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Compiled by:
Geo Pollution Technologies – Gauteng (Pty) Ltd
81 Rauch Street
Georgeville
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P.O. Box 38384
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Tel: +27 (0)12 804 8120
Fax: +27 (0)12 804 8140
HYDROGEOLOGICAL REPORT
FOR
PROPOSED VOLSPRUIT MINE (NORTH PIT)
NEAR MOKOPANE
GPT Reference Number: EaGrn-10-115
Version: Final Version 1
Date: July 2010
Compiled for:
EScience Associates (Pty) Ltd
HYDROGEOLOGICAL REPORT FOR THE PROPOSED VOLSPRUIT MINE (NORTH PIT)
GEO POLLUTION TECHNOLOGIES – GAUTENG (PTY) LTD i
Report Type: Hydrogeological Report
Project Title: Hydrogeological Report for the Proposed Volspruit Mine (North Pit) near Mokopane
Site Location: On the farm Volspruit 326KP, Mokopane
B. J. Bredenkamp (M.Sc., Pr.Sci.Nat) (Chapter 6, 7, 8, 11, report editing)
G.J. du Toit; (D.Sc., Pr.Sci.Nat) (Chapter 9, 10, quality control)
GPT Reference: EaGrn-10-115
Version: Final Version 1.0
Date: July 2010
Distribution List: PDF to EScience Associates
(Current Version)
Disclaimer:
The results and conclusions of this report are limited to the Scope of Work agreed between GPT and the Client for whom this investigation has been conducted. All assumptions made and all information contained within this report and its attachments depend on the accessibility to and reliability of relevant information, including maps, previous reports and word-of-mouth, from the Client and Contractors. All work conducted by GPT is done in accordance with the GPT Standard Operating Procedures. GPT is in the process of obtaining ISO 9001:2008 accreditation.
Copyright:
The copyright in all text and other matter (including the manner of presentation) is the exclusive property of Geo Pollution Technologies – Gauteng (Pty) Ltd, unless where referenced to external parties. It is a criminal offence to reproduce and/or use, without written consent, any matter, technical procedure and/or technique contained in this document. This document must be referenced if any information contained in it is used in any other document or presentation.
Declaration:
I hereby declare: 1. I have no vested interest (present or prospective) in the project that is the subject of this report as well as its
attachments. I have no personal interest with respect to the parties involved in this project. 2. I have no bias with regard to this project or towards the various stakeholders involved in this project. 3. I have not received, nor have I been offered, any significant form of inappropriate reward for compiling this
report.
B.J. Bredenkamp, M.Sc.,Pr.Sci.Nat Professional Natural Scientist (No 400015/09) Geo Pollution Technologies – Gauteng (Pty) Ltd
Quality Control: This report was checked by:
G.J. du Toit; D.Sc.,Pr.Sci.Nat Professional Natural Scientist (No 400043/86) Geo Pollution Technologies – Gauteng (Pty) Ltd
Customer Satisfaction: Feedback regarding the technical quality of this report (i.e. methodology used, results discussed and recommendations made), as well as other aspects, such as timeous completion of project and value of services rendered, can be posted onto GPT’s website at www.gptglobal.com.
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GEO POLLUTION TECHNOLOGIES – GAUTENG (PTY) LTD 3
cleaned one litre plastic bottles. All samples were kept on ice or in a refrigerator until delivered to
a laboratory.
A total of 50 boreholes were identified during the hydrocensus on 11 & 12 May 2010. Five additional
boreholes were also drilled and water samples were collected from these boreholes as well. In
total, 18 groundwater samples and one surface water sample from the Nyl River samples were
collected. The water samples were sent to Clean Stream Scientific Services (Pty) Ltd in Pretoria for
major ion analysis to determine water quality in the area.
3.4 GEOPHYSICAL SURVEY
A geophysical was carried out during May 2010. The survey consisted of four traverses of CSAMT
(Controlled Source Array Magneto Telluric), and five traverses of direct current resistivity profiling.
3.4.1 Objectives of Geophysical Survey
The objective of this geophysical survey was to image the subsurface resistivity / conductivity in
order to investigate the geological structure of the area.
3.4.2 Survey Methods and Instrumentation
For this survey, the Geometrics “Stratagem” EH 4 CSAMT instrument and the ABEM “Lund”
resistivity imaging system were used. Both systems measure bulk resistivity from the surface as
apparent resistivity (Rho) vs. frequency (CSAMT) and apparent resistivity vs. a geometric factor (dc
resistivity). This is then converted to true resistivity vs. depth during the interpretation process.
The details pertaining to the methodology and instrumentation can be seen in Appendix B.
3.5 BOREHOLE DRILLING
Makulu Manzi drilling contractors were commissioned to drill five monitoring wells at the project
site. The holes were drilled using the rotary percussion drilling method. Four of the boreholes were
drilled on targets identified during the geophysics (VOL17, VOL18, VOL19 VOL20). One borehole
(VOL21) was drilled in the centre of the ore body to a depth of 170 meters below ground level to be
representative of the ore body. The depth of mining is likely to approach 160m.
3.6 PUMP TESTING
GPT appointed Trans Africa Water Services to conduct pump test on the newly drilled water supply
boreholes viz. VOL17, VOL19, VOL20, VOL21 and VOL22. VOL18 was used as an observation hole for
VOL22. The tests were conducted from 1 of June 2010 to the 17 of June 2010. The pump tests were
conducted according to the SANS 10299-4 standard. A step discharge test was initially conducted on
each borehole to determine the rate at which it should be pump during the constant discharge test.
The constant discharge test was conducted for a 24 hour period followed by a 12 hour recovery to
determine the aquifer hydraulic properties (transmissivity and storativity). It was also necessary to
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identify any boundaries, which may have an effect on the long term sustainable yield of the
borehole. Observation boreholes were monitored where available.
3.6.1 Aquifer Parameters
The aquifer parameters were determined from the pump tests. The transmissivity and storativity
was estimated by curve fitting in Aquifer Test Pro2 and by calculation in FC-Method3.
Transmissivity is the rate at which water is transmitted through a unit width of an aquifer under a
unit hydraulic gradient, while storativity is the volume of water per volume of aquifer released as a
result of a change in head. Steady-state can be defined as the situation where variations of the
drawdown with time are negligible, or where the hydraulic gradient has become constant.4
3.6.1.1 Aquifer Test Pro
Curve fitting of the aquifer data was used to determine the transmissivity and storativity. The
double porosity (uniformly fractured aquifers) method by Warren and Root (1963)2 presented the
best fit. This method stipulates flow from the blocks (porous medium) to the fractures. The
fractured rock mass is assumed to consist of two interacting and overlapping continua: a continuum
of low-permeability primary porosity blocks, and a continuum of high permeability, secondary
porosity fissures or fractures.
The assumptions and conditions underlying this method are:
The aquifer is isotropic and confined
The thickness of the aquifer is uniform over the area of influence.
The extent of the aquifer is infinite (no barriers causing preferential flow paths),
Constant discharge rate were used in the pump testing
The well fully penetrates a fracture (matrix and fracture is considered as two overlapping
continuous media),
Horizontal piesometric surfaces exist prior to pumping
Pseudo steady state conditions is achieved
3.6.2 Groundwater Recharge Estimation
The groundwater recharge was estimated using the RECHARGE program5, which includes using
qualified guesses as guided by various schematic maps. The following methods/sources were used
to estimate the recharge.
Soil information
2 Aquifer Test Pro version 4.0, developed by Waterloo Hydrogeologic Inc
3 Flow Characteristic method version 2.0 developed by the University of the Free State
4 Kruseman, G.P. and de Ridder N.A. (1991), Analysis and Evaluation of Pumping Test Data, 2nd ed., ILRI publication 47,The Netherlands 5 Gerrit van Tonder, Yongxin Xu: RECHARGE program to Estimate Groundwater Recharge, June 2000. Institute for Groundwater Studies, Bloemfontein RSA.
All concentrations are presented in mg/l, EC is presented in mS/m
0 = below detection limit of analytical technique
Exceeding maximum allowable standard for domestic use
Class II
Class I
Notes:
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Figure 9: Pie Diagrams of Groundwater and Surface Water Chemistry
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Figure 10: Stiff Diagrams of Groundwater and Surface Water Chemistry
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Figure 11: Piper Diagram
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5.7.3 Discussion of groundwater chemistry and health risks
From the chemistry analysis of the water samples collected from the boreholes, magnesium, nitrate
and chloride were the chemical substances found not to comply with SANS 241 drinking water
standard. The health risks to humans20 were obtained from the DWAF water quality guidelines and
were summarised in Table 14. At the levels of magnesium found in the water, no major health or
aesthetic effects are likely.
Table 14: Summary of the Human Health Risks Posed by the Relevant Constituents
Elevated
element
Groundwater/surface
water samples
Human Health effects at current
concentrations
Nitrate
BOK1, BOK8, DEK1, GV1,
GV5, VOL13, VOLS1,
VOLS3, VOLS5, VOL17,
VOL19, VOL20, VOL21,
VOL22.
Concentrations above 20 mg/l cause
methaemoglobinaemia in infants and mucous
membrane irritation in adults.
Chloride BOK1, BOK8, DEK1, VOLS3
Water aesthetically unacceptable due to a
salty taste and may cause nausea, a
disturbance of electrolyte balance and
dehydration especially in infants ( >1200 mg/l).
20 DWAF 1996, South African water Quality Guidelines, volume 1, domestic use
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6 GROUNDWATER VULNERABILITY
According to Lynch et al. aquifer vulnerability is defined as the intrinsic characteristics that
determine the aquifer’s sensitivity to the adverse effects resulting from the imposed pollutant21.
The following factors have an effect on groundwater vulnerability:
Depth to groundwater: Indicates the distance and time required for pollutants to move
through the unsaturated zone to the aquifer.
Recharge: The primary source of groundwater is precipitation, which aids the movement of
a pollutant to the aquifer.
Aquifer media: The rock matrices and fractures which serve as water bearing units.
Soil media: The soil media (consisting of the upper portion of the vadose zone) affects the
rate at which the pollutants migrate to groundwater.
Topography: Indicates whether pollutants will run off or remain on the surface allowing for
infiltration to groundwater to occur.
Impact of the vadose zone: The part of the geological profile beneath the earth’s surface
and above the first principal water-bearing aquifer. The vadose zone can retard the
progress of the contaminants21.
The Groundwater Decision Tool (GDT) was used to quantify the vulnerability of the aquifer
underlying the site. The depth to groundwater below the site was estimated from water levels
measured during the hydrocensus and borehole drilling inferred to be ~20 mbgl at the site. A
groundwater recharge of 3.2%, a sandy clayey-loamy clay soil and a gradient of 1.15% were assumed
and used in the estimation. The GDT calculated a vulnerability value of 45%, which is moderate or
medium. This implies that the aquifer is reasonably sensitive to contamination and care should be
taken with any activities that could generate pollutants.
21 The South African Groundwater Decision Tool (SAGDT), Manual Ver. 1 (Department of Water Affairs and Forestry)
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7 AQUIFER CLASSIFICATION
The main aquifers underlying the area were classified in accordance with the Aquifer System
Management Classification document22. The aquifers were classified by using the following
definitions:
Sole Aquifer System: An aquifer which is used to supply 50% or more of domestic water for a
given area, and for which there is no reasonably available alternative sources should the
aquifer be impacted upon or depleted. Aquifer yields and natural water quality are
immaterial.
Major Aquifer System: Highly permeable formations, usually with a known or probable
presence of significant fracturing. They may be highly productive and able to support large
abstractions for public supply and other purposes. Water quality is generally very good
(Electrical Conductivity of less than 150 mS/m).
Minor Aquifer System: These can be fractured or potentially fractured rocks which do not
have a high primary permeability, or other formations of variable permeability. Aquifer
extent may be limited and water quality variable. Although these aquifers seldom produce
large quantities of water, they are important for local supplies and in supplying base flow
for rivers.
Non-Aquifer System: These are formations with negligible permeability that are regarded as
not containing groundwater in exploitable quantities. Water quality may also be such that it
renders the aquifer unusable. However, groundwater flow through such rocks, although
imperceptible, does take place, and needs to be considered when assessing the risk
associated with persistent pollutants.
Based on information collected during the hydrocensus, it can be concluded that aquifer system in
the study area can be regarded as a major aquifer, based on the reliance of the surrounding water
users on groundwater. As part of the aquifer classification, a Groundwater Quality Management
(GQM) Index is used to differentiate the degree to which an aquifer should be protected In order to
achieve the GQM Index a points scoring system as presented in Table 15 is used to calculate the
GQM.
22 Department of Water Affairs and Forestry & Water Research Commission (1995). A South African Aquifer System Management Classification. WRC Report No. KV77/95.
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Table 15: Ratings for the Groundwater Quality Management Classification System
Aquifer System Management Classification
Class Score
Sole Source Aquifer System:
Major Aquifer System:
Minor Aquifer System:
Non-Aquifer System:
Special Aquifer System:
6
4
2
0
0 – 6
Aquifer Vulnerability Classification
Class Score
High:
Medium:
Low:
3
2
1
The occurring aquifer(s) at the site, in terms of the above definitions, is classified as a sole aquifer
system, due the relative absence of surface water and heavy reliance on groundwater. The
vulnerability of the groundwater system in terms of the above is classified as medium (See section
6).
The level of groundwater protection based on the Groundwater Quality Management Classification:
GQM Index = Aquifer System Management x Aquifer Vulnerability
= 4 x 2 = 8
Table 16: GQM Index for the Study Area
GQM Index Level of Protection Study Area
<1
1 – 3
3 – 6
6 – 10
>10
Limited
Low Level
Medium Level
High Level
Strictly Non-Degradation
8
7.1 AQUIFER PROTECTION CLASSIFICATION
A Groundwater Quality Management Index of 8 was estimated for the study area from the ratings
for the Aquifer System Management Classification. Therefore the fractured aquifer should not be
negatively affected by contamination and should be protected.
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Due to the high GQM index calculated for this area, a high level (no or little degradation of
groundwater quality) of protection is needed to adhere to DWAF’s water quality objectives.
Therefore reasonable and sound groundwater protection measures are recommended to ensure that
no cumulative pollution affects the aquifer, in the long term.
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8 SITE CONCEPTUAL MODEL
The site conceptual model was developed using a risk based approach, whereby impact source
areas are identified, pathways are characterised and potential receptors identified. Both the
mining and post mining scenarios are addressed. In the mining phase, drawdown of the groundwater
level will be the main impact, while pollution emerging from the backfilled opencast is considered
the most important post mining impact.
8.1 IMPACT SOURCE
The potential impact source areas were identified as the following:
Opencast mine
Waste rock dumps
Tailings dams
Workshops and petroleum storage tanks
Septic tank
General waste facilities
The hydraulic characteristics of the source and the geochemical properties of the subsurface will
determine the behaviour of the contaminants emanating from the source. In addition, the location
and extent of the pollution source will have an effect on the extent of the contaminant plume.
8.1.1 Opencast mine
During mining the opencast will have to be dewatered to allow access to equipment and mining
personnel. Dewatering can be done by installing dewatering boreholes at the perimeter of the
mine, and/or dewatering from a sump(s) at the mine floor elevation, various other methods exist.
Regardless of the method used, the end result is that the local groundwater in and immediately
around the opencast will be at the elevation of the bottom of the ore body by end of mining.
The northern ore body is subdivided into a north-eastern section and a south-western section by a
fault with the south-western section downthrown by about 100 metres. Thus, while the bottom of
mining of the north-eastern section is predicted to be at 50 metres below ground level, the south-
western section could be as deep as 150 metres. This depression in the groundwater will result in a
cone of depression around the opencast, with the radius depending on the hydraulic conductivity of
the host material, as illustrated in Figure 12 below. Should the hydraulic conductivity be relatively
low, the cone of depression would be localised around the immediate vicinity of the mine, while a
high conductive bedrock material could result in the groundwater level being drawn down to below
the river.
Post mining, the groundwater will return to pre-mining levels, or even above pre-mining levels in
the lower sections of the opencast. This is due to the very high hydraulic conductivity of the
backfilled material in comparison to the undisturbed bedrock material that will tend to flatten the
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water level in the opencast. Should the water level in the lower sections rise above the surface
level, decanting will result. Furthermore, normal groundwater flow from the backfilled opencast to
the river will resume. If the backfilled material is sulphide containing, these outflows will most
likely be contaminated with mainly sulphate and selected metals, and could also be acidic
depending on the neutralisation potential of the material and reactivity of the sulphides.
The contaminant generation potential is pronounced where the source minerals of contaminants
are in direct contact with water and oxygen underground. The opencast mining operations expose
reactive minerals to water and oxygen. Sulphides are the main minerals which react and contribute
to the formation of acid rock drainage (ARD).
Mining sections that are not in contact with groundwater flow paths i.e. flooded or stagnant
sections are unlikely to contribute to ARD formation. ARD formation may be enhanced and continue
at high rates if there are active flow paths through sections. Where water is flowing through moist
sections, ideal conditions for sulphide mineral oxidation exist.
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Figure 12: Conceptual groundwater drawdown and flow model
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8.1.2 Waste rock dumps
The major controlling factor of ARD in waste rock dumps is the wide distribution in the particle
sizes of waste material as it governs the dominant processes responsible for ARD generation. As
rainfall infiltrates into the rock dumps, fines are either washed out or consumed through sulphide
oxidation and neutralisation, while larger particles weather to smaller particles. Preferential flow
paths in the dump (between particles) may result in rapid drainage of water and thus reduce the
effectiveness of neutralisation. Oxygen diffusing into and circulating in voids between particles
together with water films covering particles provide optimum conditions for sulphide oxidation,
especially as the dumps are unsaturated. Dust originating from the rock dumps may also settle on
surface water bodies and contribute to pollution. Rock dumps have a large potential to generate
ARD due to more exposure to the atmosphere.
In general the material (soil horizon) underlying waste rock dumps have a lower permeability than
the material of the waste rock dump itself. As a result, percolation of infiltrating water through the
rock dump into the subsurface may be limited. The leachate generated from the dump will thus
contribute to surface runoff and contribute to seepage in the soil/weathered horizon. It can be
assumed that mounding of the phreatic surface (water table) in the rock dump will be pronounced,
thereby increasing outflow from the dumps. The soil below the rock may continue to act as a
secondary source even once the all the rock has been removed (although the removal of the rock
would be unlikely).
8.1.3 Tailings dams
These facilities usually consist of an unsaturated upper zone and saturated lower zone. In the
unsaturated zone oxygen penetrate to some depth and sulphide oxidation can occur. The saturated
zone forms due to infiltration resulting in a rise in the phreatic surface in the dams. The presence
of water in this zone suppresses chemical reactions.
The tailings is characterised by small particle size, resulting in large exposed surface area of
reactive minerals (sulphides). This enhances the rate of weathering processes resulting in the
formation of ARD leachate. Due to the fine particle size of the tailings, oxygen penetration in the
dam is limited in the saturated zone. Normally these dams do have an impact on the groundwater
quality. The soil below the dam is also likely to act as a secondary source of contamination.
8.1.4 Workshops, septic tanks and domestic waste disposal sites
Workshops, fuel dispensing areas, septic tanks and waste disposal sites may contribute to the
contamination potential of the mine. Hydrocarbons may be found in elevated levels in the soil,
groundwater and surface water in the area where they are handled (workshops and fuel dispensing
areas). Although waste disposal sites and septic tanks do not contribute largely to the potential
contaminant load of the proposed mine, they may impact in localised areas around the sites. The
potential impacts include groundwater, surface water and soil. It is currently unknown whether the
above mentioned contaminant sources will existed on the site and where they will be located.
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8.2 PATHWAYS
Pathways along which contaminants may be mobilized and migrate toward groundwater receptors
include:
The vadose zone (unsaturated zone)
Groundwater (fractured aquifers)
Surface runoff in storm water or water courses (rivers and streams)
The scope of this study is however only to characterise the pathway of concern viz. groundwater.
Seepage from the tailings dams, waste rock dumps into the vadose zone and fracture systems of
deeper aquifers can lead to the contamination of groundwater and consequently water supply
boreholes. For accurate prediction of the behaviour of a contaminant plume along pathways it is
critical that the monitoring and field measurements are representative of the physical
environment. It is also important to keep seasonal and annual trends in mind as it affects the water
quality at the receptor.
8.2.1 Site specific hydrogeology
Although aquifers can vary considerably regarding geohydrological characteristics, they are seldom
observed as isolated groundwater systems. Usually they would be interconnected by means of
fractures, faults and intrusions. As mentioned in section 4.4, parts of the Northern Limb of the
Bushveld Complex is characterised by abundant faulting and elevated borehole yields. This
Volspruit project is located in one of the uncharacteristically highly fractured parts of the Northern
Limb. Therefore the hydrogeological conceptualisation of this project is different from
conventional Bushveld Complex conceptual model. Observations made by Genmin in 1990 found the
core to be heavily fractured with extensive serpentinised alteration, which may indicate the flow of
water along these structures.14 It was further mention in that study14 that water related problems
were found during the operation of the Grasvally chrome mine which lies 3km north of the project
area. An observation made in the study was that borehole yields became more reliable and
sustainable towards the south inferred to be the result of increased fracturing near the Zebediela
fault, this was however not substantiated. The study found the water table to be fairly flat in the
project, due to the high transmissivity of the subsurface and relatively shallow (~20-~37mbgl) over
abstraction of groundwater.14
The rocks appear to be fractured and weathered below most of the site to a depth of 50-60mbgl as
deduced in section 4.3.1. This finding is also substantiated by the groundwater study in 199014.
Therefore the top 50m of the rock profile defined by weathering and fracturing is characterised by
intergranular and fracture flow. Groundwater flow and storage is likely occurs in the fractures and
matrix. As the weathering decreases with depth, fracturing becomes dominant over weathering.
Flow and storage of groundwater therefore occurs in fractures in the rock mass.
The faults in the project area act as conduits and are associated with large volumes of groundwater
as substantiated by the hydrocensus and borehole drilling on the faults. A borehole targeted on a
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zone of deep weathering also had a high yield (sustainably pumped at 6l/s), this borehole was
drilled near the Nyl River.
The Nyl River and alluvial sediments are likely to be hydraulically connected to the weathered and
fractured aquifer. This could however not be substantiated by the waters geochemical signatures
due the recent rainfall at the time of sampling. Although different aquifers units are likely to be
found, they are inferred to be hydraulically connected with each other. It can said that the aquifers
are likely to be recharged by rainfall and the Nyl River.
8.3 RECEPTORS
Any user of a groundwater or surface water resource that is affected by drawdown of the
groundwater level or pollution from any of the above mentioned sources, is defined as a receptor.
The following receptors may be found:
Groundwater users by means of borehole abstraction
Water courses: water users, fauna and flora.
The main water uses in the vicinity of the mine are domestic and agricultural, while the nearby Nyl
River is a sensitive water course with several wetlands upstream and downstream.
The river is likely to be gaining and losing river depending on the season. A lowering of the
groundwater level could result in a local reduction of inflow to the river. Furthermore,
contaminated surface and groundwater is likely to impact on the river water quality. If the river is
gaining after mine closure then potential pollution emanating from the mine activities may impact
on the river.
The primary receptors of groundwater in this area are irrigation and domestic users. The potential
receptors may be impacted on in terms of groundwater quality and/or quantity can be seen in
Table 17. It must noted that no access was gained to the property immediately south of Volspruit
326 KP Portion 2 owned by Mr. M. Mazzaro.
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Table 17: Potential Receptors
Borhole Number
Owner Use Static water level
(mbgl) Farm name
ABDUL Abdulla Irrigation 6.18 Rondeboschje 295 KR
GV5 K. Kusbach Domestic 28.8 Grasvally293KR
GV6 Mine Domestic no access Grasvally293KR
GV7 Not at home Irrigation 23.42 Grasvally293KR
GV8 Niemcor Irrigation 37.75 Zoetveld 294 KR
VOLS1 D. De Beer domestic and
Irrigation 27.85 Volspruit 326 KP Portion 2
VOLS2 D. De Beer Irrigation no access Volspruit 326 KP Portion 2
VOLS3 D. De Beer Irrigation 23.81 Volspruit 326 KP Portion 2
VOLS4 D. De Beer Not in use 24.3 Volspruit 326 KP Portion 2
VOLS5 D. De Beer Irrigation 16.1 Volspruit 326 KP Portion 2
VOLS6 D. De Beer Irrigation 14.73 Volspruit 326 KP Portion 2
VOLS7 D. De Beer Irrigation no access Volspruit 326 KP Portion 2
VOLS8 D. De Beer Irrigation 9.98 Volspruit 326 KP Portion 2
VOLS9 D. De Beer Not in use 10.7 Volspruit 326 KP Portion 2
VOLS10 D. De Beer Irrigation no access Volspruit 326 KP Portion 2
VOLS11 D. De Beer Irrigation 15.17 Volspruit 326 KP Portion 2
BOK1 J. De Klerk Domestic 7.53 Bokpoort 328 KR
BOK2 J. De Klerk Domestic no access Bokpoort 328 KR
BOK3 J. De Klerk Domestic 11.88 Bokpoort 328 KR
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9 NUMERICAL MODEL
It is the aim of this chapter to determine the implication that the groundwater might have on the
proposed opencast mining, and to assess the likely hydrogeological impact that the mining
operations might have on the receiving environment.
Numerical groundwater modelling is considered to be the most reliable method of anticipating and
quantifying the likely impacts on the groundwater regime. The model construction will be
described in detail in the following paragraph, followed by predicted impacts in terms of
groundwater quality and quantity for the relevant mining phases.
The finite difference numerical model was created using the US Department of Defence
Groundwater Modelling System (GMS Version 7.0.3, build date Feb 22, 2010) as Graphical User
Interface (GUI) for the well-established Modflow and MT3DMS numerical codes.
MODFLOW is a 3D, cell-centred, finite difference, saturated flow model developed by the United
States Geological Survey. MODFLOW can perform both steady state and transient analyses and has
a wide variety of boundary conditions and input options. It was developed by McDonald and
Harbaugh of the US Geological Survey in 1984 and underwent several overall updates since. The
latest update (Modflow 2000) incorporates several improvements extending its capabilities
considerably, the most important being the introduction of the new package called the Layer-
Property Flow Package.
MT3DMS is a 3-D model for the simulation of advection, dispersion, and chemical reactions of
dissolved constituents in groundwater systems. MT3DMS uses a modular structure similar to the
structure utilized by MODFLOW, and is used in conjunction with MODFLOW in a two-step flow and
transport simulation. Heads are computed by MODFLOW during the flow simulation and utilized by
MT3DMS as the flow field for the transport portion of the simulation.
The structure of the remainder of this report is divided into the following main sections:
Construction of the numerical model, including a description of the boundaries used and
subdivision of the model into discrete finite difference cells.
Prediction of the groundwater drawdown due to dewatering at the Volspruit Northern Pit.
9.1 FLOW MODEL CONSTRUCTION
In this paragraph the setup of the flow model will be discussed in terms of the conceptual model as
envisaged for the numerical model, elevation data used, boundaries of the numerical model and
assumed initial conditions.
9.1.1 Elevation data
Elevation data is crucial for developing a credible numerical model, as the groundwater table in its
natural state tends to follow topography. The best currently available elevation data is derived
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from the SRTM (Shuttle Radar Tomography Mission) DEM (Digital Elevation Model) data. The SRTM
consisted of a specially modified radar system that flew on board the Space Shuttle Endeavour
during an 11-day mission in February of 2000, during which elevation data was obtained on a near-
global scale to generate the most complete high-resolution digital topographic database of Earth23.
Data is available on a grid of 30 metres in the USA and 90 metres in all other areas. The data points
in the Volspruit mining area are shown in Figure 14 below. It should be emphasised that this figure
is not derived from satellite photography, but is purely a 3-D presentation of the elevation data.
The accuracy and density of the data is deemed adequate in relation to the size of the Volspruit
opencast areas.
This data has been used for delineating the model area as depicted in Figure 14. Due to the density
of data it is not possible to picture the points over the whole modelled area. In total, 102 000
elevation data points define the model topography.
Several studies have been conducted to establish the accuracy of the data, and found that the data
is accurate within an absolute error of between 2 and 4 metres for Southern Africa24. Over a limited
area, as in this study, the relative error compared to neighbouring point is expected to be less than
one metre. This is adequate for the purpose of a numerical groundwater model, especially if
compared to other uncertainties; and with the wealth of data this results in a much improved
model.
23 http://www2.jpl.nasa.gov/srtm/
24 Rodriguez, E., et al, 2005. An assessment of the SRTM topographic products. Technical Report JPL D-31639, Jet Propulsion Laboratory, Pasadena, California.
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Figure 13: SRTM Elevation Points
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Figure 14: SRTM Elevation Data
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9.1.2 Lateral Boundaries
To simulate the groundwater conditions that will be affected by the proposed pit, the aquifer has
been modelled as described below. Boundaries were chosen to include the area where the
groundwater drawdown could reasonably be expected to spread and simultaneously include as
many natural groundwater boundaries as possible.
Wherever practical, natural topographical water divides are normally used as no-flow boundaries,
assuming that the groundwater elevation follows the topography. Inspection of the topography data
as described above revealed that the area is indeed bounded by well-positioned natural boundaries,
depicted in Figure 15 below. Natural water divides were used as no-flow boundaries in the eastern
and western extents of the model. In the north and south, the model was terminated with parallel
flow boundaries, on the assumption that groundwater will flow perpendicular to the surface
contours. A small inflow and outflow area in the south and north, coinciding with the Nyl River, was
used as a constant head boundary at an elevation of 5 metres below ground level.
These boundaries resulted in a modelled area of about 5 to 15 km around the proposed Volspruit
Northern Pit, which is considered far enough apart for the expected groundwater effects not to be
influenced by the boundaries.
The modelling area was discretizised by a 220 x 200 grid refined at the Volspruit Pit, resulting in
finite difference elements of about 50 x 50 metres at the Pit increasing to somewhat more than one
kilometre square. All modelled features, like mining areas, etc., are sizably larger than these
dimensions, and the grid is thus adequate for the purpose. Nevertheless, the total amount of active
cells over all layers added up to about 20 000, resulting in a relatively large model.
9.1.3 Vertical Delineation
For the purpose of this study, the subsurface was conceptually envisaged to consist of the following
hydrogeological units:
The upper few meters below surface consist of completely weathered material. This layer
is anticipated to have a reasonable high hydraulic conductivity, but in general unsaturated.
However, in the immediate vicinity of proposed tailing dams and waste rock dumps the
groundwater level is kept close to surface due to the availability of water, and this layer is
probably at least partially saturated. Furthermore, a seasonal aquifer perched on the
bedrock probably also forms in this layer, especially after high rainfall events. Flow in this
perched aquifer is expected to follow the surface contours closely and emerge as fountains
or seepage at lower elevations.
The next few tens of meters are slightly weathered, fractured bedrock with a low hydraulic
conductivity. The permanent groundwater level resides in this unit and is about 2 to 10
meters below ground level. The groundwater flow direction in this unit is influenced by
regional topography and for the site flow would be in general northerly from high lying
areas.
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Below a few tens of meters the fracturing of the aquifer is less frequent and fractures less
significant due to increased pressure. This results in an aquifer of lower hydraulic
conductivity and slower groundwater flow velocities.
Fracturing of the bedrock could consist of both major fault structures and/or minor pressure-
relieve joints. On a large enough scale (bigger than the Representative Elemental Volume) the
effect of these structures become less important and has been considered as a relative
homogeneous aquifer in this study.
The following assumptions and simplifications were made in constructing the numerical model:
The upper completely weathered aquifer perched in the bedrock is mostly unsaturated over
the study area. Although it is an important part of the hydrogeological system in this area
in the mining areas, it has not been modelled as a separate component as this will result in
abundant dry cells and consequential numerical instability. It has thus been grouped into
the upper layer of the model.
The bedrock has been modelled as five layers of decreasing hydraulic conductivity and
specific yield. Fractures in bedrock close up at depth, which result in a lowering of the
hydraulic conductivity25.
The local effect of discontinuities, such as faults, fractures and intrusions, has been
disregarded. Pumping test has indicated highly conductive areas not only at fault locations,
but also in seemingly un-faulted areas.
Laterally there is also no indication that faulting is limited to the ore body. With reference
to Figure 3, it can be seen that faults are found throughout the valley. It is therefore
unlikely that the highly transmissive aquifer is confined to the major faulting zones or the
ore body, and was therefore applied to the entire modelled area.
To ensure that the Volspruit model is not restrictive in depth, a total depth of 250 metres was
modelled, which is well below the lowest elevation of 150 metres below surface for the planned
pit. The model was subdivided in five layers of 50 metres each to fully exploit the 3D capabilities of
the modelling software and to include flow from both the sides and through the bottom of the
excavated pit.
Pump tests generally supply valuable information on the hydraulic conductivity of the subsurface.
However, unless sections of the borehole are sealed off with packers, this information is an average
value for the borehole. Thus, hydraulic conductivity (or alternatively, transmissivity) derived from
pump tests cannot be extensively used in 3D numerical simulations.
If suitable geotechnical borehole logs are available, the RQD (rock quality designation) is a very
helpful tool to derive the variation of conductivity with depth. In a study26, it was found that the
25 Barnes, S. L. et al. Coal Mine Drainage Prediction and Pollution Prevention in Pennsylvania. Pennsylvania
Department of Environmental Protection.
26 Gates, William C. B. The Hydro-Potential (HP) Value: a Rock Classification Technique for Estimating
Seepage into Excavations
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RQD is directly proportional to the hydro-potential (HP) value, which is defined as the potential for
a rock mass to hydraulically transmit groundwater.
In the Volspruit geotechnical study, a few boreholes were logged for RQD, as illustrated in Figure 1.
The calculated HP values are remarkably similar for the logged boreholes and correlates reasonably
well with the core recovery values. It thus seemed to be the best indicator of variation of hydraulic
conductivity with depth, given the available data. It follows from this graph that RQD varies rapidly
from close to zero at surface, and stabilise at a value of about 80% at a depth of about 50 metres
below ground level. This correlates very well with another finding25 that the hydraulic conductivity
due to fracturing resulting from stress release of bedrock, varies by an order of magnitude for every
50 metres below surface.
Given the RQD values calculated for the Volspruit project, it was thus decided to allocate hydraulic
conductivities to the layers as shown in Table 18 below. In essence, the hydraulic conductivity of
the middle two layers has been taken half of that of the upper layer, while that of the lower two
layers have been halved again. As no boreholes penetrated to depths exceeding 150 metres, this
further halving of the hydraulic conductivity of the lower layers is an assumption. However, it is a
reasonable assumption25 and, as no mining is anticipated below 150 metres, the assumed values will
only influence inflow through the mine floor. The ideal would be to conduct detailed packer tests
in freshly drilled core boreholes in future to increase the accuracy of this assumption.
9.2 FIXED AQUIFER PARAMETERS
Hydraulic conductivity is probably the most important aquifer parameter for which a sound value
needs to be established. For this model less reliance has been placed on calibration to determine
hydraulic conductivity, while results obtained from pump testing was considered the best to use.
This was a result of the very flat water table over the study area that did not favour calibration
techniques. However, extensive pump test were done and values obtained from these was
considered the most reliable available at the time of the study (Table 18 and Table 19).
The remaining aquifer parameters normally have to be calculated and/or judged by conventional
means. The following values were used:
Recharge = 20 mm/a ≈ 0.00006 /d. This value was calculated using the RECHARGE
program27 and the chloride method as described earlier in this report. This value relates to
a recharge percentage of 3.2%, very close to the general accepted value of 3% for the South
African Highveld28. Please note that this is not effective recharge, as evapotranspiration
was also modelled as discussed below. The result will thus be higher recharge in high
topographical areas and lower recharge where the water table is shallow, similar to the
conditions in nature.
27 Gerrit van Tonder, Yongxin Xu: RECHARGE program to Estimate Groundwater Recharge, June 2000. Institute for Groundwater Studies, Bloemfontein RSA. 28 Vermeulen P D, Usher B H: An investigation into recharge in South African underground collieries. The
Journal of the Southern African Institute of Mining and Metallurgy, Vol. 106 Nov 2006.
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Maximum Evapotranspiration = 1770 mm/a ≈ 0.0048 m/d. This value is based on the E-pan
evaporation data for this area29 as shown in Table 1. Note that this rate of
evapotranspiration is used by the modelling software only if the groundwater should rise to
the surface. For the groundwater level between the surface and the extinction depth, the
evapotranspiration is calculated proportionally.
Evapotranspiration Extinction Depth = 2 m. This depth relates to the expected average root
depth of plants in this area.
The specific yield and storage over the area was taken as 0.01 and 0.0001 respectively, as
calculated from the pump test data as documented in Table 18 below.
Hydraulic Permeability of the mined out and rehabilitated opencast area = 1 m/d. This is
two orders of magnitude larger than the pre-mining conditions, and typical that of a sandy
gravel.
Vertical Hydraulic Anisotropy (KH/KV) of the bedrock = 1. There are no layering in this
geological setting and no indication that either horizontal or vertical fracturing dominates.
Vertical Hydraulic Anisotropy (KH/KV) of the backfilled opencast = 1, as no post mining
layering is anticipated.
The effective porosity value was taken as 0.01. This value was assumed similar to the
specific yield.
Longitudinal dispersion was taken as 50 metres, which is about 10% of expected plume
dimensions, as recommended in various modelling guidelines.
Transverse and vertical dispersion was taken as 5 metres and 0.5 metre respectively.
Stream conductance = 0.01. A value close to the expected aquifer hydraulic conductivity
has been assumed as starting value.
29 http://www.dwaf.gov.za/hydrology
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Table 18: Calculated aquifer parameters
Borehole Average T (m2/d) Borehole Depth (metres) Hydraulic Conductivity (m/d) Specific Yield (Sy) Storativity(Ss)