GAS EXCHANGE IN A CONTAMINATED ESTUARY INFERRED FROM CHLOROFLUOROCARBONS

GEOPHYSICAL RESEARCH LETTERS, VOL. 19, NO. 11, PAGES 1!33-1136, JUNE 2, 1992 GAS EXCHANGE IN A CONTAMINATED ESTUARY INFERRED FROM CHLOROFLUOROCARBONS Jordan F. Clark, H. James Simpson, William M. Smethie, Jr., and Chris Toles Lamont-Doherty Geological Observatory and Department of Geological Sciences, Columbia University A new method of quantifying mean rates of gas The Hudson Estuary exchange for natural water systems using the distribution of pollutant chlorofluorocarbons (CFCs) is presented. Concentrations of dissolved CC13F (F-11), CC12F2 (F-12), and CC12FCC1F2 (F-113) in the Hudson estuary are The lower Hudson estuary (Figure 1), as defined here, extends from the seaward opening at the Narrows, northward to the upstream limit of saline water. Tidal stage changes occur for about 270 km upstream of the Narrows and the area- much as an order of magnitude. The most plausible source weighted mean water depth of the salt-intruded reach is about appears to be discharges from waste water treatment facilities. 9 m. During summer months, saline water typically extends Loss of these compounds from the estuary water to the more than 110 km upstream of the Narrows. supersaturated with respect to the remote troposphere by as atmosphere occurs both upstream and downstream of the zone of input. Using a multi-box model, gas exchange coefficients were calculated to be between 2 and 4 cm hr- 1 based on the observed concentrations. This rate of gas exchange is similar to values determined in lakes and coastal bays and substantially lower than the mean value for the open ocean. Cornwall I Kilometers Introduction Stony Point Estuary Exchange of dissolved gases across the air-water interface is an important physical process which influences many properties of natural waters. Water quality assessment often requires quantitative estimates of this parameter both for Rockland Lake 41 oo' LDGO the determination of reaeration rates [O'Conner and Dobbins, 1958] and loss of volatile pollutants [Bopp, 1983; Dyrssen et al., 1990]. Unfortunately, in many environments it is difficult to employ geochemical techniques which have been valuable in Manhattan estimating gas exchange rates for the ocean [e.g. 222Rn deficit, natural and bomb 14C methods; Peng et al., 1979; Broecker et al., 1985]. Estuaries, which often receive large quantities of municipal and industrial wastes, are such environments. A relatively new method in which a volatile tracer, sulfur hexafluoride (SF6), is purposefully added to lakes and streams has assisted in determining gas exchange rates in systems not suited for oceanographic methods [Wanninkhof et al., 1985 and 1990; Upstill-Goddard et al., 1990]. Here, we attempt to determine the spatially and temporally averaged gas exchange rate in the lower Hudson estuary using a method which is conceptually similar to the SF6 approach. Rather than monitoring a gas that has been Na•ows Fig. 1. Hudson River and estuary: sample locations are indicated by ,. LDGO indicates Lamont-Doheny Geological Observatory. The lower Hudson is a partially-mixed estuary in purposefully added to the system, we have estimated gas which the surface and bottom salinities generally differ by less of low freshwater discharge. Upper exchange rates from the distribution of chlorofluorocarbons than 5 %0 during periods (CFCs). The CFCs measured were CC13F (F-11), CC12F2 basin runoff usually reaches a maximum during early spring (F-12), and CC12FCC1F2 (F-113). As with SF6, these gases and a minimum during late summer. Extreme values of are synthetic compounds with no known natural source and monthly mean river discharge at the Battery (12 km upstream are typically 1200 m 3 s -1 and 150 m 3 s -1. are chemically very stable in solution. CFCs have a variety of of the Narrows) (>600 m 3 s-l), freshwater replacement industrial uses and thus are likely to be found in waste water. During high discharge Common uses are refrigeration (F-12), foam blowing (F-11 times are less than 15 days while during low flow conditions (150 - 250 m 3 s -1) they are between 45 and 60 days [Clark et and F-12), solvents (F-11 and F-113), and washing of semiconductors (F- 113). al., 1992]. Copyright 1992 by the American Geophysical Union. discharged into the lower Hudson estuary [Dajardiam et al., 1991] of which more than 90% presently pass through secondary treatment during dry weather conditions. Although treatment facilities are scattered along the entire tidal reach, Approximately 107 m 3 of waste water per day is Paper number 92GL00558 GL-00558503.00