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SPECIAL SECTION: SAVANNAH RIVER SITE

Spatial and Temporal Variability in Colloid Dispersion as a Function of Groundwater Injection Rate within Atlantic Coastal Plain Sediments

^a Savannah River Ecology Lab., The Univ. of Georgia, Drawer E, Aiken, SC 29802
^b Savannah River National Lab., Aiken, SC 29808
^* Corresponding author (seaman{at}srel.edu).

Received 24 March 2006.

A subsurface injection experiment was conducted on the USDOE's Savannah River Site (SRS) to determine the influence of pump-and-treat remediation activities on the generation and transport of groundwater colloids. The impact of colloid generation on formation permeability at injection rates ranging from 19 to 132 L min^–1 was monitored using a set of six sampling wells radially spaced at approximate distances of 2.0, 3.0, and 4.5 m from a central injection well. Each sampling well was further divided into three discrete sampling depths that were pumped continuously at a rate of 0.1 L min^–1 throughout the course of the injection experiment. Discrete samples were collected for turbidity and chemical analysis. Turbidity varied greatly between sampling wells and zones within a given well, ranging from <1 to 740 NTU. The two sampling wells closest to the injection well displayed the greatest response in terms of turbidity to increases in injection rate. Transient spikes in turbidity generally corresponded to incremental increases in the injection rate that were followed by a decrease in turbidity to a stable injection rate–dependent level. Mineralogical analysis of the resulting suspensions confirmed the presence of kaolinite, goethite, and to a much lesser degree, quartz and illite, with many of the particles too large (>1 µm) to be readily mobile within the formation. Turbidity measurements taken during this study indicate that colloid mobilization induced by water injection was both spatially and temporally heterogeneous. Furthermore, colloid release did not follow simple predictions based on shear force, presumably due to the complexities encountered in real heterogeneous systems. These findings have important implications to our understanding of how colloids and the co-contaminants are mobilized in the subsurface environment, as well as for the development of monitoring practices that minimize the creation of colloidal artifacts. Technical and logistical obstacles encountered in conducting such an extensive field experiment are also discussed.

Abbreviations: DO, dissolved oxygen • EDX, energy-dispersive X-ray • ICP–MS • inductively coupled plasma–mass spectrometry • ITS, injection test site • IW, injection well • SEM, scanning electron microscopy • SRS, Savannah River Site • TEM, transmission electron microscopy • XRD, X-ray diffraction.

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