Where is Your Mine-Impacted Water Going? 1

Where is Your Mine-Impacted Water Going?

Where is Your Mine-Impacted Water Going? 

A Cheat Sheet for Modelling with Hydraulic Conductivity  

 

Water quality impacts to surface water and groundwater receptors can be a major risk of mining operations. Managing that risk requires modelling, monitoring and management.  One of the major uncertainties in developing a thorough water quality model is understanding the partitioning of seepage through a mine rock or tailings landform.  Does the water report as surface water or as groundwater recharge?  

A key control of this partitioning is the hydraulic conductivity (K) of geologic media underlying mine waste landforms.  Similarly, hydraulic conductivity is an important control on mobilization and transport of contaminants through saturated mine rock and tailings.  Hydraulic conductivity is a physical property which describes the ease with which a porous material can transmit fluid (usually water) through its pore spaces. It differs from permeability in that it is dependent on both the material and the fluid characteristics.  Hydraulic conductivity is a parameter that is heavily relied on for use in numerical models to estimate travel times, velocity, and peak concentrations of contaminants of concern (COPC) reaching an environmental receptor. This article present considerations for one simple, yet effective, method to estimate in situ hydraulic conductivity and help conceptualization of water flowpaths at your site. 

The reliability of hydraulic conductivity estimates is dependent on three primary factors: 

1) How the tests were completed in the field 

2) Estimation parameters selected during data processing 

3) Appropriate interpretation by the solver  

Error introduced at any of these stages may lead to under or over-estimation at the magnitude scaleKeep reading for tips to avoid these errors in your tests and improve your model results 

In the Field

In situ horizontal hydraulic conductivity may be estimated in saturated deposits under confined or unconfined conditions.  ‘Slug tests are a common and effective method to estimate in situ hydraulic conductivity in saturated zones. Other type of K tests could include pump tests, infiltration testing, or permeameter testing.   

An in situ slug test begins by injecting or withdrawing a volume (typically a 1m3 solid slug) into the water column to generate an instantaneous change in pressure head. The change in displacement over time is monitored using a pressure transducer (logger). Appropriate test selection, range of logger, and reading interval of the logger is crucial to execute a reliable K test.  

Some common areas where error can be introduced into the test include: 

  • Logger Range: Loggers should be correctly selected based on anticipated height of water column. Ranges include 5, 10, 20, 30, 100, and 200 m head, with accuracy decreasing with increasing pressure head.  
  • Reading interval: Reading intervals should be selected based on the local lithology. This test may be limited by the soil texture and particle size. Care should also be taken in confirming adequate data points are available based on the interval chosen. The newest loggers have capacity for 100,000 readings [1]. For example, a reading interval of 5 seconds is appropriate for a slow recovery unit such as silty clay and will record measurements for approximately 5.8 days. 
  • Under-developed wells: Fine sediments clogging the screen prevents the movement of water into the well, resulting in an artificially low water table and low recovery in that location leading to under-estimated K values.  
  • Barometric effects: To account for atmospheric pressure effects on the water table, a logger capable of measuring both water level and atmospheric pressure should be used, or a second logger can be installed in the vicinity of the well. The results of the test will be compensated for measured barometric effects.  
  • Static Measurement: Accurate measurement of static water level prior to the test is crucial. Factors to keep in mind which could influence the static water level include barometric effects and underdeveloped wells as discussed above, as well as how the well is capped. If the cap on a monitoring well is secured too tight and does not allow any venting, the static water level measurement required to initiate a slug test will not be representative due to pressure buildup within the well. Best practice when time permits is to open the wells and allow them to equilibrate to static conditions before initiating the test.  

Data Processing

Parameter selection is dependent on which analytical solution is being used, under what conditions (confined vs. unconfined), and accuracy of physical measurements taken during execution of the test.  

The most common mathematical solutions for overdamped tests include Hvorslev, Bouwer-Rice (B-R), and Kansas Geological Survey (KGS). Underdamped tests are typically solved using the Butler method. Each solution has been developed for specific aquifer properties, and care should be taken in selecting which solution to use.  

  • Hvorslev was developed for use in confined aquifers, however, it can be used in unconfined aquifers provided there is sufficient distance between the water table and the well screen. A tendency to overestimate K due to omission of storativity effects in unconfined aquifers has been documented in literature [2].  
  • B-R is a modification of the Hvorslev method, developed to account for the effect of partially penetrating conditions [2], unconfined conditions, and wells that are screened across the water table.  
  • KGS is our preferred analytical solution. It is the most robust solution as it can account for skin effect, anisotropy, and specific storage. The KGS method can be applied to both confined and unconfined aquifers making it the most widely applicable solution, provided the water table does not intersect the well screen.  
  • Butler is a solution developed for underdamped tests characterized by very high K resulting in an oscillatory response (Figure 3). A pneumatic slug tester should be used in these cases to capture the data required. In these tests, recovery is too fast for manual readings to be taken with any confidence.  

Interpretation

To summarize some key consideration in the interpretation of K test, interpretation will be discussed in terms of B-R and KGS solutions. Assuming a test is completed in an unconfined aquifer in which the water table intersects the well screen, the B-R solution may be considered as a starting point. A filter pack correction is assumed for a porosity of 30%. Figure 2 illustrates the curve generated from the hypothetical test. Note the “double straight-line effect” which is an indication that B-R is an appropriate solution to use in this case.  

Interpretation of this solution involves curve matching and feasibility assessment. When curve matching to the appropriate portion of the curve, an ideal head range between normalized head 0.2 and 0.3 m/m is typically a reasonable starting point. In the below example, the ideal head range captures later time recovery, and would not be considered the best part of the curve to fit. To assess if the estimated K value is feasible, consider what is known about the site, like lithology, groundwater flow direction, and confined/unconfined conditions. 

Figure 2: B-R Example.  

Created in AQTESOLV (demo version) [3] 

Figure 3 shows an example of a reliable KGS solution in an unconfined aquifer with water level above the well screen. Curve matching in this case is typically computed by a program and involves less interpretation than the B-R solution above.  

In this example, the Ss value may be used to assess the reliability of the estimate. An Ss < 1E-6 m/s may indicate an underdeveloped well or other potential issues. Adjustments can be made to initial displacement, anisotropy ratio, and/or well radius relative to borehole radius (to account for skin effects) until Ss provides a reasonable magnitude estimate. In most programs, Ss can also be forced set to a minimum value of 1E-6 m/s. 

Figure 3: KGS Example 

Created in AQTESOLV (demo version) [3] 

Okane’s Approach 

Okane applies a comprehensive approach for developing hydrological and water quality models for greenfield and operating sites.  The hydrological and water quality performance of a mine waste landform are interpreted within the context of the site-specific hydrological and hydrogeological settings, and with appropriate characterization of materials governing COPC fate and transport.  

Okane takes great care in developing and interpretating numerical models that serve as powerful tools for understanding water flow and quality in mine-impacted watersheds.  Hydraulic conductivity is a key input in developing quantitative estimates of water flow and quality.  In the development of 2D and 3D numerical models, horizontal hydraulic conductivity is especially important as it dictates the lateral movement of water through the subsurface towards potential environmental receptors. Used in conjunction with other properties such as hydraulic gradients and COPC source and sink terms, accurate estimation of hydraulic conductivity is an important step towards developing a water quality model that informs the potential risk to water quality and effective strategies to mitigate this risk.  

References 

[1] Solinst. 2021. https://www.solinst.com/products/dataloggers-and-telemetry/3001-levelogger-series/ltc-levelogger/ 

[2] Sun, H., & Koch,M. 2014. Under -versus overestimation of Aquifer Hydraulic Conductivity from Slug Tests. Hamburg - Lehfeldt & Kopmann (eds) 

[3] AQTESOLV. 2021. https://www.aqtesolv.com  


Building Mined Rock Stockpiles for the Future 2

Building Mined Rock Stockpiles for the Future

Building Mined Rock Stockpiles for the Future

 

 

The Current State of Play

Despite the global pandemic, national economies like Australia have shown resilience due in large part to the strength of the resources sector. [1] Despite the benefits this performance brings to investors, governments, and communities, it is the sector’s environmental, social, and governance (ESG) performance which is dominating public discourse and impacting investment decisions. License-to-operate remained the leading risk to businesses within the resource sector from 2019 to 2020. [2]

The reason – Inability to predictably control the impact on environmental and heritage values of the land on which we operate.

Where then, can operators focus efforts to control these impacts? From an environmental perspective, the mine features which frequently represent the largest potential source of contaminants are mined rock stockpiles (MRS) or waste rock dumps (WRD). These constructed landforms can contain significant existing contaminant loads, and have the potential to continue producing contaminants which can be mobilized; this process is generally known as metal leaching and acid rock drainage (ML/ARD) or acid and metalliferous drainage (AMD). These landforms dominate our post-mining landscapes yet their design and construction are often only undertaken with immediate operational expenditure (OPEX) in mind. It is this singular focus which compromises not only our industry’s environmental performance, but also our legacy and long-term liability.

The Opportunity

By considering additional performance metrics such as projected seepage water quality and landform evolution during MRS / WRD design and construction, we can better understand the potential impacts of post closure liability on net present value (NPV). Understanding closure liability from an NPV perspective allows closure professionals to communicate upwards within their business and reach consensus about how full lifecycle environmental goals can be realised through smart landform engineering.

Fundamentally, limiting ML/ARD or AMD from an MRS or WRD is about managing the supply of oxygen into reactive waste, then limiting contaminant transport through surface water management and/or reduction of water through the waste (net percolation; NP). The way in which we manage or place mined rock, in combination with site-specific controls (e.g., oxygen availability, rainfall, temperature, surface water) largely drives the potential for negative impacts. Therefore, with years of research undertaken and an industry understanding of ML/ARD or AMD processes, some historically used MRS or WRD dumping and construction practices are no longer acceptable.

A Path Forward

At Okane, we couple geochemical and hydrological modelling with mine-planning processes to evaluate a range of construction scenarios in respect to closure goals and NPV. We empower operators to make smart decisions about how legacy and liability are controlled.

Our process, rooted in the observational method,[3] takes a site-specific approach through risk and opportunity analysis. Using Okane’s approach to integrated life of mine planning:

  1. We seek to understand landforms and build a Conceptual Site Model (CSM) that considers climate, hydrological setting, and material characteristics.
  2. We seek to build consensus on the site-specific environmental and social risks through collaborative risk assessment processes that includes stakeholders from across the operational setting.
  3. Our team of geotechnical engineers, geochemists, and environmental scientists address knowledge gaps to update the CSM.
  4. We develop landform design and construction options based on the CSM, considering site-specific risks and opportunities that have been agreed on.
  5. We evaluate options from a multi-criteria perspective, considering hydrological and geochemical performance through modelling.
  6. Options are then integrated into mine-planning software to project associated costs, residual risk, and conformance to regulatory requirements.

We present the range of options and their associated performance to our clients and stakeholders to inform decision-making and facilitate consensus building.

The landform option carried forward then becomes a fully integrated part of the mine plan, and operators no longer need to make unverified assumptions about the potential environmental impact of their MRS or WRD. Okane’s approach empowers operators to:

  • Control uncertainty with regards to latent water quality impacts.
  • Provision for projected schedules and cost.
  • Plan with confidence to manage liability and achieve post-mining land uses.
  • Communicate the suitability and benefits of a given approach.

Okane has partnered with clients across the mining industry for 25 years and has established a clear Roadmap to Closure by managing closure performance from feasibility assessment through to relinquishment.

 

1 Constable, T (2020) Minerals Council Australia, accessed July 2021 https://www.minerals.org.au/news/mining-largest-contributor-australian-economy-2019-20

2 Mitchel (2020) Ernest and Young, accessed July 2021. https://www.ey.com/en_au/mining-metals/10-business-risks-facing-mining-and-metals

3 Terzaghi and Peck (1967) Soil Mechanics in Engineering Practice, 3rd Edition | Wiley


8 Success Factors to Consider When Designing Your Cover System During Mine Operations or at Closure 3

8 Success Factors to Consider When Designing Your Cover System During Mine Operations or at Closure

8 Success Factors to Consider When Designing Your Cover System During Mine Operations or at Closure

 

What is a Cover System?

A cover system is a type of engineered barrier used to manage mine waste during operations or at mine closure.  A cover system may be constructed using naturally available materials, such as locally available soils and mine waste with appropriate physical and chemical properties. However, they may also be constructed from a combination of natural and synthetic materials, with synthetic materials providing physical properties that natural materials alone cannot.  A cover system’s main purpose is to provide a reclamation surface for mine waste storage facilities (e.g. waste rock dumps, tailings storage facilities) that provides a stable, reliable, and sustainable interface between the receiving environment and the mine waste (Figure 1)[1].

Figure 1:     Conceptual schematic of a cover system.

INAP, 2017[1].

Design objectives for cover systems can vary, however common objectives are typically developed to address physical stability and chemical stability. Physical stability of a cover system is related to the overall landform design and may address concerns related to surface water management, dust and erosion control. Cover designs can also address chemical stability whereby water movement through a cover system and into stockpiles of reactive mine rock is to be minimised, leading to reductions in contaminant loadings into the receiving environment.  We refer to the water movement through a cover system and into the underlying waste material as “Net Percolation”1.

Mining landscapes can be described at several scales: Region, Landscape, Landform, Landform Element and Microsite[2] (Figure 2).  A cover system design falls within the Landform Element scale and is an integral component of waste stockpile design, for example, waste rock dumps and tailings storage facilities.  Therefore, integration of cover system and landform objectives is critical, and both sets of objectives must contribute towards achieving the agreed returning land use.

Figure 2:     The mining landscape.

LDI, 2021[2].

How Can a Cover System Help You Minimise Your Environmental Impact?

Cover system functions and types are broad, and selection depends on a site’s climate and the environmental impact(s) requiring mitigation.

There are six general cover system functions, some of which include several variations for achieving the intended function (Figure 3)[1]:

  1. Reclamation/Revegetation;
  2. Erosion Protection;
  3. Store and Release Cover;
  4. Enhanced Store-and-Release Cover
    • Capillary Break Layer;
    • Seasonality Frozen Capillary Break Diversion (SFCBD);
  5. Barrier (Ksat ≤ 1 x 10-7cm/s);
    • Compacted Clay Layer (CCL);
    • Compacted Sand Bentonite (CSB);
    • Permanent Frozen Layer (or Permafrost Aggradation Layer);
    • Geosynthetic Materials; and
  6. Saturated Soil or Rock.

Figure 3:     Cover system function; a) Reclamation/revegetation; b) erosion protection; c) store-and release and enhanced store-and-release; d) enhanced store-and-release – capillary break; e) barrier; f) saturated soil or rock.

INAP, 2017[1].

Cover system designs may incorporate multiple functions.  For example, it is very common to have reclamation/revegetation, erosion protection, and store and release cover functions coupled with a primary objective, such as minimizing contaminated seepage.  In this example the cover design does not solely focus on minimising contaminated seepage but also captures the reclamation/revegetation and erosion functions providing a more robust solution. The overall solution establishes the target vegetation community and achieves acceptable erosion rates, which in turn have positive impacts to the cover systems ability to mitigate contaminated seepage.

Okane’s Approach

Okane’s approach to designing cover systems for mine waste has been developed over 25 years working on mine sites spanning the globe across all climate regimes. Unlike other geotechnical consultants we integrate mine closure with mine planning to deliver optimized cover system design solutions for our clients at all mining stages.  Over the years Okane has developed the following eight key factors that will ensure the success of your cover system:

  • Set clear returning land use expectations;
  • Define realistic cover system objectives;
  • Know your site and material characterisation;
  • Align your landform design and cover system design;
  • Watch out for your basal flow conditions;
  • Look at long term surface water management;
  • Validate the availability of desired cover material; and
  • Consider constructability.

Okane is the industry leading expert in cover system design and performance monitoring. Our multidisciplinary team can help you design, construct and monitor cover systems that will complement your final landform designs, and help you achieve your mine closure plans.

[1] International Network for Acid Protection (INAP), 2017.  Global Cover System Design--Technical Guidance Document.  November.

[2] Landform Design Institute (LDI), 2021.  Mining with the end in mind: Landform design for sustainable mining.  Position Paper 2021-01.  March.  Landform Design Institute.  Delta, BC, Canada.