Stable lead isotopes, contaminant metals and radionuclides in upper Hudson River sediment cores: implications for improved time stratigraphy and transport processes

Abstract

Radionuclide, stable lead isotope and trace metal analyses on fine-grained sediment cores collected along a 24-mile reach of the upper Hudson River were used to establish temporal trends of contaminant loadings, to establish stable lead isotopes as an additional stratigraphic tool, and as tracers for resolving particle transport fluxes over periods of decades. Very large contaminant inputs of Cd, Sb, Pb and Cr were evident in the sediment record. One potential large source for these metals was from a pigment manufacturing facility in Glens Falls, NY. The total range in stable lead isotope ratios observed in well-dated cores from about 15 miles downstream of the potential metal inputs was large (e.g., maximum difference in ²⁰⁶Pb/²⁰⁷Pb is 10%) and characterized by four major shifts occurring in the 1950s, 1960s, 1970s and 1980s. The temporal trend in ²⁰⁶Pb/²⁰⁷Pb has been used to establish precise dating of a sediment core from 24 miles further downstream. The large magnitude and abrupt shifts in stable lead isotope ratios preserved in upper Hudson sediment cores provide a way to significantly improve dating models, based only on radionuclide analyses. Cadmium, lead and antimony were identified as quite sensitive tracers of upper Hudson sediments due to the magnitude of contamination and the lack of significant additional downstream sources of these contaminant metals. Metal measurements in a pair of sediment cores located 24 miles apart were used to constrain relative fluxes of sediment entering the river between the two coring locations, with sediment sections deposited between the early 1960s and the late 1970s in these two cores suggesting that 3–4 times more sediment entered the river between the two coring sites than was transported from upstream. These dilution factors agree very well with estimates based on suspended sediment measurements during a flood event in April 1994 and with estimates based on mechanistic model of suspended sediment transport between 1977 and 1992.

Introduction

Contamination of sediments and fish with polychlorinated biphenyls (PCBs) along more than 200 miles of the Hudson River has greatly influenced public policies of this river/estuary system since the 1970s. Large-scale dredging of some of the most contaminated sediments is one option under consideration for sites that are relatively near the original PCB discharges from two capacitor manufacturing facilities. In addition to PCBs, upper Hudson River sediments are highly contaminated with several metals. This complex mixture of contaminants could possibly limit disposal/reclamation options, as well as influence legal responsibility issues. It is important to characterize the distribution and transport processes related to the entire suite of contaminants in Hudson River sediments in order to develop sound management plans for the system.

In this paper we have three major objectives:

• document temporal trends in metal levels and stable-lead isotope composition in fine-grained sediment cores along a 24-mile stretch of the upper Hudson River;

• develop the use of stable lead isotopes as an additional stratigraphic tool in upper Hudson River sediments which is a new application in this region; and

• develop a simple empirical budget model for calculating relative amounts of fine-grained sediment fluxes that have entered the Hudson between coring locations, based upon observed metal concentrations in paired sediment cores which have been dated using combined radionuclide plus stable lead isotope time stratigraphies.

Whereas all three objectives are specifically related to contaminant management issues in the upper Hudson River, the third objective provides a methodological approach to estimate relative sediment fluxes which could potentially be used in other river systems that have received large discrete influxes of particle-reactive contaminants. Fine-grained sediment fluxes can often be difficult to assess directly since data have not been commonly collected for many downstream sediment sources.

Lead ore bodies have varying Pb isotope ratios that reflect the U/Pb and Th/Pb ratios and ages of their Pb sources, as well as the ore formation ages. Very old ores, such as those from Broken Hill, Australia (ca. 2×10⁹ years) contain small amounts of radiogenic Pb isotopes (Gulson et al., 1985), while younger ores derived from high U/Pb sources, such as those mined in several regions of Missouri (ca. 2.5×10⁸ years) have much higher proportions of ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb relative to ²⁰⁴Pb. This range in stable Pb isotopic composition has been exploited in numerous studies of sources of lead to the environment, from urban centers to relatively pristine settings (Chow and Johnstone, 1965, Simpson and Catanzaro, 1978, Elbaz-Poulichet et al., 1986, Sturges and Barrie, 1987, Maring et al., 1987, Hamelin et al., 1989, Keinonen, 1992, Mukai et al., 1993, Farmer et al., 1996, Rosman et al., 1997, Weiss et al., 1999, among others). Lead isotopes have also provided sensitive tracers of Pb sources in blood Manton, 1977, Rabinowitz, 1987, Rabinowitz, 1995, sediment Shirahata and Wong, 1981, Hirao, 1986, soils Rabinowitz, 1989, Rabinowitz, 1995 water (Stukas and Wong, 1980), and paint Rabinowitz, 1987, Rabinowitz, 1995.

Although environmental studies have primarily reported ²⁰⁶Pb/²⁰⁷Pb ratios, additional information can be obtained from other Pb isotope ratios. ²⁰⁸Pb derives from the decay of ²³²Th, whereas ²⁰⁶Pb and ²⁰⁷Pb derive from decay of ²³⁸U and ²³⁵U, respectively, so that variations in U/Th ratios of Pb sources to ores, as well as their formation age, influence the ratio of ²⁰⁸Pb to the other Pb isotopes. North American ores have lead isotope ratios which differ by as much as 6–25%, depending on the isotope ratio Doe and Delavaux, 1972, Heyl et al., 1974, Doe, 1975, Sverjensky et al., 1979a, Sverjensky et al., 1979b, Sverjensky, 1981, Hart et al., 1981, Fletcher and Farquhar, 1982a, Fletcher and Farquhar, 1982b, Deloule et al., 1986. Since these ratios can be measured with a precision (2σ) of ≤0.05% per atomic mass unit (amu) by mass spectrometry, stable lead isotopes can be especially sensitive tracers. For example, the ratio of maximum potential signal to measurement precision is between ca. 500 for ²⁰⁶Pb/²⁰⁷Pb and 60 for ²⁰⁷Pb/²⁰⁴Pb.

In addition to permitting deconvolution of multiple sources of environmental lead, stable Pb isotope ratios can potentially provide sensitive time stratigraphic information. Simpson and Catanzaro (1978) reported predictions made by Claire Patterson that ²⁰⁶Pb/²⁰⁷Pb isotope ratios measured in environmental samples from the USA would increase from around 1.15 in the 1950s and 1960s to 1.35 in the 1970s, due to shifts in sources of lead ore. By the 1970s, less ore was extracted from mines in the northwest (low ²⁰⁶Pb/²⁰⁷Pb) and more ore from mines located in the Midwest (high ²⁰⁶Pb/²⁰⁷Pb). Observed temporal trends in environmental samples reflect an upward shift in ²⁰⁶Pb/²⁰⁷Pb during the 1960s to 1970s although not as large as that predicted by Patterson Rosman et al., 1994, Marcontionio et al., 2002. Hurst et al. (1996) exploited this temporal trend in Pb isotopes to establish the timing of leaks of leaded gasoline from underground tanks and to estimate the age of depth sections of sediment cores. Marcontionio et al. (2000) observed temporal trends in ²⁰⁶Pb/²⁰⁷Pb ratios in Chesapeake Bay sediments that were in excellent agreement with those observed in Bermuda corals (Shen and Boyle, 1987).

Here, we report detailed chronologies of radionuclides, metals, and stable lead isotope ratios derived from fine-grained sediment cores collected from along a 24-mile reach of the upper Hudson River.

Approximate dates of particle accumulation were assigned to individual depth sections of sediment cores based on depth distributions of ¹³⁷Cs and ⁷Be. Cesium-137 from global fallout produced measurable activities in sediments around 1954 and reached a maximum in 1963 (Ritchie and MacHenry, 1990). Measuring detectable activities of ⁷Be in the upper layers of a sediment core provides another constraint on the most recent history of particle accumulation at a particular site. Beryllium-7 is a particle-reactive cosmogenic radionuclide, produced in the atmosphere, that has a radioactive half-life of 53 days; consequently, the presence of ⁷Be at measurable levels in a core section indicates that a portion of that sediment either accumulated within 6 months to a year prior to collection of the core or was physically or biologically mixed into the section. As an initial approximation, a constant rate of particle-mass deposition was assumed between pairs of radionuclide time stratigraphic indicators (earliest presence of ¹³⁷Cs, maximum activity of ¹³⁷Cs, and measurable ⁷Be in core top section). First-order rates of accumulation can thus be assigned by simply multiplying the mean sedimentation rate by the sample depth. Accumulation rates can be independently estimated using each pair of these three sediment horizon layers, providing a first-order indication of variability of sedimentation rates throughout a core.

Uncertainties sometimes associated with the above approach to assigning ages to sediment layers include: coring artifacts, diffusion of dissolved ¹³⁷Cs in pore waters, or bioturbation (see Crusius and Anderson, 1991 and references therein). These uncertainties generally appear to be minor in areas of the Hudson River with relatively rapid sedimentation rates as supported by the following observations. Beryllium-7 has been observed in a substantial fraction of our core tops without measurable activity in sections below, indicating recent particle accumulation and lack of sediment mixing. Diffusion of ¹³⁷Cs in upper Hudson sediment samples is probably very limited since Hudson sediments have relatively high clay content and low porosities compared to values typical of many lakes and fresh water is only present. The average weight percent solids in surficial Hudson sediments is about 40–50% compared to values typical of lake sediments of 2–10%. Compaction of the upper layers of sediments relative to deeper layers during coring is also much less important than for most lakes because Hudson sediments have lower porosities. Effects of bioturbation on estimates of mean sediment accumulation rates appear to be minimal for Hudson River sites with sedimentation rates ≥1 cm/year (Olsen et al., 1981a).

Cesium-137 has previously been used to estimate sedimentation rates for many cores from the Hudson River and estuary Simpson et al., 1976, Bopp, 1979, Bopp et al., 1981, Bopp et al., 1982, Bopp et al., 1993, Olsen et al., 1981b, Bush et al., 1987, Bopp and Simpson, 1989. ²¹⁰Pb has not been used very frequently in river systems due to relatively low excess ²¹⁰Pb activities in most riverine sediments (Beasley et al., 1986, Bush et al., 1987; W. Schell, personal communication, 1994).