Abstract
Pit lakes present significant safety risks for boat-based water sampling crews. The Matrice-HydraSleeve method improves the safety of water sampling in pit lakes by eliminating the need for a boat-based sampling crew. The method connects an off-the-shelf unmanned aircraft system, or drone (DJI, Matrice 600) to an off-the-shelf water sampling device (GeoInsight, HydraSleeve). It is capable of collecting 1.75 L water samples from up to 122 m deep and is simpler than previous drone water sampling methods. To validate the method, water samples were collected from similar depths in Dillon Reservoir, Colorado, USA using the Matrice-HydraSleeve method and traditional boat sampling methods using a Van Dorn water sample bottle. Concentrations of Ca, Na, K, HCO3, SO4, Cl, and Zn showed less than 20% relative percent difference, and concentrations of Cd were within ± the detection limit, meaning variability between samples met the data quality objective for duplicate samples. The method was also used on two occasions to collect eight water samples from the 101 m deep pit lake at the Thompson Creek mine in Idaho, USA including a sample from 92 m deep. Calcium and sodium concentration profiles were nearly identical, indicating little change in water chemistry and providing confidence in the method. In situ profiles of temperature and electrical conductivity collected with a conductivity-temperature-depth probe (YSI, CastAway) suspended below the drone indicated the lake was meromictic during both sampling events. To date, the Matrice-HydraSleeve method has been used at 10 pit lakes in the USA to collect 81 samples. Use of this method at other pit lakes has the potential to improve safety while lowering sampling costs and increasing data acquisition, leading to better pit lake management.
Highlights
70% of the properties owned by the six largest mining companies exist in water-stressed regions (Beck 2018), where open pit mining often results in the formation of pit lakes following mine closure (Castendyk and Eary 2009)
Ex situ temperatures were ≈ 2 °C higher than in situ temperatures. This shift impacted the HydraSleeve samples and the Van Dorn samples suggesting that the temperature increase was caused by warming during sample retrieval. This is consistent with the findings of a review of drone water sampling by Lally et al (2019), who noted that water samples collected by drones had a different temperature than in situ water samples
Except for Cd and Mn, concentrations of each analyte exceeded 5 × contract required detection limit (CRDL), and the relative percent difference (RPD) of the analytical results was less than ± 20%
Summary
70% of the properties owned by the six largest mining companies exist in water-stressed regions (Beck 2018), where open pit mining often results in the formation of pit lakes following mine closure (Castendyk and Eary 2009). Routine water sample collection at multiple depths in the pit lake water column informs site water management; results are used to calibrate predictive water quality models, inform closure planning and trade-off studies, design appropriate water treatment plants, Golder Associates Inc., Lakewood, CO, USA 2 Thompson Creek Mining Company, Challis, ID, USA and monitor treatment progress and efficiency. Such studies factor into financial assurance estimates used in closure planning, bond calculations, and stakeholder disclosures. The benefits of pit lake characterization must be weighed against the risks of sample acquisition
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