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Effects of legacy sediment removal on hydrology and biogeochemistryin a first order stream in Pennsylvania, USA
Citation:
Mayer, P., M. Audie, Reneej Brooks, Ken Forshay, J. Hartranft, D. Merritts, R. Walter, AND J. Weitzman. Effects of legacy sediment removal on hydrology and biogeochemistryin a first order stream in Pennsylvania, USA. Ecological Society of America, Baltimore, MD, August 09 - 14, 2015.
Impact/Purpose:
A large component of the sediment and nutrient loads that flow to the Chesapeake Bay and other estuaries on the East Coast are derived from so-called legacy sediments that fill valley bottoms throughout the Mid-Atlantic region. These sediments eroded into floodplain beginning with land conversion to agriculture during the American Colonial era and the ubiquitous appearance of mill dams for hydropower. Today, the mills are gone but the sediments remain, continually eroding and moving downstream carrying nitrogen and phosphorous and tons of sediment. EPA/ORD is part of a multidisciplinary team examining the effects and feasibility of removing legacy sediments as a stream restoration approach at an intensively studied site in Lancaster County, PA. The complex interactions of hydrology, biology, and geochemistry create challenging conditions for study. However, preliminary data suggest that removing legacy sediment can be effective for limiting sediment and phosphorous loads and perhaps nitrogen. Costs of restoration may be offset by the sale of sediments as topsoil for landscaping.
Description:
Historic forest conversion to agriculture and associated stream impoundments built for hydropower led to extensive burial of valley bottoms throughout the mid-Atlantic region of the US. These so-called legacy sediments are sources of nutrient and sediment pollutant loads to downstream waters including the Chesapeake Bay. Recent efforts to remove legacy sediments to restore ecosystem function created an opportunity to assess the effect of such restoration on hydrology and biogeochemistry. We present hydrologic and biogeochemical patterns in pre- and post-restoration stages of Big Spring Run, a stream in Lancaster County, Pennsylvania, USA. Carbon and nitrogen are closely linked in both ground and surface water, suggesting that microbial processes such as denitrification may be carbon limited. Spatially variable but temporally stable hotspots and coldspots of nitrate nitrogen were detected in groundwater throughout the restoration reach watershed. Residence times of groundwater, measured using stable isotopes of water, suggested that greater nitrogen transformation occurred along more active flow paths of ground water. Denitrification potential was lowest in legacy sediments and significantly higher in buried wetland soils. Other restoration effects were dramatic; exposed buried wetland soils allowed native vegetation to regrow from the seed bank, shorebirds were observed following sediment removal in newly exposed riparian wetlands, and bank erosion from freeze-thaw processes and extreme storm events was drastically muted where incised banks were removed and stormwater could spread throughout the floodplain. A fundamental understanding of the history of sites, including legacy effects of past land use, is essential to identify feasible restoration recovery goals.