Natural Ground Water Colloids from the USGS J-13 Well in Nye County, NV: A Study Using SAXS and TEM

We report results from ultra-small-angle x-ray scattering (USAXS) and transmission electron microscopy (TEM) of dilute silicate colloids that occur naturally in ground water from the U.S. Geological Society J-13 well, located near the Yucca Mountain Site in Nevada. Also included are results from our examination of a separate sample of this groundwater that had been treated by heating to 90 oC in contact with crushed Topopah Spring Tuff from the Yucca Mountain site. The USAXS measurements were done at the UNICAT undulator beamline at the Advanced Photon Source (APS) at Argonne National Laboratory. Power-law plots (scattering intensity versus momentum transfer) were fitted to the USAXS data. Colloids in the untreated J-13 groundwater were shown to have a fractal dimension of nearly 3, whereas colloids in the treated groundwater (EJ-13) have a dimensionality of approximately 2.4 over a length scale of approximately 3 to 300 nm. Similar power-law plots with dimension 3 characterized concurrent SAXS measurements from aqueous suspensions of Na-montmorillonite and NIST Brick Clay (NBS-67). We attribute these results to the sheet-silicate-layered structure of the clay colloids present in J-13 well water, montmorillonite, and 'brick clay' systems. The differences between EJ-13 and as-received J-13 are perhaps owing to exchange of calcium for sodium with the tuff. Radionuclide incorporation into, adsorption onto, or ion exchange with existing groundwater colloids may promote colloidal transport of radionuclides in groundwater. Such radionuclide-bearing colloids could thereby increase the concentrations of actinides in groundwater and enhance migration into human-accessible aquifers. Our results demonstrate the first application of USAXS to study the physical nature of such groundwater colloids, and represent perhaps one of the most dilute systems ever studied by small-angle scattering.