MEASUREMENTS IN-SITU OF CONCENTRATION AND SIZE DISTRIBUTION OF COLLOIDAL MATTER IN DEEP GROUNDWATERS BY PHOTON-CORRELATION SPECTROSCOPY

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 Aspo 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 alpha-Fe2O3, gamma-Al(OH)(3) and SiO2. 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 Aspo (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 mi) was bubbled through the water samples. The precipitating colloidal phase was a mixture of ferric hydroxide and calcium carbonate in all three cases.