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Choosing Where to Establish a Wetland in the Coastal Chesapeake Bay Watershed

By Bob Sowers, NRCS Information Management Specialist

Conservation planners take a lot of factors into consideration before restoring a drained cropland area back to a wetland. Now, there is one more important thing to consider: the subsurface permeability of the area’s soil.

It turns out that the permeability of the subsurface soil, or lack thereof, can be important to planners, particularly if they hope to achieve a specific objective from that wetland. A recent Conservation Insight report, “Soil Influences on Water Balance in Wetlands May Impact Wetland Effectiveness in Achieving Different Restoration Objectives,” explains how subsurface permeability in the Coastal Plain of the Chesapeake Bay watershed may affect how a restored wetland functions.

WE_Wetland_PhotoA restored wetland can provide many different beneficial ecosystem outcomes for the farmer, the community, and the ecosystem. Examples include a place for water to go during extreme weather events (floodwater retention); fertilizer nutrient and pesticide removal before water can reach streams and lakes; habitat for water-loving birds, reptiles, fish, and plants; hunting, fishing, and other recreation areas; and improved groundwater recharge, which can support surface waters such as ponds and streams in lower areas during periods of drought.

The study featured in this report compares two low-lying, or “depressional,” wetlands in the Coastal Plain of the Chesapeake Bay watershed: one with a low permeability subsurface, the other with a high permeability subsurface. Permeability means the ability of water to move vertically in the soil. A highly permeable soil lets water move downward when the groundwater level is below the soil surface, and upward as the groundwater level rises through the soil profile. A low permeability subsurface, however, restricts such vertical movement.

Planners seeking floodwater retention in a wetland should choose depressional cropland areas with a highly permeable subsurface to allow surface waters to quickly move downward during heavy rains, at least until the groundwater levels rise to the surface. This retained water, in turn, may prove useful during periods of drought, because the groundwater in these wetlands contributes to surface waters such as ponds, creeks, and rivers in lower lying areas and may be the only input during a drought.

If downstream flooding is not a prime concern, however, a less permeable subsurface can provide other ecosystem services. A less permeable subsurface allows water to stay on the surface of the wetland for a longer time. Water remaining on the surface for longer periods promotes a higher carbon-holding capacity of a wetland, eventually removing more carbon from the air to help in the fight against climate change. Through a process called “denitrification,” wetlands with frequent flooding and repeated saturated soil conditions are also better at removing fertilizer nitrogen from cropland runoff water before that nitrogen can get into downstream waters.

Subsurface soils and their effect on wetland water levels also impact plants and animals, particularly fish, birds, reptiles, and amphibians. Fish always need at least some standing water, as do some plants and other animals, and therefore would be better off in a restored wetland having a low permeability subsurface soil. Plants and animals preferring shorter periods of standing water, in contrast, may be more suited to wetlands with high permeability subsurface soils.

For more information, see the full Conservation Insight report.