Arsenic and Microbial Risk in a Dual-Aquifer Village Supply
Two adjacent villages need a safe drinking-water supply of 5,000 L/day. Village A has a 45 m deep tubewell in grey micaceous sand: arsenic 0.12 mg/L, iron 8 mg/L, manganese 1.1 mg/L, sulfate 2 mg/L, dissolved oxygen 0.3 mg/L. Village B has a 12 m deep dug well: arsenic 0.01 mg/L, nitrate 48 mg/L, and E. coli 120 CFU/100 mL. A nearby 80 m deep exploratory well has arsenic 0.005 mg/L but a yield of only 0.3 L/s and unknown sustainability. The national arsenic limit is 0.05 mg/L; the nitrate limit is 45 mg/L; the WHO arsenic guideline is 0.01 mg/L. Treatment options quoted are: (i) chlorination only; (ii) aeration, oxidation and sand filtration; (iii) activated alumina for arsenic.
1. Interpret the chemistry of well A and explain why arsenic is elevated and which treatment option is appropriate. Include the fate of iron during treatment.
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Well A has low dissolved oxygen, very low sulfate, high iron and high manganese. These are signs of reducing conditions in the aquifer. Under reduction, iron(III) oxides and hydroxides are unstable and dissolve, releasing iron(II) and the arsenic adsorbed on those solids. This is a common mobilisation mechanism in grey micaceous alluvial sands, so the arsenic is probably not from a pollution source but from natural aquifer chemistry.
Aeration, oxidation and sand filtration is the appropriate option. Aeration oxidises iron(II) to iron(III), which precipitates as iron(III) hydroxide. This fresh precipitate adsorbs arsenic, especially arsenic(V), and the sand filter removes the solids. Pre-oxidation is important because arsenic(III) is less strongly adsorbed than arsenic(V).
Chlorination alone may oxidise arsenic(III) but does not remove arsenic from the water, so it is not sufficient. Activated alumina can remove arsenic but requires pH control and regeneration. It may be a polishing step, not the first choice here.
2. Compare the three sources for the 5,000 L/day supply and recommend an immediate and long-term plan.
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Village A exceeds both the national arsenic limit and the WHO guideline, and also needs iron and manganese removal. Village B is microbiologically unsafe and its nitrate is above the limit, so it should not be used for drinking without treatment. The deep exploratory well has low arsenic and a yield of 0.3 L/s, which is about 25.9 m3/day, far more than the 5 m3/day required, but its sustainability and full water quality are unproven.
Immediate plan: provide bottled or trucked water, or use the deep well only after disinfection and a confirmed arsenic result. Do not use village B for drinking. Village A water should be treated before use.
Long-term plan: test the deep well with a pump test and full chemical and microbial analysis. If sustainable, develop it as the primary source with wellhead protection and disinfection. If not, install aeration, oxidation and sand filtration at village A, followed by disinfection, and provide a separate safe source for village B. Monitor arsenic, iron, manganese, nitrate and E. coli regularly.
3. What additional data and monitoring are needed to confirm that the deep aquifer is a safe long-term source?
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Carry out a constant-rate pump test to determine sustainable yield, drawdown and recovery, and observe nearby shallow wells for interference. Measure water levels seasonally because heavy abstraction can draw shallow, contaminated water downward.
Analyse the deep water for arsenic, iron, manganese, nitrate, salinity, fluoride and major ions, and repeat sampling over at least one monsoon. Test for E. coli and total coliforms. Confirm that the well is properly sealed and that the annulus is grouted so shallow water cannot leak down the borehole.
Protect the wellhead from latrines, livestock and runoff. Establish a community monitoring schedule, with arsenic testing at least twice a year and microbial testing quarterly. If arsenic rises above 0.01 mg/L or microbial contamination appears, switch to treatment or an alternative source. The deep well should not be declared safe on one sample alone.