Chloride Rise in a Coastal Well Field
A coastal panchayat in Alappuzha district relies on 12 shallow community wells completed in a sandy aquifer capped by laterite. In 2019, chloride in wells within 300 m of the backwater averaged 45 mg/L; by 2024 it is 380 mg/L, while inland wells remain near 70 mg/L. Static water level in the affected wells fell from 2.1 m to 3.4 m below ground over the same period. Each well is pumped about 6 hours per day in summer. The panchayat proposes four new wells and a longer pumping schedule to meet a 15% rise in demand. Annual rainfall is about 3000 mm, but paving and canal drainage have reduced infiltration. The brackish backwater is 200 m away and has a tidal range of 1 m. There is no continuous salinity monitoring, and the wells are not sealed at the casing.
1. Using the chloride and water-level data, evaluate whether the salinity is more likely due to backwater/seawater intrusion or to a local geological source such as evaporite or connate water. Show a conservative mixing calculation and state its assumptions.
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Reasoning: (1) The high-chloride wells are those nearest the backwater, while inland wells stay near 70 mg/L; that spatial pattern points to a marine or backwater source, not a uniform geological source. (2) Chloride rose while the water level fell by 1.3 m, which steepens the hydraulic gradient toward the wells and can draw brackish water inland. (3) The aquifer is sandy and laterite-capped; no evaporite is reported in this coastal plain, and connate water would not usually produce such a sharp distance dependence.
Conservative mixing: C = f C_sea + (1 - f) C_fresh, so f = (C - C_fresh)/(C_sea - C_fresh). If the seawater end-member is 19,000 mg/L and fresh water is 45 mg/L, f = (380 - 45)/(19,000 - 45) = 335/18,955 = 0.018, or about 1.8% seawater. If the backwater end-member is only 10,000 mg/L, f = 335/9,955 = 0.034, or about 3.4%. Both fractions are small but enough to raise chloride above 250 mg/L. Assumptions: chloride behaves conservatively, the end-members are known, and evapoconcentration and clay-membrane effects are negligible. Conclusion: intrusion is the most likely cause, but the calculation does not by itself prove the pathway; water-level and chloride monitoring are needed.
2. Recommend a monitoring and abstraction plan to keep chloride below 250 mg/L. Give at least four specific measures and explain how each reduces risk.
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A defensible plan would combine reduction, relocation, and monitoring:
- Cut pumping in the affected wells by 30-50% and rotate abstraction among wells so that no single well sustains a large drawdown cone. This reduces the inland hydraulic gradient that pulls brackish water toward the wells.
- Relocate new wells at least 500 m inland and screen them in the freshwater part of the aquifer, not near the backwater. New wells close to the existing line would simply extend the problem.
- Seal the annular space around well casings and repair damaged aprons. Poorly sealed wells can allow shallow brackish water or tidal water to move down the casing.
- Build recharge structures such as recharge pits, trenches, and permeable paving to increase infiltration of the 3000 mm rainfall, and protect existing recharge areas from further paving.
- Monitor chloride or electrical conductivity monthly, and continuously log water level in at least three wells. Set trigger values, for example 200 mg/L chloride or a 0.3 m additional water-level decline, that require immediate pumping reductions.
- Track tidal stage and backwater salinity so that pumping can be reduced during high-tide or high-salinity periods.
Each measure either reduces the driving gradient, increases freshwater storage, or provides early warning before the 250 mg/L limit is breached.
3. A consultant argues that because the mixing fraction is only about 2%, the wells are safe to pump. Critically assess this argument.
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The argument confuses a small mixing fraction with a safe concentration. At 380 mg/L, chloride has already exceeded the 250 mg/L drinking-water guideline, so the wells are not currently safe. A 2% seawater fraction is enough to cause that exceedance because seawater chloride is about 19,000 mg/L.
More importantly, the fraction is not fixed. Continued pumping lowers the water level further, steepens the gradient, and moves the saline front inland; the mixing fraction can rise quickly. The calculation also treats chloride as the only concern, but intrusion raises sodium, sulphate, and possibly boron, and it can mobilise salts from the aquifer. Small fractions do not mean a small risk when the end-member is concentrated and the flow system is changing. The correct response is to reduce pumping and monitor, not to use the 2% figure as a safety certificate.