Development of a numerical model for unsaturated/saturated hydraulics in ash/brine systems - Menghistu, Mehari Tewolde (University of the Free State, 2010)
English: Vast quantities of coal combustion residues (ash) and effluents are produced simultaneously in the coal processing facilities of South African parastatals, Eskom and Sasol. The handling and disposal of saline effluents is a difficult and complex problem. The current practice used by these parastatals is to co-dispose the effluents with the ash in landfills; so-called ash dams. Although this practice provides a potentially elegant approach, at least from the viewpoint of the generator of both the ash and effluent, the co-disposal of ash and brine in a landfill could have dire consequences on the environment of the landfill site. This applies in particular to the release of environmentally deleterious and toxic constituents of the ash into the air, soil, surface and groundwater which can lead not only to environmental and land-use problems, but also jeopardize the health of organisms living in the surrounding ecosystem. The question therefore arises as to how Eskom and Sasol better manage their ash dams, to not only satisfy all legal requirements and possible pressure from social awareness groups, but also more importantly, prevent, or at least limit, pollution of the natural environment. The present investigation arose from a request by Eskom and Sasol to provide them with a detailed proposal for a framework with a view to increasing the competency of both organizations in the management of the co-disposal of ash and brine and the dissemination of knowledge with regard to the impacts of the sites. Two sites were selected by the organizations for this purpose: the Tutuka Power Station and the Secunda Synthetic Fuel Plant. The application of geohydrological models to assess the behaviour of a waste disposal site has historically often been viewed as an attempt to predict the future behaviour of the site. However, this would require information on relational parameters and known interactions whose behaviour far into the future cannot be determined with certainty. A geohydrological model should therefore never be viewed as an attempt to predict the future of a given waste site, but rather as an aid to assess how effectively the site is managed and controlled. The best way to achieve this is to investigate the waste site systematically, preferably utilizing wellestablished and accepted international methodology. Unfortunately, at this time, no documents exist that describe such a methodology, its implications and the steps necessary to implement it in practice in a way that can also be understood by interested members of the public. A two-dimensional transient model for flow through saturated/unsaturated porous ash dump media has been developed. This model numerically solves the governing partial differential equations, which are highly non-linear. The model code uses quadrilateral finite elements for the geometrical assembly: the bilinear Galerkin interpolation for the spatial integration, and the Gaussian elimination for the solution of the resulting matrix equations. In addition to the usual constant-flux and constanthead boundary conditions, the code is capable of applying pressure-dependent boundary conditions at the ground surface. Thus, infiltration into, or seepage from, this surface may be simulated. Each element may be assigned different material properties that allow the investigation of layered geologic formations. The results discussed in this work are all based on conclusions derived from a generic model for the Tutuka ash dumpsites. While such a model can provide valuable insight into the physical behaviour of such a study area, it can never replace field observations. Field observations and models must complement each other. An attempt was also made to provide guidelines for the development of site-specific data investigation methodology, based on the ISAM Safety Assessment Methodologies for Near Surface Disposal Facilities.

Afrikaans: Groot hoeveelhede koolstof verbrandingsreste (as) en vloeibare aflope word gelyktydig in die steenkool prosesserings-fasiliteite van die Suid-Afrikaanse semistaatsinstellings, Eskom en Sasol, gevorm. Die hantering en wegdoening van hoë southoudende aflope is 'n moeilike en komplekse probleem. Die praktyke wat tans aangewend word deur hierdie instellings, behels die gelyktydige vrylating van die afvalstowwe en as in die stortingsterreine, wat as die “as-damme” bekend staan. Alhoewel hierdie praktyk na 'n potensieel aanloklike benadering lyk, ten minste uit die oogpunt van die vervaardigers van die as-afvalstof en die afvloei, kan dit sekere nagevolge vir die omgewing in en om die stortingsterrein inhou. Hierdie is veral van toepassing op die toksiese en beskadigende elemente teenwoordig in die as wat in die atmosfeer, grond, oppervlakte- en grondwater vrygelaat word. Die nagevolge is nie slegs beperk tot die omgewingsgebruike en grondgebruik probleme nie, maar hou ook nadelige gevolge vir die gesondheid van organismes binne die omliggende ekosisteem in. Die vraag ontstaan dus hoe Eskom en Sasol hul as-damme beter kan beheer, om nie net die wetlike verpligtinge en enige moontlike druk van sosiaalbewuste groepe te hanteer nie, maar belangriker nog, die besoedeling van die natuurlike omgewing te verhoed of, ten minste te beperk. Die ondersoek wat tans onderneem word, het ontstaan op versoek van Eskom en Sasol. Die versoek behels 'n indiepte voorstel met die doel om ‘n raamwerk daar te stel om die vermoëns van dié twee organisasies te verhoog ten einde die bestuur van die wegdoening van die as en soutwater, asook die verspreiding van kennis met betrekking tot die impakte van die afvalstowwe op die stortingsterreine, te bevorder. Twee terreine is deur die organisasie vir hierdie doeleindes gekies, naamlik die Tutuka Kragstasieterrein en die Secunda Sintetiese Brandstof Aanlegterrein. Die aanwending van geo-hidrografiese modelle om die gedrag van 'n stortingsterrein te bepaal, is op 'n historiese grondslag gesien as 'n poging om die toekomstige gedrag van die betrokke terrein te voorspel. Alhoewel hierdie benadering inligting sou benodig aangaande verwante parameters en bekende interaksies, sou gedragspatrone ver in die toekoms strek wat natuurlik nie met sekerheid bepaal kan word nie. 'n Geo-hidrografiese model moet dus nooit gesien word as 'n poging om die toekomstige gedrag te voorspel van 'n gegewe stortingsterrein nie, maar verkieslik as 'n hulpmiddel vir die bepaling van die effektiwiteit van die beheer en kontrole van die terrein wat toegepas word. Die beste manier om dit te verkry is om die stortingsterrein sistematies te ondersoek, verkieslik deur die aanwending van reeds gevestigde en aanvaarde internasionale metodologieë. Tans bestaan daar geen dokument wat so 'n metodologie beskryf, of die implikasies en stappe aantoon om dit aan te wend in die praktyk op so 'n manier dat die beginsels ook verstaan kan word deur belangstellende lede van die publiek nie. 'n Twee-dimensionele oorgangsmodel vir die vloei deur 'n versadigde/onversadigde poreuse stortingsterrein media is al reeds voorheen ontwikkel. Hierdie model los die hoogs nielineêre oorheersend parsiële differensiale vergelykings numeries op. Die model kode gebruik kwadrilaterale eindige elemente vir die geometriese samestelling: die biliniêre Galerkin interpolasie vir die ruimtelike integrasie en die Gaussiese eliminasie vir die oplossing van die gevolglike matriks-vergelykings. Bykomend tot die gewone konstante vloei en konstante drukhoogte grenstoestande, is die kode in staat daartoe om druk-afhanklike grenstoestande toe te pas op die heersende toestand van die grondoppervlakte. Sodoende word infiltrasie na of die sypeling vanaf hierdie oppervlakte gesimuleer. Verskillende materiaal-eienskappe kan aan elke element toegeken word wat die ondersoek van gelaagde geologiese formasies sal toelaat. Die resultate wat bespreek word in hierdie verhandeling is almal baseer op gevolgtrekkings wat afgelei is uit 'n generiese model vir die Tutuka asstortingsterreine. Terwyl só 'n model waardevolle insig kan bied in die fisiese gedrag van die studieterrein, kan dit nooit veld waarnemings vervang nie. Veld waarnemings en modellering moet mekaar komplimenteer. 'n Poging is ook aangewend om riglyne te voorsien vir die ontwikkeling van 'n terreinspesifieke data ondersoek-metodologie wat gebaseer is op die “ISAM Safety Assessment Methodologies for Near Surface Disposal Facilities.”
Keywords: Thesis (Ph.D. (Geohydrology))--University of the Free State, 2010; Waste disposal sites -- South Africa; Groundwater flow -- Mathematical models; Environmental geotechnology; Hydrogeology -- South Africa; Coal ash sites -- South Africa; FEP methodology; Flow Equations Finite Element method; Numerical model; Ash moisture content; Moisture content; Ash dam/dumpsite saturated; Unsaturated flow zones