Measurements in situ of concentration and size distribution of colloidal matter in deep groundwaters by photon correlation spectroscopy

Abstract

Photon correlation spectroscopy (PCS) was applied for the characterization (concentration and size distribution) of colloidal matter in a deep groundwater. The measurements were performed on-line and in situ at the Äspö underground Hard Rock Laboratory, Oskarshamn, Sweden. The colloidal concentration in the rather saline groundwater was not above 0.1 mg/l and probably below 0.03 mg/l, according to the field measurements and with calibrations of the PCS-instrument with well-defined reference colloids of α-Fe 2O 3, γ-Al(OH) 3 and SiO 2. The results clearly demonstrated that the stability and concentration of a colloidal-size suspended phase in an anoxic groundwater with high Fe(II) content, like the one in Äspö (0.3 mg/l), is sensitive to exposure to atmospheric conditions during the handling of the sample. Diffusion of air into the closed measuring cuvette was enough to alter the colloidal content significantly within 6 h. A particle fraction with a size distribution range of 170–700 nm was formed within 45 min when air was allowed to diffuse into the aqueous phase from the air-filled upper part of the cuvette. The corresponding time to generate a significant colloidal precipitate was less than 1 min (size distribution range 100–600 nm) when a stream of air (1.5 ml) was bubbled through the water samples. The precipitating colloidal phase was a mixture of ferric hydroxide and calcium carbonate in all three cases.