12/8/04

Chemical evolution of groundwater

I. Major reactions and expected trends in chemical evolution. The major reactions are described in section 21.2 of the text. Dissolution of CO2 results in recharge water having a relatively low pH. Dissolution of carbonate and silicate minerals increases TDS and raises pH. The dominant anion in groundwater tends to evolve from bicarbonate to sulfate and finally to chloride because of relative solubilities of carbonate minerals, sulfate minerals, and salts. While there is not a corresponding pattern for cation evolution in general, sodium can become the dominant cation due to ion-exchange in aquifers that contain sediments originally deposited in the marine environment (and hence with abundant exchangable sodium). Redox reactions involving oxidation of organic carbon lead to decreases in dissolved oxygen followed by release of iron and manganese to solution, and finally by sulfide (see figure 21.5).

II. Open and closed system dissolution of carbonates. Figure 21.6 illustrates the expected pH evolution of water as carbonates dissolve under "open conditions" in contact with the atmosphere and under "closed conditions" (expected for water below the water table). Ground water in many carbonate aquifers appear to correspond to "open condition" pH and bicarbonate levels. This could reflect dissolution in the unsaturated zone. It could also result from addition of CO2 below the water table as a product of organic carbon oxidation.

III. The Floridan aquifer, a carbonate aquifer, was used to illustrate a case of the common ion effect leading to supersaturation with respect to both calcite and dolomite.

IV. Figures and tables in a handout were used to illustrate the effects of order of encounter

V. A case study of the Madison Aquifer in the Williston Basin (described in a 1990 paper in Wat. Res. Res. by Plummer et al.) was discussed as a more complex example of chemical evolution of groundwater. Groundwater samples were collected along flow paths that had been identified by groundwater flow modeling. Total dissolved solids increase along flow paths. The anion composition (as plotted on a Piper diagram) changes from bicarbonate dominated, to sulfate and finally to chloride along flow paths that extend deep into the basin. Cation facies evolve from calcium-magnesium to sodium. Carbon-14 age dating indicated an increase of sulfate concentration with groundwater age. Plots of saturation indices for calcite, dolomite and gypsum versus sulfate concentration (as a proxy for age) indicate a common ion effect similar to that observed in the Floridan aquifer. Mass balance modeling was used to estimate the amounts of various minerals dissolved along the flow paths. A good match between predicted and observed sulfur isotope ratios supports the results of the mass balance modeling.

VI. The Milk River Aquifer - this was not discussed in lecture, but is described in detail in the text and is a good example of how ground water chemistry can be affected by geologic processes such as uplift and erosion, glaciation etc.