Coastal groundwater salinity after abstraction increase
A coastal town of 9,000 people draws drinking water from a sand aquifer whose water table sits 2.1 m above sea level at the pumping well, 600 m inland from the shoreline. The aquifer has a horizontal hydraulic conductivity of 12 m/day and an effective porosity of 0.25. For the past decade the town pumped 1,800 ML/year; a new irrigation scheme raises total abstraction to 3,100 ML/year. Within 18 months, monitoring bores between the well and the coast show chloride rising from 90 mg/L to 640 mg/L, and two inland bores show a water-table decline of 0.8 m. Rainfall recharge is estimated at 250 mm/year over a 4 km² capture area. Council must decide within one season whether to cap abstraction at the old rate, relocate the wellfield 1.5 km inland, or build a desalination plant.
1. Using a water-balance approach, evaluate whether the increased abstraction is sustainable before considering water quality. Show your reasoning with numbers.
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Recharge available: 250 mm/year × 4,000,000 m² = 1,000,000 m³/year = 1,000 ML/year.
Abstraction: 3,100 ML/year exceeds recharge by 2,100 ML/year. Even the original 1,800 ML/year exceeded recharge by 800 ML/year.
Implication: The aquifer cannot be in steady state; the deficit must be met by storage depletion and/or by drawing in water from elsewhere - here, seawater from the coast. The 0.8 m inland water-table decline confirms storage is being mined, and the chloride rise confirms saline intrusion.
Judgement: The increased rate is not sustainable. However, the fact that the old rate also exceeded recharge means simply reverting to 1,800 ML/year will not stop intrusion - it will only slow it. A genuine fix requires reducing abstraction toward or below ~1,000 ML/year (the recharge limit), or supplementing supply. The water balance alone is decisive on sustainability; water-quality data then tell us the mechanism of failure.
2. Explain why chloride rose at the coastal bores while the water table fell inland, and predict what would happen if pumping continued unchanged for another two years.
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Mechanism: Freshwater in a coastal aquifer sits above denser seawater in a wedge whose position depends on the balance between seaward freshwater flow and inland pumping. When pumping exceeds recharge, the freshwater head near the coast falls, the seawater wedge migrates inland and upward, and chloride rises in coastal bores. Simultaneously, inland the water table drops because storage is being depleted. The two observations are therefore two expressions of the same head decline - not separate problems.
Prediction: Continued pumping would drive the wedge further inland, raising chloride at the wellfield toward or beyond drinking-water limits (commonly ~250 mg/L for chloride as a taste threshold). Once the well is salinised, abandoning it does not restore the aquifer quickly, because flushing the salt requires years of seaward freshwater flow. The inland decline would also continue, threatening shallower users and possibly causing land subsidence.
Key point: Salinity is effectively irreversible on management timescales, so the decision is urgent.
3. Recommend one option to Council and justify it, addressing both the water balance and the salinity risk.
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Recommended: cap abstraction at the old rate as an immediate measure, and commit to relocating the wellfield inland as the medium-term solution; defer desalination unless demand cannot be met.
Justification:
- Capping at 1,800 ML/year immediately reduces the head decline and slows wedge migration, buying time. It does not solve the deficit (still ~800 ML/year over recharge), so it must be paired with demand reduction or alternative supply.
- Relocating 1.5 km inland moves the well away from the wedge, giving a larger freshwater buffer and reducing salinity risk. It also spreads drawdown over a larger area, lowering local head decline. However, it does not fix the regional deficit, so abstraction must still be limited to roughly recharge.
- Desalination solves supply but is capital- and energy-intensive for a town of 9,000 and creates brine-disposal issues; it is a last resort.
Caveats: The 4 km² capture area may not be the true recharge zone; a proper groundwater model and longer monitoring are needed. Council should also meter and price water to bring demand toward the recharge limit.