Abstract
The Leadville mining district is historically one of the
most heavily mined regions in the world producing large
quantities of gold, silver, lead, zinc, copper, and manganese
since the 1860s. A multidisciplinary investigation was conducted
by the U.S. Geological Survey, in cooperation with
the Colorado Department of Public Health and Environment,
to characterize large-scale groundwater flow in a 13 square-kilometer
region encompassing the Canterbury Tunnel and the
Leadville Mine Drainage Tunnel near Leadville, Colorado.
The primary objective of the investigation was to evaluate
whether a substantial hydraulic connection is present between
the Canterbury Tunnel and Leadville Mine Drainage Tunnel
for current (2008) hydrologic conditions.
Altitude in the Leadville area ranges from about 3,018 m
(9,900 ft) along the Arkansas River valley to about 4,270 m
(14,000 ft) along the Continental Divide east of Leadville,
and the high altitude of the area results in a moderate subpolar
climate. Winter precipitation as snow was about three times
greater than summer precipitation as rain, and in general,
both winter and summer precipitation were greatest at higher
altitudes. Winter and summer precipitation have increased
since 2002 coinciding with the observed water-level rise near
the Leadville Mine Drainage Tunnel that began in 2003. The
weather patterns and hydrology exhibit strong seasonality with
an annual cycle of cold winters with large snowfall, followed
by spring snowmelt, runoff, and recharge (high-flow) conditions,
and then base-flow (low-flow) conditions in the fall prior
to the next winter. Groundwater occurs in the Paleozoic and
Precambrian fractured-rock aquifers and in a Quaternary alluvial
aquifer along the East Fork Arkansas River, and groundwater
levels also exhibit seasonal, although delayed, patterns
in response to the annual hydrologic cycle.
A three-dimensional digital representation of the extensively
faulted bedrock was developed and a geophysical direct-current
resistivity field survey was performed to evaluate the
geologic structure of the study area. The results show that the
Canterbury Tunnel is located in a downthrown structural block
that is not in direct physical connection with the Leadville
Mine Drainage Tunnel. The presence of this structural discontinuity
implies there is no direct groundwater pathway between
the tunnels along a laterally continuous bedrock unit.
Water-quality results for pH and major-ion concentrations
near the Canterbury Tunnel showed that acid mine drainage
has not affected groundwater quality. Stable-isotope ratios of
hydrogen and oxygen in water indicate that snowmelt is the
primary source of groundwater recharge. On the basis of chlorofluorocarbon
and tritium concentrations and mixing ratios
for groundwater samples, young groundwater (groundwater
recharged after 1953) was indicated at well locations upgradient
from and in a fault block separate from the Canterbury
Tunnel. Samples from sites downgradient from the Canterbury
Tunnel were mixtures of young and old (pre-1953) groundwater
and likely represent snowmelt recharge mixed with older
regional groundwater that discharges from the bedrock units
to the Arkansas River valley. Discharge from the Canterbury
Tunnel contained the greatest percentage of old (pre-1953)
groundwater with a mixture of about 25 percent young water
and about 75 percent old water.
A calibrated three-dimensional groundwater model
representing high-flow conditions was used to evaluate
large-scale flow characteristics of the groundwater and to
assess whether a substantial hydraulic connection was present
between the Canterbury Tunnel and Leadville Mine Drainage
Tunnel. As simulated, the faults restrict local flow in many
areas, but the fracture-damage zones adjacent to the faults
allow groundwater to move along faults. Water-budget results
indicate that groundwater flow across the lateral edges of the
model controlled the majority of flow in and out of the aquifer
(79 percent and 63 percent of the total water budget, respectively).
The largest contributions to the water budget were
groundwater entering from the upper reaches of the watershed
and the hydrologic interaction of the groundwater with the
East Fork Arkansas River. Potentiometric surface maps of the
simulated model results were generated for depths of 50, 100,
and 250 m. The surfaces revealed a positive trend in hydraulic
head with land-surface altitude and evidence of increased
control on fluid movement by the fault network structure at
progressively greater depths in the aquifer.
Results of advective particle-tracking simulations indicate
that the sets of simulated flow paths for the Canterbury
Tunnel and the Leadville Mine Drainage Tunnel were mutually
exclusive of one another, which also suggested that no
major hydraulic connection was present between the tunnels.
Particle-tracking simulations also revealed that although the
fault network generally restricted groundwater movement
locally, hydrologic conditions were such that groundwater
did cross the fault network at many locations. This cross-fault
movement indicates that the fault network controls regional
groundwater flow to some degree but is not a complete barrier
to flow. The cumulative distributions of adjusted age results
for the watershed indicate that approximately 30 percent of
the flow pathways transmit groundwater that was younger
than 68 years old (post-1941) and that about 70 percent of the
flow pathways transmit old groundwater. The particle-tracking
results are consistent with the apparent ages and mixing ratios
developed from the chlorofluorocarbon and tritium results.
The model simulations also indicate that approximately
50 percent of the groundwater flowing through the study
area was less than 200 years old and about 50 percent of the
groundwater flowing through the study area is old water stored
in low-permeability geologic units and fault blocks. As a
final examination of model response, the conductance parameters
of the Canterbury Tunnel and Leadville Mine Drainage
Tunnel were manually adjusted from the calibrated values to
determine if altering the flow discharge in one tunnel affects
the hydraulic behavior in the other tunnel. The examination
showed no substantial hydraulic connection.
The multidisciplinary investigation yielded an improved
understanding of groundwater characteristics near the Canterbury
Tunnel and the Leadville Mine Drainage Tunnel.
Movement of groundwater between the Canterbury Tunnel
and Leadville Mine Drainage Tunnel that was central to this
investigation could not be evaluated with strong certainty
owing to the structural complexity of the region, study simplifications,
and the absence of observation data within the upper
sections of the Canterbury Tunnel and between the Canterbury
Tunnel and the Leadville Mine Drainage Tunnel. There was,
however, collaborative agreement between all of the analyses
performed during this investigation that a substantial hydraulic
connection did not exist between the Canterbury Tunnel and
the Leadville Mine Drainage Tunnel under natural flow conditions
near the time of this investigation.