A feasibility study of in-situ iron removal in the atlantis primary aquifer, Western Cape province, South Africa - Robey, Kate (University of the Free State, 2014)
The sustainability of many wellfield is threatened due to elevated iron (Fe2+) and manganese (MN2+) concentrations in groundwater. Their presence causes aesthetic and potability problems but the greatest concern is production borehole clogging. Physicochemical processes and biological activities cause clogging by oxidation of these ions at the borehole and aquifer interface resulting in lowered borehole yields, increasing operations and maintenance costs. South African research has focused on the remediation of clogging problems, but knowledge is needed for preventative measures in controlling the source of the problems (i.e Fe2+ and Mn2+). The in-situ iron removal (ISIR) method is a technique involving periodic injection of oxygenated water into the aquifer. It has successfully been applied overseas for decades and more recently in Egypt in reducing the need for ex-situ removal of Fe2+ and Mn2+. Long-term applications maintain borehole yield by reducing Fe2+ movement towards the borehole and spreading the oxidation processes over a large surface area in the aquifer. The precipitates stabilize into crystalline oxides, inhibiting reductive dissolution and Fe2+ and Mn2+ mobilization. The study site was the Atlantis Aquifer, where clogging has reduced the wellfield capacity by 60%, necessitating surface water augmentation. The aims of investigating ISIR applicability in a South African context were: • Feasibility investigation at the Atlantis Aquifer; • Design a site-specific ISIR prototype and methodology; • Assess Fe2+ and Mn2+ removal effectiveness of the prototype. In-situ iron removal literature suggested there was a high probability of success for applying the technique at this locality. The high water table necessitated injection into surrounding well-points, rather than directly in the test borehole (G30966). The presence of organic compound and silica-rich groundwater also required the novel use of ozone. The World Health Organisation (WHO) drinking water standards guided the desired threshold, i.e. Fe < 0.3mg/l and Mn < 0.1mg/l. The study area showed nativeFe and Mn cocentrations of 0.5 mg/l and 0.2mg/l, respectively. A mobile ISIR prototype using aerated water from G30966 supplied ozonated groundwater with a high dissolved oxygen content. Initial test saw injection in a well-point 10 m from G30966 with abstraction being shut down. Entrained gases in the ozonated water prevented sufficiently high injection rates being reaches. These gases were subsequently removed using a degas column, resulting in a reasonable injection rate. Due to the previous test not showing removal, a third technique was applied with injection into a well-point 4 m from G30966. The iron and manganese concentrations remained higher than the known baselines, suggesting that intermittent pumping dislodged deposits from G30966 contributing to spurious high iron and manganese levels. A fourth methodology was tested with abstraction during injection. This resulted in iron and manganese concentrations within the baseline concentrations and desired removal of iron removal below the WHO standard. Manganese removal was not as effective because the Mn2+ oxidation takes longer compared to Fe2+ and requires a higher ph. This study operated at a smaller scale than international case studies but showed that iron and manganese removal is achievable at much lower injection parameters (i.e. <2 m3/h and <10 m3/day). The use of ozone was very effective in increasing DO concentrations, better than aeration investigations and comparable to using oxygen gas. The test also showed that DO can be increased in the subsurface with successive injections over 4 to 5 days, elevating the DO between 3 to 9 times above the baseline of 0.4mg/l in aquifer.

Die volhoubare benutting van menige produksieveld word bedreig deur die aanwesigheid van verhoogde yster (Fe2+) en mangaan (Mn2+) konsentrasies in die groundwater. Hul teenwoordigheid bring estetiesee problem mee en belemmer drinkbaarheid, maar die grootste bron van kommer is die verstopping van prduksieboorgate. Fisies-chemiese prosesse en biologiese aktiwiteit veroorsaak verstopping vanwee oksiedasie van hierdie ioone by die boorgat-waterdraer tussenvlak wat tot verlaagde boorgat-lewering, verhoogde bedryf- en instandhoudingskoste, en bevordering van ooronttrekking lei. Suid-Afrikaanse navorsing het tot dusver op die remediëring van verstoppingsprobleme gefokus, maar kennis is nodig vir voorkomende maatreëls by die bron, d.w.s. Fe2+ en Mn2+ mobilisasie. Die in-situ ysterverwyderingsmetode (in-situ iron removal, ISIR) is ‘n tegniek gebaseer op gereelde toediening van suurstofryke water in die waterdraer. Dit word reeds dekades lank suksesvol in die buiteland toegepas en meer onlangs in Egipte vir ex-situ behandeling en vermindering van Fe2+ en Mn2+. Gewoonlik handhaaf langtermyn toepassings boorgat lewerings deur die vermindering van die migrasie van Fe2+ en Mn2+ uit die akwifeer na die boorgat en die verspreiding van die oksidasieproses oor ‘n groeter oppervlakte in die waterdraer. Die neerslae stabiliseer in kristallyne oksiede, wat die reduktiewe ontbinding van die neerslag en gevolglike Fe2+ en Mn2+ mobilisasie verminder. Die studieterrein is die Atlantis-waterdraer, waar verstopping die produksieveld se kapasiteit met 60% verminder het en die toevoeging van oppervlakwater genoodsaak het. Die doel van die ondersoek na die toepaslikheid van ISIR in die Suid-Afrikaanse konteks was: - Ondersoek die haalbaarheid van ISIR in die Atlantis-waterdraer; - Ontwerp ‘n terrain-spesifieke ISIR prototype en metode; - Bepaal die doeltreffendheid van Fe2+ en Mn2+ verwydering. Die literatuurstudie oor in-situ ysterverwydering het op ‘n hoë waarskynlikheid van sukses vir die toepassing van die tegniek in hierdie omgewing gedui. Die hoë watervlak het die toediening van die suurstofryke water in omliggende boorgate genoodsaak, eerder as toediening direk in die toetsboorgat (G30966). Verder het die voorkoms van organiese verbindings en silica-ryke groundwater ook die innoverende gebruik van osoon genoodsaak. Die Wêreld Gesondheidsorganisasie (WHO) se drinkwaterstandaarde van Fe < 0,3 mg/l en Mn < 0,1 mg/l is as gewenste drempel-waardes gebruik van. Die groundwater in die studiegebied het Fe en Mn konsentrasies van onderskeidelik 0,5 mg/l en 0,2 mg/l. Grondwater uit produksieboorgat G30966 is eers belug en daarna met ‘n mobiele ISIR prototype eenheid geösoneer. Tydens die eerste toetse is die geösoneerde water op die regte diepte in ‘n boorgat 10m van G30966 toegedien met gelyktydige staking van wateronttrekking uit G30966. Ingeslote gasse in die geösoneerde water het stoedieningstempo’s belemmer, maar die gasse is later met ‘n ontgassingskolom verwyder, wat tot ‘n redelike stempo gelei het. Gegewe die feit dat die tweede toetslopie nie ysterverwydering getoon het nie, is die derde tegniek toegepas, wat toediening in ‘n boorgat 4 m van G30966 behels het. Hoewel die Fe en Mn konsentrasies by G30966 begin afneem het, was die konsentrasies hoër as die bekende basislyne, wat daarop gedui het dat die onderbroke onttrekking uit boorgat G30966 yster en mangaan aanpaksels in boorgat G30966 laat loskom het wat bygedra het tot anomale hoë Fe en Mn vlakke. ‘n Vierdie metode met wateronttrekking uit die produksieboorgat tydens toediening van die geösoneerde water het Fe en Mn konsentrasies laer as die basislyn opgelewer en die gewenste verwydering van Fe tot onder die WHO standard bereik. Mangaan-verwydering was nie so effektief nie omdat die Mn2+ oksidasie ‘n hoër pH vereis en langer neem as met Fe2+. Hierdie studie is op ‘n kleiner skaal as die internasionale gevallestudie uitgevoer, maar het getoon dat Fe en Mn verwydering haalbaar is teen baie laer toediendingstempo’s (bv. < 2 m3/h en < 10 m3/dag). Die gebruik van osoon het opgeloste suurstofkonsentrasies effektief verhoog tot waar dit vergelykbaar was met die gebruik van skoon suurstof gas en het beter gevaar as studies waar atmosferiese lug gebruik is. Die toets het ook getoon dat die opgeloste suurstof in die waterdraer 3 tot 9 keer bo die basislyn van 0,4 mg/l verhoog kan word met opeenvolgende toedienings oor ‘n tydperk van 4 tot 5 dae.
Dissertation (M.Sc. (Institute for Groundwater Studies))--University of the Free State, 2017
Keywords: In-situ groundwater treatment; Iron; Manganese; Wellfield development and management; Borehole clogging; Water quality improvement; Ozone; Atlantis aquifer; Pilot study; Groundwater