Deposition of Trace Elements Through an Undeveloped, Forested Maryland Watershed

Program Description

The Bear Branch watershed study, funded by the Maryland DNR, was conducted by the University of Delaware and the United States Geological Survey (USGS) from 1993 to 1994. The Bear Branch watershed is a 98-hectare, undeveloped, predominantly forested watershed located within Cunningham Falls State Park in north-central Maryland (39º37'15"N, 77 º26'24"W). This watershed was selected for study mainly because its hydrology and geochemistry been well-characterized in previous studies and it is representative of the dominant land use within the Chesapeake Bay watershed. The primary objective of the study was to evaluate the concentrations of atmospherically derived trace elements (Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Se, and Zn) in the Bear Branch watershed. This was conducted using three different types of samples: wet-only precipitation, canopy throughfall, and stream water. From 18 August 1993 to 13 December 1994, 64 wet-only precipitation, 47 sets of throughfall, and 38 stream grab samples were collected and analyzed.

Sample Collection and Analysis

Wet-only Precipitation

Wet-only precipitation was sampled on a weekly-integrated basis (Tuesday to Tuesday) from samplers located on a platform on top of a 7.6-meter high storage tank near the peak of Catoctin Mountain. Samples were collected using an AeroChem Metrics Model 301 precipitation collector, and were analyzed for major ions as well as trace elements.

Canopy Throughfall

Canopy throughfall was also sampled on a weekly-integrated basis, using funnels attached to a 4-liter sample bottle with watertight couplers, suspended about 1 meter from the forest floor. All materials were made from low-density polyethylene (LDPE). When samples were retrieved from each of the three sampling locations, debris was removed from the samplers with acid-washed forceps and the funnel was rinsed with 0.4% HCl so that any particulates attached to the funnel would be included in the sample. The bottle was then detached from the funnel, capped, sealed in two clean plastic bags, and sent to the laboratory for analysis.

Stream Water

A staff gauge was placed in a naturally occurring pool in the stream near the outlet of the watershed and was connected to a Fisher Porter analog-digital recorder. This device was used to record the stream stage on paper tape every 15 minutes. The tape was replaced every 6 weeks. Stream discharge was also measured. The stream stage and discharge data were used to create log-log rating curves. Grab samples were also collected to bi-weekly during the first four months of the project and weekly thereafter. Samples were taken from a point of maximum flow directly upstream from the gauging station. To analyze potential particulate content, stream substrate material was collected at six locations and analyzed in total as well as low (organic) and high (inorganic) specific-gravity fractions.

Sample analysis

All three sample types (wet-only precipitation, throughfall and stream water) were analyzed for Al, Cd, Cu, Cr, Fe, Mn, Ni, Pb, and Zn using graphite furnace atomic absorption spectrometry. As and Se were analyzed using more involved techniques (hydride generation, preconcentration by cryogenic trapping, and either gas chromatography-photoionization (As) or atomic spectroscopy (Se).

Quality Assurance (QA)

Because the purpose of the study involved measuring extremely low concentrations of trace elements, accurate data collection techniques were vital. All sample collection equipment (containers, bottles, filtering apparatus, etc.) was acid-washed. Sample collection, including equipment cleaning and sample filtering, was carried out on a clean bench, and all field personnel wore clean polyethylene gloves during the process. Field and laboratory blanks were used during each sampling event to assess potential contamination.

Related Publications/Reports

• Scudlark, J.R., K.C. Rice, K.M. Conko, O.P. Bricker and T.M. Church. 2005. Transmission of atmospherically derived trace elements through an undeveloped, forested Maryland watershed. Water, Air & Soil Pollution 63:53-79
• Thomas M. Church, Joseph R. Scudlark and Kathryn M. Conko and Owen P. Bricker and Karen C. Rice, 1999. Transmission of Atmosperically Deposited Trace Elements Through an Undeveloped, Forested Maryland Watershed. University of Delaware and U.S. Geological Survey, CBWP-MANTA-AD-98-2, NTIS No. PB99-103699.