Characterisation of pore water in crystalline rocks

Abstract

The mass of pore water present in the rock matrix of a crystalline rock mass is significant and its influence on fracture groundwater and future deep repositories needs to be understood. Yet, the rock matrix has a hydraulic transmissivity generally well below 10−10 m2 s−1 inhibiting sampling of pore water by conventional sampling techniques. Various innovative techniques for the chemical and isotopic characterisation of pore water in crystalline rocks are applied and evaluated. Direct sampling of pore water was facilitated by collecting seepage water in a specially designed borehole located at a depth of 420 m in the Äspö underground research laboratory (Äspö HRL), Sweden, over a period of 7.5 a. During the entire time span, seepage waters collected from different sampling sections showed constant, but individual chemical and isotopic compositions. Compositional differences compared to nearby fracture groundwaters indicate that the collected waters originated from the low-permeability rock mass without mutual influence via more conductive microfractures within the metre scale. They are interpreted as representing pore water in a transient state of diffusive interaction with different types of old palaeowaters which have periodically characterised the fracture network at the Äspö HRL over geological times (thousands to hundreds of thousands of years). Pore water compositions derived by indirect methods (out-diffusion and diffusive isotope equilibration experiments) from originally saturated drillcore samples collected at the Laxemar and Forsmark sites, Sweden, can be interpreted within a wider palaeohydrogeological framework. The chemical and isotope compositions of the pore water show distinct trends related to rock type and with depth, becoming more saline at greater depth at both sites. Concentration gradients established between pore water and fracture groundwater commonly coincide with higher fracture frequency and transmissivity in the host rocks. Differences developed in the attainment of near steady-state conditions between pore water and fracture water from Laxemar and Forsmark support a different hydrogeological, and therefore hydrogeochemical evolution, at least during Holocene times. The observed compositional differences between pore water and fracture groundwater support diffusion-dominated solute transport in the low-permeable rock masses, which is consistent with measured and experimentally derived hydraulic properties. From the present investigations it can be concluded that in such rock masses the diffusion-accessible porosity extends over significant distances, of at least metres to tens of metres.