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Description of Watershed
Water Canyon originates on Santa Fe National Forest lands on the
eastern flanks of the Jemez Mountains. The Water Canyon watershed
extends across the southern part of Los Alamos National Laboratory
(the Laboratory) to the Rio Grande for a total length of about 13
mi. The watershed drains a surface area of about 6 mi2. In general
the canyon floor crossing the Laboratory is relatively flat and
filled with alluvium and colluvial soils eroded from the canyon
walls. The canyon sides are steep and rocky with partial to full
cover of trees and shrubs, particularly on north-facing slopes.
Before the fire only limited sampling was carried out in Water
Canyon. To date, the Laboratory's Environmental Restoration Project
has not collected samples from the canyon itself, although monitoring
wells have been or will be installed as part of the groundwater
management and protection program. After the fire, samples of sediments,
surface water, and groundwater were collected to establish a baseline
prior to any potential flood events.
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Sources and Types of Contamination
Within the Laboratory, Water Canyon passes through or adjacent
to TAs-11, -15, -16, -36, -37, -68, and -71. Although no sources
of contamination originate within Water Canyon itself, contaminants
are known to have been transported from mesa-top TAs through discharges
and surface erosion. The predominant mechanisms that influence transport
within the canyon include sediment transport, dissolution/desorption,
and infiltration. Most contaminants released into the canyon were
retained in sediments deposited in geomorphic locations in the active
and inactive channels and floodplains. These contaminants will remain
in place for varying time periods, depending on their locations
and surface water flow patterns. Sediment transport decreases the
contaminant concentrations as the canyon extends downstream. Laboratory
environmental surveillance reports show that contaminant concentrations
from samples collected at the point where State Highway 4 crosses
Water Canyon are not elevated.
Current knowledge of contaminants in Water Canyon is limited.
Possible contaminants include those associated with high explosives
(HEs) and firing-site residues, such as barium, lead and other metals,
and TNT and other HEs. Most major drainage potential release sites
(PRSs) at TA-16 that enter Water Canyon were characterized during
investigation activities and none of these is highly contaminated.
These PRSs include 16-029(g) (the TA-16-450 drainage), 16-030(h)
(the TA-16-430 drainage), 16-003(a) (the TA-16-410 drainage), and
16-030(g) (the TA-16-380 drainage). None of these PRSs is a major
source of contamination for Water Canyon. Very few PRSs drain directly
into Water Canyon. Cañon de Valle, a tributary of Water Canyon,
has significant concentrations of barium and HEs; but these concentrations
attenuate to within background levels before the confluence with
Water Canyon. Analysis of two samples collected in 1998 at the confluence
of Water Canyon and Cañon de Valle shows no organic compounds
detected and no inorganic compounds above background levels. Because
Cañon de Valle has a relatively low potential for flooding,
significant mobilization of contaminated sediments into Water Canyon
is less likely. Waters in Martin Spring, which drains into Martin
Spring Canyon and then into Water Canyon, are HE-contaminated. However,
based on analyses of samples collected downgradient from the spring,
little contaminant mobilization in Martin Spring Canyon has occurred.
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Impacts of the Cerro Grande Fire/Mitigation
Activities
About half the total length of Water Canyon was burned by the
fire, from the upper watershed to Technical Area (TA) 49. The severely
burned area was confined to the upper part of the watershed west
of State Highway 501, while the remainder of the watershed had only
low to moderate burn severity.
Within the watershed, eight PRSs were affected by the Cerro Grande
fire. Best management practices (BMPs) for erosion control have
been implemented at all fire-impacted PRSs. BMPs are activities
designed to prevent or inhibit contamination from reaching the canyon
or migrating down the canyon and included straw bales, mulch, wattles,
and wellhead protection. As a result of these mitigation activities,
the potential for contaminant migration into the canyon has been
reduced or eliminated.
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Program Management
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