Beneath a streambed lies the hyporheic zone, where stream water and groundwater meet and mix. This mixing allows dissolved materials—such as oxygen, nitrogen, and other nutrients—to cycle between the stream and the hyporheic zone, influencing biological processes and water quality.
According to previous studies, rising or falling groundwater can reduce the circulation of water between the stream and the hyporheic zone. However, the resulting effect on nitrogen cycling remains unclear. In a new study, Azizian et al. show that groundwater also reduces the hyporheic zone’s ability to remove stream nitrate by denitrification, allowing more nitrogen to escape downstream, where it can harm human and ecosystem health.
The researchers developed a computational model to investigate nitrogen cycling in the hyporheic zone under a variety of different conditions, including stream chemistry, stream hydrology, and groundwater flow. They found that the amount of nitrate produced in or removed from the hyporheic zone depends on both stream chemistry and the scale at which measurements are taken. In general, more nitrogen cycling occurred at polluted sites and in meter-scale sequences of shallow and deeper water. Relatively little nitrate generation and removal occurred in centimeter-scale ripples.
No matter the setting, the scientists found that rising and falling groundwater diminished nitrate processing, which could disrupt downstream ecosystems. For example, without the treatment services provided by the hyporheic zone, nitrate from agricultural runoff can flow unchecked to sensitive coastal marine environments, stimulate excessive algae growth, and produce oxygen-free dead zones where fish and other aquatic organisms cannot survive.
These findings raise concerns about human activities that could affect groundwater levels, including agriculture, urbanization, groundwater mining, and climate change. In the next few decades, scientists predict, human activities will release increasing amounts of nitrogen into streams. The combination of changing groundwater levels and increased runoff could prove harmful to affected ecosystems and populations. (Water Resources Research, https://doi.org/10.1002/2016WR020048, 2017)
—Sarah Stanley, Freelance Writer