Baseline Hydrologic Monitoring for Mining Projects
Douglas
C. Parker
Noranda Minerals Corporation
Missoula, Montana
Objectives of baseline hydrologic monitoring are dependent on and derived from
the ultimate use of the data. Baseline hydrological data collection for mining
projects typically has several functions including: 1) assembly of adequate
information to obtain an understanding of hydrologic and hydrogeologic systems;
2) documentation of baseline conditions for permitting and public disclosure
purposes; and 3) documentation of baseline conditions for comparison with future
conditions to be measured during mine operation. The scope of such
investigations and level of detail required for these different objectives may
vary significantly. It is in the interest of mining project proponents, the
public and the regulatory agencies to ensure that baseline studies are adequate
to meet these objectives.
Collection of adequate information to understand the
hydrologic system typically requires an inventory of surface and ground water
features, characterization of surface and ground water quality, identification
and quantification of ground water flow regimes, depth to groundwater, aquifer
characteristics, determination of stream flow variation, and establishment of
the degree of seasonal variation in these parameters.
The baseline water resources monitoring program conducted by
ASARCO, Inc. for the proposed Rock Creek Mine near Noxon, Montana illustrates
several issues relating to baseline hydrologic monitoring for mining projects.
ASARCO has developed a comprehensive database and has collected more hydrologic
baseline data than any mining project in Montana. The proposed Rock Creek Mine
is located in the headwaters of Rock Creek, which is a tributary of the Clark
Fork River (Figure 1). The Rock Creek ore body underlies the Cabinet Mountain
Wilderness Area and will be mined by underground mining techniques from an
access point outside of the wilderness area. The proposed mine related surface
facilities would be in the Rock Creek drainage except for portions of the
tailings impoundment, which is near the Clark Fork River. Rock Creek is an
intermittent stream with a drainage area of approximately 33 square miles al its
confluence with the Clark Fork River just below the Noxon Rapids Dam, about
twenty miles east of the Montana/Idaho stale line. Baseline monitoring has
focused on surface water and ground water in the Rock Creek drainage near the
proposed facilities in the tailings impoundment area.
Baseline
Monitoring System Design
Design of a hydrologic monitoring program requires selection of monitoring
sites, physical and chemical parameters to be monitored, frequency of monitoring
and quality control. Most baseline hydrologic investigations for mining projects
require assessment of both surface and ground water characteristics. Baseline
monitoring plans are generally reviewed by the regulatory agencies early in the
project development process.
The relationship between the stage of development of a mining
project and the initiation of baseline hydrological monitoring is important in
determining the scope of baseline data collection. Development of a mine is a
multi-phased process that typically requires management decisions on whether or
not to proceed with the project at each step. The general phases of mine
development are:
Discovery
Exploration and delineation of ore body
Conceptual design
Feasibility study
Final design
Construction
The environmental baseline studies, permitting and regulatory environmental
analysis commonly begin during the latter pan of the exploration phase. At this
time the approximate size and location of at least a portion of the ore body is
known, however, the location, design and special requirements of the processing
and other project related facilities may not be known in detail. Metallurgical
testing, preliminary geotechnical investigations, property evaluation,
preliminary economic evaluation and more detailed exploration drilling/ore
delineation are required to develop a preliminary mine design. This level of
information is then used, by the company or an outside reviewer, to prepare a
detailed engineering and economic analysis of the project (the feasibility
study). The feasibility study, current economic conditions and the site
environmental and permitting requirements then determine if the project will
proceed to construction. At the time the baseline environmental studies are
initiated details of the mineral recovery process and the design and location of
all facilities may not be known. The baseline studies must then encompass a
large enough area and be broad enough in scope to include sites that may be
chosen in the detailed plan of operation. The need for baseline studies to cover
a potentially wide range of alternatives inevitably leads to comparing the costs
and benefits of the size of the study area, the number of sites monitored and
the frequency of monitoring. The extent of the study area and the scope of the
investigation are in large pan determined by the mining company which must weigh
the risks of collecting too little data and the costs of needlessly large and
more extensive baseline study boundaries. Additional changes to the mine plan
and proposed disturbance areas, which may affect the study area, invariably
occur during the development and permitting process.
Baseline surface water data collection requires measurement
of both water quality and flow; without both, chemical loads and the potential
impacts from lakes, after spills or planned discharges cannot be calculated.
Monitoring stations in potential receiving waters both upstream and downstream
of planned facilities are typical, required. Chemical parameters to be monitored
are determined based on characteristics of the stream, type of mining and
processing proposed and characteristics of the ore and waste materials that
would be produced. Historic mining problems have most often been associated with
metals, acid drainage, erosion, and in some cases, process chemicals such as
cyanide. The analytical parameter list for the ASARCQ Rock Creek project is
contained in Table 1. This list covers the common ions needed for general
characterization of water type, nutrients and selected metals for which the U
.S. Environmental Protection Agency has established human health standards or
aquatic life criteria. Other parameters associated with specific metallurgical
processes or ore types are frequently added on a case-by-case basis. After an
initial screening, often two or three samples, a reduced parameter list that
contains only those elements of particular concern is often initiated;
parameters identified during the screening that are below the detection limit
and are not expected because of the geochemical environment are often dropped.
Detection limits for laboratory analyses should be based on
approved methods and should be, when reasonably possible, at such a level as to
allow measurement of ambient water quality with respect to applicable water
quality standards or criteria. For streams with low total dissolved solids and
hardness the calculated aquatic life criteria described in the EP A Goldbook (EP
A, 1986) may be lower than present laboratory detection capabilities.
Monitoring of surface water quality and flow in the
mountainous regions of Montana and Idaho typically requires collection of
representative information from at least three characteristic flow regimes:
spring high flow, summer low flow, and fall/winter base flow. These seasonal
flow regimes may have different water quality characteristics and have different
interrelationships with instream biological communities. Spring high flow
periods are characterized by cool water temperatures, high flows and high
turbidity resulting from snowmelt runoff and spring rains. The summer low flow
season is a period of decreasing flow, increased temperature and high biological
activity. The fall/winter period in northern Rocky Mountain streams is
often
the period of lowest flow, coldest temperatures and greatest effect from ground
water on stream flow. Establishment of adequate baseline data typically includes
sampling at least once during each of these three seasonal flow regimes. More
frequent sampling is often required during critical time periods for specific
parameters, such as suspended sediment and total metals during high flow periods
or temperature and dissolved oxygen during summer low flow.
Characterization of ground water conditions is frequently
more difficult more expensive than surface water baseline monitoring. Surface
reconnaissance and existing data from wells and exploration boreholes often do
not provide sufficient information to design a detailed ground water monitoring
program. Although general criteria for ground water monitoring can usually be
defined in a baseline monitoring plan, it may not be possible to determine well
depth or even well location until additional drilling data is available. Well
locations are often chosen to provide baseline data and also to serve as
operational monitoring sites. Wells down gradient of potential contaminant
sources are generally completed in the upper 10 to 20 feet of the upper most
aquifer. This approach is generally appropriate for alluvium and bedrock systems
that behave as a porous medium, but may not be adequate for fractured rock
systems and complex geologic systems often associated with hard rock mines.
Monitoring wells in fracture-controlled systems should be designed and located
with particular consideration to local geology and to protection of areas of
existing or potential ground water use.
An important component of any hydrologic monitoring program
is quality control and data validation. Development of a reliable database that
is capable or meeting the objectives of the monitoring program require that the
data are accurate and representative of the conditions being monitored. The
level of quality control required for baseline hydrologic data for a mining
project may not be the same as required for a Superfund
site, but are generally set to meet regulatory and public disclosure
requirements. Typical components of a quality assurance/quality control program
include collections and analysis of 10 to 20 percent duplicate, blank and known
standard samples. These quality control samples are used to evaluate the
accuracy of the laboratory results and to identify possible contamination of
samples during sampling, transport or storage. Validation of laboratory and
field data requires careful documentation of all steps of the data collection
process and includes a thorough review of all laboratory data.
Rock Creek Baseline Water Resources
Monitoring ASARCO Inc. began baseline hydrologic monitoring in the Rock Creek
drainage in the fall of 1984. A monitoring program was coordinated with the
Montana Department of Stale Lands, the Montana Department of Health and
Environmental Sciences Water Quality Bureau and the U.S. Forest Service Kootenai
National Forest. Since monitoring began, data has been collected from 12
principal surface water stations, 28 monitoring wells and a number of springs
and private wells (ASARCO, 1989). In addition to ASARCO's monitoring, the
Montana Water Bureau has collected data in the Clark Fork River and selected
tributaries Quality f a lower basin monitoring program from 1984 to 1989 (Ingman
and Kerr, 1990).
Stream flow in lower Rock Creek (station RC-l; see Figure
1)
has varied from 0 to over 300 cfs (cubic feet per second) during the monitoring
period. Flow in the West Fork of Rock Creek has varied from 0 to about 10 cfs at
station WRC-l upstream of the
proposed mine facilities and from 0 to over 60 cfs at station WRC-2 downstream
of the mine site. High flows are associated with late winter and spring when
melt runoff; low flows, including complete dewatering of several stream
sections, occur during late summer, fall and winter when runoff is limited and
surface flows recharge the alluvial ground water system.
Rock Creek and West Fork Creek have
high quality, soft, calcium-bicarbonate type waters with very low concentrations
of total dissolved solids (TDS) and metals. Occasional baseline samples have
calculated exceedences of the aquatic life criteria, however, at these low TDS
and hardness levels the meaning of these exceedences is not known. Table 2 shows
a summary of water quality analysis from one site on West Fork Rock Creek below
the location of the proposed facilities (station WRC-2). Data from this site are
representative of the water quality and range of measured baseline values
documented in the Rock Creek drainage.
The baseline surface water quality data shows a high degree
of variability over the period of record at most sites. Table 3 shows the means,
standard deviations and the 90% confidence interval around cumulative sample
means for some indicator parameters from stations WRC-2 and WRC-2a. The 90%
confidence interval surrounding the means of the first year of data is broad
enough too include the means for the latter years for essentially all
parameters. These statistics indicate that after the first full year of data
collection, or when the number of samples collected exceeds about 4 to 5, that
the annual cumulative means of these parameters for subsequent years of data
acquisition fall within the 90% confidence interval of the initial baseline
year. This suggests that collection of additional baseline data after the first
full year has not contributed substantially to improving the annual mean values
of the parameters evaluated. Statification of data to evaluate seasonal water
quality variability could potentially be used to better understand the data
variability.
The laboratory analyses for the Rock Creek baseline
investigation included unusually low target detection limits for nitrate,
ammonia, total phosphorous, copper, silver, cadmium, lead and mercury (Table
1).
The purpose of these detection limits was to document very low concentrations
of these constituents, particularly metals, in the baseline sampling and to
determine if aquatic life criteria were being exceeded in these low hardness
waters, levels of mercury, cadmium.
Silver and lead measured during the baseline period were
still frequently lower than the laboratory detection limits, and the objective
of quantifying the baseline values for these parameters was not fully achieved.
Laboratory methods for measuring lower concentrations for several of these
metals are not currently available.
The ASARCO baseline monitoring program has been designed to
provide adequate information for permitting, to meet the public disclosure and
environmental assessment objectives of the regulatory authorities and to provide
a baseline for comparison with future data. The monitoring program was modified
several times during the protracted baseline period; additional surface water
and ground water sites were added and the analytical parameter list was revised.
Unlike many mining projects, there have been several years in which to collect
baseline data. The Montana Department of State Lands, the Montana Water Quality
Bureau, the U.S. Forest Service and the public have all been involved in
development and review of the Rock Creek monitoring plan. There are, however, no
standards as to what constitutes an acceptable database for a baseline hydrology
investigation. One year of baseline data collection is generally required by the
Department of State Lands for mine permit applications, but additional data is
usually developed prior to permit approval. Establishment of a longer baseline
monitoring period during the permitting and environmental analysis process
typically provides an additional one to two years of data. Without specific
compliance requirements: the need for statistical evaluation of baseline data is
difficult for companies to Justify. The complexity and large variations common
to natural hydrologic and hydrogeologic systems needs to be kept in mind by all
parties involved in the environmental assessment of mining projects.