical loads in 2010 and therefore at which NO3 leaching would potentially prevent recovery to the critical ANC value. All regional datasets show a projected increase in the relative importance of NO3 as a proportion of excess acid anions, due mainly to the projected decreases in S deposition by 2010 (Fig. 4). In the AWMN, Lake District, Snowdonia, Mournes and Northern Ireland at least 25% of exceeded sites will have NO3 as the major excess acid anion. In the Mournes and Northern Ireland datasets NO3 will be greater than xSO4 in the majority of sites. The only regional datasets without any sites projected to have dominant NO3 are northwest Scotland, the Trossachs, the south Pennines, southern England and Welsh streams. However, the AWMN site Afon Hafren lies outside the range of the Welsh streams dataset, with NO3 predicted to be the dominant excess acid anion by 2010. 3.4. Worst-case scenario for NO3 leaching
3.3. Best case scenario for NO3 leaching Projected changes in N deposition to 2010 are much smaller than for S and are not all in the same direction (Table 2). In 11 of the 22 AWMN sites, total N deposition is unchanged or even increased from mean levels in 1998e2000, by up to a maximum of 22%. Only
The FAB mass balance was used to predict concentrations of leached NO3 and xSO4 at long-term 1.0
UK
Scotland
England
NI
0.9
Wales
0.8
NO3 / (NO3 + xSO4)
persisted until the present, possibly due to changes in the NAO, which may explain the lack of chemical recovery in this site (Rose et al., 2004; Davies et al., 2005, this issue). If current declining trends in xSO4 were to continue at the same rate and NO3 concentrations to remain stable, NO3 would equal or exceed xSO4 at five sites by 2010 and 13 sites by 2016, the target year for the achievement of good ecological status under the EU Water Framework Directive (Helliwell et al., 2003; Table 2). However, since changes in the rate of xSO4 decline are expected as emissions targets are achieved, there is no basis for assuming that current trends will continue, hence the need for other modelling approaches that account for anticipated changes in deposition.
0.7 0.6 8
0.5
4
0.4
5
14
3
0.2 0.1
18
12
1
0.3
17
10
6 2
7 9
21 20
15 16
19 22
11 13
0.0 r s s s d s y ct s d d N ia M tlan orm ach wa stri ine lan moo on am rne lan i o n AW Sco irng oss all e D en Eng art owd stre Mou Ire N D a Tr G ak S P S Sn elsh L NW C W
Fig. 4. Surface water NO3 as a proportion of excess acid anions in AWMN sites and exceeded sites in regional datasets; scaled from deposition data (B numbered AWMN sites, scaled from long-term mean NO3 from 1988 to 2002 and annual mean xSO4 for 2002). See Fig. 2 caption for explanation of boxplots.
C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82 1.0
UK
Scotland
England
NO3 / (NO3 + xSO4)
0.9 9 7 8
0.8 0.7 0.6 2
14
5 6
15 16
10 11
4
Wales 18 17
NI 20 21
22 19
12 13
0.5 3
0.4
1
0.3 0.2 0.1 0.0
r s s d s d s s y ct d N ia M tlan orm ach wa stri ine lan moo on am rne lan i o n AW Sco irng oss all e D en Eng art owd stre Mou Ire N D a Tr G ak S P S Sn elsh L NW C W
Fig. 5. Surface water NO3 as a proportion of excess acid anions in AWMN sites and exceeded sites in regional datasets; FAB predictions for 2010 (B numbered AWMN sites). See Fig. 2 caption for explanation of boxplots.
steady-state with mean modelled deposition levels for S and N in 2010 under the NECD (Table 2). For these deposition levels, FAB predicts that long-term NO3 leaching would exceed xSO4 in all AWMN sites except the two in northwest Scotland, in most cases by a factor of two or three (Fig. 5). For 15 of these 20 sites, the FAB critical load is exceeded in 2010 (Table 2), so NO3 is predicted to make a dominant contribution to the prevention of recovery or possibly to the re-acidification of recovered sites over the longer term. The box plots for FAB predictions in the regional datasets starkly illustrate the potential increase in the acidifying role of NO3 (Fig. 5). NO3 is predicted to be greater than xSO4 in almost all sites, with exceptions only in the Cairngorms and south Pennines. Note that when NO3:(xSO4 CNO3) is 0.67, NO3 concentration is double that of xSO4. In general the FAB projections for the AWMN sites lie very close to the regional medians, reflecting the dominant role of regional deposition in model predictions and the selection of regional datasets to correspond spatially with the AWMN sites. Notable exceptions are the two AWMN sites in northwest Scotland, Coire nan Arr and Allt na Coire nan Con. In these remote sites, xSO4 is still predicted to be the major excess acid anion in 2010, but these sites do not exceed their critical loads at this time.
4. Discussion The dominant excess acid anion in acidified sites is clearly still xSO4 at present. NO3 exceeds xSO4 on a seasonal basis in half of the AWMN sites (see e.g. Fig. 1). Annual mean NO3 is approaching xSO4 in importance in Northern Ireland, but only a few sites
79
elsewhere, including the AWMN site Afon Hafren. At current rates of decline in xSO4 and if NO3 levels remain close to current means, then by 2010 five AWMN sites would have mean NO3 concentrations greater than xSO4. However, this situation is unlikely given an anticipated slowdown in the rate of decline in S deposition, which must be taken into account in future projections through scaling or modelling approaches. The best-case scenario, using scaling to projected deposition levels for 2010, suggests that NO3 will be of similar or greater magnitude to xSO4 in around 25% of AWMN sites and at least 25% of exceeded sites in the Lake District, Snowdonia, Mournes and Northern Ireland. Worst-case FAB model predictions indicate that this future NO3 dominance could increase greatly if terrestrial N saturation occurs and leaching increases. The timescale over which this process may occur is, however, largely unknown. There is little current evidence from acid-sensitive surface waters of long-term N saturation having led to rising trends in NO3 concentrations. Certain long-term datasets in Scandinavia (Dickson, 1986; Henriksen and Brakke, 1988; Lepisto¨, 1995) and North America (Stoddard, 1991; Murdoch and Stoddard, 1992) showed increasing trends in surface water NO3 concentrations in the 1980s and 1990s, which seemed to provide evidence of cumulative N saturation. However, few recent analyses of trends in water chemistry at sites across Europe and North America have found increasing trends in NO3 and most earlier trends have disappeared (Stoddard et al., 1999; Skjelkva˚le et al., 2001) including those observed within the AWMN (Evans and Monteith, 2001). Only in some Italian lakes has clear evidence of rising NO3 concentrations been found (Mosello et al., 2001; Rogora et al., 2001). This lack of a general trend has been suggested to be the result of opposing factors; the increasing N saturation status of catchments in the context of declining N deposition in some parts of Europe (Wright et al., 2001). For the UK there is strong statistical evidence that inter-annual variation in NO3 leaching is closely linked to climatic factors represented by the North Atlantic Oscillation (e.g. via soil freezing; Monteith et al., 2000), so trends have to be interpreted with caution. It is evident that there are very few datasets of sufficiently long timescales to say with any certainty whether there are long-term trends in surface water NO3 related to changes in atmospheric deposition, rather than other drivers such as climatic effects. However, past increases in NO3 concentrations must have occurred in those regions where high N deposition corresponds with high NO3 concentrations. Furthermore, it is clear that NO3 is already an important acid anion on a regional basis, particularly during the dormant season, and will become the dominant excess acid anion in many acidified systems in parts of the UK over the next
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C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
10e15 years, including the Lake District, Snowdonia and Northern Ireland. In most of Scotland, southern England and the south Pennines, scaling projections from current data suggest that the current dominance of xSO4 in acidified sites is likely to persist unless N saturation of terrestrial ecosystems leads to increased NO3 leaching. In the south Pennines, the continued dominance of xSO4 due to high deposition and possibly geological sources masks NO3 concentrations that are among the largest in the country, and that would certainly be sufficient to prevent recovery from acidification, even without increased NO3 leaching. This region in particular would suffer very serious re-acidification if the worst-case projections of N saturation and increased NO3 leaching were realised. By comparison, the southern England regional dataset exemplifies the potential, but as yet unrealised, importance of NO3. Here, measured NO3 currently makes up the smallest proportion of excess acid anions of all the regional datasets, but FAB predictions based on 2010 deposition indicate that this could become the region where NO3 achieves the greatest importance relative to xSO4. The large discrepancies between the scaling and FAB approaches in this lowland region may be due to much lower runoff (c. 300 mm compared with c. 900e2700 mm) and higher temperatures than in other datasets comprising mainly upland sites. Low runoff in particular may result in the leaching of only a very small proportion of N deposition through purely hydrological leaching processes, which are likely to be much more important in the uplands. The very low NO3 concentrations observed in southern England despite relatively high N deposition levels result in low projections from scaling but high projections from the FAB mass-balance assumptions. The major uncertainty for some regions appears to be not if, but rather when, NO3 will become the major agent of continued acidification. Another key uncertainty is the degree to which measured NO3 concentrations, which are still low in absolute terms in some regions, may increase to the very high levels predicted by the FAB model. However, while planned reductions in S emissions and deposition are still vital to facilitate chemical recovery of acidified waters, N deposition will become the major policy issue in the near future, even without the occurrence of N saturation. 4.1. Limitations of the modelling approach In addition to the structural differences between the models, there are additional uncertainties that relate to drivers of change other than deposition inputs. For example, climate change could have major effects on biological cycles with highly uncertain impacts on the storage and release of both S and N. Higher precipitation could result in greater hydrological losses of acid anions,
especially the less mobile NO3 anion, while a drier climate could reduce these losses. Increased severity of droughts followed by heavy rain could result in large leaching pulses of mineralised organic S and N compounds (Reynolds and Edwards, 1995), as could more severe soil freezing and thawing cycles (Monteith et al., 2000). The storage of S in soils is not taken into account by the FAB model, but could be important in certain organic soil and wetland areas, where climatic effects could be particularly pronounced. However, Cooper (2005, this issue) found no significant time lag at the annual scale between reductions in non-marine S deposition and xSO4 concentrations at most AWMN sites. Finally, the widespread trends in increasing DOC, for which the major drivers are still the subject of some debate (see Evans et al., 2005, this issue), affect the acidity and recovery status of surface waters. Organic acidity is increasing in importance as xSO4 declines, in both relative and absolute terms. The role of organic anions has not been compared with excess mineral acid anions derived from acid deposition, because of the problem of determining organic acid anion concentrations for DOC of different composition at each site, but the observed trends in DOC suggest that their role could be changing in magnitude. However, Hrusˇ ka et al. (1997) used experimental additions and monitoring data to show that organic acids were unlikely to hinder recovery of an acidified bog stream in the Czech Republic with DOC concentrations of 48.9 mg lÿ1. Further discussion on the role of DOC in recovery from acidification is provided in Evans et al. (2005, this issue).
4.2. Research needs Current research priorities aim to determine the degree to which NO3 leaching is biologically mediated through terrestrial ecosystems and the proportion of leached NO3 that is ‘‘hydrological’’, i.e. is controlled primarily by abiotic factors such as hydrological flow routing. If currently elevated NO3 concentrations observed in many acid waters are due to biological production in catchment soils, then N saturation may already be well advanced and masked only by interannual climatic variations. The ‘‘worst-case’’ scenario of the FAB model may therefore be realised within a timescale of years to decades. If leached NO3 is mainly ‘‘hydrological’’ this may suggest that terrestrial ecosystems still have much capacity to assimilate N and saturation may not occur for many decades. The lack of information on the proportion of N deposition that is retained in soils and vegetation largely accounts for the mixed results of attempts to link NO3 leaching to measures of N saturation status such as soil carbon: nitrogen ratio (Gundersen et al., 1998; Curtis et al., 2004) and catchment soil carbon pool relative to accumulated
C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
N deposition loads (Jenkins et al., 2001; Evans et al., unpublished).
Acknowledgements This work was funded under the DEFRA Freshwaters Umbrella programme (contract 1/3/183) and by the Scottish Executive Environment and Rural Affairs Department (SEERAD). The authors thank colleagues within the Freshwaters Umbrella for the provision of chemistry data, CEH Edinburgh for the provision of deposition data and CEH Monks Wood for the provision of catchment data used in FAB modelling.
References Aber, J.D., 1992. Nitrogen cycling and nitrogen saturation in temperate forest ecosystems. Trends in Ecology and Evolution 7, 220e224. A˚gren, G.I., Bosatta, E., 1988. Nitrogen saturation of terrestrial ecosystems. Environmental Pollution 54, 185e197. Allott, T.E.H., Golding, P.N.E., Harriman, R., 1995. A palaeolimnological assessment of the impact of acid deposition on surface waters in north west Scotland, a region of high sea salt deposition. Water, Air and Soil Pollution 85, 2425e2530. CLAG Deposition Fluxes, 1997. Deposition fluxes of acidifying compounds in the United Kingdom. Critical Loads Advisory Group, Sub-group report on deposition fluxes. ITE, Penicuik. Cooper, D.M., 2005. Evidence of sulphur and nitrogen deposition signals at the United Kingdom Acid Waters Monitoring Network sites. Environmental Pollution, this issue. Cooper, D.M., Jenkins, A., 2003. Response of acid lakes in the UK to reductions in atmospheric deposition of sulfur. The Science of the Total Environment 313, 91e100. Curtis, C.J., Allott, T.E.H., Reynolds, B., Harriman, R., 1998. The prediction of nitrate leaching with the first-order acidity balance (FAB) model for upland catchments in Great Britain. Water, Air and Soil Pollution 105, 205e215. Curtis, C.J., Allott, T.E.H., Hughes, M., Hall, J., Harriman, R., Helliwell, R., Kernan, M., Reynolds, B., Ullyett, J., 2000. Critical loads of sulphur and nitrogen for freshwaters in Great Britain and assessment of deposition reduction requirements with the Firstorder Acidity Balance (FAB) model. Hydrology and Earth System Sciences 4, 125e140. Curtis, C.J., Hughes, M., Kernan, M., Shilland, E., Harriman, R., 2001. Critical loads of sulphur and nitrogen for surface waters in Northern Ireland. Report to the Environment and Heritage Service of the Department of the Environment Northern Ireland, Contract No. CON4/3(14). ECRC Research Report No. 76. University College London, UK. Curtis, C.J., Emmett, B., Reynolds, B., Shilland, J., 2004. Nitrate leaching from moorland soils: can soil C/N ratios indicate N saturation? Water, Air and Soil Pollution: Focus 4, 359e369. Curtis, C.J., Simpson, G. (Eds.), 2004. Summary of Research under DEFRA Contract ‘‘Recovery of Acidified Freshwaters in the UK’’ EPG/1/3/183. ECRC Research Report No. 98. University College London, UK. Davies, J.J.L., Jenkins, A., Monteith, D.T., Evans, C.D., Cooper, D.M., 2005. Trends in surface water chemistry of acidified UK fresh waters 1988e2002. Environmental Pollution, this issue.
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Dickson, W., 1986. Critical loads for nitrogen on surface waters. In: Nilsson, J. (Ed.), Critical Loads for Nitrogen and Sulphur. Miljørapport 1986:11. Nordic Council of Ministers, Copenhagen, Denmark, pp. 199e210. Evans, C.D., Monteith, D.T., 2001. Chemical trends at lakes and streams in the UK Acid Waters Monitoring Network. Hydrology and Earth System Sciences 5, 351e366. Evans, C.D., Monteith, D.T., 2002. Natural and anthropogenic changes in the chemistry of six UK mountain lakes. Water, Air and Soil Pollution 2, 33e46. Evans, C.D., Jenkins, A., Wright, R.F., 2000. Surface water acidification in the South Pennines I. Current status and spatial variability. Environmental Pollution 109, 11e20. Evans, C., Jenkins, A., Helliwell, R., Ferrier, R., Collins, R., 2001. Freshwater acidification and recovery in the United Kingdom. CEH, Wallingford, UK. Evans, C.D., Monteith, D.T., Cooper, D.M., 2005. Long-term increases in surface water dissolved organic carbon: observations, possible causes and environmental impacts. Environmental Pollution, this issue. Evans, C.D., Reynolds, B., Curtis, C.J., Crook, H.D., Norris, D., Brittain, S.A., 2004. A conceptual model of spatially heterogeneous nitrogen leaching from a Welsh moorland catchment. Water, Air and Soil Pollution: Focus 4, 97e105. Ferrier, R.C., Helliwell, R.C., Cosby, B.J., Jenkins, A., Wright, R.F., 2001. Recovery from acidification of lochs in Galloway, south-west Scotland, UK: 1979e1998. Hydrology and Earth System Sciences 5, 421e431. Forsius, M., Ka¨ma¨ri, J., Posch, M., 1992. Critical loads for Finnish lakes: comparison of three steady-state models. Environmental Pollution 77, 185e193. Fowler, D., Smith, R., Muller, J., 2005. Changes in the atmospheric deposition of acidifying compounds in the UK between 1986 and 2001. Environmental Pollution, this issue. Gundersen, P., Callesen, I., de Vries, W., 1998. Nitrate leaching in forest ecosystems is related to forest floor C/N ratios. Environmental Pollution 102, 403e407. Helliwell, R.C., Jenkins, A., Ferrier, R.C., Cosby, B.J., 2003. Modelling the recovery of surface water chemistry and the ecological implications in the British uplands. Hydrology and Earth System Sciences 7, 456e466. Helliwell, R.C., Wright, R.F., Evans, C.D., Jenkins, A., Ferrier, R.C., 2002. Comparison of loch chemistry from 1955 and 1999 in the Cairngorms. Water, Air and Soil Pollution: Focus 2, 47e59. Henriksen, A., Brakke, D.F., 1988. Increasing contributions of nitrogen to the acidity of surface waters in Norway. Water, Air and Soil Pollution 42, 183e201. Henriksen, A., Posch, M., 2001. Steady-state models for calculating critical loads of acidity for surface waters. Water, Air and Soil Pollution: Focus 1, 375e398. Hrusˇ ka, J., Johnson, C.E., Kra´m, P., Liao, C.-Y., 1997. Organic solutes and the recovery of a bog stream from chronic acidification. Environmental Science and Technology 31, 3677e3681. INDITE, 1994. Impacts of nitrogen deposition in terrestrial ecosystems. United Kingdom Review Group on Impacts of Nitrogen Deposition on Terrestrial Ecosystems (INDITE), Department of the Environment, London, UK. Jenkins, A., Ferrier, R.C., Helliwell, R.C., 2001. Modelling nitrogen dynamics at Lochnagar, N.E. Scotland. Hydrology and Earth System Sciences 5, 273e541. Kaste, Ø., Henriksen, A., Posch, M., 2002. Present and potential nitrogen outputs from Norwegian soft water lakes e an assessment made by applying the steady-state First-order Acidity Balance (FAB) model. Hydrology and Earth System Sciences 6, 101e112. Kernan, M., Allott, T.E.H., 1999. Spatial variability of nitrate concentration in lakes in Snowdonia, North Wales, UK. Hydrology and Earth Systems Sciences 3, 395e408.
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Lepisto¨, A., 1995. Increased nitrate leaching at two forested catchments in Finland over a period of 25 years. Journal of Hydrology 171, 103e123. Monteith, D.T., Evans, C.D., 2000. UK Acid Waters Monitoring Network: 10 year report. Analysis and interpretation of results, April 1988eMarch 1998. ENSIS Publishing, London. Monteith, D.T., Evans, C.D., Reynolds, B., 2000. Are temporal variations in the nitrate content of UK upland freshwaters linked to the North Atlantic Oscillation? Hydrological Processes 14, 1745e1749. Mosello, R., Barbieri, A., Brizzio, M.C., Calderoni, A., Marchetto, A., Passera, S., Rogora, M., Tartari, G.A., 2001. Nitrogen budget of Lago Maggiore: the relative importance of atmospheric deposition and catchment sources. Journal of Limnology 60, 27e40. Murdoch, P.S., Stoddard, J.L., 1992. The role of nitrate in the acidification of streams in the Catskill Mountains of New York. Water Resources Research 28, 2707e2720. Nilsson, J., Grennfelt, P. (Eds.), 1988. Critical Loads for Sulphur and Nitrogen. UNECE/Nordic Council Workshop Report, Skokloster, Sweden, 19e24 March, 1988. Miljørapport 1988:15. Nordic Council of Ministers, Copenhagen. Reynolds, B., Edwards, A., 1995. Factors influencing dissolved nitrogen concentrations and loadings in upland streams of the UK. Agricultural Water Management 27, 181e202. Rogora, M., Marchetto, A., Mosello, R., 2001. Trends in the chemistry of atmospheric deposition and surface waters in the Lake Maggiore catchment. Hydrology and Earth System Sciences 5, 379e390. Rose, N., Monteith, D., Kettle, H., Thompson, R., Yang, H., Muir, D., 2004. A consideration of potential confounding factors limiting chemical and biological recovery at Lochnagar, a remote mountain loch in Scotland. Journal of Limnology 63, 63e76.
Singles, R., Sutton, M.A., Weston, K., 1998. A multi-layer model to describe the atmospheric transport and deposition of ammonia in Great Britain. Atmospheric Environment 32, 393e399. Skeffington, R.A., Wilson, E.J., 1988. Excess nitrogen deposition: issues for consideration. Environmental Pollution 54, 159e184. Skjelkva˚le, B.L., Olendrzynski, K., Stoddard, J., Tarrasson, L., Traaen, T.S., Tørseth, K., Windjusveen, S., Wright, R.F., 2001. Assessment of Trends and Leaching of Nitrogen at ICP Waters Sites (Europe and North America). ICP-Waters Report 54/2001. NIVA, Oslo, Norway. Stevens, P.A., Ormerod, S.J., Reynolds, B., 1997. In: Main Text (ITE Project No T07072R5). Final Report on the Acid Waters Survey for Wales, vol. 1. Institute of Terrestrial Ecology, Bangor, UK. Stoddard, J.L., 1991. Trends in Catskill stream water quality: evidence from historical data. Water Resources Research 27, 2855e2864. Stoddard, J.L., 1994. Long-term changes in watershed retention of nitrogen: its causes and aquatic consequences. In: Baker, L.A. (Ed.), Environmental Chemistry of Lakes and Reservoirs. ACS Advances in Chemistry Series No. 237. American Chemical Society, Washington, DC, pp. 223e284. Stoddard, J.L., Jeffries, D.S., Lu¨kewille, A., Clair, T.A., Dillon, P.J., Driscoll, C.T., Forsius, M., Johannessen, M., Kahl, J.S., Kellogg, J.H., Kemp, A., Mannio, J., Monteith, D.T., Murdoch, P.S., Patrick, S., Rebsdorf, A., Skjelkva˚le, B.L., Stainton, M.P., Traaen, T., van Dam, H., Webster, K.E., Wieting, J., Wilander, A., 1999. Regional trends in aquatic recovery from acidification in North America and Europe. Nature 401, 575e578. Wright, R.F., Alewell, C., Cullen, J.M., Evans, C.D., Marchetto, A., Moldan, F., Prechtel, A., Rogora, M., 2001. Trends in nitrogen deposition and leaching in acid-sensitive streams in Europe. Hydrology and Earth Systems Sciences 5, 299e310.
Nitrate leaching as a confounding factor in chemical recovery from acidification in UK upland waters C.J. Curtisa,*, C.D. Evansb, R.C. Helliwellc, D.T. Monteitha a
ECRC, University College London, 26 Bedford Way, London WC1H 0AP, UK CEH Bangor, Orton Building, Deiniol Road, Bangor, Gwynedd LL57 2UP, UK c Macaulay Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK
b
Received 31 October 2004; accepted 17 December 2004
With declining excess sulphate, nitrate will become the dominant agent of continued anthropogenic acidification in many UK upland waters within a decade. Abstract Over the period 1988e2002, data from 18 of the 22 lakes and streams in the UK Acid Waters Monitoring Network (AWMN) show clear trends of declining excess sulphate concentrations in response to reductions in sulphur deposition, but fewer trends in increasing pH or alkalinity. There has been no significant decline in the deposition of total nitrogen over the same period, and no sites show a trend in nitrate concentration. Peak nitrate concentrations have already surpassed excess sulphate on occasion in half of the AWMN sites. Furthermore, current understanding of terrestrial N saturation processes suggests that nitrate leaching from soils may increase, even under a constant N deposition load. Best-case projections indicate that nitrate will overtake sulphate as the major excess acid anion in many sites within 10 years, while worst-case predictions with steady-state models suggest that in the longer-term, nitrate could become the dominant excess acid anion in most of the UK. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Nitrogen; N saturation; Acid deposition; Sulphate; Critical load
1. Introduction The UK AWMN has provided direct evidence of rapidly declining surface water non-marine (or ‘‘excess’’) sulphate (xSO4) concentrations in 18 of 22 monitored lakes and streams in response to reductions in the deposition of anthropogenic sulphur (S) compounds since the mid-1990s (Cooper and Jenkins, 2003). However, declining trends in xSO4 correspond with increases in alkalinity or pH at only ten sites. This is in part due to a decline in base cation leaching in response to a reduced acid load. In addition, significant positive * Corresponding author. Fax: C44 207 679 7565. E-mail address: [email protected] (C.J. Curtis). 0269-7491/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2004.12.032
trends in dissolved organic carbon (DOC) confound the detection of alkalinity trends and are at least partly responsible for increasing trends in acid neutralizing capacity (ANC) (Davies et al., 2005, this issue). Other possible reasons for the lack of significant trends in alkalinity and pH at some sites are the confounding effects of hydrological influences, seasalt inputs and nitrate (NO3) leaching (Evans and Monteith, 2002). While xSO4 is generally still the dominant excess acid anion in anthropogenically acidified surface waters, NO3 contributes significantly to the total excess acid anion load in several regions and its relative role will increase as xSO4 concentrations decline. However, temporal variability in NO3 leaching into surface waters complicates assessment of its importance, and
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potentially hinders detection of longer-term trends in alkalinity and pH. NO3 leaching into surface waters follows a seasonal pattern partly because of hydrological controls and partly because of the biological demand for nutrient N in the terrestrial ecosystem (INDITE, 1994; Reynolds and Edwards, 1995). Furthermore, mean concentrations on a year-to-year basis may be affected by climatic factors (Monteith et al., 2000; Davies et al., 2005, this issue). More importantly for recovery prospects in upland regions where S deposition is declining, not only is N deposition expected to decline to a smaller degree than S, but for a given level of N deposition, NO3 leaching into surface waters may increase in the longer term as N saturation of terrestrial ecosystems occurs (A˚gren and Bosatta, 1988; Skeffington and Wilson, 1988; Aber, 1992; Stoddard, 1994). The timescale over which this process may occur is largely unknown. Only small trends confined to central and south-eastern England have been observed for N deposition (Fowler et al., 2005, this issue), while no relationship was found between N deposition and leaching of inorganic N species by Cooper (2005, this issue). Hence, while future changes in both deposition and leaching of inorganic N species are rather uncertain, major reductions in either are unlikely in the short term. This paper provides an assessment of the increasing importance of NO3 as an acidifying anion in both relative (to xSO4) and absolute terms using best-case/ worst-case models of NO3 leaching.
2. Methods This work is focussed on the impacts of acid anions derived from anthropogenic atmospheric deposition, which are assumed to be primarily xSO4 and NO3. It is assumed that anthropogenic chloride deposition is generally negligible. Furthermore, the effects of organic acid anions are beyond the scope of this work. Hereafter, the term ‘‘excess acid anions’’ is therefore used to describe the sum of xSO4 and NO3 as the main anthropogenic agents of acidification. Protocols for the collection and analysis of water chemistry data for the AWMN sites are described in Monteith and Evans (2000), while trend analysis is reported in Davies et al. (2005, this issue). Methods for the derivation and trend analysis of deposition data are provided in Fowler et al. (2005, this issue). To place the AWMN sites into a spatial context, several regional datasets have been drawn from the national critical loads mapping programme (Curtis and Simpson, 2004), which provides good regional coverage of surface water samples (commonly standing waters) from the key areas impacted by acidification. The correspondence between AWMN sites and regional
datasets is given in Table 1. AWMN site numbers are linked to site names in Table 2. Note that sampling frequency and periods represented vary between regional datasets, while seasonal variations in NO3 mean that one-off samples are very sensitive to sampling season. However, the problem of obtaining representative data with a one-off sample has generally been minimised by sampling in spring or autumn, when chemistry is assumed to most closely represent annual, flow-weighted mean values (Forsius et al., 1992). Site-specific xSO4 trends are unavailable for regional datasets other than the AWMN sites, so the scaling and FAB modelling approaches described below were applied. 2.1. Scaling from deposition trends: ‘‘best-case’’ NO3 leaching Given the strong links between xSO4 concentrations and S deposition on an annual mean basis at AWMN sites (Cooper and Jenkins, 2003; Cooper, 2005, this issue) it is possible to predict surface water xSO4 concentrations for projected future deposition levels by rescaling measured values to changes in deposition. Links between NO3 leaching and total N deposition on a mean annual basis are poor (Cooper, 2005, this issue) but a ‘‘best-case’’ NO3 leaching scenario assumes that NO3 is leached at a fixed proportion of inputs according to hydrological controls (e.g. see Evans et al., 2004), i.e. the current proportion of total N deposition leached may be determined by estimated inputeoutput budgets and applied to different deposition levels in future. It is assumed that catchment N saturation does not occur over the period under consideration so that the remaining proportion of inputs is biologically retained in catchment plants and soils over the long term. Mean surface water xSO4 and NO3 concentrations were scaled to projected changes in excess S and total N deposition by 2010 according to the FRAME model (Singles et al., 1998). Since the most recent modelled deposition data were mean values for the period 1998e 2000, mean AWMN water chemistry data from the same period were used as the starting point for scaling. For regional datasets which pre-date the period 1998e 2000, best available contemporary deposition data were used as the starting point for scaling to 2010 concentrations. Methods for the modelling of deposition data from measurements made at a national monitoring network are given in CLAG Deposition Fluxes (1997). 2.2. FAB model predictions of S and N leaching: ‘‘worst-case’’ NO3 leaching Mean water chemistry data for 2002 from the 22 AWMN sites feed into the UK national critical loads mapping programme. Critical loads and exceedances
75
C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82 Table 1 Regional datasets used in spatial comparison with AWMN sites Dataset
ntot
nex
Sampling period
Site selection criteria
Reference
AWMN site no.
AWMN, UK
22
15
1988e2002a,b
11
1
Davies et al., 2005, this issue Allott et al., 1995
All
NW Scotland
Cairngorms, NE Scotland Trossachs, central Scotland Galloway, SW Scotland
37 32 61
13 0 31
1999c 2002c 1996e8d/2002a
Undisturbed, acid-sensitive lakesa and streamsb Sea-salt input gradient (altitude, distance to sea); lochs Comprehensive e lochs Comprehensive e lochs Comprehensive e lochs
2, 4 5, 6 7, 8, 9
Lake District, NW England
53
28
2000c
Helliwell et al., 2002 Curtis and Simpson, 2004 Ferrier et al., 2001d, Helliwell et al. (unpublished)a Evans et al., 2001
S Pennines, NW England
64
56
1998c/2002a
Evans et al., 2000
12
S England Dartmoor, SW England
25 20
7 12
2002c 1991e1993c
Curtis and Simpson, 2004 Curtis et al., 2000
13 14
Snowdonia, NW Wales Welsh streams
74 102
31 34
1996c 1995b
Kernan and Allott, 1999 Stevens et al., 1997
15, 16 17, 18
Mournes, N Ireland N Ireland
8 105
8 9
2002a 2000c
Helliwell et al., unpublished Curtis et al., 2001
20, 21 19, 22
1992e1993a
Lakes>0.5 ha without improved land in catchment Reservoirs between Kinder and Ilkley with mostly unimproved catchment Acid sensitive; lakes Critical loads mapping sites !50 km from Narrator Brook; lakes and streams All in 40 km square - lakes Extensive spatial coverage; streams Comprehensive - lakes 10 km grid survey e lakes and streams
1, 3
10, 11
ntot, total number of sites; nex, number exceeding critical load in 2010. a Annual mean based on quarterly samples. b Annual mean based on monthly samples. c One-off sample in spring or autumn. d Mean of annual one-off samples in spring or autumn.
under various deposition scenarios have therefore been calculated for all AWMN sites using the most recent version of the first-order acidity balance (FAB) model (Henriksen and Posch, 2001), as well as for another 1700 surface water sites around the UK (Curtis and Simpson, 2004). The FAB model employs a simple steady-state mass balance for S and N to predict leaching under given deposition loads over the long term, i.e. assuming that N saturation is occurring and that currently elevated rates of catchment N retention cannot be maintained. Low default values for N immobilization in soils are based on chronosequence studies of N content in soil profiles. The N mass balance predicts the leaching of a large proportion of N deposition at long-term steady-state (Curtis et al., 1998; Kaste et al., 2002). The FAB mass balance was used here to provide theoretical steady-state concentrations of xSO4 and NO3 according to mean modelled deposition levels for 2010 following implementation of the EU National Emissions Ceiling Directive (NECD). It may be argued that the future dominance of NO3 as an excess acid anion is only important if surface waters have not achieved their recovery targets. The critical load provides the threshold for deposition above which
significant harmful effects will occur to a specified sensitive element of the ecosystem (Nilsson and Grennfelt, 1988). For surface waters, critical loads are set with respect to a selected critical value of ANC (Henriksen and Posch, 2001), generally taken as 20 meq lÿ1 in the UK unless site-specific evidence suggests that pre-industrial ANC may have been lower than this threshold, in which case 0 meq lÿ1 is used (Curtis and Simpson, 2004). Within the AWMN, the only site with a reconstructed preindustrial ANC of !20 meq lÿ1 for 1850, using both a palaeolimnological transfer function (Curtis and Simpson, 2004) and the dynamic model MAGIC (e.g. Evans et al., 2001) is Blue Lough. Exceedance of the critical load implies that at long-term steady-state, ANC will decline below the pre-selected threshold for a site. Critical load exceedance was calculated here for modelled deposition in 2010 (NECD). A positive value of exceedance indicates that the critical ANC threshold will be crossed at long-term steady-state. Note that for the regional data, only sites exceeding their critical loads in 2010 are included in comparisons with AWMN sites, as these are the sites at which long-term recovery cannot occur. Lowland sites with high NO3 from nonatmospheric sources are also screened out.
76 Table 2 Measured, scaled and modelled excess acid anion concentrations (meq lÿ1), deposition (keq haÿ1 yearÿ1; NECD, EU National Emissions Ceiling Directive) and critical load exceedance (keq haÿ1 yearÿ1) at AWMN sites Site name
2002 Long-term xSO4 declinea Significance Year of Mean deposition mean mean (meq lÿ1 level ( p) convergenceb 1998e2000 xSO4 NO3a yearÿ1)
a
0.42 0.42 0.42 1.04 1.46 1.25 1.88 2.71 1.25 0.42 1.25 9.38 9.17 n/a 1.67 0.83 1.04 1.67 1.88 1.88 3.33 1.04
NS !0.01 NS !0.01 !0.05 !0.05 !0.05 !0.01 !0.01 NS !0.01 !0.01 !0.01 n/a !0.01 NS !0.01 !0.01 !0.01 !0.01 !0.01 !0.05
2013 2064 e 2021 2015 2016 2010 2007 2030 e 2030 2013 2011 n/a 2008 e 2014 2011 2004 2013 2005 2011
Mean chemistry 1998e2000
Scaled 2010 concentrations
FAB predicted concentrations and exceedance (Exc.)
S
N
S
N
xSO4
NO3
xSO4
NO3
xSO4
NO3
Exc.
0.35 0.25 0.50 0.66 0.91 0.92 0.84 0.67 0.74 1.00 0.82 1.54 0.50 0.54 0.71 0.48 0.78 0.78 0.67 0.67 0.67 0.50
0.37 0.42 0.54 1.06 1.43 1.44 1.42 1.16 1.24 1.61 1.28 2.21 1.14 1.14 1.03 0.76 1.25 1.25 1.18 1.38 1.37 1.02
0.12 0.12 0.19 0.19 0.34 0.34 0.46 0.38 0.41 0.52 0.48 0.69 0.19 0.22 0.30 0.23 0.34 0.34 0.23 0.34 0.34 0.21
0.35 0.42 0.52 0.64 0.99 0.96 1.42 1.32 1.30 1.77 1.57 1.77 0.75 1.35 1.15 0.90 1.39 1.40 0.91 1.29 1.30 0.93
8.1 27.9 21.7 39.5 33.0 21.0 27.2 43.8 45.9 36.5 44.0 170.7 122.3 44.0 27.6 43.2 51.0 35.9 10.3 44.6 43.5 14.6
1.6 1.3 3.6 15.7 12.8 2.5 7.0 23.7 9.6 16.8 6.7 48.2 7.3 8.2 6.9 9.9 21.2 8.6 2.3 19.1 22.9 2.3
2.7 12.7 8.3 11.5 12.6 7.9 14.8 25.3 25.5 18.8 25.8 76.7 47.7 17.7 11.6 20.2 22.1 15.6 3.5 22.7 22.1 6.0
1.5 1.3 3.5 9.4 8.9 1.6 7.0 26.9 10.0 18.5 8.2 38.6 4.8 9.7 7.7 11.7 23.7 9.6 1.8 18.0 21.7 2.1
4.5 10.9 9.2 13.6 18.2 18.1 24.4 20.5 23.8 19.8 23.6 60.6 48.1 17.7 10.4 12.7 17.5 16.6 18.7 25.5 27.2 18.9
2.6 13.7 5.9 20.9 29.0 27.8 50.9 40.3 65.4 57.8 63.1 128.0 108.0 81.3 29.0 33.8 53.3 54.3 50.9 75.0 76.4 56.2
ÿ0.58 ÿ0.40 ÿ0.60 0.22 ÿ0.11 ÿ0.31 1.24 1.03 0.85 1.81 0.32 0.89 0.23 0.64 0.54 0.23 0.30 0.48 ÿ0.29 0.52 0.88 ÿ1.32
Calculated over the period of AWMN monitoring from 1988 to 2002. Hypothetical year of convergence based on the continuation of current xSO4 trends and the maintenance of NO3 at long-term mean concentrations. Sites with non-significant trends excluded. n/a, not applicable (no trend). b
C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
1 Loch Coire nan Arr 7.5 2.5 2 Allt a’Mharcaidh 27.7 1.5 3 Allt na Coire nan Con 13.6 4.0 4 Lochnagar 36.4 16.4 5 Loch Chon 31.0 12.3 6 Loch Tinker 20.6 2.8 7 Round Loch of Glenhead 22.3 6.9 8 Loch Grannoch 32.1 18.3 9 Dargall Lane 46.0 10.6 10 Scoat Tarn 34.0 19.6 11 Burnmoor Tarn 41.4 5.6 12 R. Etherow 149.3 46.5 13 Old Lodge 89.0 7.1 14 Narrator Brook 45.9 7.0 15 Llyn Llagi 18.2 8.2 16 Llyn cwm Mynach 34.0 9.8 17 Afon Hafren 31.2 20.8 18 Afon Gwy 22.6 8.9 19 Beaghs Burn 7.4 3.0 20 Bencrom River 48.2 27.0 21 Blue Lough 37.0 26.6 22 Coneyglen Burn 12.5 2.5
2010 deposition (NECD)
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C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
3. Results 3.1. Current situation The ratio of NO3:(xSO4CNO3) provides an index of the influence of NO3 on chronic acidification status, assuming that both anions are derived from anthropogenic acid deposition (i.e. even if NO3 was deposited initially as reduced forms of N and was subsequently converted to NO3). A value of 0.5 indicates an equal influence on surface water acidification for both NO3 and xSO4, while larger values would indicate a greater influence of NO3 (i.e. NO3>xSO4). Based on annual means for 2002, NO3 provides the greatest proportion of total excess acid anions in Scoat Tarn, the Afon Hafren and Blue Lough (Table 2). Mean NO3 concentrations also exceed half of the xSO4 concentration in Loch Grannoch and the Bencrom River. Time-series plots of NO3:(xSO4CNO3) are shown for the Afon Hafren and Scoat Tarn in Fig. 1, with the value of 0.5 indicated by a solid line. Seasonal variations in this ratio, peaking in late winter or early spring, are clearly seen in both sites. At the Afon Hafren, the ratio has exceeded 0.4 every year since 1997 but only twice in the preceding 9 years. In the winter of 2001e2002 the concentration of NO3 exceeded xSO4 for the first time over the whole period of monitoring. At Scoat Tarn, NO3 has exceeded xSO4 on numerous occasions during the period of monitoring, but there is no indication of an increase in the maximum value of this ratio. In total, 11 of the 22 AWMN sites have at some point experienced
peak NO3 concentrations that exceed xSO4: Loch Coire nan Arr, Allt na Coire nan Con, Loch Grannoch, Scoat Tarn, River Etherow, Old Lodge, Afon Hafren, Beaghs Burn, Bencrom River, Blue Lough and Coneyglen Burn (data not shown). The same ratio (NO3 as a proportion of excess acid anions) is shown both for AWMN sites using annual mean chemistry data and the distribution in regional datasets in Fig. 2. At present, mean xSO4 is greater than NO3 in all AWMN sites (numbered circles in Fig. 2). The same is generally true for the regional datasets; box plots of NO3 as a proportion of excess acid anions show only a few outlying sites at which NO3 is greater than xSO4 (Fig. 2). Northwest Scotland contains only one exceeded site, while none are exceeded in the Trossachs. The two Northern Ireland datasets are the only ones in which NO3 is approaching xSO4 in importance in a high proportion of exceeded surface waters.
3.2. Current trends The response of NO3 as a proportion of excess acid anions for AWMN lake sites with significant trends in xSO4 is shown in Fig. 3. A rising pattern is clearly seen, as expected given the declining trend in xSO4 and lack of trend in annual mean NO3 concentrations. For annual means, the increase appears to start between 1992 and 1995 (Fig. 3a), while for annual maxima, the ratio has been increasing in most of these sites throughout the period of monitoring (Fig. 3b). A similar pattern is also observed for stream sites (data not shown). An apparent step change in NO3 leaching since 1995 at Lochnagar has 1.0
0.5
0.9
0.4
0.8
NO3 / (NO3 + xSO4)
NO3 / (xSO4 + NO3).
a: Afon Hafren 0.6
0.3 0.2 0.1 0
0.6
Scotland
Wales
NI
0.6 0.5 0.4 4
0.3 0.2 0.1
0.4
0.0
5
8 7 9
17 10
16 12
1 3
18
20
21
19
14 15
6 2
0.3
England
0.7
0.5
11
22
13
r s s d s d s s y ct d N ia M tlan orm ach wa stri ine lan moo on am rne lan i o n AW Sco irng oss all e D en Eng art owd stre Mou Ire N D a Tr G ak S P S Sn elsh L NW C W
0.2 0.1 Jul-02
Jul-01
Jul-00
Jul-99
Jul-98
Jul-97
Jul-96
Jul-95
Jul-94
Jul-93
Jul-92
Jul-91
Jul-90
Jul-89
0 Jul-88
NO3 / (xSO4 + NO3).
b: Scoat Tarn
UK
Sampling date Fig. 1. NO3 as a proportion of total excess acid anions (eq eqÿ1) in (a) Afon Hafren and (b) Scoat Tarn. Bold horizontal line indicates equivalence of NO3 and xSO4.
Fig. 2. Surface water NO3 as a proportion of excess acid anions (eq eqÿ1) in AWMN sites and exceeded sites in regional datasets; measured data (B numbered AWMN sites, using long-term mean NO3 from 1988 to 2002 and annual mean xSO4 for 2002). Boxes show 25th and 75th percentiles (inter-quartile range). Whiskers show extent of data excluding outliers. *Represents outliers that are located 1.5e3! the inter-quartile range from the median (horizontal bar).
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C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
a. Annual mean ratio
NO3:(NO3+xSO4) ratio
0.6 0.5 0.4 0.3 0.2 0.1 0 1988
1990
1992
1994
1996
1998
2000
2002
2000
2002
b. Annual maximum ratio
NO3:(NO3+xSO4) ratio
0.6 0.5 0.4 0.3 0.2 0.1 0 1988
1990
1992
Lochnagar Round Loch of Glenhead Llyn Llagi
1994
1996
1998
Loch Chon Loch Grannoch Blue Lough
Loch Tinker Loch Tink Burnmoor Burnmoor Tarn
Fig. 3. Response of NO3 as a proportion of total excess acid anions to trend in xSO4 for the eight AWMN lakes with significant xSO4 trends in Table 2. (a) Annual mean ratio; (b) annual maximum ratio.
six sites experience a decrease in total N deposition by 2010 of more than 10%. The greatest predicted decline in deposition is at Lochnagar, with a 2010 level only 60% of that in 1998e2000. As a result of the minor changes in total N deposition predicted for 2010, the best case for NO3 leaching does not differ appreciably from mean 1998e2000 concentrations when scaled to deposition. Only in Loch Grannoch, Scoat Tarn, Afon Hafren and Blue Lough will NO3 concentration be almost equal (G10%) to xSO4 when scaled to deposition in 2010, although it will be approaching xSO4 in importance in Lochnagar and Bencrom River (Table 2). These are all sites that exceed their critical loads in 2010 and therefore at which NO3 leaching would potentially prevent recovery to the critical ANC value. All regional datasets show a projected increase in the relative importance of NO3 as a proportion of excess acid anions, due mainly to the projected decreases in S deposition by 2010 (Fig. 4). In the AWMN, Lake District, Snowdonia, Mournes and Northern Ireland at least 25% of exceeded sites will have NO3 as the major excess acid anion. In the Mournes and Northern Ireland datasets NO3 will be greater than xSO4 in the majority of sites. The only regional datasets without any sites projected to have dominant NO3 are northwest Scotland, the Trossachs, the south Pennines, southern England and Welsh streams. However, the AWMN site Afon Hafren lies outside the range of the Welsh streams dataset, with NO3 predicted to be the dominant excess acid anion by 2010. 3.4. Worst-case scenario for NO3 leaching
3.3. Best case scenario for NO3 leaching Projected changes in N deposition to 2010 are much smaller than for S and are not all in the same direction (Table 2). In 11 of the 22 AWMN sites, total N deposition is unchanged or even increased from mean levels in 1998e2000, by up to a maximum of 22%. Only
The FAB mass balance was used to predict concentrations of leached NO3 and xSO4 at long-term 1.0
UK
Scotland
England
NI
0.9
Wales
0.8
NO3 / (NO3 + xSO4)
persisted until the present, possibly due to changes in the NAO, which may explain the lack of chemical recovery in this site (Rose et al., 2004; Davies et al., 2005, this issue). If current declining trends in xSO4 were to continue at the same rate and NO3 concentrations to remain stable, NO3 would equal or exceed xSO4 at five sites by 2010 and 13 sites by 2016, the target year for the achievement of good ecological status under the EU Water Framework Directive (Helliwell et al., 2003; Table 2). However, since changes in the rate of xSO4 decline are expected as emissions targets are achieved, there is no basis for assuming that current trends will continue, hence the need for other modelling approaches that account for anticipated changes in deposition.
0.7 0.6 8
0.5
4
0.4
5
14
3
0.2 0.1
18
12
1
0.3
17
10
6 2
7 9
21 20
15 16
19 22
11 13
0.0 r s s s d s y ct s d d N ia M tlan orm ach wa stri ine lan moo on am rne lan i o n AW Sco irng oss all e D en Eng art owd stre Mou Ire N D a Tr G ak S P S Sn elsh L NW C W
Fig. 4. Surface water NO3 as a proportion of excess acid anions in AWMN sites and exceeded sites in regional datasets; scaled from deposition data (B numbered AWMN sites, scaled from long-term mean NO3 from 1988 to 2002 and annual mean xSO4 for 2002). See Fig. 2 caption for explanation of boxplots.
C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82 1.0
UK
Scotland
England
NO3 / (NO3 + xSO4)
0.9 9 7 8
0.8 0.7 0.6 2
14
5 6
15 16
10 11
4
Wales 18 17
NI 20 21
22 19
12 13
0.5 3
0.4
1
0.3 0.2 0.1 0.0
r s s d s d s s y ct d N ia M tlan orm ach wa stri ine lan moo on am rne lan i o n AW Sco irng oss all e D en Eng art owd stre Mou Ire N D a Tr G ak S P S Sn elsh L NW C W
Fig. 5. Surface water NO3 as a proportion of excess acid anions in AWMN sites and exceeded sites in regional datasets; FAB predictions for 2010 (B numbered AWMN sites). See Fig. 2 caption for explanation of boxplots.
steady-state with mean modelled deposition levels for S and N in 2010 under the NECD (Table 2). For these deposition levels, FAB predicts that long-term NO3 leaching would exceed xSO4 in all AWMN sites except the two in northwest Scotland, in most cases by a factor of two or three (Fig. 5). For 15 of these 20 sites, the FAB critical load is exceeded in 2010 (Table 2), so NO3 is predicted to make a dominant contribution to the prevention of recovery or possibly to the re-acidification of recovered sites over the longer term. The box plots for FAB predictions in the regional datasets starkly illustrate the potential increase in the acidifying role of NO3 (Fig. 5). NO3 is predicted to be greater than xSO4 in almost all sites, with exceptions only in the Cairngorms and south Pennines. Note that when NO3:(xSO4 CNO3) is 0.67, NO3 concentration is double that of xSO4. In general the FAB projections for the AWMN sites lie very close to the regional medians, reflecting the dominant role of regional deposition in model predictions and the selection of regional datasets to correspond spatially with the AWMN sites. Notable exceptions are the two AWMN sites in northwest Scotland, Coire nan Arr and Allt na Coire nan Con. In these remote sites, xSO4 is still predicted to be the major excess acid anion in 2010, but these sites do not exceed their critical loads at this time.
4. Discussion The dominant excess acid anion in acidified sites is clearly still xSO4 at present. NO3 exceeds xSO4 on a seasonal basis in half of the AWMN sites (see e.g. Fig. 1). Annual mean NO3 is approaching xSO4 in importance in Northern Ireland, but only a few sites
79
elsewhere, including the AWMN site Afon Hafren. At current rates of decline in xSO4 and if NO3 levels remain close to current means, then by 2010 five AWMN sites would have mean NO3 concentrations greater than xSO4. However, this situation is unlikely given an anticipated slowdown in the rate of decline in S deposition, which must be taken into account in future projections through scaling or modelling approaches. The best-case scenario, using scaling to projected deposition levels for 2010, suggests that NO3 will be of similar or greater magnitude to xSO4 in around 25% of AWMN sites and at least 25% of exceeded sites in the Lake District, Snowdonia, Mournes and Northern Ireland. Worst-case FAB model predictions indicate that this future NO3 dominance could increase greatly if terrestrial N saturation occurs and leaching increases. The timescale over which this process may occur is, however, largely unknown. There is little current evidence from acid-sensitive surface waters of long-term N saturation having led to rising trends in NO3 concentrations. Certain long-term datasets in Scandinavia (Dickson, 1986; Henriksen and Brakke, 1988; Lepisto¨, 1995) and North America (Stoddard, 1991; Murdoch and Stoddard, 1992) showed increasing trends in surface water NO3 concentrations in the 1980s and 1990s, which seemed to provide evidence of cumulative N saturation. However, few recent analyses of trends in water chemistry at sites across Europe and North America have found increasing trends in NO3 and most earlier trends have disappeared (Stoddard et al., 1999; Skjelkva˚le et al., 2001) including those observed within the AWMN (Evans and Monteith, 2001). Only in some Italian lakes has clear evidence of rising NO3 concentrations been found (Mosello et al., 2001; Rogora et al., 2001). This lack of a general trend has been suggested to be the result of opposing factors; the increasing N saturation status of catchments in the context of declining N deposition in some parts of Europe (Wright et al., 2001). For the UK there is strong statistical evidence that inter-annual variation in NO3 leaching is closely linked to climatic factors represented by the North Atlantic Oscillation (e.g. via soil freezing; Monteith et al., 2000), so trends have to be interpreted with caution. It is evident that there are very few datasets of sufficiently long timescales to say with any certainty whether there are long-term trends in surface water NO3 related to changes in atmospheric deposition, rather than other drivers such as climatic effects. However, past increases in NO3 concentrations must have occurred in those regions where high N deposition corresponds with high NO3 concentrations. Furthermore, it is clear that NO3 is already an important acid anion on a regional basis, particularly during the dormant season, and will become the dominant excess acid anion in many acidified systems in parts of the UK over the next
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C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
10e15 years, including the Lake District, Snowdonia and Northern Ireland. In most of Scotland, southern England and the south Pennines, scaling projections from current data suggest that the current dominance of xSO4 in acidified sites is likely to persist unless N saturation of terrestrial ecosystems leads to increased NO3 leaching. In the south Pennines, the continued dominance of xSO4 due to high deposition and possibly geological sources masks NO3 concentrations that are among the largest in the country, and that would certainly be sufficient to prevent recovery from acidification, even without increased NO3 leaching. This region in particular would suffer very serious re-acidification if the worst-case projections of N saturation and increased NO3 leaching were realised. By comparison, the southern England regional dataset exemplifies the potential, but as yet unrealised, importance of NO3. Here, measured NO3 currently makes up the smallest proportion of excess acid anions of all the regional datasets, but FAB predictions based on 2010 deposition indicate that this could become the region where NO3 achieves the greatest importance relative to xSO4. The large discrepancies between the scaling and FAB approaches in this lowland region may be due to much lower runoff (c. 300 mm compared with c. 900e2700 mm) and higher temperatures than in other datasets comprising mainly upland sites. Low runoff in particular may result in the leaching of only a very small proportion of N deposition through purely hydrological leaching processes, which are likely to be much more important in the uplands. The very low NO3 concentrations observed in southern England despite relatively high N deposition levels result in low projections from scaling but high projections from the FAB mass-balance assumptions. The major uncertainty for some regions appears to be not if, but rather when, NO3 will become the major agent of continued acidification. Another key uncertainty is the degree to which measured NO3 concentrations, which are still low in absolute terms in some regions, may increase to the very high levels predicted by the FAB model. However, while planned reductions in S emissions and deposition are still vital to facilitate chemical recovery of acidified waters, N deposition will become the major policy issue in the near future, even without the occurrence of N saturation. 4.1. Limitations of the modelling approach In addition to the structural differences between the models, there are additional uncertainties that relate to drivers of change other than deposition inputs. For example, climate change could have major effects on biological cycles with highly uncertain impacts on the storage and release of both S and N. Higher precipitation could result in greater hydrological losses of acid anions,
especially the less mobile NO3 anion, while a drier climate could reduce these losses. Increased severity of droughts followed by heavy rain could result in large leaching pulses of mineralised organic S and N compounds (Reynolds and Edwards, 1995), as could more severe soil freezing and thawing cycles (Monteith et al., 2000). The storage of S in soils is not taken into account by the FAB model, but could be important in certain organic soil and wetland areas, where climatic effects could be particularly pronounced. However, Cooper (2005, this issue) found no significant time lag at the annual scale between reductions in non-marine S deposition and xSO4 concentrations at most AWMN sites. Finally, the widespread trends in increasing DOC, for which the major drivers are still the subject of some debate (see Evans et al., 2005, this issue), affect the acidity and recovery status of surface waters. Organic acidity is increasing in importance as xSO4 declines, in both relative and absolute terms. The role of organic anions has not been compared with excess mineral acid anions derived from acid deposition, because of the problem of determining organic acid anion concentrations for DOC of different composition at each site, but the observed trends in DOC suggest that their role could be changing in magnitude. However, Hrusˇ ka et al. (1997) used experimental additions and monitoring data to show that organic acids were unlikely to hinder recovery of an acidified bog stream in the Czech Republic with DOC concentrations of 48.9 mg lÿ1. Further discussion on the role of DOC in recovery from acidification is provided in Evans et al. (2005, this issue).
4.2. Research needs Current research priorities aim to determine the degree to which NO3 leaching is biologically mediated through terrestrial ecosystems and the proportion of leached NO3 that is ‘‘hydrological’’, i.e. is controlled primarily by abiotic factors such as hydrological flow routing. If currently elevated NO3 concentrations observed in many acid waters are due to biological production in catchment soils, then N saturation may already be well advanced and masked only by interannual climatic variations. The ‘‘worst-case’’ scenario of the FAB model may therefore be realised within a timescale of years to decades. If leached NO3 is mainly ‘‘hydrological’’ this may suggest that terrestrial ecosystems still have much capacity to assimilate N and saturation may not occur for many decades. The lack of information on the proportion of N deposition that is retained in soils and vegetation largely accounts for the mixed results of attempts to link NO3 leaching to measures of N saturation status such as soil carbon: nitrogen ratio (Gundersen et al., 1998; Curtis et al., 2004) and catchment soil carbon pool relative to accumulated
C.J. Curtis et al. / Environmental Pollution 137 (2005) 73e82
N deposition loads (Jenkins et al., 2001; Evans et al., unpublished).
Acknowledgements This work was funded under the DEFRA Freshwaters Umbrella programme (contract 1/3/183) and by the Scottish Executive Environment and Rural Affairs Department (SEERAD). The authors thank colleagues within the Freshwaters Umbrella for the provision of chemistry data, CEH Edinburgh for the provision of deposition data and CEH Monks Wood for the provision of catchment data used in FAB modelling.
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