Nitrogen Removal from Mining Dewatering and the Fate of Water Release to a River

Abstract

Seepage water in underground mining voids arises from interception with aquifers that may become contaminated with nitrogen (N)-based explosive during underground blasting operations. The nitrogen-contaminated water may be pumped from underground mining voids and stored at the surface or discharged to a nearby river without treatment if sufficient dilution is achieved to minimize effects on the aquatic environment and pastoral activities. Maximum N concentration in dewatering water was observed to be 200 times the receiving water of the nearby river when compared against ANZECC/ARMCANZ (2000) water quality guidelines. This study sought to identify sources of N contamination of underground seepage water, pumped to the surface for discharge to the nearby river. The level of protection (for an aquatic ecosystem) specified the water quality objectives to be achieved following water release. The ANZECC/ARMCANZ (2000) has three levels of aquatic ecosystem protection, and ’Level 2 Aquatic Ecosystem’ was selected being described as applying to slightly-moderately disturbed systems including rural streams receiving runoff from land disturbed to varying degrees by grazing or pastoralism, nearby or adjacent to urban areas. The study considered river flow data, underground dewatering production, discharge rates, and river water quality for upstream and downstream of the discharge point, as well as quantities of explosives used underground at specified % N in explosives estimated to remain following blasting. The most important nitrogen form present in the mine discharge wastewater was found to be oxidized-N (>93%). Only 0.54 % of end-of-pipe discharge was measurable in the downstream river and indicated that rapid dilution and removal of oxidized-N (nitrate/nitrite) by biota could be achieved. The level of oxidized-N expressed as load discharged to the river showed that the quantity of N discharged was similar in magnitude to the spillage of 4% explosive.