Sources of Dissolved Materials in River Water

Sources of Major Elements

Mobility during weathering. Some elements are weathered out of rocks more easily than others. The degree of mobility is related to the degree to which elements form secondary minerals from their primary silicate parents.

Ca > Na > Mg > Si > K>> Al ~ Fe

Ca, Na, and Mg come from rapidly weathered silicate minerals such as feldspars and Mg rich silicates such as olivine, pyroxene, and amphibole. Ca, Na, and Mg do not generally form secondary minerals. In contrast, Al, Fe, and Si form secondary minerals such as clay minerals, and iron oxides. K is contained in minerals that do not weather as rapidly as the Ca, Na, and Mg bearing minerals. K is concentrated in biotite, muscovite and K feldspar.

Ca is found in:

1. plagioclase feldspar (Ca,Na) Al₂Si₃O₈
2. calcium carbonate CaCO₃
3. dolomite CaMg (CO₃)₂
4. gypsum CaSO₄.2H₂O
5. anhydrite CaSO₄

Na is found in:

1. plagioclase feldspar (Ca,Na)Al₂Si₃O₈
2. halite (sodium chloride) NaCl

Mg is found in:

1. olivine (Mg, Fe)Si₂O₆
2. pyroxene Ca(Mg,Fe)Si₂O₆ or (Mg,Fe)SiO₃
3. amphibole Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
4. dolomite CaMg(CO₃)₂

Al is found in:

1. plagioclase feldspar (Ca,Na)Al₂Si₃O₈
2. kaolinite clay 3Al₂Si₂O₅(OH)₄
3. gibbsite clay Al(OH)₃
4. smectite clay (1/2 Ca,Na)Al₃MgSi₈O₂₀(OH)4.nH₂O)
5. vermiculite (biotite or muscovite with K+ replaced by hydrated cations)

K is found in:

1. biotite K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂
2. muscovite KAl₂(AlSi₃O₁₀)(OH)₂
3. K-feldspar KAlSi₃O₈

Iron is found in:

1. pyrite FeS₂
2. olivine (Mg,Fe)Si₂O₆
3. pyroxene Ca(Mg,Fe)Si₂O₆ or (MgFe)SiO₃
4. amphibole Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
5. hematite Fe₂O₃
6. goethite HFeO₂

Major contributors to Groundwater

Na

• Halite dissolution from sedimentary rocks and from road salt and other pollution
• Weathering of plagioclase feldspar
• sea salt dissolved in rainwater

K

• Weathering of biotite mica
• Weathering of K-feldspar
• Decreases in biomass (K is abundant in leaves)
• Dissolution of trapped aerosols

Mg

• Weathering of amphibole and pyroxene
• Weathering of biotite (and chlorite)
• Weathering of dolomite
• Weathering of olivine
• From rainwater

Ca

• Weathering of calcite
• Weathering of plagioclase feldspar
• Weathering of dolomite
• Dissolution of trapped aerosols

SO₄

• Pyrite weathering (especially from mine tailings)
• Gypsum (CaSO₄.2H₂0) dissolution
• From Rainwater

Cl

• Halite (NaCl) dissolution (especially road salt)
• From rainwater

Si

• Silicate weathering

Sources of Ions in Rainwater

 

Soil Dust, Burning Vegetation, Biogenic Aerosols, Absorbed on Sea SaltSoil Dust

Ion	Marine Input	Terrestrial Input	Pollution Inputs
Na+1	Sea Salt	Soil Dust	Burning Vegetation
Mg+2	Sea Salt	Soil Dust	Burning Vegetation
K+1	Sea Salt	Biogenic aerosols	Burning Vegetation, Soil Dust, Fertilizer
Ca+2	Sea Salt	Soil Dust	Manufacture of Cement, Burning of Fossil Fuels
Cl-1	Sea Salt	Volcanism	Industrial Cl chemicals, Gas Release, Road Salt
SO4-2	Sea Salt	H2S from Decay	Burning of Fossil Fuels, Marine Gases
NO3	-	Lightning, Decay of Organic Matter	Auto and Truck Emissions, Burning of Fossil Fuels
NH4	-	Bacterial Decay	Ammonia Fertilizer with Ammonia, Decomposition of Sewage, Combustion
PO4	-	-	Fertilizers
SiO2, Al, Fe	-	-	Land Clearing

Source: Berner and Berner, 1987