Ranking Sites’ Erosion Potential at Los Alamos National Laboratory

Los Alamos National Laboratory (LANL) covers 27,500 ac. in north central New Mexico about 25 mi. northwest of Santa Fe. Established in 1943 as part of the Manhattan Project to create the first atomic weapons during World War II, LANL is owned by the Department of Energy and has been operated from the beginning by the University of California. The laboratory is situated on the Pajarito Plateau, which consists of a series of fingerlike mesas separated by deep east-west canyons cut by intermittent streams.

In its 57-year history, LANL has been the site of nuclear testing, and areas of the laboratory have been used for conducting research and development in basic and applied chemistry, biology, and physics; fabricating and testing explosives; cleaning chemically contaminated equipment; and working with radioactive materials. The laboratory’s Environmental Restoration (ER) Project has the monumental task of investigating and cleaning up, if necessary, potentially contaminated areas. The current ER Project investigation is responsible for defining the presence or absence of hazardous and radioactive materials and addressing any sites where such materials exist at levels that might adversely affect human health or the environment.

About 1,400 sites need to be investigated. Limited resources make it impossible to conduct sediment or surface-water migration studies of every site, yet it’s critical to identify those that pose the greatest risks. In 1997, members of LANL’s Water Quality and Hydrology Group, the ER Project, and the New Mexico State Environment Department devised a scoring system to rank each site’s erosion potential, based on existing knowledge of each site and a rapid visual evaluation.

Much of the investigative phase of the work has been completed; the first sites were assessed in 1997, and 1,100 have been evaluated to date. As a result, many sites have been found not to be contaminated and are being removed from the total list of sites without further action. At many of the remaining sites, accelerated cleanup has begun or has already been completed. (Accelerated cleanup is an abbreviated version of the full corrective-action process that regulators authorize for sites where levels of contamination are known and the cleanup processes are obvious.) A small percentage of sites, currently estimated at less than 10%, will need to go through the entire corrective-action process, a task that is expected to take until 2009 to complete. Erosion potential of about 300 more sites will be assessed this year.

The approach has three goals: (1) to assess whether any potentially dangerous constituents are present at the site in high enough concentrations to pose a threat to human health or the environment, (2) to see whether the site’s erosion potential could cause those constituents to migrate, and (3) to determine the necessary corrective action.

Using this scoring system, field teams from LANL’s Water Quality and Hydrology Group completed more than 900 site assessments in just seven months, between June and December 1997. Although the system isn’t intended to replace sound professional judgment, it speeds the process of comparing and prioritizing sites with vastly different physical characteristics.

Finding Out What’s There: Constituent Assessments

The first step is to compile existing information for each site, such as site maps and any sample data from soil, sediment, or surface water. If surface-water samples represent runoff from more than one site, the other sites and their constituents are also identified. Information about past site conditions is not included, and data are requested only for surface soils and sediments less than 12 in. deep. Constituents vary from site to site but generally consist of a combination of radionuclides, metals, and organics.

How Far Can It Go? Surface-Water Site Assessments

Next, two-person teams determine the erosion and sediment transport potential of each site. They fill out standard assessment forms with information on three categories for each site: site setting, runoff factors, and run-on factors. A matrix is later used to assign a numerical score to each site.

The first category, site setting, includes three factors: geographic setting, percentage of ground or canopy cover, and percentage of slope. Each site fits into one of four geographic settings, listed here in order of increasing concern: mesa tops,bench or submesa tops, floodplains, and well-defined drainage channels. As with all other assessment criteria, if more than one setting applies to a site, the most conservative setting is used. For example, an inactive septic tank on a mesa top that discharges over the mesa edge onto a defined slope or bench is listed as belonging to the more serious “bench or submesa top” setting.

Percentage of ground and canopy is the next site setting consideration. Each site is described as having 0-25%, 25-75%, or 75-100% cover.

Percent of slope at the site is the third consideration. Each site is described as less than 10% slope, greater than 10 but less than 30%, or greater than 30%.

For the second surface-water site assessment category, runoff factors, the teams answer four questions for each site: (1) Is there visible evidence of water or sediment discharging from the site? (2) Are the runoff discharges channelized, is channelization naturally occurring or manmade, or is there evidence of sheet-flow processes? (3) Where does the evidence of runoff terminate-a well-defined drainage or wetland, a bench or submesa top, or a meadow or retention area? (The matrix weights runoff termination points in or near watercourses more heavily.) (4) Has runoff caused visible erosion at the site, and if so, is it sheet erosion, rill erosion, or gully erosion?

Teams assess run-on factors, the third assessment category, by answering three questions: (1) Are structures (e.g., buildings, roof drains, parking lots) creating run-on to the site? (2) Are current operations causing stormwater run-on to the site? (3) Are natural drainage patterns directing stormwater onto the site?

The weight given to each factor reflects its relative importance for erosion potential. Runoff factors have the greatest weight (46%), followed by site setting (43%). Run-on factors have a relatively low weight (11%) because run-on is of little concern unless it increases runoff. Surface-water site assessment scores for each site-ranging from 0 to 100-are calculated automatically using Microsoft Access 7.0 once all the data are entered. Relative weights of the various factors do not appear on the field forms so that the process of data collection is separate, as much as possible, from the data analysis.

Recommending Corrective Actions

In the third step of the LANL approach, data from the constituent assessments and scores from the surface-water site assessments are combined to determine which sites to address first. Although the ER Project can take care of specific concerns at any site, the high-priority sites get first consideration.

Of the first 900 sites assessed, 10% had scores greater than 60 on the surface-water site assessment, making them high-priority sites. They were further ranked according to the constituent assessment data, and those having bioaccumulators or high concentrations of other constituents were given the highest priority. Another 16% were moderate-priority sites, receiving scores of more than 40 but less than 60. The majority, 74%, scored less than 40; 95% of these low-priority sites required no corrective action. Distribution of the surface-water site assessment scores showed three peaks.

Best management practices (BMPs) were applied in many cases to temporarily stabilize the site and inhibit erosion and constituent migration until a final remedy could be applied. BMPs have been applied at more than 200 of the 1,100 sites assessed so far.

This systematic evaluation process proved effective for consistently screening and prioritizing high-erosion-potential sites. LANL is using the same approach as part of a new watershed management program, and environmental professionals may be able to adapt the approach for other characterization activities beyond erosion control and constituent transport.