Critical Considerations About Groundwater Flow in Closure Planning

Hidden beneath the surface, groundwater makes up approximately 99% of the Earth’s liquid freshwater and provides numerous social, economic, and environmental benefits and opportunities to our society (UNESCO, 2022). While groundwater is inextricably linked to surface water, its importance is often overlooked and underestimated. As water demand continues to grow across all sectors, so too does our reliance on groundwater resources (UNESCO, 2022). This demand highlights the importance of understanding hydrogeology, the study of groundwater, and developing effective strategies for sustainable groundwater resource management.

The Commonwealth Scientific and Industrial Research Organization (CSIRO)’s new Guidelines for Open Pit and Waste Dump Closure were developed to address a key gap as identified by de Graaf et al. (2021), which is the lack of industry standards for geotechnical and hydrogeological assessment of closure options for open pits and mine rock stockpiles (MRS). The release of these guidelines provides an opportunity to have an important conversation about groundwater and mine closure.

In this month’s Conversation on Closure, we explore a couple of critical groundwater considerations when planning for closure and land transition.

Are we accounting for groundwater data collection needs in our closure strategy and planning?

Mine closure requires both big-picture thinking and long-term planning. For groundwater, this means understanding the regional hydrogeological setting. While regional hydrogeologic settings can be managed during operations, implementing controls for closure can become challenging as mining activities (especially from dewatering) can lead to geological depressions, altered flow paths, and impacts to water quality (Gresswell et al., 2019; INAP, 2012).

Supporting a sustainable mine closure requires a clear understanding of surface water and groundwater systems at both local and watershed scales to facilitate effective closure mitigation. However, groundwater characterization to build this understanding can often be complex, and caution is needed when incorporating groundwater data into closure planning.

In the early stages of the mine lifecycle, groundwater data to support closure planning is typically limited. Comprehensive groundwater data is required to inform and guide the optimal siting of site domains to minimize potential groundwater impacts. Proper siting can minimize effects on shallow aquifers, dewatering impacts (Sloan et al., 2023), and critical domain liabilities such as a tailings facility failure. A common challenge at the early stages of the mine lifecycle is overreliance on preliminary groundwater data, which can produce inflexible, ineffective, or impractical closure plans and can lead to costly delays during closure.

Once a mine is approved, project monitoring priorities often shift to operations and permit compliance, which can lead to missed opportunities to understand surface water and groundwater behaviour and interaction at the site or watershed scale. Without ongoing evaluation of surface water and groundwater monitoring programs and their data, important knowledge gaps may remain unaddressed until the later stages of a mine’s life, where collecting the necessary data may be costly to conduct or no longer possible due to human impacts on pre-disturbance conditions or limited-to-no access to affected areas. Effective closure planning requires a groundwater-focused approach that collects the right data at the right time, often including data beyond what is critical for safe operations and compliance.

A groundwater-focused closure plan may vary for each operator, but it should provide enough detail to demonstrate the mine’s conceptual hydrogeologic setting (sources, groundwater pathways, and receptors), identify closure knowledge gaps, determine the risk profile, and outline mitigation measures and controls.

Closure plans should be flexible, iterative, and informed by multiple disciplines, including mine planning, geotechnical engineering, hydrology, and community engagement. The closure plan also serves as a communication tool to demonstrate understanding of the groundwater system to rightsholders, regulators, and other project stakeholders.

We are collecting groundwater data, but are we collecting the right data at the right time?

To facilitate this conversation, consider a mine site located in a mountainous region with a ridge-top open pit, a mine rock stockpile (MRS), and a tailings storage facility (TSF) in the valley bottom. The valley is in-filled with complex glaciofluvial or fluvial deposits, overlain by a river and wetlands. These deposits form shallow and deep aquifers that supply municipal water to downstream communities.

This mine operates pit dewatering and depressurization programs to safely mine. In accordance with permit requirements, a groundwater monitoring program is in place to assess groundwater seepage, where monitoring wells are installed downgradient of the MRS within shallow glaciofluvial deposits. Each well is instrumented with a pressure transducer to measure fluid pressure and is sampled quarterly, generating a substantial groundwater dataset over the life of mine. However, review of this dataset does not elucidate that the wells were installed without fully accounting for the complex depositional environment of the glaciofluvial deposits, and, as a result, physical gaps in aquitard layers were overlooked. Physical gaps in aquitard layers can create pathways for MRS-contact groundwater to migrate into a deeper aquifer (Bradbury et al., 2006). MRS-contact water in deeper aquifers is a critical consideration in closure planning as it can potentially impact municipal water supply to downstream communities.

In another example, a site in Australia developed a groundwater flow model based on existing data prior to ceasing pumping operations (Wels & Findlater, 2022). However, the model lacked early recovery data for calibration and, as a result, significantly underpredicted the rate of groundwater rebound at closure (Wels & Findlater, 2022). This case highlights that existing groundwater data collected during operations does not always address the objectives of closure.

The challenge in both mine sites is not the lack of hydrogeologic data, but rather the incompleteness of the data. Data incompleteness can be grouped into the following areas: validation, complexity, timing, biases, and siloing.

Validation

Ongoing validation of model predictions through groundwater monitoring is essential to develop an accurate knowledge base of site conditions over time to minimize the risk of unforeseen impacts and costly mitigation measures during operations, closure, and post-closure. For example, if a numerical model demonstrates a reduction in hydraulic head around the pit, it could suggest that the site’s bedrock has lower horizontal hydraulic conductivity. When horizontal conductivity is limited, water may preferentially flow through fractured bedrock in unanticipated directions (Impact Assessment Agency of Canada), potentially affecting deeper aquifers or surface water bodies. Depending on the jurisdiction, some permit conditions may not always require specific bedrock monitoring, so model predictions cannot be validated beyond operational dewatering requirements.

Complexity

The complexities of natural systems must be acknowledged, and suitable assumptions need to be accounted for to make informed decisions. Since groundwater is largely unseen, details of groundwater flow and interactions with surface water can be incredibly complex. Examples of groundwater complexity include:

A geological fault adjacent to a site’s open pit can block groundwater flow by separating permeable geologic units, but a fault beneath an MRS can enhance groundwater flow where sufficient fault gouge has created a zone of permeability.
Aquifers beneath the MRS can exhibit both upwards and downwards gradients.
Each watershed in and around a mine footprint can have different natural variations in groundwater quality due to changes in geology in each catchment.
Complex glacial and fluvial deposits in a valley bottom can result in varying aquitard distribution and thickness, which can cause complex interactions between shallow and deeper aquifers.

Timing

The timing to collect groundwater data is critical. Groundwater operates on a different temporal scale, where groundwater flow can occur quickly (within hours or days) or slowly (over decades or centuries). Understanding these temporal dynamics is vital to closure planning.

For instance, mine-impacted water may move through a shallow aquifer within days to weeks, while in a deeper aquifer, mine-impacted water could take decades to reach a downstream receptor such as a lake or a municipal water supply well. This difference demonstrates the importance of understanding groundwater flow timing when developing monitoring programs and well networks.

Trend analysis is another component of timing that requires a certain amount of data to identify and assess trends in groundwater variability. For instance, at a typical mine site, the minimum requirements often call for at least one year of quarterly sampling to establish existing groundwater quality and quantity conditions. However, once mine operations begin, the window for collecting pre-disturbance groundwater data from various catchments closes, and without early planning, mine sites risk developing conceptual models and closure plans based on incomplete baseline data.

Biased

Data bias is important to acknowledge so that mined materials in different domains are properly characterized for their variability. Spatial distribution in monitoring should reflect the site’s hydrogeological setting and the appropriate representative scale of measurement (Government of British Columbia, 2016).

Monitoring wells should span across varying hydrostratigraphic units to understand how each responds to changes (Government of British Columbia, 2016). For example, if more monitoring wells are installed in the shallowest aquifer while fewer are installed in deeper aquifers, aquitard characterization data may be limited. In our experience, this poses a significant risk, as aquitard distribution and characteristics control groundwater movement by influencing recharge and discharge between hydrostratigraphic units.

These biases often carry over into conceptual groundwater models developed for closure. Therefore, an important task in developing conceptual groundwater flow models is to evaluate their vulnerability to biased data.

Siloing

Like a mine closure plan, understanding groundwater closure relies on effective communication and interdisciplinary collaboration among permitting, operations, engineering, geology, geotechnical, mine planning, rightsholders, community relations, and other subject matter experts. Disciplinary silos can result in important groundwater data, observations, or Traditional Knowledge being unintentionally omitted from the closure planning process.

Okane’s Approach

At Okane, we believe that the earlier in the mine lifecycle you include considerations for groundwater, the smoother the pathway to closure becomes. Our approach to groundwater and closure is to conceptualize both the current and future behaviour of water onsite. Our conceptual closure models incorporate material characterization from saturated and unsaturated zones, water movement within the soil-plant-atmosphere continuum, landform-based and site-wide water balance, and interactions between groundwater and surface water.

Okane’s commitment to Water Stewardship prioritizes an integrated surface water and groundwater approach, grounded in catchment-based water management. Our team can help assess a site’s current operational and closure plans, and risk profile to help facilitate groundwater closure discussions at any stage of the life of mine.

To learn more about how Okane can support your groundwater management and closure planning, please contact us at info@okaneconsultants.com.

References

Bradbury, K.R., Gotkowitz, M.B., Hart, D.J., Eaton, T.T., Cherry, J.A., Parker, B.L., & Borchardt, M.A. (2006). Contaminant transport through aquitards: Technical guidance for aquitard assessment. Awwa Research Foundation. https://gw-project.org/books/contaminant-transport-through-aquitards-2/

Government of British Columbia. (2016). Water and air baseline monitoring guidance document for mine proponents and operators. Government of British Columbia, Ministry of Environment. https://www2.gov.bc.ca/assets/gov/environment/waste-management/industrial-waste/industrial-waste/water_air_baseline_monitoring.pdf

de Graaf, P.J.H., Beale, G., Carter, T.G., & Dixon, J. (2021). Geotechnical guidelines for open pit closure – A new publication by the Large Open Pit (LOP) project. In A.B. Fourie, M. Tibbett, & A. Sharkuu (eds), Mine Closure 2021: Proceedings of the 14th International Conference on Mine Closure, QMC Group, Ulaanbaatar. https://doi.org/10.36487/ACG_repo/2152_120

Gresswell, R., Foley, G., & Faithful, J. (2019). Hydrogeological modelling to inform closure planning for Hazelwood Mine. In A.B. Fourie & M. Tibbett (eds), Mine Closure 2019: Proceedings of the 13th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1353-1366. https://doi.org/10.36487/ACG_rep/1915_106_Gresswell

Impact Assessment Agency of Canada. (n.d.). Justify the ratio of horizontal to vertical hydraulic conductivity used for groundwater modelling. Impact Assessment Agency of Canada. https://iaac-aeic.gc.ca/050/documents/p54755/86656E.pdf

The International Network for Acid Prevention (INAP). 2012. Global Acid Rock Drainage Guide (GARD Guide). http://www.gardguide.com/

Sloan, S., Cook, P.G., & Wallis, I. (2023). Managed aquifer recharge in mining: A review. Groundwater, 61(3). https://doi.org/10.1111/gwat.13311

UNESCO World Water Assessment Programme. (2022). Groundwater: making the invisible visible. The United Nations World Water Development Report 2022. https://unesdoc.unesco.org/ark:/48223/pf0000380721.locale=en

Wels, C. & Findlater, L. (2022). Groundwater modeling as a tool for closure planning: Prediction of zinc transport for alternative cover scenarios. https://rgc.ca/wp-content/uploads/2022/03/Wels_Woodcutter.pdf