Which of the following is not an environmental problem?

Soil erosion is the loss of protective vegetation through deforestation, over grazing, ploughing and fire makes soil vulnerable to being swept away by wind and water. Without soil and plants, the land becomes desert like and unable to support life known as desertification. Irrigation without proper drainage of water leads to water logging in the soil. But reforestation is a method of conservation of forest by developing forest cover in the area which has been damaged or cleared during exploitation.