CHROMATOGRAPHY IN THE AQUIA AQUIFER

The schematic profile of the Aquia aquifer (Maryland, USA) shows a 30 m thick aquifer that extends over 90 km from Washington to Chesapeake Bay. The aquifer is confined by clay layers that were sliced by Pleistocene glaciers, creating an upward outlet for the groundwater. As the result, fresh water started to infiltrate and displace the resident saline water, and a chromatographic pattern developed. Chloride has been flushed already, but cations that are displaced from exchange sites testify of the former, more saline water in the aquifer.
From the recharge along the flowpath, Ca(HCO3)2 water quality merges in chromatographic sequence into Mg(HCO3)2, KHCO3 and NaHCO3 water types.
The modeled pattern (lines in the graphs) beautifully confirms the chromatographic origin of the water qualities.


The model (PHREEQC input file aquia) subdivides the aquifer in two parts. The upstream part (cells 1-11, keyword TRANSPORT) is defined as a regular column. The downstream half (cells 13-21) is modeled with option '-stagnant'. For this part, mixing factors are defined to simulate that the aquifer gradually loses water by upward flow through the confining layers, until horizontal flow stops altogether in cell 21. With option -stagnant, the calculation order of the cells does not follow the cell numbers, but is determined by the association of stagnant and mobile cells. For output after 93000 years of freshening (70 shifts), the cell-number order is restored by redefining the column to consist of all the 21 cells.
The original (Palaeocene) water is estimated to be 18% seawater from the Na+ /Mg2+ ratio in the dilution zone in-between 45 and 55 miles (defined in SOLUTION 4-21).
The model uses active fractions to calculate H+ and Na+ exchange (see active fraction exchange).

Details of the model are in Appelo, 1994, Cation and proton exchange, pH variations, and carbonate reactions in a freshening aquifer, Water Resour. Res. 30, 2793-2805.

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