During the past several decades, the amount of SO2 emission in China has steadily increased. The big forest areas in northeast China, located in Hei Longjiang and Ji Lin provinces, are hereby receiving an increasing amount of acid deposition both from local emissions and long-range transported pollutants. The critical loads have previously been modelled to be relatively low in most regions of these two provinces compared to other parts of China. It is therefore hypothesized the surface water acidification may be a problem in these regions.
It is a strong need in China to collect data from different regions to assess the potential for possible water acidification from atmospheric deposition of sulphur and nitrogen. This study contributes to such data collection and assessment.
In this study, four sampling sites were selected in remote forest areas, and low-order streams water samples were collected in July, August and September. Twenty-four chemical parameters including pH, conductivity, alkalinity, UV 254, major ions, total nitrogen, total phosphorous, dissolved organic carbon, and metals were measured and analyzed.
Assessment of sampled water quality indicates that the surface water at the four sampled sites is generally undisturbed in respect to other anthropogenic pressures than through air pollution.
The streams investigated in this study appeared not acidified based on pH and ANC value. Median ANC value is 1082 µeq L-1, with range of 273 4950 µeq L-1, which is well above the limits required to sustain the natural indigenous fauna. Furthermore, the water has a large capability to resist future acidification.
The presence of carbonates in the soils was clearly implied by our study. The complementary soil investigation indicates that CEC and BS% are both at a high level and that the soils in studied areas are not sensitive to acidification. Ground truthing has shown the contradictory to the prior hypothesis. The acidification potential of the sampled stream waters is accordingly assessed to be low.
Calcium is the dominant cation and carbonate is the dominant anion, implying high content of calcium carbonate in soils and/or bedrocks. Differences in water chemistry between the sites and downstream at the sites are likely due to differences in soil depth, the content of carbonates in the bedrock and amount of peat in the watershed. The amount of potassium in surface waters at Chang Bai site is relatively high. The organic charge in the two northernmost sites is a bit higher than in the other two sites. Ionic strength varies among surface water samples from different sites.
Ionic strength increases downstream due to more evapotranspiration, more weathering in deeper soil and/or more weatherable bedrocks at lower elevations that likely contain more carbonate. Some tributary streams cause ion concentration to drop along the studied streams.
Lacking relevant basic data is usually a major limitation for assessment of current status and prediction of future change. Regular monitoring of low order streams in forest areas is needed to see chemistry change in a long term basis. Detailed information on soils should be acquired. A regional scale model with higher resolution should be developed to provide detailed and updated information in this area, for instance, mapping of acid sensitive areas.
Based on the presented data, water acidification does not seem to be of major concern in the region. Soils must be studied further in order to assess the potential for enhanced soil acidification due to atmospheric deposition.