Rainwater not lost to evaporation or uptake by plants percolates through a layer of unsaturated subsoil, the vadose zone. Beneath this layer at depths ranging from a few centimeters to several meters the soil is saturated with water. This is the topmost surface, the "water table", of the aquifer, an underground water reservoir. Water in the aquifer (groundwater) flows slowly down-gradient toward the estuary at a speed usually less than 1 meter per day. Almost two-thirds of the nitrogen that traverses the vadose layer from atmospheric sources and fertilizer may be lost during passage into the aquifer. Septic systems discharge wastewater directly into this unsaturated soil layer from whence it crosses the water table and travels in groundwater toward the sea. Further losses of nitrogen (about one-third of that remaining) occur during groundwater transport, apparently due to microbial uptake and transformation. (The rates and mechanisms of nitrogen loss in the vadose zone and aquifer comprise an active area of research, and much remains to be learned.) Freshwater ponds and wetlands can intercept groundwater and retain a significant amount of its nitrogen (60 to 75% on average). These natural features act as effective "filters" of the nitrogen entering the up-gradient watershed. Where larger ponds and wetlands occur, the input of nitrogen into estuaries from up-gradient sources is likely to be minimal. As the groundwater with its load of nitrogen nears the estuary it rises above the denser salt water that is penetrating landward from the sea, and seeps out diffusely into the intertidal zone of the estuary.