Saltwater Intrusion in a Coastal Well Field
A coastal farming region pumps groundwater from 14 wells completed in a sand aquifer. Pumping averages 2.5 million m³ per year. Long-term recharge is estimated at 1.9 million m³ per year. Over eight years, chloride in three monitoring wells within 500 m of the coast rose from about 40 mg/L to 420 mg/L. Hydraulic head in the pumped aquifer declined 1.8 m. Sea level has been stable, and no major storm-surge event occurred during the period. Seawater has a density of about 1025 kg/m³; fresh groundwater is about 1000 kg/m³. A consultant proposes either reducing pumping by 20% or building two infiltration basins that would add 0.4 million m³ per year of recharge. The regional water board wants a decision within one year and asks for the reasoning, not just a choice.
1. Explain how the pumping could cause chloride to rise in wells near the coast, using the water-budget and density data.
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The water budget is in deficit: pumping is 2.5 million m³/yr and recharge is 1.9 million m³/yr, a gap of 0.6 million m³/yr. That gap must be made up by removal from storage, so hydraulic head falls; the observed 1.8 m decline is consistent with that.
Near a coast, fresh water and seawater meet at an interface where pressures balance. Because seawater is denser (1025 vs 1000 kg/m³), a thin freshwater lens can hold back seawater only if fresh head is high enough. Lowering the fresh head reduces the outward pressure, so the interface moves landward and upward. Wells near the coast then draw water that contains a growing fraction of seawater, raising chloride from 40 to 420 mg/L. The stable sea level and lack of storm surge make pumping the most likely driver.
2. Evaluate the two proposals. Which is more likely to stabilize chloride within a year, and what uncertainties remain?
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Reducing pumping by 20% cuts abstraction by 0.5 million m³/yr, leaving 2.0 million m³/yr against 1.9 million m³/yr recharge. That nearly balances the budget, but it does not create a surplus to push the interface seaward; chloride may stabilize rather than quickly decline. It acts directly on head and can show effects within a year.
Two infiltration basins add 0.4 million m³/yr, giving 2.3 million m³/yr total supply against 2.5 million m³/yr pumping, still a 0.2 million m³/yr deficit. Basins may also take time to recharge the aquifer, may lose water to evapotranspiration, and may introduce contaminants. Neither option alone is clearly sufficient. A combination, larger pumping reduction plus recharge, is more defensible, but the one-year deadline makes pumping reduction the more direct first step. Uncertainties include aquifer connectivity, offshore extent of the freshwater lens, and actual recharge.
3. List three monitoring measurements that should be collected to judge whether either action is working, and explain what pattern would indicate success.
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- Chloride concentration in nested monitoring wells at several distances and depths near the coast. Success would be a plateau or decline in chloride, especially in the shallowest wells nearest the shore.
- Hydraulic head in the pumped aquifer and in a shallower unit. Success would be recovery of head toward pre-pumping levels, indicating the freshwater lens is thickening.
- Pumping rates and recharge volumes, to confirm the water budget is actually improving.
Additional useful data include geophysical logs or resistivity surveys to track the interface, and water-level response to tides. A stable or landward-retreating interface, not just one low chloride reading, would indicate success.