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_ mobility of nitrogen. However, when nitrogen reaches subsoils and the saturated aquifer materials, the predominant form of nitrogen is nitrate. At depths greater than 10 feet bgs, the level of biologic activity is low, and there is little continued change in the form of nitrogen. SOURCES OF NITRATE There are a variety of sources of nitrate. These sources include agricultural fertilizer, animal waste, septic systems, and the natural environments. The most common source of nitrate is agricultural fertilizers containing nitrogen. Excess application of fertilizers to crops, orchards, lawns, parks and golf courses may cause nitrates to reach groundwater. Fertilizer application to heavily irrigated soils or soils with a high water table are an environmental concern and a subject of study throughout the United States and Europe. Animal production is also a source of nitrate. Feedlots with poultry, livestock, swine, and other animals may create substantial nitrate especially when animal density is high and feedlots are not vegetated. The organic nitrogen in manure is converted into ammonium and then into nitrate in soils. Rainwater then flushes nitrates through the surface soils to subsoils and, potentially, to groundwater. Septic systems and wastewater treatment may also be potential sources of nitrate. Historically, septic and wastewater treatment systems have not attempted to remove nitrogen prior to wastewater discharge. Therefore, human septage (composed of approximately 5% to 10% organic nitrogen) is converted to ammonium and then to nitrate near the surface soils. After septage is converted to nitrate, plant uptake of nitrogen or denitrification may take place in the upper 10 feet of soils. Alternately, rainfall infiltration or subsequent discharge of leachate may move the nitrate into deeper soils. In nature, the majority of nitrogen is recycled within the vadose zone. In the temperate .. climates in North America, organic nitrogen accumulates during the fall and winter as trees lose their leaves and annual plants set seed and die. Low temperatures inhibit decomposition of organic nitrogen, the production of nitrate during the winter, but small amounts of soil nitrate may be leached past the roots of dead or dormant plant roots with winter rains. In the spring, organic matter decomposition increases with the increasing soil temperatures, and nitrate production from organic nitrogen reaches its annual peak. Despite active tree roots and young annual vegetation, some nitrate may be flushed into the groundwater, especially during high-intensity rainstorms. As annual plant root systems become more extensive, little nitrate moves past the root zone in summer and early fall. This pattern is highly dependent on the rainfall distribution pattern throughout the year. Contrary to earlier belief, recent studies have - 2 .