Trends in the chemistry of atmospheric deposition and surface waters in Lake Maggiore catchment

M. Rogora, A. Marchetto and R. Mosello
C.N.R. Istituto Italiano di Idrobiologia, L.go Tonolli 50-52, 28922 Verbania Pallanza, Italy

Full Reference

Rogora, M. Marchetto, A. and Mosello, R. (2001). Trends in the chemistry of atmospheric deposition and surface waters in the Lake Maggiore catchment. Hydrology and Earth System Sciences Vol. 5, No. 3, 379-390.

Summary of Research

The Lago Maggiore catchment is the area of Italy most affected by acid deposition. Trend analysis was performed on long-term (15-30 years) series of chemical analyses of atmospheric deposition, four small rivers draining forested catchments and four high mountain lakes. An improvement in the quality of atmospheric deposition was detected, due to decreasing sulphate concentration and increasing pH. Similar trends were also found in high mountain lakes and in small rivers. Atmospheric deposition, however, is still providing a large and steady flux of nitrogen compounds (nitrate and ammonium), which is causing increasing nitrogen saturation in forest ecosystems and increasing nitrate levels in rivers. Besides atmospheric deposition, an important factor controlling water acidification and recovery is the weathering of rocks and soils, which may be influenced by climate warming. A further factor is the episodic deposition of Saharan calcareous dust, which contributes significantly to base cation deposition.

Fig.1.Trends in concentrations of SO4 NO3 NH4

The regular gradient of pollutant deposition in the Lake Maggiore catchment is easily explained by the meteorology of the study area, which receives air masses mainly from the south-east, i.e. from the Po Plain. In effect, if the sampling stations located in the north-western (Italian) part of the Lake Maggiore catchment, which are relatively far (40-110 km) from the emission sources, SO₄ and NO₃ concentrations are negatively correlated with distance from the city of Milan, which may be taken as a reference point for industry in the Po Plain (Figure 1). The gradient of SO₄ deposition related to the distance from Milan was more pronounced in the 1980s but it is still evident in 2000. Nitrate concentration also decreases with the increasing distance from Milan. The pattern for NH4 deposition is more complex, because of the greater importance of local sources. A decreasing gradient from south to north is still present, and was quite evident in the 1980s (Figure 1).

Table 1.Yearly percent contribution of saharan dust The contribution of alkaline events to the total annual fluxes of ionic species is shown in Table 1. In spite of the low amount of precipitation (3-13% of the annual amount), these events carry from 20% to 70% of the Ca deposition and can buffer the mineral acidity of atmospheric deposition.

Fig.2 Input and output N fluxes 1984-1999
In terms of input and output fluxes for each river (Figure 2), during the whole study period, River Cannobino received a distinctly lower N flux (mean value 240 meq m^-2 y^-1) than the other catchments (270-290 meq m^-2 y^-1). This difference is reflected in the output fluxes, which average 70 and 170-190 meq m^-2 y^-1, respectively, indicating that the southernmost basins have reached a higher stage of N saturation than the Cannobino catchment.

Fig 3.Trends in the percentage of total N input
Retention is clearly related to the input flux (Figure 3) and in the study period the average retention in River Cannobino was 70%, while in the southernmost basins it ranged between 34 and 42%. Furthermore, during the study period, N retention in River Cannobino remained stable, while in the basins receiving higher N flux it clearly decreased, indicating conditions of increasing N saturation.

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

The James Hutton Institute

THE REGIONS

RESULTS

Trends in the chemistry of atmospheric deposition and surface waters in Lake Maggiore catchment