The
sediments and nutrients (phosphorus and nitrogen) that are washing into the Chesapeake
Bay have largely been attributed to non-point sources generated from agricultural
and urban stormwater runoff. LandStudies, an environmental consulting in Pennsylvania,
has undertaken a study in Lititz, Pa., along Santo Domingo Creek (2.4 square mile
watershed) in the Lititz Run watershed, to help quantify the amount of sediment
and nutrients being conveyed downstream because of stream channel evolution. This
study points to a third, and possibly greater, source of these pollutants: “legacy” sediments
and pollutants that accumulated along stream banks throughout the valleys of the
Piedmont region during the post-settlement era. Up to two centuries of impacts
such as forest clearing, poor land-use practices, stream channel straightening,
uncontrolled wastewater discharges, and the construction of thousands of milldams
along our waterways have filled the floodplains with nutrient-laden sediments.
As watershed sediment yields lessened over the last 30 to 50 years because of urbanization
and improved land-use practices, stream channel evolution began to move from an
aggrading to a degrading evolutionary stage. Streams are now cutting through the
post-settlement alluvial deposits toward their historical streambed elevations,
and are also moving laterally, trying to create new, accessible floodplains. Both
actions create highly unstable, erosive streambed and bank conditions, leading
to the current high influx of sediment and pollutants into the Chesapeake Bay.Based on measurements from monumented cross sections, one four-month monitoring period of 193 linear feet of stream channel yielded the release of 27.8 tons of sediment that contained 34.6 pounds of phosphorus and 96.3 pound of nitrogen. Variation in nutrient concentrations within the watershed was also assessed by land-use, by location in the stream bank profile, and by multiple soil testing methods. By extrapolating the nutrient concentrations from the Santo Domingo to other watersheds where stream channel erosion has been measured, the study strongly suggests the widespread nature and significance of this environmental issue. Therefore, LandStudies believes that these findings might help explain why, despite our significant efforts to implement stormwater management plans to control site-development runoff, estuaries like the Chesapeake Bay continue to decline in environmental quality.
Stream Restoration Project
To
help eliminate this source of sediment and nutrients, a 900-foot section of the
Santo Domingo Creek within the study area was restored in August/September 2004.
The restoration of the straightened channel included re-creating a more natural
meandering pattern that focuses on the development of a new floodplain at the proper
elevations relative to site-specific variables, including the size and quantity
of the bed/sediment load, downstream base-level controls, and stream bank materials.
The removal of three to four feet in floodplain height, to reattach the active
stream channel to a floodplain area, has removed from the watershed over
7,800 tons of sediment that contains more
than 8,930 pounds of phosphorus and 26,080 pounds of nitrogen; the creation of
wetland pockets in the floodplain will help trap incoming sediments, vegetatively
filter incoming nutrients, and promote groundwater recharge. Restoring our waterways
using this methodology has the potential to stem the tide of the current evolutionary
path of environmental degradation in the Northeast.
