The Wet Deposition of Trace Elements on Delmarva and Their Utility as Emission Source Indicators

Program Description

Trace element composition of precipitation was used to examine sources of atmospheric emissions that contribute to the composition of wet deposition on Delmarva. Atmospheric emissions of trace elements and contaminants such as nitrogen and semivolatile compounds are co-emitted with acid components as a result of fossil fuel combustion may provide significant inputs to aquatic systems. In order to create reasonable and effective control strategies, this study was undertaken to evaluate the relative contribution of power production emissions and their geographical origin.

This study used a combination of three types of models (meteorological, chemical and statistical) to evaluate a combination of major ion/trace element precipitation database. Both aerosol-based and precipitation-based receptor models were used to examine the use of trace elements to identify the emissions sources of acidic components and trace element in wet deposition in the Mid-Atlantic region; particular emphasis was placed on identifying the role of emissions originating from electric power generating stations. This study was implemented in two phases: review of long-term trace metal data collected at Lewes, Delaware and direct comparison of synoptically collected aerosol and precipitation samples collected at two locations- Lewes, Delaware and Beltsville Maryland. Three complementary receptor-based approaches were applied to the long-term and intensive data: factor analysis, chemical mass balance, and cluster analysis based on air mass back-trajectories.

Precipitation/Air Sampling and Analysis

The long-term phase of the study involved analysis of archived precipitation event data collected between 1986 and 1989 for trace metal analytes. The intensive phase of the study focused on event basis (i.e., associated with the passage of a major meteorological frontal system). Intensive sampling at the Lewes site was conducted in 1989 in two modes: sampling of aerosols continuously over a six hour period, with no regard for precipitation, based on anticipated diel cycles; and aerosol sampling coordinated with anticipated precipitation events. The Beltsville intensive was conducted in 1990 in a similar manner as the second mode at Lewes. However, the aerosol sampling was more closely coupled with the precipitation collection using an automatic switching device that allowed better coordination than was possible based on the manual method employed at Lewes.

The Lewes, Delaware sampling site is located on the 4,000 acre Cape Henlopen State Park at the southern junction of Delaware Bay and the Atlantic Ocean (38° 46'N by 75° 02'W, elevation less than 10m). The sampling site is located in an isolated area of the Park where there is no camping or vehicular traffic within 0.5 km. The Atlantic Ocean is located about 0.8 km to the east and the Delaware Bay is about 2 km to the northeast. The landuse in southern Delaware is primarily rural-agriculture with no major industries near by. Washington, D.C./Baltimore metropolitan area is approximately 150 km to the west-northwest and there are two fossil fuel fired power plants in the vicinity: Indian River (coal-fired) 25 km southwest of the site and Vienna (oil-fired) 60 km southwest of the site. In addition, there is considerable year-around ship traffic in the nearby coastal waters. Except for the marine component, precipitation at this site is typical of the northeastern United States.

The Beltsville, Maryland sampling station is located on the grounds of the United States Department of Agriculture Experiment Station. Sampling was carried out in a large, flat grassy clearing on the site of an abandoned airstrip (39° 1'N by 76° 49'W, elevation 46m). This site has been employed since 1988 as one of the EPA/National Dry Deposition Network sites. Landuse in the vicinity of the site is primarily suburban/rural-agriculture; the Patauxent Federal Wildlife Area provides a large buffer zone. Even though the Beltsville sampling location is located in close proximity to the Washington, D.C. metropolitan area, it is relatively isolated with no major emissions sources in the immediate vicinity.

Precipitation sampling for trace elements, including the long-term samples, was performed according to rigorous "trace metal clean" sampling and handling procedures. Collections utilized a commercially available, automated wet-only collector (Model 301, Aerochem Metrics, Inc. Bushnell, Fla), factory modified to include a polycarbonate lid and Teflon-coated lid support arms. Collection efficiency, determined by comparison with the predicted collected volume based on Belfort rain gauge data, was >95%. Precipitation samples for major ion analysis were collected in accordance with standard MAP3S Network procedures using a similar collector. Samples were tested for field pH and cooled for storage. Neither the trace element samples, nor the ion samples were filtered prior to analysis. Bulk (wet+dry) deposition samples were collected using an acid-washed polyethylene funnel/bottle sampler continuously exposed to the atmosphere. The funnel was approximately 25 cm in diameter and possessed extended vertical sides (32 cm) intended to limit the aerodynamic effects on dry deposition sampling efficiency. The funnel opening was located about 2 meters above the surface of the ground, approximately the same elevation as the wet-only collector. Bulk deposition was collected only from June 1985 to October 1986.

Aerosol sampling employed filter heads consisting of five stacked 110 cm filter cassettes, separated by silicone rubber gaskets and mounted in series in an air-tight housing. The first upstream cassette contained a 90 mm Fluoropore (PTFE Millipore type FA) particulate filter with a 1 micron pore size. Downstream of the particulate filter were placed three KOH impregnated Whatman-41 filters to trap vapor-phase species (e.g., SO₂). A rain shield was employed around the filter head to allow for continual sampling during precipitation. In the field, two parallel filter systems (filter cassette, high-volume pump, gas meter and HEPA filter) were deployed. The filter cassette was connected to the inlet of the pump by means of non-contaminated polyethylene tubing. The pumps employed were Gast rotary vane, oil-less pumps driven by a ½ HP 1725 RPM General Electric motor. Sample air volumes were measured using a temperature compensated dry gas meter. Sampling was conducted at a flow rate of 150 l/min that was nearly constant due to the relatively short sampling times. Exhaust from the pumps was vented through a HEPA filter to remove impurities from the pump.

Analysis of precipitation samples was performed using GFAA methods. Arsenic and selenium were quantified using non-traditional methods. These analytes were measured using a method that involved hydride generation, cryogenic trapping, and selective volatilization. Arsenic detection was accomplished using gas-chromatograph-photoionization, while selenium used an air/hydrogen quartz burner with an atomic absorption detection. Major ion analysis was performed in accordance with MAP3S protocols.

Aerosol analysis was performed using ion chromatography for the major ions. Trace elements were analyzed by irradiating a portion of the Fluoropore filter and counted at the University of Maryland using ORTEC and Canberra high-purity germanium gamma-ray detectors. Sodium and sulfur were analyzed using ICP-AE spectroscopy.

Quality Assurance

To establish and maintain data accuracy, the QA program for this study encompassed all components, from site selection to final data validation. Essential elements of this program included:

• Conducting and evaluating operational blanks on a regular basis;
• Use of externally-certified analytical reference solutions;
• Intra-laboratory analytical comparisons using redundant methods (GFAA and ICP-AE); and
• Inter-laboratory analytical comparisons.

Three types of blank comparisons were employed to reliably quantify background contamination from materials and methods. Process blanks were bottles that were filled with ultra-pure water and preserved using HCl. These bottles traveled from the laboratory to the field with sample bottles and back to the lab. They were utilized to detect contamination from the water, acid and leaching from bottleware. Laboratory blanks were obtained by pouring ultra-pure water through a clean collection bucket, preserved and analyzed in the same manner as the precipitation samples; this blank was intended to measure similar contamination as the process blank and metal contamination from the sample bucket. Field blanks were collected by pouring ultra-pure water into collectors in the field that had not obtained a sample as a result of dry conditions. This blank measures the impact of fugitive dust during deployment, transport, and recovery. Field blanks were collected at least quarterly basis during the long-term study. And, during the intensive phase, two sets of blanks were collected.

Results

Based on average annual wet deposition on the Delmarva Peninsula, the contribution of acid species and trace elements from the southeastern United States source region are comparable to that provided by the traditionally implicated midwestern United States source region. Dry deposition appears to be the major mode of atmospheric flux of crustally derived elements (e.g., Al and Fe), while for elements of anthropogenic origin, wet and dry flux are similar in magnitude. Principal components analysis indicates that four dominant factors influence the precipitation chemistry at these sites: sea-salt, soil, incineration and or smelting and an acid component. An annual pro-rated Chemical Mass Balance profile indicates that on a mass basis, the relative source strengths contributing to the Lewes precipitation are: secondary (regional) sulfate>sea salt> lime kiln, soil>motor vehicle, coal combustion>incinerators>steel manufacturing. Based on synoptic sampling, receptor modeling (chemical mass balance and air mass trajectory analysis) indicates that precipitation and aerosols do not reflect scavenging of the same air masses. Precipitation reflects a greater contribution of long-range transport of more distant upwind sources, while the ground-level aerosol composition is more indicative of local sources.

Related Publications/Reports

• The Wet Deposition of Trace Elements on Delmarva and Their Utility as Emission Source Indicators. J. Scudlark, T. Church and K., Conko, 1994. CBRM-AD-94-3
• Estimation of Annual Trace Element Deposition to the Chesapeake Bay Watershed. P. Miller. 1995

Go to the Delmarva Trace Element Study Data Page

For more information, e-mail Dr. John Sherwell at the PPRP, or call him at 410-260-8660