Regional and temporal variation in pH, alkalinity and carbon dioxide in Danish streams, related to soil type and land use

Abstract

SUMMARY 1. The Weichsel glaciation has divided Denmark into two regions with different susceptibility to acidification. East of the Weichselian terminal moraine, soils are usually clayey and calcareous, and the streams are alkaline (mean alkalinity 2.24 mmol 1‐‐1) and resistant to inputs of acidifying substances.2. Trend analysis of pH and alkalinity of water samples taken over 15 years in two streams with alkalinities above 1.5 mmol 1−1 in eastern Jutland, showed no trends of acidification.3. West of the terminal moraine the soils are sandy and leached and alkalinity is lower (mean 0.59 mmol 1−1). Although such streams with medium alkalinity are believed not to be vulnerable to acidification, we have documented significant decreases in their pH and alkalinity over 12 years.4. Trends of pH and alkalinity in four western streams with mean alkalinities between 0.05 and 0.79 mmol 1−1 showed annual decreases of 0.027 pH units and 4.7 nmol 1−1 in alkalinity.5. Overall, Danish streams contain about 7.9 times more calculated free CO2 (pCO2=10−2.6 atm) than water in equilibrium with air (pCO2= 10−3.5 atm). The calculated free CO2 content has increased significantly in western Danish streams over the study period (6.9 μmol 1−1 yr‐1). This increase cannot be explained by the prevailing global increase in atmospheric pCO2 which only can account for 0.54 pmol 1−1 yr−1 at maximum.6. Reasons for the ongoing stream acidification in the western part of Denmark are discussed. We suggest that atmospheric deposition causes stream acidification in a heath‐covered catchment without agriculture. In heavily cultivated regions the main acidification factor is argued to be proton production in the soil through nitrification of ammonium‐containing fertilizers.