COLLOID PROPERTIES IN GRANITIC GROUNDWATER SYSTEMS .1. SAMPLING AND CHARACTERIZATION

Colloids are present in all groundwaters. The role they may play in th e migration of contaminants in the geosphere must be studied. Colloid sampling and characterisation campaigns have been carried out in Switz erland. On the basis of the results obtained from studies in the Grims el area (Grimsel Test Site, Transitgas Tunnel), Northern Switzerland ( Leuggern, Zurzach) and the Black Forest (Bad Sackingen, Menzenschwand) , a consistent picture is emerging. The groundwater colloids in granit ic systems are predominantly composed of phyllosilicates and silica or iginating from the aquifer material. Under constant hydrogeochemical c onditions, the colloid concentration does not exceed 100 ng . ml(-1) w hen the calcium concentration is larger than 10(-4) M and the sodium c oncentration is larger than 10(-2) M. However, under transient chemica l or physical conditions, such as geothermal or tectonic activity, col loid generation may be enhanced and the colloid concentration may reac h 10 mu g . ml(-1) or more if both calcium and sodium concentrations a re low (i.e. <10(-4) M and <10 M respectively). The colloid size distr ibution yields information about their stability. For a representative deep crystalline water ([Ca] 3.5 x 10(-4) M, [Na] = 1.4 x 10(-2) M, [ TOC] = 3 x 10(-6) M), the colloid concentration is <100 ng . ml(-1) an d around 10 ng . ml(-1) in Zurzach water for sizes ranging from 100 to 1000 nm. Copyright (C) 1996 Elsevier Science Ltd