A dune forest in SW France composed of maritime pines was irrigated with treated wastewater for a decade in an experiment (including irrigated plots versus control plots) to evaluate the environmental impact of applying wastewater on the water table, soil properties, and plants. The amount of treated wastewater (1921 mm yr−1) applied was twice the annual precipitation. Nutrient inputs were also very high, particularly nitrogen (N: 539 kg-N ha−1 yr−1), phosphorus (P: 102 kg-P ha−1 yr−1), and calcium (Ca: 577 kg-Ca ha−1 yr−1). Irrigation caused a rise in the water table, and increased its sodium (Na), NO3−, potassium (K), and calcium concentrations. Soil properties were affected by irrigation at least down to a depth of 1.2 m. After eight years of irrigation, soil pH had increased by 1.4 units, and soil available P content (POlsen) increased nearly 8-fold. In the short-term (i.e. 1–3 years), irrigation with treated wastewater improved growth, standing biomass, and the nutritional status of the vegetation. But tree dieback started in the fourth year of irrigation and worsened until the end of the monitoring period when almost all the irrigated trees were dead or moribund. The understory composition was drastically modified by irrigation, with an increase in α-biodiversity and in the biomass of herbaceous species, and a reduction in woody species abundance. The factor that best explained tree dieback was manganese nutrition (Mn): (i) the Mn content of the tree foliage was negatively affected by irrigation and below the deficiency values reported for pine species, and (ii) soil available Mn (CaCl2 extraction) decreased by half in the topsoil layer. Manganese deficiency was probably the consequence of the increase in soil pH, which in turn reduced soil Mn availability. Tree dieback was not related to either to a macronutrient deficiency or to toxicity caused by a trace element.