Landfill siting over a volcanic aquifer in the Güímar valley
A waste-management company proposes a 120,000 m² non-hazardous landfill in the Güímar valley, Tenerife, to accept 250,000 t/yr of construction and commercial waste for 15 years. The site lies 30 m above the water table on 18 m of lapilli and volcanic ash overlying fractured phonolitic lava. A spring 1.2 km downslope supplies 3,000 people and irrigation; nitrate in the spring is already 42 mg/L, close to the 50 mg/L drinking-water limit, and tritium indicates recent recharge. The environmental impact statement states that a compacted clay liner and a leachate collection system will prevent contamination, and that monitoring will be periodic. The regional water authority has asked you to review the proposal and recommend whether to approve it, approve it with conditions, or reject it. The company argues that rejecting it will force waste to be exported by ship, with a higher carbon footprint.
1. Assess the aquifer vulnerability using the site geology, water-table depth and spring data. Is the EIS claim that a clay liner will prevent contamination sufficient?
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The aquifer is highly vulnerable. Lapilli and volcanic ash are porous and permeable, and the fractured phonolite beneath provides fast preferential pathways; 30 m of vadose zone is not a reliable barrier in such material. The spring 1.2 km downslope is hydraulically connected, and tritium shows recent recharge, so contamination could travel quickly. Nitrate at 42 mg/L already indicates that the aquifer is stressed, leaving little headroom before the 50 mg/L limit. A compacted clay liner can reduce leakage, but it is not a guarantee: volcanic terrain causes differential settlement, desiccation cracking and liner punctures, and leachate collection systems can clog. The EIS provides no hydrogeological model, no contaminant transport calculation and only vague periodic monitoring. The claim that the liner will prevent contamination is therefore not sufficient. The correct question is not whether the liner works in principle but what happens when it fails, and whether the spring can be protected.
2. Recommend approve, approve with conditions, or reject. Justify with reference to the precautionary principle and the waste hierarchy.
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I would reject the site as proposed. The precautionary principle is relevant because the potential harm-contamination of a drinking-water source for 3,000 people-is serious and difficult to reverse, while the scientific evidence of aquifer vulnerability is strong. The company's shipping argument is a real trade-off, but carbon footprint does not outweigh a drinking-water risk when the aquifer is already near the nitrate limit. Applying the waste hierarchy, construction and commercial waste should first be reduced, reused and recycled; landfill is the last option and should not be sited over a vulnerable aquifer. If the regional authority nonetheless approves, conditions would need to include a double composite liner, a leak-detection system, full leachate collection and treatment, a two-year baseline groundwater study, sentinel wells between the site and the spring, a financial guarantee and a closure plan. Even then, I would prefer an alternative site with a thicker, lower-permeability vadose zone and no downslope spring.
3. Design a monitoring plan that would detect leachate leakage before it reaches the spring.
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The plan should use nested piezometers completed in the lapilli and in the fractured phonolite, both up-gradient and down-gradient of the landfill, plus at least two sentinel wells between the site and the spring. Baseline monitoring should run for two years before operation and include water level, pH, electrical conductivity, nitrate, ammonium, chloride, major ions, trace metals, stable isotopes and tritium. Continuous electrical-conductivity sensors in the sentinel wells would give an early warning of leachate, which typically raises conductivity and ammonium. Sampling should be quarterly, with storm-event sampling because recharge pulses may mobilise contaminants. Trigger levels should be defined statistically from baseline, and exceedance should require investigation, not just more monitoring. The plan must also include a leachate balance for the landfill, surface-water sampling and a commitment to stop accepting waste if a trigger is confirmed. Detection before the spring is the only way to protect the supply.