Colloid properties in granitic groundwater systems. I: Sampling and characterisation

Colloids are present in all groundwaters. The role they may play in the migration of contaminants in the geosphere must be studied. Colloid sampling and characterisation campaigns have been carried out in Switzerland. On the basis of the results obtained from studies in the Grimsel area (Grimsel Test Site, Transitgas Tunnel), Northern Switzerland (Leuggern, Zurzach) and the Black Forest (Bad Säckingen, Menzenschwand), a consistent picture is emerging. The groundwater colloids in granitic systems are predominantly composed of phyllosilicates and silica originating from the aquifer material. Under constant hydrogeochemical conditions, the colloid concentration does not exceed 100 ng·ml−1 when the calcium concentration is larger than 10−4 M and the sodium concentration is larger than 10−2 M. However, under transient chemical or physical conditions, such as geothermal or tectonic activity, colloid generation may be enhanced and the colloid concentration may reach 10 μg·ml−1 or more if both calcium and sodium concentrations are low (i.e. < 10−4 M and < 10−2 M respectively). The colloid size distribution yields information about their stability. For a representative deep crystalline water ([Ca] = 3.5 × 10−4 M, [Na] = 1.4 × 10−2 M, [TOC] = 3 × 10−6 M), the colloid concentration is < 100 ng · ml−1 and around 10 ng · ml−1 in Zurzach water for sizes ranging from 100 to 1000 nm.