Chemicals in the Environment - American Chemical Society

Chapter 13

Atmospheric and Fluvial Sources of Trace Elements to the Delaware Inland Bays

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0806.ch013

Thomas M. Church, Joseph R. Scudlark, and Kathryn M. Conko Graduate College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958

The primary objective of this study was to compare the fluvial versus atmospheric importance of trace element fluxes to the Delaware Inland Bays. The dissolved fluvial loading of selected trace metals was determined based on seasonal sampling (1992-95) of the major streams. Atmospheric wet metal deposition was gauged based on long-term (1982-present) continuous collections at a nearby site. Atmospheric dry deposition was modeled based on measured aerosol concentrations and assumed bi-model deposition velocities. Atmospheric dry flux appears to be more significant for crustal elements (Al, Fe, and Mn), while wet flux is more important for non-crustal trace elements (Cd, Cr, Cu, Pb and Zn). Overall, atmospheric deposition provides at least 5% (Mn) to 30% (Zn) of the total loading to the Inland Bays, with direct flux to the surface waters comprising about half of the total aeolian input. The Inland Bays watershed serves as a reservoir for atmospherically deposited Al, Cr, Cu and Zn. A n examination of basin yields (kg/m ) for the major tributaries reveals the accelerated weathering of uniquely bog iron deposits in two marshy tributaries, while another exhibits exceptionally large export of anthropogenic metals Cd, Cr, Cu and Pb. 2

© 2002 American Chemical Society Lipnick et al.; Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction Within the past two decades, atmospheric deposition is increasingly recognized as a biogeochemically important and quantitatively significant non-point source of sulfur and nitrogen oxides to surface waters (7,2,5). Many trace metals are coemitted from similar high temperature emission processes. Thus atmospheric mobilization and deposition of other contaminants in acid precipitation may have equally important ecological implications. For example, the atmospheric input of trace elements to mid-Atlantic estuaries and shelf waters has been shown to rival those from other sources (4,5). Largely as the result of such studies, the 1990 Clean A i r Act Amendments contain specific provisions (Section 112m, the so-called "Great Waters" section) which legislatively mandate an examination of atmospheric contaminant deposition to coastal and inland waters. This includes smaller east coast estuaries such as Delaware's Inland Bays (Rehoboth and Indian River) since their combined output to local shelf waters can be equivalent. With this in mind, the primary objective of this study was to accurately assess the relative atmospheric and fluvial loading of trace elements to the Inland Bays.

Study Area The Delaware Inland Bays are comprised of three interconnected estuaries: Rehoboth Bay, Indian River Bay and Little Assawoman Bay, located on the midAtlantic coast of Delaware (Figure 1 ). Although dwarfed in size by large east coast estuaries such as the Chesapeake and Delaware Bays, the Delaware Inland Bays are more typical of the numerous small, shallow, poorly-flushed systems that are found along the Atlantic and Gulf coasts. For the purposes of this study we have focused on the two primary bays, Rehoboth and Indian River. Freshwater inputs are derived from a number of lateral sources (total discharge of 8.6 m /s), 80% of which originates from groundwater. Overall, the Inland Bays encompass a surface area of 8.81 χ 10 m , and drain a watershed of 6.81 χ 10 m . Thus they possess a large watershed: open water area ratio (8:1) typical of coastal plain estuaries. Land use is primarily agricultural (40%) and forest (38%), and the underlying geology is predominantly quartz sand and silts as part of the Omar coastal plain formation. Seasonally, urban areas comprise only 10% of the watershed, but have a major impact on water quality. The bays are highly eutrophic due to nutrient loading resulting from the transient summer population density, and surrounding agricultural activities. The coal-fired Indian River power plant impacts both the atmospheric as well as fluvial loading 3

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Lipnick et al.; Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Figure 7. Map of the Inland Bays, showing the stream sampling locations and atmospheric collection site

1. Love Creek 2. Herring Creek 3. Hopkins Prong 4. Guinea Creek 5. Swan Creek 6. Millsboro Pond 7. Iron Branch 8. Whartons Branch 9. Pepper Creek 10. Vines Creek 11. Blackwater Creek 12. White Creek

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246 to the bays in the form of fugative blowoff and runoff from the surrounding storage piles of fly ash.

Methods Atmospheric Wet and D r y Deposition Precipitation was sampled at a long-term atmospheric research site near Lewes, D E in a remote area of the 4000 acre Cape Henlopen State Park (Figure 1). Major ions ( H , N a , K , C a , M g , CI", S 0 ' , and N 0 ) have been sampled on an event/daily basis continually since 1977. The trace elements (Al, As, Ba, Cd, Cu, Cr, Fe, Mn, N i , Pb, Se, Sb, and Zn) have been sampled in parallel since 1982, using rigorous "ultra-clean" trace metal sampling and handling techniques. The precipitation trace elements were all analyzed by graphite furnace atomic absorption spectrophotometry (GFAAS), as verified using EPA standard reference materials as described elsewhere (6,7). Precipitation amount was determined using a continuously-recording Belfort (gravimetric) rain gauge. Based on a comparison of predicted (rain gauge) with collected precipitation depths, the overall collection efficiency was >95%. There does not exist a widely-accepted method for the routine determination of dry deposition by direct means. Thus, this study adopted the traditional approach of estimating dry flux based on aerosol concentrations measured at Lewes times modeled dry deposition velocities. Aerosol data collected at the Lewes site during independent studies was utilized for this purpose(#,9). To derive dry flux rates, the average aerosol concentrations were first apportioned into crustal and non-crustal components, based on published soil compositions (10), and using A l as a crustal normalizer. Deposition velocities representative of coarse and accumulation mode particles were then applied to the crustal and non-crustal fractions (0.5 and 0.1 cm/s, respectively) (77). +

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Stream Water The major streams discharging to the Rehoboth and Indian River Bays (based on freshwater discharge) were sampled quarterly, under variable flow conditions. Sampling was conducted during all four seasons during each of the three years, starting 4/92 and ending 3/95. During the first year of this study, the 12 streams, indicated in Figure 1 were sampled. Because our preliminary results indicated that the trace metal loading is dominated (>90%) by seven of these streams, during

Lipnick et al.; Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


247 Years 2 and 3 our sampling concentrated on just those systems. In addition, to ensure that our sampling location for Swan Creek was not tidally influenced, during Years 2 and 3 it was moved a short distance inland, above an impoundment. However, this did not appear to significantly alter the observed high metal concentrations or loading fluxes. The sampling sites were chosen to be above tidal influence and wellmixed, at locations which facilitated the measurements for stream discharge. This was usually a straight stream segment with a uniform cross-section, such as a culvert, which was surveyed and marked for future reference. A t Millsboro Pond, sampling was conducted near the USGS gauging station. Grab samples of surface stream water were manually obtained in acidwashed 2 liter L D P E bottles and stored refrigerated until processed (within 4 hours of collection). In the laboratory, the samples were filtered through a 0.45 pm Gelman Mini-Capsule® filter, using a peristaltic pump and employing trace metal clean techniques. Aliquots for trace metal analyzes were acidified to 0.4% (v/v) using ultra-pure (doubly quartz re-distilled) HC1, and stored frozen. Analyzes for "dissolved" (

Chemicals in the Environment - American Chemical Society

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