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`. .i shown that nitrate can impact groundwater in natural forest environments, especially in the spring. TRANSPORT OF NITRATE In soil, organic nitrogen and ammonium are found primarily in shallow soils because these compounds are strongly adsorbed to soil particles. When organic nitrogen and ammonium are oxidized to nitrate by soil microorganisms, the nitrate may be adsorbed by plants, converted to nitrogen gas by denitrifying soil microorganisms, or may simply move with rainwater through the vadose zone and into groundwater. Once nitrate passes through the _ upper 10 feet of soil, soil microbial populations and microbial activity decreases sharply. At depths below 15 feet, nitrate is the primary form of nitrogen, and it moves through soil with infiltrating water. For all sources of nitrate in the upper 10 feet of soils, the nitrate concentration is highest in soil water near the source and steadily declines with depth. This occurs because of the dilution effect in the vadose zone as soil water with dissolved nitrate moves downward and mixes with existing soil water. As nitrate moves downward with soil water, mechanical disper- sion and diffusion causes the nitrate impacted water to spread laterally as well as vertically. As water moves through unsaturated soils in the vadose zone, nitrate enters the groundwater. Upon entering the groundwater, mixing, dilution and diffusion causes further reduction of nitrate concentrations. When the nitrate in groundwater moves either downward to a deeper aquifer, or horizontally downgradient from the source, nitrate concentration is reduced again by dilution in the saturated zone. In summary, nitrate concentrations in soil water in the vadose zone and saturated zone decreases steadily with distance from the original source. The extent of nitrate impacted water will increase laterally and vertically because of mechanical dispersion and hydraulic and gravitational pressure. The resulting dilution and diffusion of nitrate result in a decrease in nitrate concentrations in soil water and groundwater. - 3 -