Because nitrogen, in the form of ammonia and nitrates, acts as a plant nutrient, it also causes eutrophication. As you learned in the Total Phosphate section, eutrophication promotes more plant growth and decay, which in turn increases biochemical oxygen demand. However, unlike phosphorus, nitrogen rarely limits plant growth, so plants are not as sensitive to increases in ammonia and nitrate levels. Sources of Nitrates Sewage is the main source of nitrates added by humans to rivers. Sewage enters waterways in inadequately treated wastewater from sewage treatment plants, in the effluent from illegal sanitary sewer connections, and from poorly functioning septic systems.
Septic systems are common in rural areas.
Unlike large, centralized urban sewer systems that collect waste from many households, septic systems are generally used to treat the waste from only a single household. Water containing high nitrate levels can cause a serious condition called methemoglobinemia if it is used for infant milk formula. Two other important sources of nitrates in water are fertilizers, and the runoff from cattle feedlots, dairies, and barnyards. High nitrate levels have been discovered in groundwater beneath croplands due to excessive fertilizer use, especially in heavily irrigated areas with sandy soils. Stormwater runoff can carry nitrate-containing fertilizers from farms and lawns into waterways.