Natural hydrochemical variations in small catchments with thin soil ...

ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002Natural hydrochemical variations in small catchments with thin soillayers and crystalline bedrock in SwedenAbstractSiri Knape 1 , Otto Graffner 1The hydrochemistry of small catchments, with thin soil layers and crystalline bedrock,was investigated during 1997 and 1998 on northern Äspö Island (the east coast ofSweden), and in Gårdsjön (the west coast of Sweden). Monthly data on precipitation,groundwater and runoff were compiled and evaluated. Groundwater was collected to adepth of 70 meters below the surface. In the small catchments (< 0.057 km 2 ), inorganicanalyses were conducted to study the temporal and spatial hydrochemical effects causedby natural drop of the groundwater table. Parallel to the hydrochemical recording,hydrological measurements were also carried out. To generalise the complexity of thecatchments and enable a comparison of the effects studied, hydrogeochemicalconceptual models were set up. The conceptual models were based on the geology,hydrogeology, and hydrochemistry of the catchments (Knape, 2001).There are natural groundwater chemistry variations in time and space, due to thechemical composition of precipitation, the groundwater recharge, the flow patternswithin an aquifer, and the chemical and biological processes within a geologicalmedium. A normal hydrochemical transformation in the direction of groundwater flowtypically leads to changes, such as a shift from acid to basic and from aerobic toanaerobic groundwater. Saether and De Caritat (1997) and Stuyfzand (1999), amongothers, have presented overviews of hydrogeochemical processes along the flow path incatchments. In general there are large hydrochemistry fluctuations (in both time andspace) in the upper part of an aquifer, while the lower part of it has more or lessconstant hydrochemistry.The precipitation at Gårdsjön (1145 mm/y), was almost twice as high as at Äspö (675mm/y). Peaks of chloride (up to 44 mg/l) in the precipitation, due to seaspray, wererecorded in Gårdsjön. In addition to the precipitation, the relationship to the highestshoreline has a major impact on the hydrochemistry in the ground. The temporalhydrochemical variations of the groundwater and runoff caused by a natural drop of thegroundwater table were more obvious at Äspö, an area that is situated below the highestshoreline, than at Gårdsjön. The Gårdsjön area, which has been situated just above thehighest shoreline since the ice age, do not leach marine chloride and sulphur. Duringdry periods in the summer at Äspö, sulphur pulses of maximum 100 mg/l, followed byhigh concentrations of cations, were recorded in the soil groundwater (Fig. 1 & 2).1 Department of Geology, Chalmers University of Technology,Göteborg, Sweden, 2002

Belowthe highestshorelineAbovethe highestshorelineUnsaturated zone8Mostly retardation8Dissolution of O 2 (g) and CO 2 (g):(CH 2 O) + O 2 (g) = CO 2 (g) + H 2 O =H 2 CO 3 = HCO 3-+ H +CO 2 (g) + H 2 O = H 2 CO 3 = HCO 3-+ H +8Carbonate and silicate dissolution, for example,CaCO 3 (s) + H + = Ca 2+ + HCO 3-8Cation exchange, for example,Ca 2+ + 2Na-X(s) = 2Na + + Ca-X(s)8Oxidation of the sulphur pool = SO 42-+ 2H +OrganicClaysoilRunoff8Mostly advection and hydrodynamic dispersion8Cation exchange8CO 2 degassed8Fe 2 O 3 (s) precipitateTillBedrockSaturated zone8Mostly advection and hydrodynamic dispersion8Carbonate and silicate dissolution8Dissolution of soluble salts, for example,NaCl(s) = Na + + Cl -8Redox reactions, for example,½ Fe 2 O 3 (s) + 3H + + e - => Fe 2+ + 3/2 H 2 O1/8 SO 42-+ 9/8 H + + e - => 1/8 HS - + ½ H 2 O8Cation exchangeFig. 1. Qualitative conceptual model of the hydrochemical processes taking placeduring low recharge in the catchments studied. Obvious effects are the appearanceof pulses of sulphate, cation and chloride, as well as the low redox potential in thegroundwater.Belowthe highestshorelineAbovethe highestshorelineUnsaturated zoneSaturated zone8Mostly retardation8Dissolution of O 2 (g) and CO 2 (g):(CH 2 O) + O 2 (g) = CO 2 (g) + H 2 O =H 2 CO 3 = HCO 3-+ H +CO 2 (g) + H 2 O = H 2 CO 3 = HCO 3-+ H +8Carbonate and silicate dissolution, for example,CaCO 3 (s) + H + = Ca 2+ + HCO 3-8Cation exchange, for example,Ca 2+ + 2Na-X(s) = 2Na + + Ca-X(s)Organic soilClayRunoff8Mostly advection and hydrodynamic dispersion8Cation exchange8CO 2 degassed8Fe 2 O 3 (s) precipitateTillBedrockSaturated zone8Mostly advection and hydrodynamic dispersion8Carbonate and silicate dissolution8Dissolution of soluble salts, for example,NaCl(s) = Na + + Cl -8Redox reactions, for example,½ Fe 2 O 3 (s) + 3H + + e -

The deeper groundwater in the bedrock, at both Äspö and Gårdsjön, did not showseasonal variations, since the groundwater table fluctuated approximately one meterover the year. Hydrogeological and hydrochemical studies indicated that there is lowhydraulic connectivity between fractures in the bedrock aquifer at Äspö (Knape, 1999);the aquifer in Gårdsjön was the opposite. The concentration of dissolved organic carbonand iron increased in correlation with drought periods (i.e. low groundwater rechargeperiods), in both groundwater and runoff. At Äspö the concentration rose to 190 mgDOC/l in the runoff.The study and the resulting hydrogeochemical conceptual models can also be applied asa support to predicting a groundwater abstraction scenario in the catchments studied.ReferencesCaritat de, P. and Saether, O. M., 1997. Chemical changes attending water cyclingthrough a catchment – An overview. In: Saether, O. M. and de Caritat, P. (eds.),Geochemical Processes, Weathering and Groundwater Recharge in Catchments. A.A. Balkema, Rotterdam, Netherlands, pp. 305 – 328.Knape, S., 1999. Shallow groundwater quality variations within a hardrock aquifer. InKnutsson, G. (ed.); Proceedings of the Workshop on Hardrock Hydrogeology,Äspö, Sweden. NHP Report, No. 45.Knape, S., 2001. Natural Hydrochemical Variations in Small Catchments with Thin SoilLayers and Crystalline Bedrock. A Two-year Study of Catchments on the East andWest Coasts of Sweden. Chalmers University of Technology, Department ofGeology, Licentiate thesis, Göteborg, Sweden.Stuyfzand, P. J., 1999. Patterns in groundwater chemistry resulting from groundwaterflow. Hydrogeology Journal, Vol. 7, pp. 15 – 27.