Marsh sediments as records of sedimentation, eutrophication and metal pollution in the urban Delaware Estuary
Résumé
The tidal freshwater portion of the Delaware Estuary has historically been an area of intense urban and industrial activities including dredging, point discharges, petroleum refineries, and shipyards. Multiple cores were collected in fringing marshes across from the urban area and used here to investigate historical records of these associated activities. Two cores in the most industrialized portion exhibited regular and congruent geochronology (0.6 cm/year downstream and an average of 1.2 cm/year upstream) based on both natural 210-Pb and fallout 137-Cs radionuclides. Thus, these sedimentary records should reflect sedimentary pollution histories over much of the latter past century. Recorded in the freshwater marsh sediment upstream is a dramatic increase in total phosphorus (TP) starting in 1950-1960, and, as in the Delaware River water, tracks the introduction of P detergent use. Although this might include increased use of P fertilizers, there is a substantial decrease after removal of the P detergent source in the mid-1970s. Carbon stable isotopes (δ13C) track P changes after 1955. The heavier carbon isotope (13C) corresponds to higher levels of P in the sediments (and water), while the lighter carbon (12C) isotope in recent times corresponds to decreased use or discharge of P. In more recent times since the 1970s, there is a significant relationship (p < 0.05) between δ13C and sediment P, while before 1965 there is a significant but different relationship. As such, the lower δ13C of the sediment organic matter may record decreased growth/eutrophication when P loadings and concentrations are reduced. The N stable isotope record shows a marked increase in δ15N (ca. 3.5‰ to 7.5‰) starting in the early 1960s. This corresponds to a substantial increase in the concentration of dissolved nitrogen (mainly as nitrate) from population growth, fertilizer applications, or changes in the processing of wastewater leading to reduction in chemical oxygen demand. The industrial metals fall into at least two transient records:(1)The Ag, As, Co, Cd, Cr, and Co show 2- to 4-fold increases after 1950, with steady inventories over the past 20-30 years. This reflects periods of increased industrialization, followed by better sewage treatment, industrial stagnation, relict sources, or continued urban run-off from atmospheric deposition. Episodic remobilization of sedimentary inventories is not indicated in the regular geochronologies and lack of porosity change.(2)The Pb and Sn show similar, but a much larger (10-fold) increase after 1950. This reflects the use and then legislative controls of anthropogenic organo-metallic compounds. The lead transient peak corresponds to the use (and local manufacture) of tetra ethyl lead as a gasoline additive, but phased out starting 30 years ago. Likewise, industrial organo-tins include tri-butyl tin (TBT), anti-fouling additives used in ship bottom paints. The sustained modern levels of organo-tins indicate some continued relict inputs. The decreasing order of butyl-tin concentrations is mono- to di- to tri-butyl species, implicating degradation of TBT via trans-alkylation. This also suggests the order of product stability, rather than that of independent source loading. The conservative correlation of linear butyl tins (linear order mono→di→ tri-) with both total and butyl-tins, also strongly implicates TBT as the unique input. Although TBT use was banned about 20 years ago, there appear to be continued sources and diagenesis of buried TBT with mobilization of butyl-tin products into the tidal Delaware Estuary. © 2006.