By Alisha A. Renfro, Coastal Scientist, National Wildlife Federation
Estuaries are some of the productive – and in many ways, some of the most complex – ecosystems in the world. The abundance and distribution of fish and wildlife within an estuary largely depends on the amount, location and frequency of freshwater inflow. The construction of flood control levees along the lower Mississippi River has severely constricted the surface freshwater flow into the surrounding basins. However, a new study published in the Journal of Hydrology, “Pathways and processes associated with the transport of groundwater in deltaic systems,” led by Alex Kolker, Ph.D., suggests that additional fresh water from the Mississippi River is introduced into these estuaries as submarine groundwater discharge through sandy paleochannels buried beneath the muddy delta surface. These findings may have important implications for the ecology of the estuaries adjacent to the Mississippi River as well as coastal restoration projects in the Mississippi River Delta.
Submarine groundwater discharge refers to the release of groundwater directly into marine waters. Much like rivers, submarine groundwater delivers fresh water, nutrients and metals to coastal waters. However, it is often invisible, as the water seeps through permeable sediments, rather than moving through confined channels on the earth’s surface. In the study, the researchers investigated several different lines of evidence to determine if submarine groundwater flux from the Mississippi River was a significant source of fresh water to Barataria Bay, a basin adjacent to the west bank of the river.
The researchers found that there were localized sources of fresh groundwater into the bay and the flux of groundwater varied over time. Investigation into the unseen geology of the bay indicated that the input of submarine groundwater into Barataria Bay was associated with more permeable sandy sediments that were remnants of distributary channels and other sandy deposits built prior to the leveeing of the river system. The results also suggest that the variable nature of the submarine freshwater discharge into the bay is linked to river stage, where high water levels in the river are associated with increased groundwater flow. Overall, the study evidence suggests that while the flux of groundwater into Barataria Bay may be limited by river stage and the presence of sandy paleochannels, submarine groundwater was a major source of freshwater into the bay, with results suggesting fresh groundwater flux exceeds the limited surface water inputs.
The importance of submarine groundwater as a freshwater source in the delta may have very important implications for the ecology of the estuaries adjacent to the river. The transitional environments between freshwater and saltwater, estuaries – influenced by a combination of freshwater flow, rainfall, tides, winds, waves and storm events – are some of the most dynamic ecosystems on earth. This is undeniably the case in the Mississippi River Delta, where estuarine conditions vary day to day, week to week and year to year. In addition to these factors that drive estuarine conditions throughout the world, the Mississippi River Delta’s estuaries are also affected by devastating wetland loss and encroaching salt water from the Gulf of Mexico which, when combined with the severely limited freshwater inputs, has shifted the abundance and distribution of the assortment of fish and wildlife that depend on the estuary.