Spatial and Temporal Variations of the Chemical Composition in Acid Deposition in the Peak District, Northern England
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Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8 Spatial and temporal variations of the chemical composition in acid deposition in the Peak District, Northern England D. Driejana^, D.W. Rape/, I.L. Gee' & A.F.R. Watson' 'aric, Department of Environmental and Geographical Sciences, Manchester Metropolitan University, United Kingdom * Department of Environmental Engineering, Institute of Technology Bandung, Indonesia Abstract Six monitoring sites operated on a weekly basis have been established to examine the impact of current emission reduction strategies on ionic concentrations and depositions in an acid sensitive sub-area of the Peak District. Present concentration and deposition values were compared to the monitoring results in the same area in 1988. On average, H* and nss-SO/" ion concentrations have dropped by approximately 60%, compared to their levels 10 years ago, and NCV has decreased by 30%. However, as the present precipitation amount is higher than that of 1988, I-T and nss-SO/" wet depositions have not decreased in similar proportion as their concentrations. Nitrate deposition shows very little decrease. H\ nss-SO/" and NOg" concentrations are generally higher in summer when the precipitation amount is lower. Significant spatial variability with an increasing gradient from north to south is observed for calcium and sulphate concentrations. 1. Introduction As a response to economic, cultural and environmental factors in the past decade, acidic precursors have been reduced by the introduction of more stringent industrial emission controls and exhaust emission standards. Sulphur dioxide emissions in the U.K. were reduced by 68% in 1997, relative to 1970 [1]. However, emission of nitrogen oxides were only reduced by 24% from 1970 to 1997, despite a gradual decrease in vehicle emissions with the application of catalytic converters for new cars. Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8 656 Air Pollution VIII Power stations remain the main sources for sulphur dioxide emissions and contributed about 62% of the total UK emission in 1997. It is likely that emission reductions from electricity generation were offset by increases in emissions from the growth in the transport sector. However, a substantial drop in the total mass of coal burnt and the installation of FGD at coal-fired power stations have had a direct effect on hydrogen chloride emissions in the UK, which have decreased from 335 ktonnes in 1970 to about 91 ktonnes in 1997 [1]. This paper discusses spatial and temporal variation and the impact of current emission reduction strategies on ionic concentrations and depositions in an acid sensitive sub-area of the Peak District. 2. Methods The study area in the Peak District is bordered by the major cities of Manchester to the west and Sheffield to the east (see Fig.l). Consequently, it has a long history of acid deposition dating back to the industrial revolution. Landuse within the Peak District is dominated by agricultural activity such as sheep rearing and designated National Trust protected land. Six monitoring sites were located in an area of approximately 20km by 30km. The network comprises bulk collectors fabricated to the UK National Acid Deposition Network standard [2] and co-located meteorological office rain gauges. Although the term wet deposition is used in this paper, the rain water collectors were collecting bulk precipitation which includes some dry deposition. Site locations were chosen as far as possible to have little influence from local sources as well as being secure and accessible. To prevent the effect of seeder-feeder mechanisms which is known to happen in elevated locations, sites were also located at an elevation below 500m. Sampling Sites 20 Kilometer* Figure 1. Location of the sampling sites and the two major conurbations Site 1 is located near the Hurst Reservoir in Glossop. However, due to persistent vandalism, in August 1999 the site was moved to Padfield which is located within 2 km from the original monitoring site. Site 2 Kinder and Site Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8 Air Pollution VIII 657 3 Lockerbrook Farm are located at an altitude of 320m and 420m, respectively. Geology of the area for the first three sites is characterised by Namurian shales and grits and soil types belong to the Wilcock and Winter Hill Series. Site 4 Oxlow House lies on a distinctively different geological features. The site is less than 1 km from the Namurian shales and grits in the north but is underlain by Monsal Dale Limestone. Site 5 located in Bamford at an elevation of 210m. The Bee Low Limestone underlies the site 6 in Harpur Hill, with Namurian shales and grits bordering the west and south west sides. Table 1. Site Locations Location Grid Reference Altitude (m) 1 Hurst Reservoir SK 054 938 250 Padfield SK 033 965 230 2 Kinder Reservoir SK 062 881 320 3 Lockerbrook Farm SK 164892 420 4 Oxlow House Farm SKI 27 824 320 5 Bamford SK 214 832 210 6 Harpur Hill SK 057 708 380 Rainwater samples were collected on weekly basis. The chemical species measured include H\ Cl", N and SO ^" ^a^, Mg^, Na*, K* and NH/. Anions (CT, NCV, and ]>4") were analysed using ion chromatography. Ca^, Mg^, Na ^ were measured using Atomic Absorption Spectrophotometry, while NH/ was analysed in an external laboratory using a continuous flow analysis method. Non sea salt- sulphate (nss-SO/") was calculated by assuming that all Na* is derived from the sea. Details of sampling and analytical methodology have been given by Driejana era/[3]. 3. Results and Discussion 3.1. Characteristics of precipitation chemistry and rainfall amount Quality assurance was applied by duplicate analyses in the laboratory and calculation of ionic balance values. On average, cations comprise 51% of the precipitation chemistry. This might suggest the presence of organic acid [4] in the sample, e.g., in the form of acetic and formic acids [5]. The presence of organic-acid was supported by unidentified chromatograph peaks, which occasionally appeared in the ion chromatograph output. The retention time of this unknowns in relation to the other identified analytes, suggests that the unidentified analyte is acetate. The mean absolute-ion balance value for the overall data set is 8.76%. However, a 15% ion balance cut off had been chosen for data screening to take into account the unaccounted acids in the samples. All ions are log-normal transformed to meet the assumption criteria of normal distributions prior to any statistical analysis. Statistical analysis was performed using SPSS software version 9.0. Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8 658 Air Pollution VIII A student t-test analysis was applied to compare the rainfall amount collected from both bulk collectors and the rain gauges. The statistical test found that there was no significant difference between the rainfall amount collected in the bulk collectors and the rain gauges in the spring, summer and autumn. However, winter precipitation showed a significant difference in the rainfall volume collected by the two types of rain collectors. It is suggested that higher wind velocities in the winter affect the collection efficiency of rainwater collected above the ground, in agreement with previous findings of other authors [6,7]. 3.2. Spatial Variability The precipitation-weighted mean concentrations and wet depositions were calculated using precipitation volumes in co-located rain gauges, for the whole period of sampling. The results of precipitation-weighted mean concentrations are presented in Table 2. Hydrogen ion concentration ranged between 9.\/jeq ^ and 15.\/jeq /"'. Calcium has the most varied concentration, which ranged from about 8.0//eg /"* to \92/ueq /"', followed by nss-SO/", with a concentration ranging from 25.Kfjeq f* to 39.8 jjeq /"*. Nitrate is less varied with a concentration range of 20.8/jeg /"* to 29.9jueq /"'. Ammonium had a concentration range of 30.%peq /"* to 43.\fjeq /"'. In general, the concentrations of NCV (Fig.3), nss- SO/" (Fig.4), and Ca^ (Fig.5) seem to have almost similar patterns, with the highest concentrations found in the southern part of the study area. Hydrogen concentration (Fig.6) displays no similarity with the spatial pattern of other ions, but contrary to other ions, it seems to be higher in the northern part of the study area. Table 2. Precipitation-weighted Mean Concentrations (peq /"^) Sites H Cl NO, SO4 nss- Ca Mg Na K NH, SO, Hurst/Padfield 10.8 107.6 20.8 36.2 25.8 8.5 19.1 86.6 6.7 38.3 Kinder 12.7 82.7 23.2 36.1 28.1 8.4 14.8 66.5 5.1 37.2 Lockerbrook 15.1 84.4 21.2 35.2 26.9 8.0 15.3 68.8 5.4 30.8 Oxlow House 13.0 71.7 29.9 47.2 39.8 16.0 13.2 60.9 6.1 43.1 Bamford 14.1 90.6 25.4 47.2 38.5 19.2 17.1 72.1 4.2 36.3 Harpur Hill 9.1 73.9 20.9 36.6 29.3 10.7 13.3 60.1 4.2 38.2 Deposition patterns (Fig.8-Fig.13) closely resemble the ionic concentration patterns. The H* deposition (Fig. 12) also shows higher values in the northern region. Beside showing similarity with concentration patterns, non- anthropogenic ions such as Na*, Mg^ (figures are not shown), and Cl" (Fig.8), depositions also exhibit similarity in shape with the precipitation (figure is not shown).