There are scale differences in the chemical composition of groundwater both laterally (recharge/discharge areas) and vertically (shallow oxidation and deep reduction zones), which are typical particularly for groundwater in sedimentary basins. Generally, groundwater in recharge areas and shallow aquifers has a lower total dissolved solids (TDS) content than groundwater in discharge areas and in deeper aquifers. The increase in total dissolved solids and the anion dominance evolution sequence HCO3-
→
SO42-→
Cl-, reflecting the change from oxidising conditions (shallow zone) into reducing conditions (deep zone), are to be seen in the vertical profile of a groundwater system expressed by Chebotarev (1955):In crystalline or pure siliceous sedimentary terrain, the Chebotarev sequence might hold for ionic ratios with depth, but the dissolved solids concentrations might even be reversed (Verhagen 1992).
Based on the Chebotarev anion evolution sequence, Domenico (1972) identified three main hydro -chemical zones in large and deep groundwater basins, which correlate in a general way with depth.
Mineral dissolution and molecular diffusion control in particular the gradual changes in anion composition of groundwater.
Recharge zones and near-surface inland aquifers are characterised by active groundwater circulation, lower temperature and brief contact of groundwater with leached rock materials. Groundwater is low in total dissolved solids and HCO3-is the dominant anion. In deeper intermediate zones temperature, pressure, contact time and surface with reactive rock minerals gradually increase as groundwater flow velocity decreases. This usually leads to increases in groundwater TDS with depth and sulphate ion dominance. In deep groundwater systems where flushing by groundwater is very low and residence time long, chloride gradually becomes the dominant anion, calcium is replaced by sodium and groundwater is often high in total dissolved solids. At the base of the aquifers in deep basins highly saline brines are found. The sequence of a gradual transition along the flow paths from fresh bicarbonate groundwater through sulphate water to mineralised chloride water many millennia old at the deep downstream end is shown in Fig. 4.3.1.
The HCO3- content in groundwater is mostly derived from biogenic CO2 in the soil zone (soil microorganisms and organic matter), dissolution of calcite and dolomite, or decomposition of igneous feldspars. The origin of sulphate in groundwater depends on the presence of soluble sulphate bearing minerals (gypsum CaSO4. 2H2O, anhydrite CaSO4or potash salt deposits), metallic sulphide minerals and the deposition of marine aerosols. High chloride contents in deep groundwater depends primarily
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G R O U N D W A T E R R E S O U R C E S F O R E M E R G E N C Y S I T U A T I O N S - A M e t h o d o l o g i c a l G u i d eFigure 4.3.1. Example of flow, age and hydrochemical patterns in groundwater (adapted from British Geological Survey, 2003)
Travel along flow path
HCO3-
→
HCO3-+ SO42 -→
SO42 -+ HCO3-→
SO42 -+ Cl-→
Cl-+ SO42 -→
Cl-Increasing age
on the presence of chloride-bearing sedimentary rocks and their soluble halite (NaCl) and sylvite (KCl) contents, and quite strongly on groundwater contact surface and time with these rocks.
The chemical composition of groundwater in some deep aquifers may be modified by gases -particularly by carbon dioxide (CO2), originating from volcanic processes and present in high concentrations in deeper parts of aquifers. Dissolved CO2 is an acid that keeps pH low, hydrolyses silicates, and releases solutes to the groundwater (Bucher and Stober, 2000).
Ground water in the different zones can be roughly classified in terms of its age. In sedimentary basins, groundwater in the upper zone may be years to tens of years old, whereas in deep basins ages of hundreds to thousands of years are common. Saline, chloride-rich connate water in the deep zone is usually very old, the ages varying from thousands to millions of years.
The groundwater evolution sequence and groundwater zoning described above could be disturbed in geological structures affected by tectonics that interconnect aquifers carrying water of different age and origin. In coastal regions where groundwater composition is under the influence of saline water intrusion, the chemical zoning of groundwater does not apply as high groundwater salinity in shallow groundwater aquifers is produced by mixing of fresh water with sea water or through salination by evaporation in arid environments.
The potential usefulness of groundwater in emergency situations needs to be carefully studied as it is controlled by its chemistry and quality. This is illustrated by an example from the Kanto Plain, the largest groundwater basin in Japan containing groundwater of different origin, age and chemistry. A distinct vertical hydrochemical zonation is developed in this basin that is clearly visible in the cross section from the Kanto Plain to Tokyo Bay (Fig 4.3.2). In the central part of the Plain groundwater with different dissolved solids content and different chemical type is present at various depths. Most prominent here is shallow groundwater of low dissolved solids and Na-HCO3 type derived from hills, plains and from shallow aquifers of less than 100 m deep (e.g. sampling sites no. 21, 22, 43, 72 in Table 4.3.1). High groundwater salinisation in shallow aquifers in Tokyo Bay is the result of sea water intrusion. In Tokyo Bay and the deep parts of Kozo lowland electric conductivity (EC) is high and groundwater of Na-SO4or Na-Cl type. The chemical composition of high residence time groundwater
Figure 4.3.2. Vertical distribution of groundwater quality in the cross section across the Kanto Plain to Tokyo Bay in Japan displayed in Stiff diagrams
in such deeper aquifers is influenced by intrusion of fossil saltwater that contains gases (mainly methane) or by salts eluted from the sediments. The chemical composition of groundwater in various depths of shallow and deep aquifers and coastal regions of the Kanto Plain and with influence of fossil water is given in the Table 4.3.1. Stiff and Piper diagrams of the four typical groundwater chemistry sites selected in this table can be seen in Fig. 4.3.3.
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G R O U N D W A T E R R E S O U R C E S F O R E M E R G E N C Y S I T U A T I O N S - A M e t h o d o l o g i c a l G u i d eTable 4.3.1. Chemical composition of groundwater of the Kanto Plain and Tokyo Bay at various depths (compiled by Yoshioka, based on the data of Seki et al., 2001)
Site 31 M 1,200 6.6 3,390 8,663 138 111 104 12,800 106 2,115 NaCl
36 M 600 9.5 40 98 1.7 0.7 2.3 3.1 17 239 NaHCO3
41 P 1,500 7.6 3,090 6,441 77 50 440 11,060 76 127 NaCl
43 M 83 9.1 30 63 1.2 0.7 3.7 2.1 23 142 NaHCO3
45 C 1,066 8.0 3,400 6,674 76 41 1,454 13,150 78 49 NaCl 47 C 100 8.4 160 407 17 3.1 3.0 31 0.0 1,103 NaHCO3 56 P 1,200 7.5 3,170 6,770 91 166 352 11,510 79 317 NaCl*
60 M 1,000 8.4 140 221 3.6 0.0 58 64 472 22 Na2SO4
72 P 95 7.4 50 104 8.9 8.2 12 4.2 0.1 356 NaHCO3
78 P 1,500 7.4 3,570 7,308 92 180 364 12,660 156 550 NaCl*
79 P 1,500 7.5 2,370 4,602 95 276 276 8,556 8.1 310 NaCl 80 P 1,500 7.5 3,610 9,989 353 249 880 18,270 181 254 NaCl*
81 P 1,300 7.4 4,710 13,600 623 204 927 23,410 216 204 NaCl*
86 P 200 8.5 100 201 14 2.4 5.9 13 0.0 630 NaHCO3
M: Mountainous P: Plain C: Coastal * Fossil Water
Figure 4.3.3. Stiff and Piper diagram of groundwater chemistry in the 4 typical sites
of the Kanto Plain and Tokyo Bay. Position of shallow groundwater of Na – HCO3type (site No. 72) in the diagram significantly differs from the other three samples
of groundwater from deep aquifers
High salinity groundwater is observed in arid and semi-arid regions. Evapotranspiration at high rates over a long time periods leads to the build up of high groundwater salinity (Chapman, 1992). This is seen in seepages or salt marches with distinctive vegetation, known as Salinas, or in sabkhas or pans mostly without vegetation due to locally endorheic conditions.
A cation evolution sequence in the groundwater system similar to the Chebotarev anion sequence is difficult to identify because there is a larger variation in cation contents. The presence of major cations (Ca2+, Mg2 +, Na+ + K+) depends on the solubility of the source minerals and on the type, extent and velocity of cation exchange processes. Matthess (1982) identified the following vertical hydrogeo -chemical zonation based on the characteristic cations:
Influence of biological processes on groundwater chemistry
Biological processes enhance the extent and rate of geochemical processes, stimulate or control many redox processes occurring in groundwater systems, have significant influence on the solubility of salts (particularly in the soil environment), on oxidation processes, and on oxygen and carbon dioxide content. The occurrence of the latter involves the active participation of bacteria and fungi. According to Fairbridge (1967) the subsurface environment is always modified to some degree by organic metabolic processes. Biological processes are particularly intensive in regions with warm and humid climates in the uppermost soil and root domain of the unsaturated zone, where dissolved oxygen is usually available supporting organisms which break down organic matter. Organic material often present in these sediments may produce reducing conditions favourable for the mobilization of iron and manganese and their higher content in groundwater. Biochemical processes in the soil produce large amounts of inorganic and organic acids which render the groundwater aggressive, initiating the hydrochemical process. Biological processes affect groundwater composition and quality particularly in shallow aquifers which are usually not considered as a safe source of groundwater in emergency situations. However, living bacteria were identified also in deep groundwater hundreds of meters below ground (Gurewisch, 1962, Davis 1967, and others). Their occurrence is controlled mainly by nutrient supply, pH, Eh, salt content, groundwater temperature, and permeability of the aquifer (Matthess, 1982). It should be stressed that such bacteria as are part of natural evolutionary processes of mineralisation and purification of groundwater, are not in themselves harmful or pathogenic.
Modeling of hydrogeochemical processes
Geochemical modeling may be applied to study the chemical and isotopic evolution of groundwater and helps in evaluating the suitability of groundwater as a source of drinking water in emergency situations. The implementation of inverse modeling requires the identification of the extent of specific chemical reactions that control groundwater chemical evolution. In addition one needs to know the hydrogeological conditions, i.e. the minerals which can be dissolved or precipitate, to define processes which may occur in the groundwater system and which are kinetically and thermo -dynamically possible. Furthermore a conceptual groundwater flow model is desirable with which to calculate travel time and to support the establishment of a relation between initial and final chemical composition of the studied groundwater system. Forward modeling is based on a definition of initial conditions in the groundwater system and calculation and quantification of the extent of a series of reactions imposed on that system. Incorporating thermodynamic databases is desirable in order to quantify and predict the extent of reactions (Glynn and Plummer, 2005).
The further development of geochemical modeling strongly depends on the quality and Ca2+ → Ca2++ Mg2+→ Na
-consistency of groundwater chemical and isotopic data. The use of statistical methods – particularly factor and cluster analysis – can provide reliable data needed for geochemical modeling and ground -water chemistry studies. According to Griffionen (2004) present-day hydrochemical modeling of a groundwater system has to deal with mineralogical constraints, limitations in the knowledge of thermo dynamic and kinetic reactions and uncertainties in the knowledge of the groundwater system of large aquifers. More details about modeling of geochemical processes can be found in Glynn and Plummer (2005). Some hydrogeochemical models are freely available from the U.S. Geological Survey.