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Highlights
- One of the largest gold producing mines in Indonesia.
- Several areas of concern including a large potentially
acid forming (PAF) waste rock storage impoundment, an
equally large tailings impoundment and associated containment
facilities, a complex series of polishing ponds, and a
series of uncovered stockpiles comprising both barren
and potentially PAF producing rock.
- Geologically complex terrain located in the centre of
Kalimantan; part of the Central Kalimantan Arc and Kutai
Basin deposits which have locally been intruded by a series
of andesitic bodies.
- Equally complex hydrogeological model.
- Potential for onsite activities to impact on the nearby
Kelian River, an important source of drinking water for
several local communities.
- Introduction of a relatively complex yet cost effective
system of groundwater sampling into a very remote and
difficult area.
- Part of an ongoing collage of projects designed to meet
environmental compliance set by the East Kalimantan Regency
and Indonesian Government as part of the mine closure
program.
Background
C. M. Jewell & Associates was commissioned by Kelian
Equatorial Mining (KEM) to undertake a review of a pre-existing
groundwater monitoring network and provide comment as to
whether the network was suitable to ensure that environmental
compliance was able to be sufficiently monitored and met,
or whether deficiencies in system existed that would allow
for the undetected migration of leachate impacted waters
from the site and into nearby receptors such as the Kelian
River, which is an important source of drinking water for
several local communities.
The appraisal identified a number of areas, in areas of
both natural abutments and constructed containment facilities,
where containment of known and potentially impacted waters
could not be guaranteed. As a result, a plan detailing the
enhancement of the monitoring network via the installation
of a series of new and replacement monitoring wells was
submitted to KEM, and the proposed scope of works commissioned
by KEM.
Geological Environment
The geology encountered at the site consists of a complex
sequence of silicic pyroclastics and sediments which have
been intruded by a number of andesitic bodies. The pyroclastics
range from fine-grained ash tuff to coarse lithic tuffs,
and grade upwards into a sequence of siltstones, sandstones,
minor limestones and carbonaceous units. Economic mineralisation
is believed to have been introduced in the area following
the emplacement of the andesites, whilst younger basalts
occur as dykes and sills within the north-western corner
of the current extraction pit, and top a series of surrounding
plateaus to the west of the site.
Hydrogeological Environment
The site is situated predominantly on hard-rock terrain
encompassing the lower foothills and ridgelines which dominate
the western embankment of the Kelian River. Groundwater
movement beneath the site occurs predominantly through a
series of secondary features such as fracturing associated
with the network of joints and features, whilst in comparison
only a relatively small volume of water is thought to migrate
via primary porosity (i.e. through the interconnected pore
spaces between individual grains) within areas dominated
by sedimentary sequences. Secondary porosity, formed by
joints, fractures and fault zones within the rock mass,
is considered to be the major component of the effective
porosity of the aquifer. Thus secondary porosity and permeability
constitute the major component of the aquifer's transmissivity,
and a significant component of the aquifer's storage. The
joints and fractures within the sedimentary rock pile are
predominantly sub-vertical whilst the bedding plane openings
and fractures are predominantly sub-horizontal.
The site forms part of the regional groundwater recharge
zone, with recharge to the aquifer occurring by direct rainfall
infiltration through the various soil horizons before entering
the secondary fractures within the aquifer. Discharge from
the aquifer occurs primarily through natural dewatering
via springs and base flow into local watercourses which
predominantly form first and second order tributaries of
the Kelian River. The groundwater flow volume (i.e. groundwater
flux) at the site is relatively small in comparison with
surface runoff volume, however groundwater flow occurs in
most places around the site, and is considered to have a
significant potential role in contaminant transport.
Scope of Work
Following the acceptance of the proposed network, a drilling
program was undertaken at the site where twenty-seven additional
piezometers were installed piezometers at preselected locations
within and around the waste-rock impoundment, mine site,
and tailings storage facility, which was promptly followed
by the installation of a dedicated groundwater-sampling
pump within each of the boreholes. This was followed by
a hands-on training session designed to allow field technicians
from the mine site to collect groundwater samples from each
of the newly installed wells as part of the ongoing commitment
to the groundwater-monitoring program.
On completion of the fieldworks, a detailed report describing
the works undertaken, geological and hydrogeological conditions
observed within each borehole, an appraisal of groundwater
data, and mathematical modelling of solute transport in
groundwater (which was modelled using the specialist packages
PHREEQC and PRINCE), was prepared and submitted to KEM as
an ongoing appraisal of the hydrogeological conditions at
the site. 
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