Contract EVK1-1999-00087 - RECOVER:2010

Part of the 'Sustainable Management and Quality of Water'

Ecosystem Functioning

Directorate General Research

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Land use influences on acidification and recovery of freshwaters in Galloway, south-west Scotland

R.C. Helliwell1, R.C. Ferrier1, L. Johnston1, J. Goodwin2 and R. Doughty2
1Macaulay Land Use Research Institute, Aberdeen, AB15 8QH, UK
2SEPA WEST, 5 Redwood Crescent, Peel Park, East Kilbride, G74 5PP,UK

Full Reference

Helliwell, R.C., Ferrier, R.C., Johnston, L. and Goodwin, J. (2001). Land use influences on acidification and recovery of freshwaters in Galloway, south-west Scotland. Hydrology and Earth System Sciences Vol. 5, No. 3, 451-458.

Summary of Research

The long term response of surface waters to changes in sulphur deposition, and afforestation is investigated for three upland river systems in the Galloway region of south-west Scotland. From 1984 to 1999 these rivers exhibited a statistically significant decline in non-marine sulphate concentrations in response to reduced acid deposition. This reduction in non-marine sulphate was however, insufficient to induce a pH recovery over the period. A statistically significant increase in river pH was observed between 1956-1970 (0.05 yr-1) when subsidised agricultural lime payments were at a maximum. In 1976 this subsidy ceased and surface waters have progressively acidified. In addition, climatic change was found to influence long-term trends in pH. Mean annual pH was greatest during a dry period between 1969 and 1973 when annual total discharge was low. Thereafter, pH declined gradually in response to higher rainfall and increased total annual discharge. Overall surface water drainin g the afforested catchments of the Rivers Cree and Bladnoch are more acid than those draining the moorland catchment of the Luce. These results indicate that in afforested catchments, current reductions in sulphur emissions have not led to an observed improvement in the acid status of surface waters. Forestry, therefore, represents a confounding factor with regard to chemical recovery from acidification in this region.

Fig.1 Map of galloway and study catchments

Collectively, the Rivers Luce, Cree and Bladnoch drain a large area of 1102 km2 from the Galloway uplands to the Solway Firth. The drainage areas of the head-water rivers included in this analysis are 62 km2 (Cree at Arnimean), 54 km2 (Bladnoch at Glassoch) and 32 km2(Luce at Penwhirn) (Figure 1). The Cree and Bladnoch sub-catchments are dominated by coniferous plantations (80%-71% respectively), whereas the Luce is predominantly moorland

Fig.2(a)Land use in 3 catchments
The Cree and Bladnoch sub-catchments are dominated by coniferous plantations (80%-71% respectively), the remaining area is comprised of rough grassland and heather moorland (Fig. 2a). These sub-catchments were afforested during the 1950s, 60s and 70s. Since the 1980s, more forest has been planted although there is now significant felling as much of the first plantings reach maturity.

 Fig.2(b)Geology of 3 catchments

The soils in the Cree and Bladnoch sub-catchments are mineral in nature and derived from greywackes and shales (Ettrick association) (Fig. 2b), although, 7% of the Cree sub-catchment is composed of soils derived from granite (Dalbeattie association).

Fig.2(c)Soil types in 3 catchmentsPeat, brown forest soil and peaty podzols derived from greywackes and shales are the major soils types in the Bladnoch catchment. In contrast, the Penwhirn sub-catchment is dominated by blanket peat (80%), with a small area of peaty gleyed soil derived from greywackes and shales (Ettrick association) surrounding the lower river channel (Fig. 2b or c). A large area of the catchment comprises of rough grassland and heather moorland (89%), only 9% of the Penwhirn sub-catchment is afforested (Fig. 2 a).

A comparison of mean annual (1984 to 1999) surface water pH (derived from mean H+ concentration, Table 1)) shows that samples collected from the River Cree (pH 5.4) and the River Bladnoch (pH 5.7) are more acid than the River Luce (pH 6.0). Alkalinity measurements replicate this pattern with the lowest mean data recorded at the River Cree (88 µeql-1), and the highest at the least acid site, the River Luce (259 µeql-1).The highest mean annual concentrations of nmSO4 and NO3 for 1984-1999 were recorded for the River Bladnoch (76 µeq l-1 and 12 µeq l-1 respectively). Nm SO4 concentrations measured at the River Luce were slightly lower than the concentrations in the Cree and Bladnoch, with a mean concentration of 54 µeql-1 for 1984-1999. Surface water Cl concentrations are high, and range from 336 µeql-1 at the River Cree to 384 µeql-1 at the River Bladnoch, reflecting the close proximity of these river systems to the coast.

Table 1.Mean Chemistry of rivers

Table 2 Annual trends in water chemistryResults from the regression analysis for linear trend and annual cycle representing 1984-1999 are shown in Table 2. Significant trends were only observed in the River Cree for SO4, nmSO4, Cl, and Al (Table 2). There has been a pronounced and highly significant decline in nmSO4 at this site of an estimated 20 µeq l-1 over the reported 15 years (-1.34 µeq l-1yr-1). Surface water trends observed in the River Bladnoch are similar to those in the River Cree, where SO4 (-3.08 µeq l-1yr-1**) and nmSO4 (-2.32 µeq l-1 yr-1**) have decreased significantly throughout the record. In contrast to trends identified in the Rivers Cree and Luce (which have lower concentrations of NO3), a seasonal NO3 cycle can be identified in the River Bladnoch data where samples collected during the winter months have higher concentrations of nitrate compared to those samples collected in the summer. Nonetheless, the long-term NO3 trend is not statistically significant (-0.32 µeq l-1 yr-1). Trend analysis from the River Luce reveals very different results to those observed in the River Cree and Bladnoch. Regression analysis shows a slight improvement in surface water pH but this decline is not statistically significant (0.007 yr-1). The clear decline in total S deposition has resulted in a small decrease in stream concentrations of nmSO4, but this decline was not significant at the 5% level (Table 2).

Fig 3 UK agricultural lime useHistorical records of surface water pH are limited to two sites in this study, records began in 1956 for the River Cree at Arnimean, and in 1964 for the River Bladnoch at Shennanton. These data reveal very different trends in surface water acidity compared with the shorter record (1984-1999) described previously (Figure 3). Through detailed analysis of surface water chemistry from the Cree, it is apparent that pH significantly increased from 1956-1970 (0.05 yr-1*). A similar increase in pH was evident at Bladnoch (1964-1970), although the increase was not statistically significant (0.07 yr-1 n.s). From the mid 1970s evidence points to a decline in surface water pH for both the Cree and Bladnoch (Figure 3). The total decline in pH measured at the Cree and Bladnoch from 1976-2000 was 0.76 and 0.64 respectively. Statistical analysis of the entire dataset from the Cree (1956-2000) and Bladnoch (1964-2000) show the long term trend in pH is declining (-0.02 yr-1***; -0.03 yr-1*** respectively).

Fig 4 Relationship between flow and pHThere is a strong relationship between pH and annual climatic variability. The relationship between river flow at Newton Stewart (down stream gauging station on the River Cree) and pH is clearly presented in Figure 4. Low annual total discharge records during 1969-1973 coincide with a period when pH measurements were greater than 6. Since 1973 total annual discharge has increased resulting in a gradual decline in surface water pH.