30 research outputs found
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PASSIVE FLUXMETERS: APPLICATION AS A CHARACTERIZATION MONITORING TOOL FOR MONITORED NATURAL ATTENUATION OF CHLORINATED SOLVENTS
Rainfall-Runoff Time Lags from Saltwater Interface Interactions in Atlantic Coastal Plain Basins
The dynamic behavior of the freshwater-saltwater interface (FSI) in coastal aquifers can introduce unexpected lags between recharge and stream discharge, especially when recharge is forced by long-term cyclical precipitation patterns. This work seeks to assess these FSI impacts at the watershed scale. Recharge-discharge time lags were evaluated in 68 watersheds overlying the Floridan Aquifer System in the coastal region of the southeastern United States (Florida, Georgia, and South Carolina). Utilizing the strength of the Atlantic multidecadal oscillation (AMO) signal in this region, 10–20 year averaged recharge and discharge time series were used for the selected watersheds. Lags of 10–25 years between recharge and discharge were found in 16% of the basins considered, possibly induced by a dynamic FSI which responded slowly to the AMO-scale recharge signal. Freshwater storage coefficients (S) were estimated from time series of change-in-storage and groundwater level, with 11 basins showing S>1.5 indicating water storage well above that expected for unconfined aquifers. These 11 basins with both multidecadal recharge-discharge time lags and high S values showed a positive linear relationship between time lag and FSI depth with slope 0.016 yr/m (R-squared = 0.30). These large time lags may be directly impacting the management of these basins as they obscure water and solute mass balances in the southeastern US
Back Diffusion from Thin Low Permeability Zones
Aquitards
can serve as long-term contaminant sources to aquifers
when contaminant mass diffuses from the aquitard following aquifer
source mass depletion. This study describes analytical and experimental
approaches to understand reactive and nonreactive solute transport
in a thin aquitard bounded by an adjacent aquifer. A series of well-controlled
laboratory experiments were conducted in a two-dimensional flow chamber
to quantify solute diffusion from a high-permeability sand into and
subsequently out of kaolinite clay layers of vertical thickness 15
mm, 20 mm, and 60 mm. One-dimensional analytical solutions were developed
for diffusion in a finite aquitard with mass exchange with an adjacent
aquifer using the method of images. The analytical solutions showed
very good agreement with measured breakthrough curves and aquitard
concentration distributions measured in situ by light reflection visualization.
Solutes with low retardation accumulated more stored mass with greater
penetration distance in the aquitard compared to high-retardation
solutes. However, because the duration of aquitard mass release was
much longer, high-retardation solutes have a greater long-term back
diffusion risk. The error associated with applying a semi-infinite
domain analytical solution to a finite diffusion domain increases
as a function of the system relative diffusion length scale, suggesting
that the solutions using image sources should be applied in cases
with rapid solute diffusion and/or thin clay layers. The solutions
presented here can be extended to multilayer aquifer/low-permeability
systems to assess the significance of back diffusion from thin layers