Coastal Groundwater Salinity and Borefield Management
A coastal town pumps water from an unconfined sand aquifer for irrigation. Monitoring bores sit at three distances inland from the shoreline: bore A at 120 m, bore B at 350 m, and bore C at 800 m. Baseline electrical conductivity (EC) values are 620 µS/cm at A, 480 µS/cm at B and 450 µS/cm at C. Over two dry years, pumping from a wellfield 200 m inland increases. EC at bore A rises to 1450 µS/cm, while bores B and C remain near baseline. Rainfall recharge over the same period is 40% below the long-term average. The aquifer is 18 m thick and underlain by a confining clay layer, so there is no upward leakage from depth. The council must decide whether to reduce pumping, install a desalination plant, or do nothing.
1. Using the bore data, identify the most likely salinity source and explain the mechanism. Then state which of the three options is best supported and why.
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Source and mechanism. Only bore A, closest to the coast, shows a large rise (620 to 1450 µS/cm). Bores B and C are essentially unchanged. Because the aquifer is confined below by clay, there is no deep saline upwelling pathway. The pattern is therefore a seawater intrusion wedge moving inland as the freshwater lens thins. Two factors combine: increased pumping lowers the water table near the wellfield, reducing freshwater head, and reduced recharge (40% below average) lowers the lens volume. The freshwater-saltwater interface migrates landward, and bore A is the first to intercept the intruding saline water. The gradient of change with distance (large at A, negligible at B and C) is the diagnostic signal.
Best option. Reducing pumping is the best-supported first action. It directly addresses the head decline driving intrusion, is reversible, and is cheap compared with desalination. Desalination would treat the symptom while allowing the intrusion to continue, and doing nothing would let the wedge advance toward the wellfield. A defensible plan is to cut abstraction, monitor EC at all three bores, and only consider desalination if EC continues to rise after pumping is reduced.
2. The council proposes a target of keeping bore A below 800 µS/cm. Explain one limitation of using EC alone as the monitoring indicator.
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EC measures total dissolved ions, not which ions are present. A rise in EC near a coast is consistent with seawater intrusion, but it could also reflect agricultural return water, fertiliser leaching, or dissolution of salts in the unsaturated zone flushed by the first rains after drought. EC alone cannot distinguish these sources, so a reading of 800 µS/cm could be triggered by a non-marine source and lead to the wrong management response. Supporting indicators would help: chloride-to-bicarbonate ratios, sodium and chloride concentrations, and stable isotope signatures differ between seawater and agricultural sources. EC is also temperature-dependent, so readings must be compensated to a standard temperature or trends can be distorted. Finally, EC responds to concentration changes but lags behind the physical movement of the interface, so a stable EC does not prove the wedge has stopped advancing.