Figure 1: A researcher collects data at the West Bear Creek Valley Tracer Test Site. New groundwater tracing techniques, used to improve scientists' understanding of contaminant fate and transport, are tested at this unique field site.
Characterization of the heterogeneous, fractured saprolite which is ubiquitous to the Oak Ridge Reservation (ORR) is difficult using conventional tracing techniques and thus, there is a paucity of measurments of aquifer parameters to be used in groundwater models for optimizing remediation. To improve our understanding of contaminant fate and transport, the West Bear Creek Valley Tracer Test Site was created for the purpose of developing new and innovative groundwater tracing techniques that will provide information on matrix diffusion and fracture flow velocities under natural gradient conditions and determine the effects of transient events on long term migration. The site is located in an uncontaminated area that has a similar hydrogeology as many of the waste sites on the ORR. Instrumentation on site includes over 70 piezometers (both single well and multilevel), a data logger which has been used to record data from three pressure transducers and a thermistor at 30-minute intervals for nearly 2 years, and a rain gauge.
Figure 1: A researcher collects data at the West Bear Creek Valley Tracer Test Site. New groundwater tracing techniques, used to improve scientists' understanding of contaminant fate and transport, are tested at this unique field site.
A summary of the tracer work performed at this site over the past three years includes the development of the use of dissolved gas tracers in groundwater with simplified injection, sampling and analysis techniques that were used for a long term (1 year) injection and sampling to simulate contaminant releases into the fractured shale saprolite. During this test, multiple inert tracers (Helium and Neon) were used to examine the effects of matrix diffusion. The saprolite has a matrix porosity as great as 50%; therefore, as He and Ne are advected together along the fractures, the mass transfer of helium from the fracture to the matrix is greater than that of neon due the greater diffusivity of He relative to Ne. As a result, the ratio of He to Ne at any given location is related to the matrix porosity, fracture aperture and spacing, and velocity. A second test involved the injection of colloidal-sized tracers (bacteriophage, microbes, and microspheres) under a natural gradient to measure fracture flow velocities. First arrival velocities at a well 14 m from the source were 45 m/day for the bacteriophage and microbes and 150 m/day for the microspheres. These velocities are in marked contrast to the measured first arrival velocity of the dissolved gases at 14 m of 5 m/day. This large difference between first arrival velocities of the dissolved tracers and the colloidal tracers can be attributed to the apparent retardation of the dissolved tracers by matrix diffusion. A third tracer experiment was a successful field test of microspheres labeled with unique DNA patterns. These DNA-labeled microspheres can be used to determine fracture interconnectedness and capture zones in fractured rocks.
These tracing techniques have been directly applied to several contaminated sites on the ORR with great success. One example is from a site within Waste Area Grouping 5 on the ORR. This site is contaminated with tritium, strontium-90, and various volatile organic compounds (VOCs). A six-month continuous injection of He, Ne, and Br was made under natural gradient conditions into an isolated high permeability zone located in the less weathered fractured shale bedrock. Tracer mobility was monitored temporally and spatially within the fractured zone and unfractured matrix using multi-level sampling wells located down-gradient from the injection well. One component of the tracer plume moved preferentially along bedding strike as quick-flow in fractures, while another component of the plume migrated slowly into the surrounding matrix. The breakthrough data of multiple, inert tracers with differing aqueous diffusion coefficients confirmed that matrix diffusion was a significant process controlling solute mobility. A comparison of the shapes of the breakthrough curves and the relative concentrations of the three tracers at any location can be used to determine if the sampling well was located within a fracture or within the matrix. Also, the three tracers can be used to constrain the physical parameters of the aquifer that are important for estimating mass transfer between the fractures and the matrix.
The plots are simulated breakthrough curves at 23 m from the source for
three non-reactive tracers (Br, Ne, He) injected simultaneously for 120
days. The model used was CRAFLUSH, a 1-dimensional fracture flow and
transport code developed by Sudicky. For
each of the three plots, the following parameters were used: 2-m
fracture spacing, 0.0001-m fracture aperture; 30% matrix porosity; and
100 m/day fracture flow velocity. These parameters are those
determined at the WAG5 site discussed earlier. The only difference
between the transport of the 3 tracers is their molecular diffusion
coefficients (D), with DHe > DNe >
DBr. Figure 2 shows the BTCs within the fracture. Note
that the arrival of the tracers is in reverse order of the magnitude of
their diffusion coefficients. This is because the tracers with the
larger D diffuse faster into the matrix along the flow path.
Figure 3 is of the BTCs measured 0.4 m into the matrix. Note that all
three tracers arrive at nearly the same time. At this distance in the
matrix, the mass fluxes of all three tracers are nearly identical
because the tracers with the larger D's can ``catch-up with'' the
tracers with the lower D's. Figure 4 shows the BTCs at 0.5 m into the
matrix. At this distance, the arrivals of the tracers are in the same
order as the magnitude of their diffusion coefficients. This far into
the matrix, the flux of the tracers with the larger D becomes greater.
As a result of the relative behavior of the three tracers, the distance
in the matrix that the sampling location is relative to the fracture
that the tracers are injected into can be determined. This information
is of great importance for interpreting tracer tests and
characterization studies in fractured rock aquifers.
