Spatial and Temporal Variability in Colloid Dispersion as a Function of Groundwater Injection Rate within Atlantic Coastal Plain Sediments
J.
C. Seaman1, P. M. Bertsch1, and D. I. Kaplan2
1Savannah River Ecology Laboratory, The University of Georgia,
Aiken, SC 29802, USA
2Savannah River National Laboratory, Aiken SC 29808
Abstract
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.
SREL Reprint #3039
Seaman, J.
C., P. M. Bertsch, and D. I. Kaplan 2007. Spatial and Temporal Variability
in Colloid Dispersion as a Function of Groundwater Injection Rate within
Atlantic Coastal Plain Sediments. Vadose Zone Journal 6:363-372.
