Open pit flooding as a post-closure option: a geochemical approach

Abstract

English: Australasian, European, Canadian and American geo-environmental specialists have long been involved with pit lake studies in the physical processes and dynamics as well as the geochemical aspects of these mitigation methods and events. The use of pit flooding as an environmental post-closure option has however been studied and used to a limited extent in Southern Africa and Africa to a whole, with some recorded cases mostly being by accident rather than a planned mitigation. This document is a written paper on a hydrogeochemical investigation on the feasibility of flooding an open pit platinum mine in the Bushveld Igneous Complex as a post-closure mitigation option. Various data sets and sources were compiled and processed and served as input data into a hydrogeochemical model of the expected impacts on the conceptual meromictic pit lake environment. Weathering and speciation models allowed the evaluation of the current system with thermodynamic and chemical reaction processes leading to the hydrochemical composition of the groundwater as we observe it in the field. Furthermore transient mixing simulations between groundwater, surface water and rain water was allowed to occur under various reducing and evaporative conditions. The modelled lake chemistry was then evaluated against recreational, agricultural and domestic water use standards to investigate the feasibility of the lake to be used post-closure. Final recommendations and mitigation methods were proposed with the flooding of the open pit as a post-closure option declared feasible. A final deliverable of the study was a system thinking and modelling template for hydrogeochemical modelling in various scenarios to guide other scientist through the process of modelling fluid-rock, fluid-waste and fluid-fluid interaction.

Afrikaans: Australasiese, Europese, Amerikaanse en Kanadese Geo-omgewingswetenskap spesialiste is al lank betrokke by put meer studies met navorsing op die fisiese prosesse en dinamika asook die geochemiese aspekte van hierdie omgewings versagtende metodes en gebeure. Die gebruik van put oorstromings en vloede as n omgewings post-operasionele opsie word egter min nagevors en ook gebruik tot n beperkte mate in Suider-Afrika en Afrika tot n geheel, met n paar opgetekende gevalle meestal per toeval eerder as n beplande versagtings metode. Hierdie dokument is n skriftelike opstel van n hidrogeochemiese ondersoek na die haalbaarheid van die oorstroming van n oopgroef-platinummyn in die Bosveld Stollings Kompleks as n post-operasionele omgewings versagtings opsie. 191 Verskeie datastelle en bronne is saamgestel en verwerk en het gedien as invoer data in n hidrogeochemiese model van die verwagte impak op die konseptueie put meer omgewing. Verwering en spesiasie modelle het toegelaat dat die huidige stelsel bestudeer kan word met termodinamiese en chemiese reaksie wat lei tot die hidrochemiese samestelling van die grondwater soos ons dit in die veld waarneem. Verdere meng simulasies tussen grondwater, oppervlakwater en reën water is toegelaat om plaas te vind onder verskillende vermindering en verdamping voorwaardes om die put meer water chemie te simuleer. Die gemodelleerde meer chemie is toe geëvalueer teen ontspannings-, landbou-en huishoudelike watergebruik standaarde om die haalbaarheid van die meer postsluiting as water bron te kan klasifiseer. Finale aanbevelings en versagtende metodes is voorgestel met die oorstroming van die oop put as n post-operasionele omgewings opsie as haalbaar verklaar. N finale doel en produk van die studie was n stelsel denk wyse en modellering gids vir hidrogeochemiese modellering in verskeie scenarios en dus ander wetenskaplike te kan lei deur die prosesse van hierdie tipe modellering.