Nitrogen is found everywhere. Air is approximately 78% nitrogen gas. Nitrogen is necessary for protein structure and as such is a key element for life. However, too much misplaced nitrogen, such as in drinking and groundwater, causes concern. How much nitrogen is too much? That is under debate, but one convenient cut off from a public health perspective is 10 milligrams per liter. Concentrations greater than this are considered a potential health hazard to infants. Concentrations lower than this may be hazardous to environmentally sensitive areas. Groundwater usually does not contain sufficient concentrations of nitrates (NO3) to warrant concern over human health. However, if a groundwater aquifer is susceptible to contamination, excess nitrate may accumulate and become a problem.
Household wastewater usually contains both nitrogen and phosphorous. These nutrients can contribute to a process known as eutrophication. In eutrophication, increased concentrations of the limiting nutrient for growth is added to the ecosystem. This results in an overgrowth of algae, weeds, and other aquatic plants; shifting the population from the more stable resident plant species to "opportunist species." When the residents or opportunists die, the amount of organic matter available for decomposition by microorganisms increases. The majority of the microorganisms involved in degradation of organic matter are aerobic, meaning they require oxygen to respire. Microbial respiration lowers the dissolved oxygen concentration in the water, creating anaerobic conditions. Aquatic organisms, such as fish, that require oxygen in the water to "breathe" may then be injured or killed by a lack of oxygen. Nitrogen is more often the limiting nutrient in salt water, while phosphorous is more often limiting in fresh wate
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