Field, Laboratory, and Modeling Study of Reactive Transport of Groundwater Arsenic in a Coastal Aquifer
Introduction
Introduction
Advantage of Kd : easy to use.
Disadvantage of Kd : become inaccurate when deals wide range of geochemical conditions.
When develop reactive transport model with Kd, Langmuir isotherm is more suitable for most contaminant sorption reactions compared to linear isotherm.
Reason: It accounts for the decrease in Kd value as the adsorbing surface is increasing occupied by adsorbed species.
Surface complexation model (SCM) is claimed to be better than Kd.
Reason: Previous studies have shown than SCM captured the dynamic behavior of uramium than approach with Kd .
Apart from the inaccuracy of Kd reactive transport modeling also seldom used for As.
Objective
To determine what geochemical parameters are important for reactive transport modeling of As.
Compare which is better: SCM or Kd in modeling distribution of As in groundwater.
Method
Study Site: the upper permeable layer of Cape Cod aquifer which flows toward Waquoit Bay.
Sampling and Analysis
When: 18th-20th June 2007
Sample: three pore water profile up to 8m deep along a 12 m transect perpendicular to the shore of Waquoit Bay.
The in-line filtered pore water samples were assayed immediately on site for dissolved Fe(II) and As(III).
3 sediment cores (2-4 m depth) were sampled in June 2007 along the same transect perpendicular to the shore.
The samples were kept within a N2 atmosphere for HCl-leachable sediment Fe (II) and Fe.
Another aliquot was extracted with anerobic phosphate solution for As(III) and As.
One Sediment core which was penetrated up to 6 m dept was obtained in June 2006.
HCl-leachable Fe and phosphate-extractable As were obtained on samples stored wet and cold after 4 months of sample collection.
Batch Adsorption Experiment
As was added into three serum vials containing sediments sampled from 3 sampling site.
The concentrations of As (III) and As in the supernatant and sorbed on the sediment were quantified after 1 week and again after 2 weeks of equilibration time.
Geochemical Modeling of Sorption Isotherms
Sorption capacity (Smax) and KLa(a constant representing the binding strength) were calculated with the experimental data fitted to Langmuir equation.
With the Smax and KLa, the Kd. Value was determined.
Geochemical Modeling Of Sorption Isotherms
A semimechanistic SCM also developed with the experimental data and the acidity constants and equilibrium constants (K) as in a diffuse layer surface complexation model of ferrihydrite.
Reactive Multicomponent Transport Modeling
A 2D reactive transport of As in the discharging fresh groundwater of the upper aquifer was simulated
Simulation time was 110 years and 6400 steps.
Result And Discussion
Chemistry of Groundwater
The As-containing groundwater plume moves upward as the water flows all the way to Waquoit Bay.
The center of the plume also rises from one site to another over a distance of 6 m.
Chemistry Of Aquifer Sediment
Oxidation and Sorption of As by Fe (III) Oxides at Multiple Redox Interfaces
Langmuir Sorption Isotherms And Kd
Semimechanistic Surface Complexatin Modeling of sorption Experiment
