Factors influencing atmospheric deposition,stream export,and landscape accumulation of trace metals in forested watersheds

Wet and dry deposition inputs and streamflow output of Cd, Mn, Pb, Zn, and Al were measured intermittently at four deciduous forested watersheds in the southeastern United States between 1976 and 1982. Atmospheric inputs to each site were similar, varying by factors of 1.1 to 2.2 for the different metals. Metal levels in precipitation indicate that these sites are representative of rural, continental areas. Metals in rain exhibit significant temporal and spatial trends, with concentrations generally higher during summer than winter at all sites and generally lowest at the more remote site. The concentrations of Cd and Pb in both wet and dry deposition decreased between the period 1976 to 1977 and 1981 to 1982. Ion ratios and enrichment factors suggest that Mn is largely soil derived in atmospheric samples while Cd, Pb, and Zn are enriched over typical soil levels. Factor analysis indicates that soil components influence both Al and Mn while fine aerosol components influence Cd in wet and dry deposition. Both components influence the behavior of Pb. Dry deposition dominated the input of Mn and Al to each site, while wet deposition was the major input process for the other metals (54 to 85% of total). On an annual basis, deposited Cd, Pb, and Zn are strongly retained in each watershed: 2% of the Pb, 8 to 29% of the Cd, and 8 to 34% of the Zn inputs were transported in stream flow. Deposited Mn and Al are retained to a lesser degree and show a net loss from two sites. Metal export is controlled by stream pH, organic carbon, bedrock geology, and hydrologic characteristics of each site.