Abstract:

Nitrate leaching into groundwater is an emerging issue of increasing environmental concern around the world. These nitrates may come from a range of natural and anthropogenic sources^[1] including “synthetic” fertilisers, animal manure, sewage plants and septic tanks, direct mineral and coal emissions and other societal sources, such as automobile use. The chemistry of Nitrogen compounds in groundwater is complex, with up to nine possible oxidation states reported^[2], however the most stable ionic forms (nitrate, NO₃^-; nitrite, NO₂^-; and ammonium, NH₄⁺) are well known in industrial fields of concern. In the Canadian mining industry (e.g. ^[3]), there is increasing focus for control of these compounds that may either partially or completely originate from the use of bulk (Ammonium Nitrate-based) explosives, with the possibility of nitrates being dissolved or leached from the bulk product following contact with water.

This paper reports on an internal study that has aimed to better understand the nature and factors affecting nitrate leaching into groundwaters around mine sites and then provide mitigation strategies through a holistic product management system. A three-stage nitrate risk reduction framework has been developed that in the first stage, concentrates on best-in-class on-bench practices to monitor and minimise losses and spillage of materials during: storage of Ammonium Nitrate components; filling of loading equipment; actual loading of blastholes; and then effective management of any excess bulk products. In conjunction with these processes, the framework secondly highlights the optimisation of blast design and on bench operational practices to minimise the amount of poorly- or un-detonated explosive residue remaining following blasting, and taking due account of site-specific factors such as fractured ground and stemming design. The third, complementary stage focuses on the use of specialist bulk explosives. These products: i) act as an enabler for the above stages by expanding the options for blast engineers to confidently design for high detonation efficiency across a broad spectrum of ground conditions and design constraints; and ii) remove or minimise the leaching of nitrogen compounds following loading and during in-ground “sleep time”. Through appropriate product design of the physical, mechanical, rheological, and surface and general chemical material properties, these optimised explosives can be tailored for high performance in a wide range of hydrogeological ground conditions including fractured ground, and seasonally-affected static and dynamic ground water conditions in blast holes of varying diameter and depth.

The role of specialist bulk explosives as an effective component within Best Available Technology and Techniques Economically Achievable (BATEA or BAT) programs in Mine Site Environmental Monitoring Plans for Explosives and Nitrogen Management is also described.

Footnotes:

^[1] Fowler, et al, (2013), The Global Nitrogen Cycle in the Twenty-First Century, Philos Trans R Soc Lond B Biol Sci., 368, 1621, 1-13.

^[2] Jermakka, et al, (2015), Potential Technologies for the Removal and Recovery of Nitrogen Compounds From Mine and Quarry Waters in Subarctic Conditions, Critical Reviews in Environmental Science and Technology, 45, 7, 703-748.

^[3] British Columbia Ministry Environment, Climate Change, (2018), Technical Guidance 9; Guidance on preparing Nitrogen Management Plans for Mines using ANFO for Blasting, V1.0 February 2018.

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Charles Chromicky