The main areas of concern are nitrate pollution in surface and groundwater, phosphorus levels in surface water and contamination by pesticides released during farming activities. These chemicals act as nutrients for unwanted algal growth, which causes oxygen depletion within river bodies. This phenomenon is called eutrophication: “increased nitrogen and phosphate promote excessive growth of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen” (Art, 1993). The nitrate ion (NO3-) is the most common form of nitrogen found in natural waters. It may be biochemically reduced to nitrite (NO2-), usually under anaerobic conditions. The nitrite ion is rapidly oxidized to nitrate (Chapman, 1992). Natural levels of nitrate in surface waters seldom exceed 0.1 mg/l as N, but waters influenced by human activity normally contain up to 5 mg/l as N with levels over 5 mg/l as N indicating pollution by animal or human waste or fertilizer runoff. National drinking water standards for nitrates are 10 mg/l as N .
Proposals for a strategy to control nutrient enrichment published by DOE (NI) 1999 stated that: Eutrophication is considered to pose the most widespread single threat to good water quality in Northern Ireland (AFBNI) . One area in particular which has seen adverse effects associated with changes in water composition as a result of human activities is Lough Neagh. It has been estimated that the nitrate inputs to rivers flowing into Lough Neagh have increased by more than 70% in the past three decades. Such increase has caused a number of problems associated with eutrophication including: unusual algal blooms (dominant species of algae; Oscillatoria agardhii), disturbed filtration in cleaning beds, and depletion of oxygen near mud.