Reconstruction of palaeoinfiltration during the Holocene using porewater data (Laxemar, Sweden)

Abstract

The interaction between fracture groundwater and crystalline rock matrix was explored by extracting the porewater residing today in the low-permeability, saturated rock matrix. About 100m below surface, concentrations of the natural tracers Cl−, δ18O, δ2H in the porewater along a 11m long profile from a water-conducting, fractured deformation zone into the intact rock matrix are the result of superimposed changes in the fracture groundwater composition. The tracer profiles have evolved by diffusive exchange of the porewater in the rock matrix with groundwater flowing in fractures of high and low transmissivity since the retreat of the last Weichselian glacier from the Laxemar investigation area, Sweden. Numerical simulations indicate the necessity of a diachronic arrival of surface infiltration signals in the two fractures at the sampling location and of heterogeneous transport properties in the rock matrix as determined for the individual rock samples. Stringent conclusions regarding the palaeoinfiltration conditions can only be obtained when at least two tracers with independent source terms (i.e. salinity and temperature/moisture source) are simulated simultaneously. Quantification of the porewater–fracture groundwater interaction allows modification of the different evolutionary stages of the palaeoinfiltration conditions over Holocene times. Most prominently, this suggests the absence of any influence of brackish water from the Yoldia Sea and Littornia Sea in the area and the requirement for warmer climate precipitation from 8ka to 4.5ka BP coinciding with the Holocene thermal maximum.The ability to interpret the natural tracer profiles in accordance with independent geological, hydrological and climatic constraints indicates that matrix diffusion in crystalline rocks is effective over decametres within timescales of ∼10–14ka. The impact of matrix diffusion on the fracture groundwater composition will become measurable over these timescales, especially in low transmissivity fractures. These data demonstrate that porewater indeed acts as an archive for palaeohydrologic variations although the resolution is still limited.