The Seybouse Wadi is one of the most important water courses in Algeria, extending over a length of 225 km and irrigating some of the most fertile land. This wadi supplies drinking water to more than 1.5 million people (66% of the wadi water), water for industry (8%, supplied to more than 71 factories) and irrigation water (26%). Its vast drainage basin (6500 km²) is the site of major human activities (including chemical and heavy industry, agricultural and household activities). A multitude of by-products of these activities are rejected into the wadi, without pretreatment. Most of this waste material is of agricultural and domestic origin and is highly charged in nutrient salts and organic matter. The aim of this study was to carry out a physicochemical characterisation of the waters of the Seybouse Wadi, to evaluate their impact on the environment. The analyses focused on the variables likely to characterise these waters: suspended matter, BOD5 (biological oxygen demand over five days), dissolved oxygen content, COD (chemical oxygen demand), pH, temperature, electrical conductivity and nitrate content. We found that the wastewater was highly contaminated, and that the use of this water for irrigation might have environmental repercussions.