Case Study 5: Interpreting a Coastal Basin Core
A 120 m core from a coastal basin is logged as follows: 0–25 m grey clay with root traces and lignite; 25–55 m coarse sandstone with gravel and clay clasts, fining upward; 55–80 m fine sandstone with glauconite, shell fragments and ripple cross-lamination; 80–120 m dark shale with ammonites. Porosity is 30% in the fine sandstone, 18% in the coarse sandstone, and 5% in the shale. A village at the coast needs 0.3 L/s. Water samples from test wells give chloride 40 mg/L at 40 m, 900 mg/L at 70 m, and 3,000 mg/L at 100 m. The sea is 2 km away, and pumping will lower the water table by 5 m. The drillers propose screening the coarse sandstone because it is the most permeable. The water engineer asks you to interpret the sequence, choose a screen interval, and explain the risks.
1. Interpret the depositional environment of each unit and state whether the sequence shows overall sea-level rise or fall. Give reasons.
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From bottom to top:
- 80–120 m dark shale with ammonites: offshore marine, low energy, fine suspension settling; ammonites are marine pelagic fossils.
- 55–80 m fine sandstone with glauconite, shells and ripple cross-lamination: shoreface or shallow marine, wave and current reworking; glauconite forms in shallow marine conditions.
- 25–55 m coarse sandstone with gravel and clay clasts, fining upward: fluvial or delta-plain channel; gravel and clay clasts indicate stronger currents and erosion of bank material; fining upward is a channel-fill signature.
- 0–25 m clay with root traces and lignite: lagoon or coastal swamp, with subaerial exposure and plant growth. Overall the sequence shallows upward, so it records a regression (sea-level fall or delta progradation), not a transgression.
2. Choose the screen interval and justify it using porosity, permeability and chloride. Explain why the drillers' proposal to screen the coarse sandstone is partly right but incomplete.
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Screen the coarse sandstone at about 30–50 m. Reasons:
- Chloride is 40 mg/L at 40 m, well below the 250 mg/L drinking-water guideline; at 70 m it is 900 mg/L and at 100 m 3,000 mg/L.
- The coarse gravelly sandstone is likely to have high permeability because of large interconnected pores, even though its porosity is 18%. The fine sandstone has 30% porosity but finer pores, lower permeability, and chloride 900 mg/L at 70 m.
- The shale is an aquitard (5% porosity). The drillers are partly right: coarse sandstone is the best permeability target, but they must avoid the saline lower part and confirm the screen is above the freshwater–saline interface. Porosity alone is misleading; connected pore throats and water quality decide the yield.
3. Pumping will lower the water table by 5 m. Assess the risk of saline intrusion and describe a monitoring plan. What would you do if chloride at 40 m rises above 250 mg/L?
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Risk: 5 m of drawdown can reverse the hydraulic gradient, pulling deeper saline water upward or seawater from 2 km inland. The basin already has saline water at 100 m (3,000 mg/L), so a cone of depression could mobilise it. Monitoring plan:
- Install an observation piezometer between the village and the sea and one below the screen.
- Measure chloride, electrical conductivity, water level and pumping rate monthly, and after heavy pumping.
- Keep a pumping log; test at 0.3 L/s for 24 hours before commissioning. If chloride at 40 m exceeds 250 mg/L: stop or reduce pumping; relocate the screen shallower if a freshwater lens exists; or develop an alternative source. Do not simply increase pumping.