B43B-01 INVITED
Role of Aquifer Heterogeneity in Salinity Persistence in Tsunami Affected Coastal Aquifers - Need for Better Characterization
The 2004 Indian Ocean tsunami caused extensive contamination of coastal aquifers. Seawater entered the
aquifers through open dug wells and also via direct infiltration. In Sri Lanka alone, an estimated 40,000 drinking
water wells were either destroyed or contaminated. Based on an initial conceptual model, it was hypothesized
that salinity should decrease with recharge from seasonal rains associated with monsoons within a couple of
seasons. However, monitoring data collected at some sites for more than two years after the tsunami show that
concentrations still have not returned to pre-tsunami levels. This persistence is attributed to a combination of
factors that were not taken into consideration in the earlier model. These factors include sub-normal recharge,
subsurface heterogeneity, preferential flushing of freshwater through permeable channels, inappropriate
cleaning and pumping activities from wells, bathymetry, coastal land topography and local hydrologic and
geohydrologic conditions. This paper presents the results from a laboratory study conducted in a 2D intermediate
scale test tank to generate both qualitative and quantitative data for preliminary validation of a new conceptual
model that takes these factors into account. The focus was to evaluate the effects of heterogeneity on the
propagation of an infiltrating salt-water plume and the effectiveness of subsequent flushing with freshwater.
Tsunami flooding events were simulated in a large soil tank packed to represent various heterogeneous
configurations. Both the tsunami event and flushing from ambient fresh groundwater and infiltrating rainwater
were simulated. In addition to qualitative observations, the spatial and temporal variations of salinity were
monitored using a set of in-situ electrical conductivity probes. The data strongly suggest that the plume
development and persistence is highly controlled by the geological heterogeneity that needs to be characterized
using methods other than conventional coring (e.g. geophysical methods).
http:www.cesep.mines.edu
B43B-02
Sources and Transport of Arsenic Contaminating Shallow Groundwater in the Historical Industrial Zone of Monterrey City, Nuevo León, México
Arsenic contamination has been detected in a shallow aquifer of the geographical center of Monterrey City, in the Northeast of Mexico, and where the historical industrial zone of the city is located, reaching concentrations of up to 3.1 mg/L. A metallurgical plant that operated more than a 100 years and closed down in 1999 is identified as a possible pollution source because it deposited arsenic-rich wastes in the fields adjoining it, showing total arsenic contents that reach up to 1-2 % As in weight. We deem arsenic transport through the soil profile as highly unlikely based on the following experimental and field evidence: water-soluble arsenic contents in 1:20 soil:water extracts of both surface and subsurface samples is very low, showing values in a majority of samples below 1 mg As/L; the unsatured zone under the inactive plant is characterized by the presence of two impermeable clay layers (hydraulic conductivities of 2.1 x 10-8 cm-1 to 8.4 x 10-8 cm-1), the first one occurring at 0.6 to 1 m below the surface, and the second one at approximately 17 m below the surface; arsenic analyses of several core samples from boreholes 40 m deep show that As released at the soil surface is completely retained in the upper clay layer. Nevertheless, the highest dissolved arsenic concentrations (1.1 to 3.1 mg/L) were found in wells from shallow groundwater from active industries located to the north of the inactive plant. The groundwater flux in the area goes in a direction west to east, therefore, the source of these arsenic levels cannot come from the inactive plant premises and indicates the occurrence of additional sources of arsenic groundwater contamination. The evidence points to the fact that the most likely cause of shallow groundwater contamination inside the inactive plant is via surface runoff of deposited wastes during the rainy season, into drilled boreholes in the area. This is supported by a 10-year semester analysis database. Therefore, the authors recommend a suitable and immediate closure of these boreholes.
B43B-03
Development of Physical Systems for Fate and Transport Measurements of TNT and DNT in Variably-Saturated Soils
Transport of chemicals from buried explosives in soils influences their detection at the surface. The transport of these chemicals in soils is controlled by fate and transport processes, which are influenced by environmental and soil factor. These processes are often studied though the development of models that simulate the conditions found in the field. Laboratory-scale physical models are attractive for this purpose because they can be controlled to isolate specific parameters and minimize interferences from unknown conditions. These model, however, must be developed properly to be representative of processes and conditions found in the field. This research focuses on the development of physical transport models to simulate the fate and transport processes that occur in the field under variable environmental conditions. To achieve this, field lysimeters (soil tanks) have been designed and developed. The lysimeters are exposed to outdoor conditions, which are variable and uncontrolled but measurable. Transport experiments in these lysimeters are supported by a wide range of laboratory-scale models, ranging in size and dimensions, that simulate transport of DNT and TNT subjected to controlled environmental conditions. The development of the scalable systems and methods involved proper reproduction of soil composition, lithology, and structures, appropriate placement of boundary conditions, suitable simulation of representative environmental conditions, and the use of representative sampling systems. All systems developed use the same field soils, which are packed to similar properties. Flow and transport boundaries were developed to simulate free or no flow, capillary-controlled, constant concentration or temporally- variable point source conditions. Representative environmental conditions in the laboratory-scale models are imposed through temperature-controlled transfer, lighting systems, and rainfall simulators. Simulated processes include advective and dispersive transport under infiltration and evaporation conditions, rate-limited dissolution and vapor transfer, surface retention, degradation, vegetation uptake, and daily radiation and precipitation. This presentation addresses the development of the physical models and the results obtained on the experimental work conducted to determine and quantify the effect of environmental factors on fate and transport of TNT and DNT from buried explosive devices.
B43B-04 INVITED
A new paradigm in DNAPL source zone characterization: 3D imaging of contaminant distributions through hydraulic and tracer tomography
Dense Nonaqueous Phase Liquids (DNAPLs) are prevalent at a large number of sites throughout the world. The
variable release history and geologic heterogeneity make the spatial distribution of DNAPLs in the source zone
complex. This causes difficulties in cleanup and can contribute to long-term groundwater contamination for
decades to centuries. Therefore, information on the spatial distribution, mass, and composition of DNAPLs
present in the source zone need to be obtained so that efficient remediation schemes can be designed. Existing
approaches of site characterization, however, are considered to be invasive and costly. We present here an
alternative to traditional characterization approaches based on hydraulic and tracer tomography which require far
fewer boreholes than the traditional coring techniques to image the DNAPL source zone. Specifically, it first
analyzes the information derived from hydraulic tomography, a new type of aquifer test, to identify the three-
dimensional hydraulic heterogeneity of the aquifer. The newly derived knowledge of heterogeneity is then used to
design partitioning tracer tomography tests to accurately depict the spatial distribution of DNAPL saturation in the
source zone. The proposed techniques have been tested numerically and currently are being validated in
laboratory sandbox experiments.
http:www.iihr.uiowa.edu/~illman