Results

Acidification and chemical recovery

Deposition of S and N at Langtjern peaked during the late 1960s at 9.8 kg S ha-1 yr-1 (61 meq m-2 yr-1) and 8.8 kg N ha-1 yr-1 (63 meq m-2 yr-1) (averaged over 1965-1969) (Figure 2 ). After the 1960s, S deposition gradually declined towards the current level of 1.1 kg S ha-1yr-1 (6.6 meq m-2yr-1) (2018-2022 average) and is expected to remain more or less constant towards 2100 under the emission scenario of current legislation (CLE). Deposition of N was more variable, showing two other peaks after the 1960s, and is currently (2018-2022) at 3.7 kg N ha-1 yr-1 (26.5 meq m-2 yr-1).
Streamwater chemistry follows patterns that are typical for acidified, but recovering, streams (Figure 3): strong declines in SO4 and SAA, following the temporal change in S deposition; declines in base cations (and SBC), in particular in Ca and Mg, and increases in pH and TOC. Labile Al shows the strongest decline. The other strong acid anions Cl and NO3 also decline, where Cl is currently at a fairly constant level of circa 10 µeq l-1, similar to SO4. Nitrate is present in very low concentrations relative to SO4because of high retention in the catchment. pH, ANC and ANCoaa are currently at 5.16, 61.5 µeq/L and 24.0 µeq/L, respectively (averaged over 2018-2022), which is below MAGIC-simulated preindustrial levels (5.38 (pH), 73.6 µeq/L (ANC) and 34.7 µeq/L (ANCoaa), respectively) (Table 1 ).
Organic acidity, calculated from the charge balance between strong acid anions and major cations, increased markedly over time, balancing 33% of the base cations during the end of the 1980s and 73% for 2018 to 2022 (Figure 4). On an equivalent basis, organic acids have dominated SAA at Langtjern since 2000-2004. Charge density of TOC increased from 4.3 to 5.7 µeq/mg C since the end of the 1980s (Figure 5 ).

MAGIC hindcasts of historical acidification and recovery

Model performance for individual and compound variables were evaluated for a set of performance statistics (Table 2). MAGIC described levels and variations for major cations, major anions, pH and ANC well (Figure 3). The variation was described best (r2>0.7) for the SAA and its constituents, dominating base cations Ca and Mg, ANC, ANCoaa, organic acids and pH. These variables also had low normalized RMSE’s (<0.7), except for variables associated with, or affected by, organic acidity such as ANCoaa, labile Al, and organic matter charge density – these variables also had relatively low values for NSE, illustrating systematic overestimation (organic acids, ANCoaa, charge density). The overestimation of organic acidity was primarily related to an overestimation of the TOC charge density with on average ca 1 µeq mg-1 TOC (Table 1; Figure 5). The variables that were described with least bias (|bias| <1) were those that showed least temporal variation (Figure 3), while those described with a large bias (|bias|> |5|) were ANC, ANCoaa and organic acids.
Since organic acidity is such an important part of the total charge balance, model simulation of other ions is very sensitive to its level. Decreasing of organic acidity in MAGIC leads to lowering of the base cation concentrations, so optimizing organic acidity in MAGIC can come at the cost of good optimization of SBC and therefore pH and ANC. However, the upward trend in organic acidity was described well, which is a novel aspect of MAGIC modelling.
The hindcast of all the elements, from 1850 to 1973, was driven by the change in deposition (Figure 2 ). Preindustrial pH, ANC, ANCoaa, and TOC were higher than current-day values and much higher than their values during the period when acidification peaked (Table 1 ). Base saturation decreased from 23.2% (preindustrial) to 19.4% (present-day).

MAGIC projections of future acidification and recovery

Deposition of S and N in 2050 under the CLE deposition was 56% and 67% of 2015 deposition, respectively, where 2015 was used as the reference year. The catchment became warmer and wetter during 1974-2022, as indicated by trends (Sen slopes, MK test (Sen 1968)) in annual temperature, precipitation and discharge, which are +0.38oC decade-1 (p<0.0001), +39 mm decade-1 (p<0.1) and + 40 mm decade-1 (p<0.01), respectively. The climate scenario RCP8.5 predicted less climate change than these empirical climate data, but the locally adapted climate scenario RCP8.5* gave a higher mean discharge (+80mm) compared with average discharge for 1974 to 2022 (Figure 6 ).
The Constant Climate scenario did not result in more chemical recovery in 2050 and 2100 than observed for 2018-2022 (Figure 3 ), while locally adapted RCP8.5* and RCP8.5 resulted in a slight reacidification (Table 1 ). RCP8.5* had a slightly stronger effect on acidification than RCP8.5, as a consequence of higher discharge and thus higher base cation export. Chemical acidification status in 2100 was markedly below the preindustrial water quality indicators pH, ANC and ANCoaa. The reacidification is driven by the depletion of base cation stores from the catchment soils due to increased element export related to the increased discharge. The three future scenarios give rather similar results, with organic acidity dominating over mineral acidity (Figure 4 ) and where the strongest climate change scenario results in a slight reacidification (reduction of ANC with 5.2 µeq/l (Table 1)).

Sensitivity analysis

We tested the necessary increase in weathering rates to obtain preindustrial base saturation and ANC in 2100 in a sensitivity analysis (Table 3) where the weathering rates were increased with a fixed percentage from year 2000 for three climate scenarios. The uncertainty intervals for both RCPs are related to uncertainty intervals for future precipitation, illustrating the importance of precipitation for estimation of weathering rates. For the ‘constant climate’ scenario, the annual increase was 0.54%, and the two RCPs had median increases in weathering rates of 0.59% and 0.96%, resulting in weathering rates in 2100 that were a factor 1.7 to 2.5 higher than in 2000. Interestingly, it took relatively less weathering to reach preindustrial ANC than for preindustrial BS% as a consequence of lower expected acid deposition in the future compared to pre-industrial. The cumulative excess contribution of base cations from enhanced weathering were 33% (ConstantClim), 36% (RCP8.5) and 68% (RCP8.5*), to reach preindustrial base saturation. To reach preindustrial ANC, these numbers were 35% (RCP8.5) and 59% (RCP8.5*).

Discussion

The monitoring data demonstrate strong chemical recovery from acidification at Langtjern, similar to other strongly acidified surface waters in Norway (De Wit et al. 2023), elsewhere in Europe and North America (Bukaveckas 2021; Garmo et al. 2014; Houle et al. 2022; Kopacek et al. 2021; Lawrence et al. 2021; Sterling et al. 2022) and in Japan (Sase et al. 2021). Simultaneously, DOC at Langtjern has increased and now contributes more to the total anion charge in the streamwater than the strong acid anions. The site is thus moving towards a state dominated by natural-, rather than anthropogenic acidification. Browning is a common feature for surface waters in boreal regions, primarily related to reduced acid deposition (de Wit et al. 2021; Monteith et al. 2007). To what extent and at what time scale organic acidity will dominate anthropogenic acidification is not well-known, however. Assumptions about natural levels of organic acidity have considerable implications for assessments of the anthropogenic contribution to surface water acidity (Erlandsson et al. 2011) and the necessity of liming to accelerate recovery (Laudon et al. 2021). Humic charge density for DOM increased, as found earlier (De Wit et al. 2007), suggesting that both increased solubility and decreased deprotonation of humic and fulvic acids control the return to naturally acidified systems and pre-industrial water quality.
The last MAGIC application for Langtjern was done based on the period 1974 to 2003 (Larssen 2005). We used the same soil and historical deposition data. In our study, we optimized for 1974 to 2022 and captured the trend in chemical recovery with hardly any bias, which is considerably better than Larssen (2005), where MAGIC underestimated the positive trend in ANC, overestimating peak acidification ANC and underestimating ANC in the early 2000s. Larssen (2005) reported relatively poor model performance for K, Na, Mg and Cl compared with MAGIC applications for two other acidified catchments in southernmost Norway, which could indicate that the relatively poor simulation of ANC levels at Langtjern were related to other major cations and anions than SO4 and Ca. Also, an internal source of S in the Langtjern catchment (6 meq m-2 yr-1) was assumed by Larssen, contrary to our study, which could lead to overemphasizing the importance of background SO4 at the cost of anthropogenic SO4 and anthropogenic acidification. This is illustrated by the substantially higher hindcast for preindustrial ANC in our study (e.g. ANC of 80 µeq l-1 (Figure 3 ) than in Larssen (2005) (e.g. ANC of 40 µeq l-1). Furthermore, the longer time series in our study presents a stronger dataset for model calibration. The inclusion of changing organic acidity in our study, where Larssen assumed constant organic acidity, cannot explain the poorer description of recovery by Larssen since organic acidity counteracts changes in mineral acidity, leading to a lower response in ANC to changes in SAA.
The increases in DOC and organic acidity were described tolerably well by MAGIC albeit more poorly than mineral acidity. The levels of organic acidity at Langtjern were somewhat overestimated possibly indicating that organic acid properties at Langtjern are outside the range for values for organic acid deprotonation reported by Hruska et al. (2003). The organic charge density estimated in our study was between 4.4 and 5.7 µeq g-1 C, which agrees with the values previously calculated for Langtjern (De Wit et al. 2007).
The MAGIC-forecasted pH, ANC and ANCoaa under reduced S deposition and current climate were not yet back at preindustrial levels in 2100, which to a great degree was related to depleted base cation stores. Climate change acted through increasing discharge and thereby further depleting base cation stores and thereby leading to a slight reacidification. Such tendencies have already been observed in humus-rich acidified lakes in Eastern Norway (de Wit et al. 2023), which was attributed to organic acidity rather than climate change. However, TOC and organic acidity are also climate-sensitive, in particular to precipitation (de Wit et al. 2016), a feature that is currently not included in MAGIC. Increased catchment base cation export related to discharge is textbook knowledge, but changes in soil base cation stores for periods shorter than 3 to 5 decades are typically difficult to measure (Ahrends et al. 2022).
We showed in a sensitivity analysis, using new features of MAGIC-Forest, that a doubling in weathering rates would be required to achieve preindustrial base saturation and associated water quality in 2100. An even higher weathering would be necessary to compensate for enhanced soil base cation loss under higher runoff from climate change. While it is clear that mineral weathering rates are higher under forested versus non-forested sites (Berner 1997), generally higher in moist versus dry climates (West et al. 2005), and are promoted by higher pCO2 and organic acids (Bargrizan et al. 2020), it is difficult to constrain weathering rates based on empirical measurements (Koseva et al. 2010; Kronnas et al. 2019). However, unexpected widespread increases in dissolved silicates and calcium have been observed in Norwegian lakes (de Wit et al. 2023), possibly providing indirect evidence of recent, enhanced weathering rates. We conclude that chemical recovery at Langtjern may be impacted – positively and negatively – by climate change. Continued monitoring is needed to evaluate which drivers will prove to be strongest.
Our study illustrates that a combination of long-term monitoring and process-oriented modelling is key for predictions of water quality under changing environmental conditions. Our assessment points towards an ongoing transition from anthropogenic to natural acidification in surface waters, due to reduced acid deposition and stronger impact of climate, humic compounds, and possibly land use change as drivers determining the surface water acidity status.

Acknowledgements

This work was conducted as part of the CatchCaN project (The fate and future of carbon in forests), funded by the Technology Agency of the Czech Republic (TA CR) project number TO 01000220. We acknowledge EMEP/MSC-W (http://emep.int/mscw) for the provision of historical deposition data and projections of future deposition. Arne Stuanes, Dick Wright and Kari Austnes are gratefully acknowledged for helpful comments on the manuscript.

Conflict of interest

The authors declare no conflicts of interest

List of Figures

Figure 1 Map of the Langtjern catchment
Figure 2 Total atmospheric deposition of SO4 and N (sum of NOx and NHy) at the Langtjern catchment in meq m-2 yr-1. Historical (1850-1973) and future deposition, following current legislation policy (CLE), provided by EMEP. Deposition of SO4 is corrected for seasalt contribution.
Figure 3 Empirical flow-weighted mean annual element concentrations from 1974 to 2022 (open circles) and MAGIC-simulations for 1850 to 2100, for SO4, Cl, NO3, pH (and H+), Na, K, TOC (expressed on an equivalent basis), ANC, ANCoaa, labile Aluminum (lAl) SBC and SAA. The future scenarios include EMEP deposition according to current legislation (CLe) in combination with constant climate (dotted line), RCP8.5 (blue line) and locally adapted RCP8.5* (red line).
Figure 4 Five-year averages of the equivalent sum of strong acid anions (SAA), organic acids and sum of base cations (SBC) in µeq/l. Hatched and plain bars show simulated data by MAGIC in a preindustrial decade, at peak acidification and for future (2090-2100) scenarios (ConstantClim: CLe deposition, no climate change; CLe+RCP8.5, CLe+RCP8.5*).
Figure 5 Simulated and empirical TOC charge density (µeq/mg TOC, top panel) and organic acidity (in µeq/l) (bottom panel). Open circles show empirical estimates, open squares and lines show MAGIC simulation.
Figure 6 Annual runoff (mm) at Langtjern for 1850-2100. Open circles: empirical data; 1850-1973: modelled runoff using averaged climate for 1971–2000. Future runoff 2023-2100: dotted line, constant climate (discharge equal to pre-1974); blue line, RCP 8.5 (change relative to predefined period 1971-2000); red line, RCP8.5* (change relative to empirical runoff for 2000 to 2022).

List of Tables

Table 1 Key MAGIC model results for preindustrial time (mean 1850-1860), peak acidification (1965-1974 ), current situation (2018-2022), and future (2090-2100), and for the different climate scenarios. All with CLe deposition (constant climate; RCP8.5, RCP8.5*), all with constant weathering. Last column shows results sensitivity analysis (for RCP8.5* only, increased weathering).
Table 2 MAGIC calibration performance statistics over 1974-2022 including Nash-Sutcliffe efficiency (NSE), root mean squared error (RMSE), coefficient of determination (r2), standardized root mean squared error (RMSE/std) and bias (the mean signed difference between modeled and observed values).
Table 3 Estimation of required increase in weathering rates (in %) and resulting cumulative additional base cation input from weathering over 2000–2100 (expressed as % of base cation inputs from weathering for 2000-2100 under constant weathering), to return soil base saturation, ANC or pH back to their pre-industrial levels, for three climate scenarios. The intervals show interquartile range based on the interquartile range of predicted precipitation under the RCPs.

References

Ahrends B, Fortmann H, Meesenburg H (2022) The Influence of Tree Species on the Recovery of Forest Soils from Acidification in Lower Saxony, Germany. Soil Syst. 6(2): 25
Augustin F, Houle D, Gagnon C, Couture S, Courchesne F (2015) Partitioning the impact of environmental factors on lake concentrations and catchment budgets for base cations in forested ecosystems. Applied Geochemistry 53: 1-12
Bargrizan S, Smernik RJ, Mosley LM (2020) Constraining the carbonate system in soils via testing the internal consistency of pH, pCO(2) and alkalinity measurements. Geochem. Trans. 21(1): 10
Berner RA (1997) Paleoclimate - The rise of plants and their effect on weathering and atmospheric CO2. Science 276(5312): 544-546
Bukaveckas PA (2021) Changes in acidity, DOC, and water clarity of Adirondack lakes over a 30-year span. Aquatic Sciences 83(3): 11
Clark JM, Chapman PJ, Heathwaite AL, Adamson JK (2006) Suppression of dissolved organic carbon by sulfate induced acidification during simulated droughts. Environmental Science & Technology 40(6): 1776-1783
Cosby BJ, Ferrier RC, Jenkins A, Wright RF (2001) Modelling the effects of acid deposition: refinements, adjustments and inclusion of nitrogen dynamics in the MAGIC model. Hydrology and Earth System Sciences 5(3): 499-517
Cosby BJ, Hornberger GM, Galloway JN, Wright RF (1985) Modeling The Effects Of Acid Deposition - Assessment Of A Lumped Parameter Model Of Soil-Water And Streamwater Chemistry. Water Resources Research 21(1): 51-63
de Wit HA, Couture RM, Jackson-Blake L, Futter MN, Valinia S, Austnes K, Guerrero JL, Lin Y (2018) Pipes or chimneys? For carbon cycling in small boreal lakes, precipitation matters most. Limnology and Oceanography Letters 3(3): 275-284
de Wit HA, Garmo Ø, Jackson-Blake L, Clayer F, Vogt RD, Austnes K, Kaste Ø, Gundersen CB, Guerrerro JL, Hindar A (2023) Changing water chemistry in one thousand Norwegian lakes during three decades of cleaner air and climate change. Global Biogeochemical Cycles:
De Wit HA, Mulder J, Hindar A, Hole L (2007) Long-term increase in dissolved organic carbon in streamwaters in Norway is response to reduced acid deposition. Environmental Science & Technology 41(22): 7706-7713
de Wit HA, Stoddard JL, Monteith DT, Sample JE, Austnes K, Couture S, Folster J, Higgins SN, Houle D, Hruska J, Kram P, Kopacek J, Paterson AM, Valinia S, Van Dam H, Vuorenmaa J, Evans CD (2021) Cleaner air reveals growing influence of climate on dissolved organic carbon trends in northern headwaters. Environmental Research Letters 16(10): 13
de Wit HA, Valinia S, Weyhenmeyer GA, Futter MN, Kortelainen P, Austnes K, Hessen DO, Raike A, Laudon H, Vuorenmaa J (2016) Current Browning of Surface Waters Will Be Further Promoted by Wetter Climate. Environmental Science & Technology Letters 3(12): 430-435
Erlandsson M, Cory N, Folster J, Kohler S, Laudon H, Weyhenmeyer GA, Bishop K (2011) Increasing Dissolved Organic Carbon Redefines the Extent of Surface Water Acidification and Helps Resolve a Classic Controversy. Bioscience 61(8): 614-618
Evans CD, Monteith DT, Reynolds B, Clark JM (2008a) Buffering of recovery from acidification by organic acids. Science of the Total Environment 404(2-3): 316-325
Evans CD, Reynolds B, Hinton C, Hughes S, Norris D, Grant S, Williams B (2008b) Effects of decreasing acid deposition and climate change on acid extremes in an upland stream. Hydrology and Earth System Sciences 12(2): 337-351
Garmo ØA, Skjelkvale BL, de Wit HA, Colombo L, Curtis C, Folster J, Hoffmann A, Hruska J, Hogasen T, Jeffries DS, Keller WB, Kram P, Majer V, Monteith DT, Paterson AM, Rogora M, Rzychon D, Steingruber S, Stoddard JL, Vuorenmaa J, Worsztynowicz A (2014) Trends in Surface Water Chemistry in Acidified Areas in Europe and North America from 1990 to 2008. Water Air and Soil Pollution 225(3):
Grennfelt P, Engleryd A, Forsius M, Hov O, Rodhe H, Cowling E (2020) Acid rain and air pollution: 50 years of progress in environmental science and policy. Ambio 49(4): 849-864
Hanssen-Bauer I, Førland EJ, Haddeland I, Hisdal H, Mayer S, Nesje A, Nilsen JEØ, Sandven S, Sandø AB, Sorteberg A, Ådlandsvik B (2015) Klima i Norge.Kunnskapsgrunnlag for Klimatilpasning, oppdatert i 2015.NCCS report 2/2015. In: NCCS report 2/2015, NCCS, Oslo, Norway. Norwegian Centre for Climate Services, Oslo, Norway. p 203
Helliwell RC, Simpson GL (2010) The present is the key to the past, but what does the future hold for the recovery of surface waters from acidification? Water Research 44(10): 3166-3180
Hindar A, Torseth K, Henriksen A, Orsolini Y (2004) The significance of the North Atlantic Oscillation (NAO) for sea-salt episodes and acidification-related effects in Norwegian rivers. Environmental Science & Technology 38(1): 26-33
Houle D, Augustin F, Couture S (2022) Rapid improvement of lake acid-base status in Atlantic Canada following steep decline in precipitation acidity. Canadian Journal of Fisheries and Aquatic Sciences 79(12): 2126-2137
Hruska J, Kohler S, Laudon H, Bishop K (2003) Is a universal model of organic acidity possible: Comparison of the acid/base properties of dissolved organic carbon in the boreal and temperate zones. Environmental Science & Technology 37(9): 1726-1730
Kopacek J, Hejzlar J, Kana J, Porcal P, Turek J (2016) The sensitivity of water chemistry to climate in a forested, nitrogen-saturated catchment recovering from acidification. Ecological Indicators 63: 196-208
Kopacek J, Kana J, Porcal P, Stuchlik E (2021) Diverse effects of accelerating climate change on chemical recovery of alpine lakes from acidic deposition in soil-rich versus scree-rich catchments*. Environmental Pollution 284: 10
Koseva IS, Watmough SA, Aherne J (2010) Estimating base cation weathering rates in Canadian forest soils using a simple texture-based model. Biogeochemistry 101(1-3): 183-196
Kronnas V, Akselsson C, Belyazid S (2019) Dynamic modelling of weathering rates - the benefit over steady-state modelling. Soil 5(1): 33-47
Larssen T (2005) Model prognoses for future acidification recovery of surface waters in Norway using long-term monitoring data. Environmental Science & Technology 39(20): 7970-7979
Laudon H, Sponseller RA, Bishop K (2021) From legacy effects of acid deposition in boreal streams to future environmental threats. Environmental Research Letters 16(1): 10
Lawrence GB, Baldigo BP, Roy KM, George SD (2021) Trends and current status of aluminum chemistry in Adirondack headwater streams 30 Years after the Clean Air Act Amendments of 1990. Atmospheric Environment 249: 13
Lund E, Garmo OA, de Wit HA, Kristensen T, Hawley KL, Wright RF (2018) Reduced Acid Deposition Leads to a New Start for Brown Trout (Salmo trutta) in an Acidified Lake in Southern Norway. Water Air and Soil Pollution 229(11): 12
Lydersen E, Larssen T, Fjeld E (2004) The influence of total organic carbon (TOC) on the relationship between acid neutralizing capacity (ANC) and fish status in Norwegian lakes. Science of the Total Environment 326(1-3): 63-69
Monteith DT, Stoddard JL, Evans CD, de Wit HA, Forsius M, Hogasen T, Wilander A, Skjelkvale BL, Jeffries DS, Vuorenmaa J, Keller B, Kopacek J, Vesely J (2007) Dissolved organic carbon trends resulting from changes in atmospheric deposition chemistry. Nature 450(7169): 537-540
Norling MD, Jackson-Blake LA, Calidonio JLG, Sample JE (2021) Rapid development of fast and flexible environmental models: the Mobius framework v1.0. Geosci. Model Dev. 14(4): 1885-1897
Norling MN, Kaste Ø, Wright RF (this issue) A biogeochemical model of acidification: MAGIC is alive and well.
Oulehle F, Fischer M, Hruska J, Chuman T, Kram P, Navratil T, Tesar M, Trnka M (2021) The GEOMON network of Czech catchments provides long-term insights into altered forest biogeochemistry: From acid atmospheric deposition to climate change. Hydrological Processes 35(5): 18
Posch M, Aherne J, Moldan F, Evans CD, Forsius M, Larssen T, Helliwell R, Cosby BJ (2019) Dynamic Modeling and Target Loads of Sulfur and Nitrogen for Surface Waters in Finland, Norway, Sweden, and the United Kingdom. Environmental Science & Technology 53(9): 5062-5070
Reuss JO (1990) Critical loads for soils in Norway. Analyses of soils in eight Norwegian catchments. In: NIVA report. NIVA, Oslo. p 78
Sase H, Saito T, Takahashi M, Morohashi M, Yamashita N, Inomata Y, Ohizumi T, Nakata M (2021) Transboundary air pollution reduction rapidly reflected in stream water chemistry in forested catchment on the sea of Japan coast in central Japan. Atmospheric Environment 248: 10
Schopp W, Posch M, Mylona S, Johansson M (2003) Long-term development of acid deposition (1880-2030) in sensitive freshwater regions in Europe. Hydrology and Earth System Sciences 7(4): 436-446
Sen PK (1968) Estimates Of Regression Coefficient Based On Kendalls Tau. Journal Of The American Statistical Association 63(324): 1379-&
SFT (2001) Overvåking av langtransportert forurenset luft og nedbør. Årsrapport - Effekter 2000. SFT-rapport 834/01, TA-1830/2001. p83-87.
Skjelkvale BL, Stoddard JL, Jeffries DS, Torseth K, Hogasen T, Bowman J, Mannio J, Monteith DT, Mosello R, Rogora M, Rzychon D, Vesely J, Wieting J, Wilander A, Worsztynowicz A (2005) Regional scale evidence for improvements in surface water chemistry 1990-2001. Environmental Pollution 137(1): 165-176
Sterling S, Clair TA, Rotteveel L, O’Driscoll N, Houle D, Halfyard E, Keys K (2022) Kejimkujik calibrated catchments: A benchmark dataset for long-term impacts of terrestrial and freshwater acidification. Hydrological Processes 36(2): 8
Stuanes AO, Abrahamsen G, Rosberg I (1995) Acidification of soils in five catchments in Norway. Water Air and Soil Pollution 85(2): 635-640
Valinia S, Kaste O, Wright RF (2021) Intensified forestry as a climate mitigation measure alters surface water quality in low intensity managed forests. Scandinavian Journal of Forest Research 36(1): 15-31
Vogt RD, Skancke LB (2022) Overvåking av langtransportert forurenset luft og nedbør. Årsrapport–Vannkjemiske effekter 2021. NIVA report 7778-2022; Miljødirektoratet report 2347-2022:
Vuorenmaa J, Augustaitis A, Beudert B, Clarke N, de Wit HA, Dirnbock T, Frey J, Forsius M, Indriksone I, Kleemola S, Kobler J, Kram P, Lindroos AJ, Lundin L, Ruoho-Airola T, Ukonmaanaho L, Vana M (2017) Long-term sulphate and inorganic nitrogen mass balance budgets in European ICP Integrated Monitoring catchments (1990-2012). Ecological Indicators 76: 15-29
Watmough SA, Aherne J, Alewell C, Arp P, Bailey S, Clair T, Dillon P, Duchesne L, Eimers C, Fernandez I, Foster N, Larssen T, Miller E, Mitchell M, Page S (2005) Sulphate, nitrogen and base cation budgets at 21 forested catchments in Canada, the United States and Europe. Environmental Monitoring And Assessment 109(1-3): 1-36
West AJ, Galy A, Bickle M (2005) Tectonic and climatic controls on silicate weathering. Earth and Planetary Science Letters 235(1-2): 211-228