Yields of Dissolved Organic C,N, and P. from Three High-Elevation Catchments,Colorado Front Range,U.S.A.

Ecosystem dynamics in high-elevation watersheds are extremely sensitive to changes in chemical, energy, and water fluxes. Here we report information on yields of dissolved organic C, N, and P for the 1999 snowmelt runoff season from three high-elevation catchments in the Colorado Front Range, U.S.A.: Green Lake 4 (GL4) and Albion townsite (ALB) on North Boulder Creek and the Saddle Stream (SS), a tributary catchment dominated by alpine tundra. Dissolved organic carbon (DOC) concentrations in stream waters ranged from <1 to 10 mg C L^-1, with the highest values occurring at the SS site. Dissolved organic nitrogen (DON) concentrations ranged from below detection limits to 0.28 mg N L^-1 and were again highest at the tundra-dominatedsite. Dissolved organic phosphorus (DOP) concentrations were at or near detection limits throughout the season in all three catchments indicating a strong terrestrial retention of P. OnlyDOC showed a significant relationship to discharge. Yields of DOC in the three catchments ranged from 10.6 to 11.8 kg C ha^-1 while yields of DON and DOP ranged from 0.32 to 0.41 and 0.02 to 0.08 kg ha^-1, respectively. The relatively highyield of organic N and P relative to C from the highest elevationsite (GL4) was somewhat surprising and points to either: (1) a source of dissolved organic material (DOM) in the upper reaches of the catchment that is enriched in these nutrients or (2) theselective uptake and processing of organic N and P downstream ofthe sampling site. Additionally, seasonal changes in the relativeimportance of DOM precursor materials appear to result in changesin the N content of DOM at both the GL4 and ALB sites.     

Active capping demonstration in the Anacostia river,Washington, D.C.

An active capping demonstration project in Washington, D.C., is testing the ability to place sequestering agents on contaminated sediments using conventional equipment and evaluating their subsequent effectiveness relative to conventional passive sand sediment caps. Selected active capping materials include: (1) AquaBlokTM, a clay material for permeability control; (2) apatite, a phosphate mineral for metals control; (3) coke, an organic sequestration agent; and (4) sand material for a control cap. All of the materials, except coke, were placed in 8,000‐ft test plots by a conventional clamshell method during March and April 2004. Coke was placed as a 1.25‐cm layer in a laminated mat due to concerns related to settling of the material. Postcap sampling and analysis were conducted during the first, sixth, and eighteenth months after placement. Although postcap sampling is expected to continue for at least an additional 24 months, this article summarizes the results of the demonstration project and postcap sampling efforts up to 18 months. Conventional clamshell placement was found to be effective for placing relatively thin (six‐inch) layers of active material. The viability of placing high‐value or difficult‐to‐place material in a controlled manner was successfully demonstrated with the laminated mat. Postcap monitoring indicates that all cap materials effectively isolated contaminants, but it is not yet possible to differentiate between conventional sand and active cap layer performance. Monitoring of the permeability control layer indicated effective reductions in groundwater seepage rates through the cap, but also showed the potential for gas accumulation and irregular release. All of the cap materials show deposition of new contaminated sediment onto the surface of the caps, illustrating the importance of source control in maintaining sediment quality. © 2006 Wiley Periodicals, Inc.