Atmospheric pollution histories of three Cumbrian surface waters

Abstract

1. The CHemistry of the Uplands Model (CHUM), driven by measured and estimated atmospheric deposition, was used to simulate the chemical compositions of three upland Lake District surface waters, Devoke Water (DW), Levers Water (LW) and Mosedale Beck (MB) over several hundred years. 2. “Natural acidification” combined with human activities, notably forest clearance, was assumed to have brought about chemically stable acid moorlands by the period 1000–1500 A.D. Deposition of sulphur, nitrogen, chlorine and heavy metals, released into the atmosphere by coal-burning and industrial processes, then took place, gradually increasing to maximum levels in the late 20th century. 3. Surface water concentrations of chloride are consistent with depositional inputs, whereas the transfer of atmospherically deposited sulphur to the surface waters is delayed by temporary retention processes within the catchment. Over the last 40 years, concentrations of pollutant (non-marine) sulphate in the surface waters declined to one-third of their maximum levels. Atmospherically deposited pollutant N continues to accumulate in catchment soils, although a significant fraction appears in surface waters as nitrate. Annual average surface water bicarbonate concentrations were 20 - 60 µmol L-1 in the pristine past, fell nearly to zero in all three waters when acidification was most intense, but now are increasing. 4. Combined data from several Lake District upland waters suggest substantial recent increases in concentrations of dissolved organic carbon (DOC). If this can be attributed to acidification reversal, a corresponding decline in DOC concentrations would have occurred as acidification intensified, and pristine surface waters would have been comparatively rich in DOC. 5. Major cationic elements enter the soil-water system in deposition (H+, Na, Mg, K, Ca), from organic matter decomposition (H+), or by chemical weathering (Mg, Al, Ca), and are much affected by sorption to soil organic matter (SOM). The surface soils of all three catchments are acid (current pH ~ 4.5) and so variations in surface water chemistry among sites reflect differences in mineral dissolution rates deeper in the soil-rock profile. The simulations indicate pH values of 6.9, 6.1 and 6.4 for DW, LW and MB respectively in the period up to 1800, followed by declines to minima of c. 6.0, 4.7 and 5.0 in around 1980, then acidification reversal in agreement with observations. 6. The transfer of atmospherically deposited heavy metals to surface waters depends upon their sorption by SOM. Nickel, zinc and cadmium adsorb relatively weakly and therefore are quite readily leached, and sensitive to changes in acidification status. The higher affinities of organic matter to Cu and Pb promote retention and these two metals are continuing to accumulate in soil, despite major declines in deposition over the past several decades. 7. The WHAM-FTOX model was used to estimate the maximum number of Ephemeroptera, Plecoptera and Trichoptera species in MB through time, as influenced by chemical variability. The maximum number is estimated to have fallen from 14-15 under pristine conditions to 9-10 when acidification was greatest and a modest recovery to 10-11 species since then.