Is the Coastal Well Contaminated by Seawater or Road Salt?
Since 2018, a coastal town has pumped 2.1 million m³ per year from a confined sand aquifer 45–65 m below sea level, 350 m inland. Chloride in well MW-4 rose from 42 mg/L in January to 285 mg/L in September; bromide rose from 0.08 to 0.11 mg/L; the Cl/Br mass ratio is about 2,600. Seawater has Cl/Br near 290; road salt in the region has Cl/Br greater than 3,000. Static water level fell 1.2 m over the same period. A nearby irrigation well pumped 0.8 million m³ during the summer. Tidal efficiency in the aquifer is 0.35. δ18O in MW-4 is -6.8 per mil, similar to local recharge at -7.0 per mil and heavier than seawater at -0.5 per mil. The utility proposes drilling deeper.
1. Determine the most likely source of the chloride rise using the chemical and hydrologic evidence, and state whether deeper drilling is justified.
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Use two independent tracers. Cl/Br = 285 / 0.11 ≈ 2,600 by mass. Seawater is about 290; regional road salt is greater than 3,000. The value is far from seawater and much closer to road salt, but not identical-mixing with low-chloride recharge would lower both Cl and Br, so a road-salt source is plausible. δ18O = -6.8 per mil is close to local recharge at -7.0 per mil and far from seawater at -0.5 per mil; simple seawater intrusion should shift the isotopic composition toward seawater as chloride rises. The falling water level and summer irrigation are real stresses, but they do not prove seawater is the source.
Judgement. Do not drill deeper yet. The chemical evidence favors road salt or a mixed source, while the hydrologic evidence shows vulnerability. Sample a depth profile in MW-4 and nearby wells, analyze Cl/Br and δ18O, and check road-salt storage and septic inputs. If a deeper screen is installed, it could tap a more saline confined layer or increase upconing, making the problem worse.
2. Explain why deepening the well could worsen the problem even if seawater is not currently the main source.
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Deeper drilling can worsen because the deeper aquifer may be more saline, and pumping from a deeper screen changes the vertical pressure gradient. In a density-stratified coastal aquifer, fresh water floats on saltwater; a well screened near the interface causes upconing. Deepening without a hydraulic analysis can turn a shallow contamination problem into a larger saltwater problem. It also commits capital before the source is identified. If road salt is the source, a deeper well may avoid shallow contamination but will not solve the salinization risk if pumping continues to lower heads. The decision should depend on a depth-specific sampling program and a pumping test, not on the single chloride number.
3. Design a monitoring program that could distinguish among road salt, seawater intrusion, and connate brine within one year.
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Monitor pumping and water levels continuously; sample chloride, bromide, and δ18O quarterly at nested depths; log conductivity in situ; install a tidal-efficiency station; map road-salt application. Use a conservative mixing model: seawater should plot near Cl/Br 290 and δ18O -0.5 per mil; road salt should plot at high Cl/Br and recharge-like δ18O; connate brine would typically be enriched in chloride with a distinctive Br and strontium signature. Add 87Sr/86Sr if the first samples are ambiguous. A year of quarterly depth profiles will show whether the plume is advancing from the shore, from the surface, or from below. Set an action threshold, such as chloride above 250 mg/L at the shallow screen, that triggers reduced pumping or alternative supply.