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78 Chapter Three

Table 3.3 Simple classification of groundwater based on total bicarbonate waters at outcrop to saline waters at
dissolved solids (TDS) content. After Freeze and Cherry (1979). depth in the Earth’s crust.
In a later paper, Hanshaw and Back (1979)
−1
Category TDS (mg L )
described the chemistry of groundwater as a result of
Freshwater 0–1000 the intimate relationship between mineralogy and
Brackish water 1000–10,000 flow regime since these determine the occurrence,
Saline water (seawater) 10,000–100,000 (35,000) sequence, rates and progress of reactions. With refer-
Brine water >100,000
ence to carbonate aquifers, Hanshaw and Back (1979)
presented the conceptual model shown in Fig. 3.3 to
−1
Note: TDS > 2000–3000 mg L is too salty to drink.
depict the changes in groundwater chemistry from
the time of formation of a carbonate aquifer through
to the development of the aquifer system. When
3.4 Sequence of hydrochemical evolution of carbonate sediments first emerge from the marine
groundwater environment, they undergo flushing of seawater by
freshwater during which time the salinity decreases
In a series of three landmark papers, and based and the hydrochemical facies becomes dominated
on nearly ten thousand chemical analyses of natural by Ca-HCO . At this time, the carbonate sediments
3
waters, Chebotarev (1955) put forward the concept are selectively dissolved, recrystallized, cemented
that the salinity distribution of groundwaters obeys and perhaps dolomitized to form the rock aquifer.
a definite hydrological and geochemical law which Gradually, as recharge moves downgradient (R → D
2+
can be formulated as the cycle of metamorphism of in Fig. 3.3), Mg increases due to dissolution of
2+
natural waters in the crust of weathering. Chebotarev dolomite and high-magnesium calcite while Ca
(1955) recognized that the distribution of groundwa- remains relatively constant. With this chemical evolu-
2− −
ters with different hydrochemical facies depended on tion, SO increases as gypsum dissolves and HCO
4 3
rock–water interaction in relation to hydrogeolo- remains relatively constant. For coastal situations or
gical environment, with groundwaters evolving from where extensive accumulations of evaporite minerals

Fig. 3.3 Schematic model showing the evolution of chemical character of groundwater in carbonate aquifers. In areas of recharge (R), the
high concentrations of CO and low dissolved solids content cause solution of calcite, dolomite and gypsum. As the concentrations of ions
2
increase and their ratios change downgradient (D), groundwater becomes saturated with respect to calcite which begins to precipitate.
Dedolomitization (dissolution of dolomite to form calcite with a crystalline structure similar to dolomite) occurs in response to gypsum
solution with calcite precipitation. Where extensive accumulations of evaporite minerals occur, their dissolution results in highly saline
brines (B). Another common pathway is caused by mixing with seawater (M) that has intruded the deeper parts of coastal aquifers. After
Hanshaw and Back (1979).