Sediment & Contaminant Transport

GGI has extensive experience in evaluating sediment transport, in particular through canyons draining the Pajarito Plateau from Los Alamos National Laboratory (LANL). GGI has worked with LANL personnel to calculate inventories of radionuclide, heavy metal, semivolatile, and volatile organic contaminants adsorbed to sediments. (Ephemeral Stream Contaminant Distribution Publication).

Selected Projects:

GGI has performed detailed geomorphic mapping, soil stratigraphic descriptions, dendrochronology and soil sampling in canyons draining Los Alamos National Laboratory (LANL), a Department of Energy (DOE) facility. GGI conducted radiological surveys, and collected sediment samples to determine the inventory of contaminants in sediments in the vicinity of LANL to delineate and target post-1942 sediment deposits for sampling campaigns . In conjunction with LANL, GGI developed stratigraphy-based sampling approach and methodology for geomorphic characterization of canyons draining LANL to determine contaminant inventories in individual watersheds. GGI conducted soil sampling for a wide range of contaminants (including radionuclides, PCB congeners, HE, semivolatile and volatile organics, and metals) in Los Alamos, Pueblo and Mortandad Canyons, Cañada del Buey, Pajarito, Rendija, Sandia, Portrillo, Fence, Ancho, Chaquehui, and Water Canyons, Cañon de Valle and tributary drainages. These studies have been used to design sediment retention structures and implement sediment transport mitigation and cleanup programs for the RCRA Subtitle C facility. Project has been on-going from 1998 to the present.

Details of Work Performed:

Radiological surveys, including alpha, beta, and gamma radiation, were utilized for site investigations in canyons with elevated radionuclide inventories in sediments, and surveys for key metals (e.g. chromium) were conducted using an X-Ray Fluorescence analyzer. Gamma-radiation surveys included collection of gamma-vs-depth profiles at several hundred locations in Mortandad Canyon, both upstream and downstream of the outfall from the radioactive liquid waste treatment facility at TA-50. Gamma-radiation surveys were used to help target sample collection and define areal extent of high-gamma emitting deposits, which correlated to areas of elevated-concentration radionuclides in sediments (including cesium, strontium and americium). Radiological surveys, in conjunction with geomorphology and dendrochronology analyses and collection of samples from discrete stratigraphic intervals in sediment deposits, were used to constrain the temporal context of deposition of the most highly contaminated sediments in LA-Pueblo, Acid Canyon, and Mortandad Canyon, and to evaluate contaminant sources, fate and transport of contaminants in watersheds draining LANL.

WatersLANL geomorphic maphed investigations typically included an initial phase that involved sampling for a wide range of potential contaminants, and a second phase that targeted chemicals of potential concern (COPCs). A series of investigation reaches were selected within each watershed, and each reach was mapped at a scale of 1:200. The area of post-1942 sediment deposition was determined based on a detailed field assessment of deposits, and a series of geomorphic map units keyed to channel and floodplain depositional environments, were delineated. Average contaminant concentrations in coarse and fine facies in each geomorphic unit, combined with unit areas, average thickness and density were used to calculate an inventory of COPCs within each reach in a given watershed. Contaminant concentrations in each sample reach were normalized to kg/km, and the contaminant inventory in nonsampled reaches used the average normalized inventory of bounding reaches. The total contaminant inventory was then calculated based on the sum of all sampled and nonsampled reaches.

GGI’s watershed-scale investigations also identified areas of higher-concentrations of COPCs in Mortandad, Sandia, Los Alamos (LA), and Pueblo Canyons, and were used to develop a conceptual model of contaminant transport (Drakos et al., 1996; Reneau et al., 2004). Mitigation activities have been undertaken in these canyons, resulting in construction of sediment retention structures in LA, Pueblo, DP, Mortandad, and Sandia Canyons designed to mitigate the migration of contaminants to the Rio Grande. GGI has utilized repeat geomorphic surveys using total station and high precision GPS surveying methods to evaluate the effectiveness of the sediment transport mitigation sites in drainages upstream of the City of Santa Fe Buckman Diversion for the past five years.  In addition to the construction of sediment retention structures, GGI’s investigations identified and delineated an area of PCB contamination in LA Canyon that required cleanup by sediment_samplingLANL, and GGI conducted sampling to confirm that PCB-contaminated soils had been removed and remaining soils were within acceptable risk-based PCB concentrations (see Reneau et al., 2010b). GGI also participated in an earlier investigation of plutonium contamination in Acid Canyon that resulted in a cleanup effort by LANL.

To monitor on-going fate and transport of contaminants, GGI conducts annual sampling of sediment deposits in canyons draining LANL and along the Rio Grande. Sediment deposits from winter runoff and monsoonal flood events of the current year are targeted in sampling conducted in November-December of each year. Unexploded Ordinance (UXO) surveys are completed each year prior to the start of sampling activities in areas that could contain UXO. Geomorphic settings conducive to sediment deposition (e.g., point bars, back channels, scour holes) are targeted for sampling (Figure 5). Contaminants such as PCBs, radionuclides, and metals can be traced from LANL and non-LANL sources to the Rio Grande and downstream through White Rock Canyon. Analysis of PCB Congener Homologs has been used to evaluate PCB sources in watersheds draining LANL, and to assess PCB concentrations in sediment deposits along the Rio Grande (Reneau et al., 2010).

The effect of the 2011 Las Conchas fire in the Jemez Mountains on contaminant transport, erosion, and sediment deposition in the Water-Valle watershed was evaluated using repeat cross section surveys, repeat assessment of sediment sample locations, and sediment sample collection (Drakos et al., 2012). This investigation focused on an examination of geomorphic response to the Las Conchas Fire and the effect on contaminant transport and storage (including HE, PCBs, radionuclides, and metals), in sediment deposits over a three-year time period. Prior to starting each field campaign, GGI conducted surveys with LANL ordinance technicians to guide UXO clearance activities and field screened samples for HE during sampling campaigns.

la-weir-002Description of detailed stratigraphic section in preparation for sampling, LA Canyon Weir sediment trap.

weir_strat_2007_sampleid

Stratigraphic sections with sample locations, LA Canyon Weir sediment trap.