Hydrology and surface water chemistry in a small forested catchment : which factors influence surface water acidity?

Abstract: Water chemistry alterations such as acidification and eutrophication represent major threats to the running waters of the world, and Swedish waters are among the most vulnerable. In this study the small forested catchment Storskogen, part of the SWETHRO monitoring network, was analysed to examine the links between deposition, catchment hydrological properties and surface water chemistry. Storskogen was characterised in terms of physical and chemical hydrology, and compared to the well-studied catchment Gårdsjön. Both catchments were inventoried to document the distribution of wetlands in order to examine to what extent they influence surface water chemistry. The water balance model FyrisQ was used to model discharge from Storskogen and was validated using measurement data collected by IVL from 2014 to 2017. The stream in Storskogen had an observed median discharge of 0.7 l s-1 and responded quickly to precipitation, which could cause a tenfold increase in discharge. For this reason, the model FyrisQ, was found to be inappropriate because it could not adequately recreate the dynamics of the discharge. Surface water was found to be acidic (pH< 5) with significantly lower acid neutralising capacity and pH upstream in Storskogen than downstream. Surface water in Gårdsjön was more acidic than in Storskogen. Even though the surface waters’ acid neutralising capacity was found to be significantly correlated with deposition, none of the examined factors (deposition, mineralogy and wetland extent) could account for the observed differences in acidity between the upper and lower dam in Storskogen, or between Storskogen and Gårdsjön. The pH of both catchments was correlated with deposition of sea salt however, the amounts did not differ significantly between sites. It is likely that the difference in acidity between the upper and lower part of the catchment in Storskogen can be explained by different transit times but to prove this, further monitoring is encouraged.