River Sediment
Geochemistry
Unlocking the environmental archives recorded in the sediments of Bangladesh’s mighty rivers: Padma, Meghna, Jamuna, and the coastal Pasur-Rupsha systems.
The Triad of Sediment Controls
Sediment geochemistry is fundamentally governed by three interlocking geological and environmental processes.
Provenance
Source rock tracing using REE patterns, Th/Sc, and La/Sc ratios. Himalayan active margin vs. passive margin signatures.
Weathering
Intense chemical weathering in tropical monsoons, quantified by the Chemical Index of Alteration (CIA).
Tectonic Setting
Influence of the India-Eurasia collision zone. Active margin settings yielding immature, lithic-rich sediments.
Laboratory Techniques & Software
X-ray Fluorescence (XRF)
Major & Trace Elements
A non-destructive spectroscopic technique ideal for rapid determination of major oxides (\(\text{SiO}_2\), \(\text{Al}_2\text{O}_3\)) and trace metals (Zr, Sr, Rb) without dissolving the sample.
- High-energy X-ray tube bombards pressed pellets or fused glass beads.
- Inner-shell electron ejection produces characteristic fluorescent X-rays.
- Matrix corrections applied via Fundamental Parameters method.
Neutron Activation (INAA)
Ultra-Trace Elements & REEs
An extremely sensitive nuclear technique used for elements difficult to measure by XRF (e.g., Rare Earth Elements, Th, U, Sc, Hf).
- Thermal neutron irradiation in a nuclear research reactor.
- Neutron capture creates radioactive isotopes.
- Gamma rays measured using High-Purity Germanium (HPGe) detectors.
Python Ecosystem
pandas, numpy, seaborn, scikit-learn for PCA & data viz.
Excel / Sheets
Template-based index calculations and error propagation.
OriginPro
Publication-quality ternary plots and spider diagrams.
Zotero
Advanced reference management and bibliography generation.
Heavy Metals in River Sediments
Heavy metals (Pb, Cd, Cr, Cu, Zn, Ni, As) accumulate preferentially in fine-grained (<63 µm) river sediments due to high surface area and organic matter content. In Bangladesh rivers, concentrations often exceed crustal averages due to industrial effluents (tanneries, textiles, ship-breaking), agricultural runoff, and urban sewage.
Typical ranges observed in recent studies (Rupsha, Buriganga, Pashur rivers):
| Metal | Concentration (mg/kg) | Primary Anthropogenic Source |
|---|---|---|
| Cd | 0.001–9.9 | Batteries, electroplating, dyes |
| Pb | 0.98–183 | Vehicle exhaust, paints, battery recycling |
| Cr | 2.44–233 | Tanneries and textile industries |
| Cu | 0.45–192 | Pesticides, fungicides, wiring waste |
| Zn | 15–420 | Galvanizing, rubber industries |
| As | 1.2–28 | Groundwater irrigation and pesticides |
Ecological Significance
Metals are bound to Fe-Mn oxides, organic matter, and clay minerals. Redox changes or acidification can remobilize them into the water column, leading to bioaccumulation in fish and severe human health risks including neurological and carcinogenic effects.
Environmental Pollution Indices
Standardized mathematical formulas used globally to assess the anthropogenic enrichment of heavy metals in sediments.
Contamination Factor (CF)
- CF < 1: Low contamination
- 1 ≤ CF < 3: Moderate
- 3 ≤ CF < 6: Considerable
- CF ≥ 6: Very high
Enrichment Factor (EF)
Normalizes against a conservative reference element (Fe, Al, Sc). EF > 2 indicates anthropogenic sources; EF > 40 implies extreme enrichment.
Geo-accumulation Index (\(I_{geo}\))
The 1.5 factor accounts for natural lithogenic variations. Divided into 7 classes (Class 0: unpolluted → Class 6: extremely polluted).
Pollution Load Index (PLI)
Provides a comprehensive assessment of overall toxicity. PLI > 1 indicates baseline pollution exists.
Radiological Hazard Indices
Natural radioactivity in sediments originates from primordial radionuclides (²³⁸U, ²³²Th, ⁴⁰K). The following equations are utilized globally to assess safety thresholds, particularly for sediments used in construction.
| Index | Mathematical Expression | Threshold | References |
|---|---|---|---|
| Absorbed Gamma Dose Rate (\(D\)) |
\( D \text{ (nGyh}^{-1}) = 0.462A_{\text{Ra}} + 0.604A_{\text{Th}} + 0.0417A_{\text{K}} \)
\(A\) = activity conc. (Bq/kg). Constants are conversion factors.
|
55 nGy h⁻¹ | Isinkaye & Emelue (2015); UNSCEAR (2000) |
| Radium Equivalent Activity (\(Ra_{eq}\)) |
\( Ra_{eq} = \left(\frac{A_{\text{Ra}}}{370} + \frac{A_{\text{Th}}}{259} + \frac{A_{\text{K}}}{4810}\right) \times 370 \)
|
370 Bq kg⁻¹ | Darwish et al. (2015); OECD (1979) |
| External Hazard Index (\(H_{ex}\)) |
\( H_{ex} = \frac{A_{\text{Ra}}}{370} + \frac{A_{\text{Th}}}{259} + \frac{A_{\text{K}}}{4810} \)
|
< 1 | Ravisankar et al. (2014) |
| Internal Hazard Index (\(H_{in}\)) |
\( H_{in} = \frac{A_{\text{Ra}}}{185} + \frac{A_{\text{Th}}}{259} + \frac{A_{\text{K}}}{4810} \)
|
< 1 | Kolo et al. (2015) |
| Gamma Representative Level Index (\(I_\gamma\)) |
\( I_\gamma = \frac{A_{\text{Ra}}}{150} + \frac{A_{\text{Th}}}{100} + \frac{A_{\text{K}}}{1500} \)
|
≤ 1 | Kolo et al. (2015) |
| Annual Effective Dose Rate (\(E_{aed}\)) |
\( E_{aed} = D \times 8760 \times 0.2 \times 0.7 \times 10^{-6} \)
0.2 = outdoor occupancy, 0.7 Sv/Gy = conversion factor
|
0.46 mSv y⁻¹ | Khan et al. (2019); UNSCEAR (2010) |
| Excess Lifetime Cancer Risk (\(ELCR\)) |
\( ELCR = E_{aed} \times 70 \times (0.5 \times 10^{-4}) \)
Life expectancy = 70 yr, Risk factor = 0.5×10⁻⁴ Sv⁻¹
|
0.000290 | ICRP (1990) |
Essential Databases & Literature
GEOROC Database
Geochemistry of Rocks of the Oceans and Continents. Essential for baseline UCC/source rock comparisons.
EarthChem Portal
Provides open access to compiled, structured geochemical, petrological, and geochronological data.
Principles of Isotope Geology
by G. Faure. The definitive guide for understanding radioactive decay, mixing theory, and provenance tracking.
Principles of Environmental Geochemistry
by G. Nelson Eby. Excellent resource for aqueous geochemistry, pollution transport, and heavy metal cycling.
IAEA NUCLEUS
International Atomic Energy Agency information portal. Crucial for standardizing radiological limits and methodologies.