PALEOENVIRONMENTAL RECONSTRUCTION OF THE CHANGING WATER QUALITY OF THE TIDAL CHRISTINA RIVER, DELAWARE FROM 1600 YBP TO PRESENT

The Christina River watershed of Delaware has been impacted by rapid land-use changes since the start of European settlement in the 1600s. Initial deforestation for wood and farmland and later industrialization and urbanization sent sediments, nutrients, heavy metals, and other pollutants into the watershed. The variety of land uses and well-documented history in the region make the Christina River an excellent study site for identifying anthropogenic versus natural impacts on water quality during the last millennium.

Vibracores were collected from three locations in the marshes surrounding the river. Pb-210 and Cs-137 samples from the top 100 cm of the cores indicated that the sediment at 100 cm in Sites 1 and 2 is approximately 140±28 years old. Pollen analysis shows an increase in the amount of Ambrosia-group pollen relative to Quercus pollen to post-settlement levels between 140 and 150 cm for Site 1 and 130 and 150 cm at Sites 3. The ratio of Quercus to Ambrosia-group pollen is tied to historical records of land clearance starting when European settlers arrived in the mid-1600s, intensifying by the 1730s and peaking in the late-1700s. Two radiocarbon samples were collected from the core from Site 1 and three samples were collected from Sites 2 and 3. Samples were taken from 150 cm to the base of the core. The base of cores from Site 1 (226 cm), Site 2 (248 cm), and Site 3 (261 cm) date to 1320±30 years, 1640 ±75 years, and 1092.5±87 years, respectively. These dates suggest that sedimentation rates increased by an order of magnitude following settlement. Statistical analysis of several diatom species from all three sites indicates an increase in nutrients and variations in salinity over the timescale studied. The ratio of centrics to pennates generally increases following settlement, indicating greater productivity. Species richness does not change between pre- and post-settlement samples, but this may reflect an overall change from sensitive species to tolerant ones. Soil chemistry relating to phosphorous, nitrogen, carbon, and heavy metals also documents increases in nutrients and heavy metals and declining water quality.