Harald Grip

The origins of acid runoff in a hillslope during storm events.

Abstract At the Svartberget Research Catchment in northern Sweden, stream pH had risen to 5.7 after a period of dry summer weather in 1987 before it dropped to below 4.4 during three ensuing storm events. To clarify the relationship between flow pathways and episodic acidity, the sources of storm runoff and its acidity in one of the catchment hillslopes were investigated. Hydrological considerations locate the origins of the runoff within the upper four decimeters of the soil in a swath some 50m wide along the stream. Groundwater remained alkaline throughout the episodes. Increases in the total organic carbon content of runoff appeared to play a central role in the stream pH decline. The episodes acidity and distinctive chemical ‘fingerprint’ originated in the runoffs passage through the organic-rich forest mor and/or streambank vegetation. Results from a column-leaching experiment supported this hypothesis. Such localized origins of runoff and especially of acidity may be of significance when calculating the rate at which acid episodes in other catchments respond to changes in acid deposition. A better understanding of the interaction between runoff and organic material is needed to determine whether the acidity at Svartberget is natural or if it has been affected by atmospheric inputs of anthropogenic origin.

Effect of climatic variability from 1980 to 1997 on simulated methane emission from a boreal mixed mire in northern Sweden

The objective of this study was to evaluate the interannual variation during 1981 - 1997 in methane emission from an oligotrophic lawn plant community in a boreal mire. Daily mean air temperatures and daily accumulated precipitation figures were used as forcing variables for a mechanistic soil physical model, to generate estimates for the depths of snow, soil frost, and water table, together with soil temperature profiles. Methane emissions were then simulated with a mechanistic mixed mire methane model, described in this paper. The methane model simulated total methane emission rates separated into diffusion, ebullition, and plant-mediated transport. The climate record for 1981-1997 represented almost the total range in growing season temperature sums in northern Sweden during the twentieth century. The average temperature sum for 1980-1997 was 735 (range 553-981) degree days. The average accumulated annual precipitation during the same period was 283 (range 140-397) mm. The integrated simulated annual methane emission during 1981-1997 varied almost threefold among years, with a mean of 17.4 ± 1.1 (SE) and a range of 1 1 to 27 g m -2 . The simulated annual emissions for the three calibration years 1995-1997 were 18, 14, and 22 g m -2 , respectively, slightly higher than the integrated measured emissions over the growing season (May-September), which were 16, 13, and 18 g m -2 y -1 , respectively. Given the model formulation and parameterization, the single most important climatic predictor of simulated annual methane emission was mean water table position (r 2 = 0.58). Adding annual soil temperature sum at 26 cm in a multiple regression solution significantly increased the explained variance (R 2 = 0.85). The study emphasizes that interannual variability in methane emission may be large, and therefore extrapolations of annual methane emissions must be based on time series that adequately span the interannual variability in the local climate.

Effect of organic manure and chemical fertilizer on nitrogen uptake and nitrate leaching in a Eum-orthic anthrosols profile

Distribution and accumulation of NO3-N down to 4 m depth in the soil profile of a long term fertilization experiment with organic manure and N and P chemical fertilizer were studied after 12 years, wheat and corn were planted in each year. The apparent N recovery decreased with increased N and P fertilizer. NO3-N was mainly accumulated in 0-1.2 m depth of the soil profile with a maximum of 34 mg N kg-1 for the treatment with 120 kg N and 26 kg P per hectare, a secondary maximum of 7.2 mg N kg-1 was found at 3.2 m depth in the same treatment. NO3-N accumulation in the soil profile was minimized in the trials with highest manure application. Nitrogen that was not recovered was leached as NO3-N deeper than 4 m depth, was immobilized in the profile or was lost by denitrification.

The origin and dynamics of 137Cs discharge from a coniferous forest catchment

The turnover of radioactive caesium was studied experimentally and theoretically in a forested catchment that was covered by snow during the wet deposition of radioactive nuclides from the Chernobyl accident. The study spans from 1 week before to 8 years after the deposition event. A fraction of the catchment is covered by a mire (16010). From the edge of the mire a stream channel runs to the outlet of the catchment. Two phases of decreasing activity concentration in the stream water were found in addition to a positive influence of runoff on the activity concentration in the stream. The half-lives for 137CS in the stream water corresponding to the early and the later phase were estimated by non-linear regression to be 6.5 days and 4 years, respectively. During the first phase, which corresponded to the initial snow melt in 1986, 6.8% of the deposition was lost from the catchment, whereas the slow secondary loss during the following 8 years was 1.8%. The main contribution to the yearly discharge of 137Cs occurred during spring and autumn when the areal contribution to saturated surface runoff was highest. The remaining deposition in soil a few years after the fallout was significantly lower in the mire than in the surrounding forest. By using the calculated activity concentration of 137Cs in the stream water together with the remaining deposition in the different biotopes and information on stream flow for the catchment it was concluded that the loss originated from the mire. During the initial phase 44% of the deposition was lost from the mire, and during the following years the yearly loss was 30% from the fraction that constantly undergoes saturated surface runoff and 2% from the drier fractions of the mire. Until the end of the study it was not possible to demonstrate any loss from the recharge areas (podzol and cambisol soils), which means that physical decay will govern the decrease in activity in these areas.

Hydrochemical modelling of a stream dominated by organic acids and organically bound aluminium

An existing hydrogeochemical model, the Birkenes model (BIM), has been extended to include production of dissolved organic carbon (DOC) in the soil/water system and its chemical interactions with H+ and A13+. The model has been calibrated and verified using precipitation and runoff data from the Svartberget catchment in northern Sweden. The catchment is impacted by moderate amounts of sulphur deposition (6 kg S ha−1 a−1; the stream is high in DOC (7–35 mg L−1) and experiences episodically low pH-values (∼4.0). The refined BIM is able to simulate the main variations in major chemical components of the stream water (H+, Ca2+, Mg2+, Ali, Al0, Cl−, SO4−2 and DOC), as well as stream discharge and 18O, while Na+ is not well reproduced. Although very simplified relationships are used for the chemical interactions between DOC and H+ and aluminium, the model shows that these interactions are essential to stream acidity and aluminiumspecies content. Some of the model parameters are poorly determined by calibration with runoff data only. Soil and soil water investigations may contribute to the evaluation and development of the model structure and the representation of chemical processes. Further improvements of the model should emphasize DOC-production/absorption, detailed studies on DOC chemical behaviour and hydrological structure.