A recent scientific study shows new, important information about how groundwater can both contribute nutrients such as nitrogen to lakes, and can also carry it away. Nitrogen is an important nutrient but harmful when over-supplied. The fate and transport of nitrogen are critically important issues for human and aquatic ecosystem health because discharging nitrogen-contaminated groundwater can cause harmful algal blooms, foul drinking water, kill fish and other aquatic organisms, release toxins, and diminish the aesthetic and recreational value of lakes and streams.

While streams and surface runoff can be one source of excess nitrogen into and out of lakes, there is an additional, hidden source of nutrients: discharge of groundwater contaminated with nutrients from septic systems and land-disposal of sewage. The new study outlines the connections and bio-geochemical processes that happen between groundwater and lakes.

“The link between hydrology, chemistry, biology in both the laboratory and the field makes this research exciting, and may even point to natural processes that can help mitigate nutrient contamination from groundwater under certain conditions,” said U.S. Geological Survey hydrologist and lead author of the study, Deborah Stoliker.

Much of the nitrate carried into lakes by groundwater is transformed by microbes living in the shallow sediments. Natural microbial processes convert harmful forms of nitrogen (such as nitrate) to harmless nitrogen gas. The scientists found microbes with genes responsible for these processes to be ubiquitous in sediments beneath a lake on western Cape Cod, Massachusetts, where the study took place.

The direction of groundwater flow into or out of the lake and the chemistry of the water, especially dissolved oxygen and carbon, control how the nitrogen is processed. The microbes could actively remove harmful forms of nitrogen flowing into the lake when dissolved oxygen in groundwater was low. At a location where the groundwater contained high levels of dissolved oxygen and little nitrogen flowing into the lake, these processes were not active.

In contrast, when water flowed out of the lake, down through the sediments and into the aquifer, a different microbial process, nitrification, was at work. During nitrification, nitrogen trapped in the sediments, primarily resulting from decaying algae, gets converted into a chemical form (nitrate) that can be transported out of the lake by flowing groundwater. Lakes on Cape Cod can act as scrubbers of water that flow into them, yet at the same time, can act as a source of nitrogen to downstream aquifers. Understanding these processes is important for maintaining water quality and environmental health.

This research was conducted by an international collaboration of scientists from the USGS, Korea Institute of Geoscience and Mineral Resources, and College of William and Mary Virginia Institute of Marine Sciences. The paper, “Hydrologic controls on nitrogen cycling processes and functional gene abundance in sediments of a groundwater flow-through lake,” was just published in the journal Environmental Science & Technology.

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