PREDICTING RECOVERY IN ACIDIFIED FRESHWATERS BY THE YEAR 2010, AND BEYOND

Contract EVK1-1999-00087 - RECOVER:2010

Part of the 'Sustainable Management and Quality of Water'

Ecosystem Functioning

Directorate General Research
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THE REGIONS

RESULTS

Recovery from acidification of lochs in Galloway, south-west Scotland, UK: 1979-1998

R.C. Ferrier1, R.C. Helliwell1, B.J. Cosby2, A. Jenkins3 and R.F. Wright4
1Macaulay Land Use Research Institute, Aberdeen, AB15 8QH, UK
2 Department of Environmental Science, University of Virginia, Charlottesville, VA 22901,USA
3Centre for Ecology and Hydrology, Wallingford, Oxon OX10 8BB,UK
4Norwegian Institute for Water Research, P.O. Box 173, Kjelsås, N-0411 Oslo, Norway

Full Reference

Ferrier, R.C., Helliwell, R.C., Cosby, B.J., Jenkins, A. and Wright, R.F. (2001). Recovery from acidification of lochs in Galloway, south-west Scotland, UK:1979-1998. Hydrology and Earth System Sciences Vol. 5, No. 3, 421-431.

Summary of Research

The Galloway region of south -west Scotland has historically been subject to long -term deposition of acidic precipitation which has resulted in acidification of soils and surface waters and subsequent damage to aquatic ecology. Since the end of the 1970s however, acidic deposition has decreased substantially. The general pattern is for a rapid decline in non-marine sulphate in rainwater over the period 1978-1988 followed by stable concentrations to the mid-1990s. Concentrations of nitrate and ammonium in deposition have remained constant between 1980 and 1998. Seven water quality surveys of 48 lochs in the Galloway region have been conducted between 1979 and 1998. During the first 10 years, from 1979, there was a major decline in regional sulphate concentrations in the lochs, which was expected to have produced a decline in base cations and an increase in the acid neutralising capacity. But sea-salt levels (as indicated by chloride concentrations) were approximately 25% higher in 1988 than in 1979 and thus short-term acidification due to sea-salts offset much of the long-term recovery trend expected in the lochs. During the next 10 years however, the chloride concentrations returned to 1979 levels and the lochs showed large increases in acid neutralising capacity despite little change in sulphate concentrations. From the observed decline in sulphate deposition and concentrations of sulphate in the lochs, it appears that approximately 75% of the possible improvement in acid neutralising capacity has already occurred over the 20-year period (1979-1998). The role of acid deposition as a driving factor for change in water chemistry in the Galloway lochs is confounded by concurrent changes in other driving variables, most notably, factors related to episodic and year-to-year variations in climate. In addition to inputs of sea-salts, climate probably also influences other chemical signals such as peaks in regional nitrate concentrations and the sharp increase in dissolved organic carbon during the 1990s.

Fig.1. Wet deposition of non-marine SO4 at Eskdalemuir and Loch Dee.

The deposition data from the Eskdalemuir collector follows two different protocols; mean annual concentrations are calculated from both mean daily concentrations and mean weekly concentrations, the latter forming the basis of the UKADMN data (RGAR, 1983, 1987, 1993, 1997). Comparison of the wet deposited non-marine SO4 concentration from the two methods at Eskdalemuir (Figure 1) shows no difference.

The Loch Dee data is based on mean weekly concentrations and demonstrates greater annual variability in concentrations but the same overall trend as at Eskdalemuir. The general pattern is for a rapid decline in nmSO4 over the period 1978-1988 approximating to a >50% reduction, followed by more stable concentrations during the 1980s and early 1990s, and a subsequent reduction (10% of 1978 values) until 1998.

Fig.2. Concentrations of NO3 and NH4






Concentrations of NO3 and ammonium (NH4) at both Eskdalemuir and Loch Dee have remained constant between 1980 and 1998 (Figure 2). Similarly, concentrations of Ca and Mg have remained relatively low and constant between 1980 and 1998 (Figure 3). Chloride concentrations, however, have been extremely variable at Loch Dee reflecting the proximity of this site to the coast.






Fig. 3 Base cation and Cl concentrations

Table 1.Median regional concentration of major ions in 1998.

Concentrations of sea-salt-derived ions, particularly Cl, in the lochs varied greatly, but did not show any major trend over time for the region. Levels were relatively low in the first survey, higher in 1988 and then lower again during the late 1990s (Table 1).

As a consequence of the changes in SO4, NO3 and Cl, the median sum of strong acid anions (SAA) decreased by about 1 µeq l-1 per year over the period 1979 to 1988; the large decrease in SO4 was offset by an increase in Cl. Over the second 10 year period (1988-1998), the median SAA decreased further by 6 µeq l-1 per year, largely due to lower Cl concentrations (Table 1). For the entire 20 years the median change in SAA was -2 µeq l-1. All but four of the lochs showed negative change in SAA over the entire period.

Charge balance necessitates that decreases in SAA are compensated by either a decrease in base cations or an increase in ANC. Median change in sum of base cations was +1 µeq l-1 per year over the entire 20 years (Table 1). Trend analyses of Ca and SO4 are very similar. Loch data from the first 10 years showed a decline in Ca (median slope of -1.4 µeq l-1per year), with little change during the next 10 years. Trends in Mg and K are similar to those for Ca.

During the first and second 10 year period, increases in ANC were concurrent with decreases in SAA concentrations, whereby the median slope of ANC in the first 10 years equalled +1 µeq l-1 per year and +4 µeq l-1 per year during the second 10 year period (Table 1). The increases in ANC were due to decreased concentrations of hydrogen (H) ion and Al. Dissolved organic carbon (DOC) appears to have increased from 1994 to 1998.



Fig. 4. Box and whisker plots of selected determinands for the seven regional surveys in Galloway since 1979.

Concentrations of NO3 in the lochs, on the other hand, show no systematic regional pattern during the 20 years (Figure 4). Concentrations were somewhat lower in 1988 relative to 1979, but then increased again during the period 1988-1998. In addition, deposition of inorganic N in the region has not changed systematically over this period. Samples collected in spring 1996 had exceptionally high concentrations of NO3 (Figure 4).

Concentrations of sea-salt-derived ions, particularly Cl, in the lochs varied greatly, but did not show any major trend over time for the region (Figure 4).






References

RGAR, 1983. Acid deposition in the UK. Report to DOE. Warren Spring Laboratory, Herts, 72 pp.

RGAR, 1987. Acid deposition in the UK 1981-1985. Report to DOE. Warren Spring Laboratory, Herts, 101 pp.

RGAR, 1993. Acid deposition in the UK 1986-1988. Report to DOE. Warren Spring Laboratory, Herts, 124 pp.

RGAR, 1997. Acid deposition in the UK 1992-1994. Report to DOE. AEA Technology, Abingdon, 174 pp.