What factors affect exchanges between groundwater and streams?
Natural processes—including physical, chemical, and biological—can affect exchanges between groundwater and streams. In stream settings containing organic-rich sediments and low dissolved-oxygen concentrations, bacteria convert nitrate in groundwater to nitrogen gas through the process of denitrification. Nutrients also can be removed by plants in riparian or buffer zones adjacent to streams. The effectiveness of these processes depends on the geometry of the local groundwater flow system. For example, riparian or buffer zones are most effective in settings with thin surficial aquifers (aquifers near the land’s surface) underlain by a shallow confining unit, with organic-rich soils that extend down to the confining layer. These buffer zones can remove 100 percent of nitrate in groundwater discharging to surface water where shallow aquifers and sediments are rich in organic matter and low in dissolved oxygen, conditions which promote denitrification. In contrast, in settings where natur
