Anthropogenic anoxification (“eutrophication”) of the water table region of a deep phreatic aquifer

Depletion of dissolved oxygen was detected at the water table region (upper 2.5 m) of a 30‐m‐deep, sandy, phreatic aquifer containing high oxygen concentrations (7.5 mg L⁻¹) in bulk groundwater. Samples spaced at 3‐cm intervals disclosed very sharp oxygen gradients of up to 1.9 mg O₂ L⁻¹ cm⁻¹ between two consecutive samples. High concentrations of labile organic matter arrive at the water table region after a transport period of more than 15 years through the unsaturated zone. Part of the organic matter oxidizes at the water table region, as evidenced by the decrease in the dissolved oxygen content. The concentration of the remaining dissolved organic matter can be as high as 8 mg C L⁻¹, decreasing with depth. The dissolved organic carbon derives from a large pool of water soluble organic matter found in the unsaturated zone. The average oxygen and organic carbon fluxes at the unsaturated‐saturated interface are 8.5 × 10⁻² mg O₂ and 3.1 × 10 ⁻² mg C_org cm ⁻² yr ⁻¹, respectively. Anaerobiosis may result either from the high organic load (140 kg C_org ha⁻¹ yr⁻¹) provided by irrigation with sewage effluent since the 1960s or from the retarded influx of organic matter mobilized at the onset of the intensive agricultural development of the area in the 1930s. The high oxygen content in deep pumping wells and the positive correlation found between the depth of the reduced zone and the organic loads from sewage effluents supports the first model.