5.1 Groundwater Geochemistry and Water-Rock Interactions

Groundwater composition is shaped by complex chemical processes. As water moves through aquifers, it interacts with minerals, dissolving them and exchanging ions. These interactions determine water hardness, pH, and the presence of trace elements.

Chemical weathering plays a crucial role in groundwater chemistry. Minerals dissolve and precipitate based on their solubility, influenced by factors like pH and redox potential. Understanding these processes is key to managing groundwater quality and predicting contaminant behavior.

Groundwater Composition and Chemical Processes

Chemical constituents of groundwater

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• Major cations include calcium, magnesium, sodium, and potassium drive water hardness and salinity
• Principal anions encompass bicarbonate, chloride, sulfate, and nitrate influence pH and buffering capacity
• Trace elements like iron, manganese, fluoride, and arsenic impact water quality and potential health effects
• Dissolution of minerals releases ions into solution alters groundwater chemistry over time
• Ion exchange processes on clay surfaces modify cation ratios affect water hardness
• Redox reactions transform chemical species control mobility of metals (iron, manganese)
• Microbial activity mediates biogeochemical processes influences nutrient cycling (nitrogen, sulfur)
• Aquifer lithology determines available minerals for dissolution shapes overall water composition
• Residence time allows for increased water-rock interactions leads to higher dissolved solids
• Recharge sources introduce different chemical signatures affect groundwater quality (surface water, precipitation)
• Climate and precipitation patterns influence dissolution rates impact mineral weathering intensity

Water-rock interactions in groundwater

• Carbonate minerals (calcite, dolomite) dissolution buffers pH increases water hardness
• Silicate minerals (feldspars, quartz) weathering releases cations contributes to overall dissolved solids
• Evaporite minerals (gypsum, halite) dissolution increases salinity affects water quality
• Clay minerals facilitate cation exchange processes modify groundwater composition
• Ion exchange impacts water hardness by replacing calcium and magnesium with sodium
• Surface complexation on mineral surfaces adsorbs trace elements affects contaminant mobility
• pH-dependent adsorption of trace elements influences their concentration in groundwater
• Iron and manganese oxidation/reduction alters water color and taste affects water treatment needs
• Sulfate reduction in anaerobic environments produces hydrogen sulfide causes odor issues

Chemical Weathering and Mineral Solubility

Chemical weathering in groundwater

• Hydrolysis of silicate minerals breaks down feldspars forms clay minerals (kaolinite, smectite)
• Silicate weathering releases cations (Na+, K+, Ca2+, Mg2+) into solution increases total dissolved solids
• Carbonation reactions dissolve carbonate rocks (limestone, dolomite) enhance aquifer porosity
• Dissolved CO2 forms carbonic acid accelerates weathering of both carbonate and silicate minerals
• Oxidation of sulfide minerals (pyrite) releases metals and acidity can lead to acid mine drainage
• Acid-base reactions neutralize acidic waters through dissolution of carbonate minerals
• pH buffering in groundwater systems maintains stable chemical conditions affects metal solubility

Mineral solubility in groundwater

• Solubility product constant ($K_{sp}$) determines mineral dissolution equilibrium predicts saturation states
• Ion activities in solution control mineral precipitation or dissolution affects water chemistry
• Common ion effect reduces mineral solubility when ions are already present (calcite in gypsum-rich waters)
• pH influences mineral solubility by affecting protonation state of dissolved species
• Acid-base equilibria in groundwater control carbonate system speciation (CO2, HCO3-, CO32-)
• pH-dependent solubility of metal hydroxides affects mobility of trace metals (aluminum, iron)
• Complexation reactions form aqueous complexes increase apparent solubility of metals
• Metal-organic complexes enhance mobility of trace elements in groundwater
• Redox potential (Eh) controls speciation of redox-sensitive elements (iron, manganese, sulfur)
• Eh-pH diagrams predict mineral stability and dominant aqueous species in different environments
• Temperature affects mineral solubility constants typically increases solubility with rising temperature
• Geothermal systems exhibit unique water chemistry due to elevated temperatures and pressure