Site characterisation methodologies for DNAPLs in fractured South African aquifers - Gebrekristos, Robel Amine (University of the Free State, 2007)
English: The objective of this research is to characterise fractured aquifers contaminated with DNAPLs in South Africa. The site characterisation approach is conducted in two case studies by integrating a range of site assessment techniques. It started with the noninvasive methodologies that provide rapid and cost-effective ways for screening. After developing the initial conceptual model, the investigation proceeded to invasive techniques that provide more information on DNAPLs and fractures on a local scale. This research indicated that fracture “hunting” is extremely difficult with the current technology, making DNAPL characterization very challenging. It also indicated that no single technique provides an unequivocal indication of fracture position and orientation, but that best results were obtained when several invasive techniques were used in a toolbox approach. These tools were most appropriate when integrated with hydraulic testing methods. The site characterisation was done in a continuous and iterative process, and each phase of investigation was used to refine the conceptual model of the site. The non-invasive techniques applied included: • General assessment of Test Site 1 by site inspection, interviewing employees, documentation studies and aerial photo analysis. • Hydrocensus • Direct observing for DNAPL contamination using UV light and Sudan IV shake tests. • Soil gas surveys by using various models of PIDs to delineate the organic vapour plumes. • Surface and airborne geophysics for outlining major structures that may play an important role for DNAPL migration pathways. This did not yield useful results at Test Site 1 due to noise interference from industrial activities and infrastructure. The non-invasive techniques were not applicable for direct evaluation of DNAPLs migrating in fractured aquifers; however, they were essential in understanding DNAPL-release zones and vapour plumes. They were also found to be relatively cost and time efficient. The invasive techniques applied include: • Test pits (excavations on the top 3 m soil) • Drilling using augur, percussion and coring methods. (An outside-in approach was followed during the drilling to avoid unnecessary DNAPL mobilisation). • Several different borehole geophysical and geochemical logging methods. Results from this diverse range of activities were integrated to construct conceptual models of the preferential fracture pathways of the two test sites. Riemann (2002) found that early time drawdown data could be used to estimate the Tvalue of the fracture zone with the Cooper-Jacop 2 method. However, in this research, the method did not yield results that were consistent with other observations. The tracer experiment was extended to evaluate the effective matrix diffusion coefficient of the mudstone in the Campus Test Site. The values at depths of 27 and 36 m were estimated as 3.4 x 10-6 and 7.8 x 10-6 m2/day, respectively. This means that the rate of dissolved mass disappearance from the fracture to the matrix is relatively high compared to other geologic formations. An in situ method of effective porosity estimation using tracer tests was employed instead of the traditionally measurements on rock/soil samples from the aquifer. In this way, uncertainty in porosity estimation due to the inherent alteration during sampling was eliminated. Column experiments were conducted to evaluate the accuracy of PITT for residual DNAPL saturation estimation. The saturation was estimated with an accuracy of 95%. However, the experiment was conducted in unconsolidated, homogenous sand and the experiment may not be accurate as such when applied to heterogeneous aquifer systems usually encountered in South Africa. With the current technologies available, PITT is believed to have limited applicability in fractured rock aquifers. Finally, based on the findings, a methodology has been proposed for the characterisation strategy of DNAPL contaminated sites in fractured South African aquifers

Afrikaans: Die doelwit van hierdie navorsing is om Suid Afrikaanse fraktuurakwifere, wat besoedel is met DNAPLs, te karakteriseer. Die die terrein karakterisering is benader deur die integrasie en toepassing van verskillended ondersoek metodes by twee gevalle studies. Eerstens is nie-binnedringend en vinnige, effektiewe ondersoek tegnieke gebruik. Nadat die aanvangs konseptuele model gekonstrueer is, is daar voort gegaan met binnedringende tegnieke om meer inligting te verskaf oor DNAPLs en frakture op ‘n lokale skaal. Hierdie navorsing het gewys dat fraktuur opsporing baie moelik is met beskikbare tegnologie, wat DNAPL karakterisering baie uitdagend maak. Dit het ook bewys dat geen enkele tegniek die fraktuur posisie en orientasie kan aandui nie, en dat binnedringende tegnieke wat saam gebruik word, die beste resultate gelewer het. Hierdie tegnieke is mees geskik as dit geintegreer word saam met hidroliese toets metodes. Die terrain karakterisering was gedoen as ‘n aaneenlopended en herhalende proses, waar die konseptuele model tydens elke fase opgedateer is. Die nie-binnedringende tegnieke wat toegepas is sluit die volgende in: •Algemene ondersoek tegnieke soos terrein inspeksie, onderhoude met werknemers, dokument studies en lugfoto interpretasie. •Hidrosensus •Direkte obserwering van DNAPL besoedeling met die hulp van UV-lig en Sudan IV kleurstof skud toetse, van die boonste grondlaag. •Die organiese gas pluim is gedelinieer, deur grond gas opnames met verskillende modelle “PIDs”. •Grond en lug geofisika is gebruik om belangrike geologiese strukture en kenmerke op te spoor wat ‘n rol kan speel in die beweging van DNAPLs. (Hierdie tegnieke was egter nie suksesvol op Toets Terrein 1 nie, aangesien daar baie agtergrond geraas was weens die industriële aktiwiteite en infrastrukstuur.) Die nie-binnedringende tegnieke was nie toepasbaar vir direkte evaluering van DNAPL migrasie in die gefraktureerde akwifere nie, maar het bygedra tot die insig in die DNAPL bron areas en die gepaardgaande gas pluime. Dit was ook bevind dat dit koste en tyd effektief is. Die binnedringende tegnieke wat toegepas is sluit in: •Toets gate (tot 3 m diep) •Boor van gate met verskillende tegnieke (‘n Buite-na-binne metodiek is gevolg om onnodige mobilisering van DNAPL bron areas te verhoed.) •Verskillende boorgat geofisika en geochemiese opnames is gebruik. Riemann (2002) het verduidelik dat vroeë tyd watervlak afname data, gebruik kan word om die T-waarde van die fraktuur sone te bepaal met die “Cooper-Jacop 2“ metode. Hierdie navorsing het egter bewys dat die T wat bepaal word op hierdie manier, geen fisiese waarde het nie. ‘n Spoorder eksperimente is uitgebrei om die effektiewe matriks diffusie koëffisiënt van die modderstene op die Kampus terrein te bepaal. Die waardes by dieptes 27 and 36 m, was bepaal as 3.4 x 10-6 and 7.8 x 10-6 m2/dag, onderskeidelik. Dit impliseer dat die tempo waarteen opgeloste massa verdwyn uit die fraktuur na die matriks is heelwat hoër as voorheen bepaal vir ander gesteentes volgens literatuur. Die effektiewe porositeit word tradisioneel gemeet deur rots of grond monsters te neem van die akwifeer. Tydens hierdie navorsing is in-situ effektiewe porositeit bepaal deur punt verdunning spoorder eksperimente. Op hierdie manier is die porositeit bepaal sonder om die monster bloot te stel aan verandering tydens monsterneming. Kolom eksperimente was gedoen om die akkuraatheid van PITT vir die bepaling van residuele DNAPL versadiging te bepaal. Die versadiging was bepaal tot ‘n akkuraatheid van 95%. Die eksperiment is egter in homogene ongekonsolideerde sand gedoen. Dus mag dit nie toepasbaar wees of dieselfde resultate lewer, indien dit toegepas word in tipiese heterogene akwifere van Suid Afrika, nie. Dus met die huidige beskikbare tegnologie word daar nie geglo dat PITT toebasbaar is in fraktuur akwifere nie. Deur die bevindinge van hierdie navorsing is daar `n strategie voorgestel vir karakterisering van DNAPL besoedelde terreine in Suid Afrikaanse fraktuurakwifere
Keywords: Dense nonaqueous phase liquids -- South Africa; Groundwater -- Pollution; Pollutants; Thesis (Ph.D. (Institute for Groundwater Studies))--University of the Free State, 2007