Historical Reconstruction of Chemical Loadings and Ecosystem Effects in Delaware Bay Tidal Marshes

Introduction

Loadings of nutrients and contaminants have resulted in substantial impacts to estuarine systems such as eutrophication and high levels of lipophilic chemicals in finfish. Since the passage of the Clean Water Act and other legislation, loadings have been reduced and ecological health often improved. However, long-term monitoring is lacking and in many systems and ecological responses are not well quantified.

Because of their ability to adsorb trace metals and organic contaminants, fine-grained sediments trapped in tidal marshes represent a major repository for contaminants and ecosystem change. Sediment cores from these marshes provide an excellent means for documenting long-term (e.g., decadal scales) changes in land use, nutrient and contaminant loadings, and related ecological changes. Importantly, sediment chronologies derived from sediment cores help provide an understanding of whether or not source reduction programs (e.g., wastewater treatment, waste site remediation projects, non-point source controls) are successful, and under what time scale a river/estuarine-wide response can be detected. The response time is an important parameter in many water quality models (i.e., TMDL) when source reduction and load allocations will be determined.

Sediment cores were obtained from freshwater tidal and estuarine marshes in the Delaware Estuary to estimate historic loadings of chemical contaminants, nutrients and their potential ecosystem impacts. Chronologies were determined with ²¹⁰Pb and ¹³⁷Cs isotopes and sediment pollution records extending back through much of the past century. This work is a collaboration of the College of Marine and Earth Studies, University of Delaware, and the Patrick Center for Environmental Research of The Academy of Natural Sciences.

The objectives of this study are to address these questions:

• Have nutrient and contaminant pollution controls been successful in the tidal Delaware River?
• Have there been ecosystem level responses?
• How will estuarine marshes respond to potential increases in sea level rise over time?

Methods

Sediment cores were obtained from freshwater tidal and estuarine marshes in the Delaware Estuary to estimate historic loadings of chemical contaminants, nutrients, and their potential ecosystem impacts. Chronologies were determined with ²¹⁰Pb and ¹³⁷Cs isotopes and sediment pollution records extending back through much of the past century.

Study sites were selected to represent mid-marsh areas spanning a range of salinities. Three cores were taken at each site.

Push-piston cores of approximately 1 to 1.5 m in length were retrieved by a tripod/pulley system. The coring system is designed to minimize the shortening and compaction of the sediment column. The cores were sectioned into specific intervals (e.g., 1 or 2 cm) depending on the length of core and visual inspection, and stored at -20°C or 4°C in the appropriate container (e.g., pre-cleaned ICHEM III jars or whirl-pak bags).

Sediment cores are being analyzed for:

• Activities of ²¹⁰Pb and ¹³⁷Cs
• Organic C, total N, total P (and forms) and biogenic Si
• PCBs (congener specific ca. 110 congeners)
• PAHs (ca. 39 parent cmpds)
• PBDEs (ca. 38 congeners)
• OC Pest (ca. 17 cmpds)
• Diatom species composition
• Stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N)
• Grain size (< 63 mm; clay+silt)

Results

Preliminary results show some interesting trends for many parameters. Sediment accumulation rates, based on ¹³⁷Cs only at this time, are generally similar across marshes in the lower portion of the estuary. Uncorrected rates range from 0.56–0.83 cm/yr (n =12). However, samples from the upper estuary have slightly higher rates (> 1 cm/yr).

Organic carbon concentrations ranged from 1.2–14%, and increased slightly towards the surface in most cores, while total nitrogen ranged from 0.2–0.8% and exhibited a larger increase towards the surface. The carbon to nitrogen ratio, an indicator of source material and biogeochemical processes, varied among cores but was similar or decreased towards the surface within each core.

The nitrogen isotopic composition of the sediments (δ¹⁵N), an indicator of the source(s) of nitrogen, increased towards the surface from approximately 2‰ to as high as 8‰ (in Churchmans Marsh). The increase started around the early 1960s and may indicate a change in the processing of N either in the estuary or in wastewater treatment facilities (e.g., secondary treatment and nitrification).

Total PCBs in all cores exhibited peak concentrations in the 1960s and 1970s. Concentrations were lowest in the southern portion of the Bay increasing upstream. Interestingly, the three cores from the tidal Christina and Brandywine Rivers show two different sources over time, and is reflected in the sediments from Dravo Marsh situated between the two sites. PCB congener 209 is observed in all cores and follows a similar trend as total PCBs. There is a significant relationship between the PCB209 and total PCBs (p <0.001; r² = 0.883).

Discussion

Sediment Geochronology

Upon finalizing the ²¹⁰Pb and ¹³⁷Cs measurements, linear accretion rates (length/time) and mass accumulation rates (mass/area/time) will be determined for each coring site. Accretion rates will be computed using down-core porosity data to account for compaction of the sediment column over time. This correction is necessary when converting sediment depths to age dates, and when relating marsh accretion rates to rates of local sea-level rise. Separate mass accumulation rates for organic and inorganic sediment fractions will be computed using bulk density and loss-on-ignition data. These rates will be used to assess natural and anthropogenic influences on marsh sedimentation, and to refine the regional sediment budget for the estuary.

Contaminant Chemistry and Ecosystem Tracers

Contaminant chemistry indicates that source load reductions over the last 30 to 40 years have reduced contaminants in the sediments throughout the estuary. PCB congener patterns from the lower bay to upper bay will be used to assess atmospheric versus runoff sources and its impact within each zone. Ecosystem tracers such as diatom assemblages, stable isotopes, nutrient relationships, biogenic Si, and others are currently being analyzed and, with the sediment geochronology information, will be used to generate a system-wide assessment of historical changes in the estuary and how they may change in the future.

Download the EPA Star Final Report:
Linking Impacts of Climate Change to Carbon and Phosphorus Dynamics Along a Salinity Gradient in Tidal Marshes

Acknowledgments

This work is a collaboration of the College of Marine and Earth Studies, University of Delaware (UD), and the Pactrick Center for Environmental Research (PCER) at The Academy of Natural Sciences. The PIs are Christopher Sommerfield (UD), Don Charles (PCER), and David Velinsky (PCER).

Many thanks to Susanne Moskalski, Michael Schafer, Linda Zaoudeh, Rosemary Malfi, Paul Kiry, Carl Natter, Jeff Ashley, Roger Thomas, and various NSF Research Experience for Undergraduates (REU) students for help in the field and laboratory.

Funding for this study was obtained from the Delaware River Basin Commission (DRBC), Delaware Department of Natural Resources and Environmental Control (DNREC), National Science Foundation REU Program, and the Environmental Associates of The Academy of Natural Sciences.

Environmental Research