Nutrient, Trace Metal and Particle Release from Sediments in the Great Bay Estuary and Riverine System

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Project Type: 
Research
Project Number: 
R/CE-141
Inception Date: 
2010
Completion Date: 
2011

Participants:

Linda Kalnejais UNH - Department of Earth Sciences Principal Investigator
Diane Foster UNH - Department of Mechanical Engineering Co-Principal Investigator

Students Involved:

Laurent Officer UNH - Department of Earth Sciences
Vincent Percuoco UNH - Department of Earth Sciences
Meagan Wengrove UNH - Department of Mechanical Engineering
Sophia Burke UNH - Department of Natural Resources & the Environment
Abstract: 

The goal of this project is to determine the chemical and physical mechanisms that release nutrients and trace-metals from the fine-grained sediments of the rivers and estuary of the Great Bay, and to assess if the sediments are a significant source of these contaminants to the Great Bay aquatic ecosystem. The conditions that lead to the erosion of fine particles from sediments will also be determined and the importance of river sediments as a source of silt to the Great Bay will be established. These questions will be addressed through a series of detailed field and laboratory observations. In the field, the geochemistry will be sampled concomitantly with observations of the fluid stresses, sediment suspension, and bed elevation at three locations within the Great Bay. The laboratory investigations will allow for a more detailed correlation between fluid stresses and nutrient fluxes and trace metal release. This study will provide regional and local resource managers with concrete information on the magnitude of nutrient, metal and particle release from sediments in the estuary and from the sediment pools behind dams.

Objectives: 

Geochemical and physical mechanisms that release nutrients and trace-metals from the finegrained sediments of the rivers and estuary of the Great Bay will be examined. The sediments will be examined to determine if they are a significant source of contaminants to the Great Bay aquatic ecosystem. The conditions that lead to the erosion of fine particles from sediments will also be determined and the importance of river sediments as a source of silt to the Great Bay will be established. The magnitude of nutrient, trace-metals and particle release from sediments in the estuary and from the sediment pools behind dams relative to other contributions will be quantified.

Methodology: 
Detailed field and laboratory geochemical and physical observations will be used to quantify the local nutrient fluxes and metals release rates. In the field, the geochemistry will be sampled concomitantly with observations of the fluid stresses, sediment suspension, and bed elevation at three locations within the Great Bay. The first site will be at the location of previous investigations; the second and third sites are upstream and downstream of a tidal dam. The laboratory investigations will allow for a more detailed correlation between fluid stresses and nutrient fluxes and trace metal release such that empirical inferences regarding the benthic exchanges may be drawn.
Rationale: 
Material accumulated in the benthic sediments is not necessarily permanently sequestered in the sediments, but can be re-released back into the water column and adversely impact water quality due to an increase in turbidity and the release of bioavailable species, especially dissolved nutrients and trace metals. A recent effort in nearby Boston Harbor suggests that resuspension events mobilize significantly more metals than any other source of metals and enhances the release of dissolved nutrients and metals to the water column. This study will provide regional and local resource managers with concrete information regarding the influence of benthic sediments on water quality.
Accomplishments: 
2012

Sediments a Significant Source of Nutrient Inputs into Great Bay

New Hampshire’s Great Bay Estuary is considered a nitrogen “hot spot” and much of the current local research has focused on determining where the nutrients are coming from. Researchers funded by NHSG conducted studies to determine if estuarine sediments are a significant source of the nutrients. In 2012, they found that the springtime loads of dissolved inorganic nitrogen and phosphate from sediments are 50-80% of the dissolved load from rivers running into Great Bay, while in the autumn the dissolved loads from sediments are greater than riverine inputs. These results provide a better understanding of nutrient sources and could impact management decisions regarding the amount of input from wastewater treatment plants and other sources in the future.

Method Developed to Collect Data During Storm Events
Researchers developed an instrumentation deployment method for equipment used to investigate fluid flow and sediment erosion in a water body. This deployment method allows the instruments to be securely mounted in the water column and allows researchers to collect data during storm events when it might not otherwise be possible to physically do so, thus providing more accurate and up-to-date information about changes in a water body during extreme weather conditions.

Trace Metals Regularly Eroded into Water Column of Great Bay
The fine-grained sediments in coastal areas close to population centers are often large stores of contaminants that have built up over centuries. These sediments do not permanently store contaminants however, and the quantity of trace metals that are released from sediments into the Great Bay was previously unknown. With funding provided by NHSG, researchers conducted erosion chamber experiments and physical field observations in 2012 to produce estimates of fluxes from the sediment in the bay. Based on their experiments, researchers determined that trace metal-enriched particles are eroded into the water column regularly — at least 20% of the time — thus providing an important method of trace metal transport in the bay. These results provide more accurate information for scientists to assess trace metal inputs into the estuary.
Ammonium Resuspension a Critical Aspect of Nutrient Load Calculation for Great Bay
Nutrients in Great Bay Estuary come from a variety of sources, but estimating the nutrients that are released due to particle resuspension in the water column can be challenging. In 2012, NHSG-funded researchers used erosion chamber experiments to determine how much ammonium in the estuary is released due to resuspension. Researchers found that the ammonium input due to resuspension in the fall is comparable to the ammonium input from rivers during that same time of year. This indicates that resuspension of ammonium is an important aspect of the nutrient load calculation for the estuary. These results help scientists and resource managers to better understand the controls on coastal water quality.

First Field Verification of Erosion Chamber Gives Confidence to Measurements of Sediment Erosion
The quantity of nutrients and trace metals that are released from sediments during resuspension events in the Great Bay Estuary has not been previously studied. In 2012, researchers combined two different datasets — in situ instrument measurements and erosion chamber experiments — to provide a better understanding of the mechanisms of sediment resuspension operating in coastal waters. This comparison resulted in the first field verification of any erosion chamber, thus giving confidence to geochemical measurements on the impacts of sediment erosion. Results from this research will enable scientists to use this technology for effective and accurate estimates of sediment resuspension in coastal waterways.

2011

 
Researchers Determine that Sediments are a Significant Source of Great Bay’s Nitrogen
New Hampshire’s Great Bay Estuary is considered a nitrogen “hot spot,” and much of the current local research has focused on determining where the nutrients are coming from. Researchers funded by NHSG conducted studies to determine the quantity of nutrients and trace metals that are released from the sediments of Great Bay. The porewater and erosion chamber data they collected suggest that sediments are a significant source of the nutrients — specifically, ammonium and silica — to the bay. These results have the potential to impact management decisions regarding the amount of nitrogen input from wastewater treatment plants and other sources in the future.
 
Using New Tool, Researchers Detect Great Bay’s Viscous Sublayer
NHSG-funded researchers deployed a new instrument for investigating fluid flow—the Vectrino II—in the Great Bay Estuary to help determine the quantity of nutrients and trace metals released from the sediments of the bay. This instrument detected the bay’s viscous sublayer, only the second time this layer has been detected in an aquatic system. The viscous sublayer is the layer of water just above the bottom that has very different properties from the overlying water because it so close to the bottom.   This layer is important to the fluid dynamics of a water body and because all solutes from the sediments need to diffuse through this layer to be introduced into the water column. Other researchers have expressed an interest in applying this novel technology to obtain direct measurements at their own field sites, so there is potential for more widespread use of this instrument in the future.

Publications

Available from the National Sea Grant Library (use NHU number to search) or NH Sea Grant

Journal Article

  • Wengrove, M. and D. Foster (2014). Field evidence of the viscous sublayer in a tidally forced developing boundary layer. Geophysical Research Letters 41(14):5084-5090, July 2014.
  • Wengrove, M., D. Foster, L. Kalnejais, V. Percuoco and T. Lippmann (2015). Field and laboratory observations of bed stress and associated sediment nutrient release in a tidal estuary. Estuarine, Coastal and Shelf Science 161:11-24, August 2015.
  • Percuoco, V., L. Kalnejais and L. Officer (2015). Nutrient release from the sediments of the Great Bay Estuary, N.H. USA. Estuarine, Coastal and Shelf Science 161:76-87, August 2015.

Thesis/Dissertation

  • Wengrove, M. (2012). Observations of a developing boundary layer in a tidally forced estuary. Master's Thesis, University of New Hampshire.
  • Percuoco, V. (2012). Geochemistry and release of contaminants from cohesive sediments of the Great Bay Estuary, New Hampshire. Master's Thesis, University of New Hampshire.

Reports