IN-SITU IRON REMOVAL: INNOVATIVE OPTION FOR PREVENTING PRODUCTION BOREHOLE CLOGGING K. Robey1 and G. Tredoux2 1
Council for Geoscience, Western Cape Unit Bellville, Western Cape Province, South Africa; email:
[email protected] 2
Private consultant Durbanville, Western Cape Province, South Africa; email:
[email protected]
Abstract Many aquifer systems worldwide are subject to hydrochemical and biogeochemical reactions involving iron which limit the sustainability of groundwater schemes. This mainly manifests itself in clogging of the screen and immediate aquifer with iron oxyhydroxides resulting in loss of production capacity. Clogging is caused by chemical precipitation and biofouling processes which also manifests in South African well fields such as in Atlantis and the Klein Karoo. Both well fields have the potential to provide a sufficient, good quality water supply to rural communities, however clogging of the production boreholes has threatened the sustainability of the schemes as quality and quantity of water is affected. Rehabilitation of the affected boreholes using techniques such as the Blended Chemical Heat Treatment method does not provide a long-term solution. Such treatments are costly with varying restoration of original yields achieved and clogging recurs with time. Currently the research, management and treatment options in South Africa have focused on the clogging processes which are complex and site specific making it extremely difficult to treat and rectify. This project attempts to eliminate elevated concentrations of dissolved iron, the cause of the clogging. High iron concentrations in groundwater are associated with reducing conditions in the aquifer allowing for the dissolution of iron from the aquifer matrix. These conditions can be natural or human-induced. Attempts to circumvent iron clogging of boreholes have focussed on increasing the redox potential in the aquifer, by injection of oxygen-rich water into the system, to prevent dissolution and to facilitate fixation of iron in the aquifer matrix. Various in-situ treatment systems have been implemented successfully overseas for some time. In South Africa thus far in-situ treatment of iron has only been proposed as a solution for production borehole clogging. Based on experience from abroad the most viable option to research the elimination of ferrous iron in South African aquifer systems would be through the in-situ iron removal treatment. Different techniques of increasing the dissolved oxygen concentration in the injected water to intensifying the redox change in the aquifer can be applied however, the use of ozone as the oxidant is a new approach. Its effectiveness is evaluated by the results in iron removal in surface water treatment for drinking water supply.
1. INTRODUCTION Iron-related clogging of production boreholes is a worldwide problem which manifests in aquifers and can have a limiting effect on the sustainability of well field schemes as seen in South Africa (e.g. Cavé et al., 2004; Smith and Roychoudhury, 2013) and internationally (e.g. Mettler et al., 2001). Aquifers with reducing conditions and low pH often have elevated concentrations of soluble ferrous iron (Fe2+) due to dissolution and mobilisation from the aquifer rocks (Appelo and Postma, 2007). The conditions which allow mobilisation could be natural or human-induced (Mansuy et al., 1990). The presence of Fe2+ has the potential to limit water use and supply as levels as low as 0.2 mg/ℓ can impart an astringent metallic taste, stain walls and laundry and cause clogging of borehole screens and delivery pipes (Mackintosh and de Villiers, 2002). Oxidation of Fe2+ to ferric iron (Fe3+) occurs rapidly when anoxic groundwater comes into contact with oxygen at pH ≥ 7 (Van Duuren, 1997 cited in Cavé et al., 2004). This redox reaction causes precipitation of iron (III) oxyhydroxide which clogs borehole screens, pumps and distribution
systems (Cavé et al., 2004). Generally oxygen is introduced into the anoxic aquifer through the production borehole by water table fluctuations initiated during drilling, pumping or artificial recharge (Cavé et al., 2004). Globally extensive research into iron-related chemical precipitation and biological fouling in production borehole clogging has been done in anoxic primary aquifers (e.g. Mansuy et al., 1990; Sommerfeld, 1999) but only in the last two decades has it received some attention in South Africa (e.g. Flower and Bishop, 2003). This is because of the severe impacts clogging has had on the management and economic sustainability of groundwater supply schemes such as the Atlantis Water Supply Scheme (AWSS), in the primary Atlantis aquifer and the Klein Karoo Rural Water Supply Scheme (KKRWSS) within the fractured Table Mountain Group (TMG) aquifer. Currently these two well fields are experiencing loss of production capacity and poor performance in water supply as a result of clogged production boreholes due to elevated iron in the groundwater. The KKRWSS was designed to produce 4.7 x 106 m3 per annum in 1987 but in 2000 production has been reduced to less than a quarter as none of the production boreholes can be sustainably pumped at higher than 20 % of their original yield (Jolly, 2000 cited in Smith and Roychoudhury, 2013). Similarly in 2000, the AWSS had to augment the water supply to the Atlantis community with surface water due to poor performance of its Witzand and Silwerstroom well fields as a result of clogged boreholes after close to three decades of sustainably supplying high potable water to a population of approximately 50 000 (Flower and Bishop, 2003). Both the AWSS and KKRWSS well fields have shown the potential to provide sufficient, good quality water to rural communities but the iron-related clogging of the production boreholes, ex-situ treatment options to remove iron before distribution and skills and management required for these well field is threatening the sustainability of these water supply schemes. Even though there are no health concerns, the World Health Organisation (2008) recommends that iron removal must occur when concentrations exceed 0.3 mg/ℓ due to associated problems. Uncomplexed iron can be easily removed after abstraction by aeration followed by conventional coagulation and flocculation techniques with sedimentation in a settling tank followed by filtration (Mackintosh and de Villiers, 2002). If there is high dissolved organic carbon content, which forms complexed iron molecules, stronger chemical oxidants than oxygen are required and lime treatment is necessary to remove these compounds from the water (Mackintosh and de Villiers, 2002). This ex-situ treatment approach is expensive, due to the required space, equipment, chemicals, and expertise required which are often not feasible for small (rural) well fields (Mackintosh and de Villiers, 2002). In addition, it also does not resolve the issue of clogging of the production boreholes.Management options, such as longer duration pumping at lower rates instead of high rate stop-start approaches, can substantially reduce iron clogging by limiting the amount of oxygen and nutrients drawn into the system (Cavé et al., 2004). Rehabilitation of affected production boreholes using techniques such as the Blended Chemical Heat Treatment (BCHT) have been applied at the AWSS and KKRWSS boreholes (More Water, 2001; Smith, 2002). Such treatment is costly costing from R 50 000 per borehole in 2001, with varying restoration of original yields achieved and since clogging recurs with time, treatment may be required every 8 to 12 months (Smith and Roychoudhury, 2013). In South Africa such management and rehabilitation options have focused on treating the clogging processes which are complex and site specific thus extremely difficult to rectify and do not provide a long-term solution in removing the source of the problem, i.e., elevated iron in the groundwater. Ferruginous minerals are common in the Earth’s upper crust and its presence in groundwater is unavoidable. The focus must shift to preventative options e.g., increasing the redox potential in the aquifer to prevent dissolution and facilitate the fixation of the iron in the aquifer matrix thus circumventing iron clogging of production boreholes. In view of the extensive iron-related clogging problems in many South African aquifers and the recent recommendations in the Groundwater Strategy (DWA, 2011) for more groundwater supply schemes to be developed in the areas prone to such iron issues, e.g., the TMG and Cape Flats aquifers (Jolly, 2002). More research is needed into prevention options. Abroad various in-situ iron removal treatment systems have been implemented successfully since the 1970’s (e.g. Hallberg and Martinell, 1976; Appelo et al., 1999; Mettler et al., 2001). However in South Africa in-situ treatment of iron has only been proposed and the need for research into its application and feasibility has already been highlighted (Tredoux et al.,
2004). The objectives of this paper is to set out the experience from abroad and to outline the feasibility of this treatment in a South African context including an operational pilot plant of this treatment at the Witzand well field of the Atlantis primary aquifer. Atlantis has been identified as a good test site as the primary aquifers are well understood and plagued by iron-related clogging.
2.
IN-SITU IRON REMOVAL: A REVIEW
The in-situ iron removal methodology involves periodic cycles of injecting a volume of oxygen-rich water into an anoxic aquifer and subsequent abstraction of a larger volume of water with iron concentrations much lower than that found in the native groundwater from the production borehole (Appelo et al., 1999). Oxygen-rich water is injected into the aquifer through a number of injection wellpoints surrounding the production borehole and is known as the “daisy wheel” configuration (e.g. Hallberg and Martinell, 1976) or alternatively directly in the production borehole (e.g. Diliūnas et al., 2006). In the latter case, the ratio of abstraction volume to injection volume varies according to local circumstances but generally injection volumes can be 10 to 30 % of the abstraction volume (Sommerfeld, 1999). Subsequently it is limiting iron-related problems in production boreholes as it targets the raw groundwater before it is abstracted (Braester and Martinell, 1988). In addition, it has been found that removal or reduction of manganese, phosphate, arsenic, organic and inorganic carbon and sulphide and with modifications of the system treatment can also include the removal of nitrates and nitrites (Braester and Martinell, 1988) and hydrocarbons (Radčenko and Hauskrecht, 1982). It has been proven to be an effective technique for semi-consolidated and unconsolidated gravel-sand aquifers (e.g. Mettler et al., 2001) and glacial aquifers (e.g. Diliūnas et al., 2006). The injected water is recommended to be derived from a nearby production borehole from the same aquifer and has to be devoid of dissolved iron and degassed to remove carbon dioxide and methane before oxygenated (Hallberg and Martinell, 1976). The water pH should also be ≥ pH 7, since the rate of Fe2+ oxidation decreases considerable at lower pH (Van Duuren, 1997 cited in Cavé et al., 2004). Oxygen is derived from either aeration, the most cost-effective option, but lower dissolved oxygen (DO) in the injected water of approximately 6 – 8 mg/ℓ is achieved (e.g. Diliūnas et al., 2006), or by dosing with oxygen gas which can give between 20 – 30 mg/ℓ DO (e.g. Hinkamp et al., 2004). The method is as follows: abstraction in a production borehole requiring treatment is stopped and water abstracted from another production borehole is oxygenated. This oxygen-rich water is then injected into the aquifer. The injection phase can take between 20 to 30 hours, but is site specific, and is followed by a recommended 4 to 10 hours of contact time (Braester and Martinell, 1988). Abstraction from the production borehole is re-started and after some time, when the monitored concentration of iron in the production borehole starts to rise, abstraction is stopped and injection of oxygenated water is repeated (Tredoux et al., 2004). The efficiency of this treatment is determined by the ratio of water pumped with iron levels below the threshold (volume V) over the injected oxygenated volume (Vi) (Hallberg and Martinell, 1976). Another major advantage of this treatment is that the efficiency of the iron removal increases with successive runs and the different physical, chemical, and biological mechanisms proposed for the extended iron removal and increase in treatment efficiency is well described in Appelo et al. (1999). The current accepted theory is that when the oxygen-rich water is injected, the groundwater is partially displaced and both homogenous and heterogeneous oxidation of Fe2+ occurs in the subsurface. Homogenous oxidation takes place in solution predominately at the interface of injected water and displaced anoxic groundwater and when the adsorbed Fe2+ ions exchanges with cations in the injected water (Appelo et al., 1999; van Halem et al., 2011). Heterogeneous oxidation occurs at the interface between the aquifer rocks and the injected water and is proposed to be the dominant mechanism for in-situ iron removal where adsorbed Fe2+ on the soil grains is oxidised (van Halem et al., 2011). The freshly-precipitated ferric iron oxyhydroxide forms a layer on the existing iron oxyhydroxide; or coatings on quartz, feldspar; or clay minerals providing new adsorption sites (Appelo et al., 1999). During the pumping stage, the native groundwater with Fe2+ is filtered through the newly-formed depleted ion exchangers and more Fe2+ is adsorbed again onto the aquifer matrix (Appelo et al., 1999). Hence more iron-free water is abstracted than injected as the iron front is retarded due to the adsorption of Fe2+ and the increase in efficiency is related to the initial consumption of the oxidant by other reactants than Fe2+ and to the increases of iron oxyhydroxide during
successive cycles (Appelo et al., 1999). It is given that after 15 injection–abstraction cycles the efficiency stabilises (Hallberg and Martinell, 1976). This treatment is also highly effective as it further reduces the potential for iron dissolution from the aquifer when the anoxic conditions resume over time. As the iron oxides increase in the subsurface they have a limiting effect on the rate of iron reduction in anoxic conditions even in the presence of organic acids (Appelo and Postma, 2007). In aerated water at pH 7 and 20 °C the half-life of iron oxidation is 15 minutes which is too rapid for nuisance iron-bacteria to compete (Emerson and Moyer, 1997 cited in Cavé et al., 2004). Thus iron biofouling is also reduced. Previous research by Diliūnas and Jurevičius (1998) established that iron concentrations were reduced by 80 to 90 % and iron-bacterial growth slowed down by 40 to 50 % when the pH 7.5, Eh > 190 mV, DO > 0.4 mg/ℓ and CO2 < 50 mg/ℓ was established in the aquifer after injecting of aerated water (cited in Diliūnas et al., 2006). The greatest concern brought up when in-situ iron removal is presented as a treatment option is the potential clogging of the aquifer as a result of the precipitation and sedimentation of the iron oxyhydroxides in the aquifer matrix since this is the cause of the problems with the production boreholes and ex-situ iron removal. However this is not a limiting factor in the application of this technique as negligible effects on the hydraulic properties of the aquifer have been documented in the literature. Two full-scale plants, after a decade of application, found the precipitated iron accumulate as compact crystalline forms of iron hydroxides which suggest that with time the amorphous iron precipitates stabilised into these more crystalline forms (e.g. Mettler et al., 2001; van Halem et al., 2011). In both case studies the iron oxides accumulated at a specific depth near the injection well points which could have been due to preferred flow paths (e.g. coarser sands, which permit a better flow) or geochemical/mineralogy conditions (e.g. carbonate minerals). Although there are a number of advantages to this technique certain conditions are required within the subsurface to reduce potential operational flaws such as the section of the aquifer injected should be as homogenous as possible without extremely coarse layers to prevent preferential flow of the injected water and low exchange capacity with the iron oxyhydroxide (Appelo et al, 1999). The presence of other soluble ions must also be taken into account such as the presence of organic carbon, silicates, sulphides and/or ammonia which could interfere with the iron removal process by either forming complexes with Fe2+ that do not precipitate out or form less thermodynamically-stable minerals like ferrihydrite rather than lepidocrocite or preferentially consuming the oxygen before it reaches the iron (Sommerfeld, 1999; Cavé et al., 2004).
3. FEASIBILITY IN A SOUTH AFRICAN CONTEXT Primary Aquifer Application South Africa only has a small number of significant primary, unconsolidated aquifers e.g., the Atlantis and Zululand aquifers on the Western and Eastern coastlines, respectively (Woodford et al., 2006). The Atlantis Aquifer is a perfect study area to try out the treatment because the extensive clogging problems has plagued most of the production boreholes in both the Witzand and Silwerstroom well fields for over two decades. The decline in production has been a limiting factor in the success and sustainability of the well field which supplied the town’s bulk water demand for close to three decades. The case study to evaluate the feasibility of the in-situ iron removal treatment at a primary aquifer is taking place at a production borehole in the Witzand well field.
Fractured Aquifer Application Over 80 % of South Africa is underlain by fractured aquifers (Woodford et al., 2006) with the TMG aquifer one of the most important aquifers in the Western and Eastern Cape Provinces of South Africa being affected by iron clogging (Smith and Roychoudhury, 2013). The feasibility of this treatment in a South African fractured aquifer was initially proposed in this Water Research Commission funded project however due to worldwide iron-related problems generally being associated with anoxic primary aquifers (Smith and Roychoudhury, 2013) there is a lack of literature on the application of the in-situ iron removal method in a fractured aquifer. In view of the complexity of understanding the hydrodynamics of a
fractured system to control iron removal treatment, lack of aboard experience and the associated costs expected for such a study to be undertaken, this project only focuses on the testing of the local applicability in a primary aquifer context. Once the technique has been demonstrated in a primary aquifer further studies will explore its feasibility in a fractured aquifer context. However, other pro-active predictive approaches to controlling iron-related clogging of production boreholes in TMG aquifers are being explored (e.g. Jolly, 2002; Mackintosh and de Villiers, 2002; Smith and Roychoudhury, 2013).
4. IN-SITU IRON REMOVAL APPLICATION AT ATLANTIS Site Description The primary coastal aquifer system in the Atlantis area is formed by semi- to unconsolidated Cenozoic sediments of the Sandveld Group (Roberts, 2006). The Sandveld Group is a succession, predominantly of quartz sands derived from fluvial, aeolian, and shallow marine depositional environments that unconformably overlie the Malmesbury metasediments and granites of the Cape Granite Suite (Stapelberg, 2005). The Sandveld Group is stratigraphically divided into seven formations: Elandsfontyn, Prospect Hill, Varswater, Velddrift, Langebaan, Springfontein, and Witzand Formations that extend along the southern African coastline from Elands Bay to Cape Town but not all formations are present in along the coastline. The study site within the Witzand well field only the Varswater, Springfrontein and Witzand Formations are present. The basal Varswater Formation comprises of well sorted, chiefly fine-grained quartz sands which are shell- and phosphate-rich overlying a muddy-gravel base. Within the Witzand well field in various production boreholes, this formation is intersected at >35 m depth with a thickness ranging between 3 to 8 meters before intersecting the basement rock (CSIR, 1997). Its depositional environment is suggested to be a marine inner shelf and estuarine setting. The Springfontein Formation covers the majority of the study area and represents a marine deposit but has no shell fragments. The quartz sands are relatively well-sorted, ranging from fine- to coarse-grained and with low mud/silt content except for a peaty-sand base and peat layers inter-fingering the formation (Tredoux and Cavé, 2002). This is the targeted formation in the Witzand well field in which the majority of the boreholes screened, it is intersected in all production boreholes within the Witzand well field around 15 meter below ground level (mbgl) and has an average sand thickness of 22 meters within the well field (n = 16). The overlying Witzand Formation which is an aeolian deposition of calcareous, well-sorted quartz-rich, fine to medium grained sands that contain shell fragments and discontinuous calcrete horizons. This unit is characterised as unvegetated, elevated white sand dunes in the central parts of Atlantis and in the Witzand well field has an average sand thickness of 14 meters (n = 16) (CSIR, 1997). The Springfontein and basal 3 – 5 meters of the Witzand Formations are the main water-bearing units in the area and the saturated parts of these formations constitute the Atlantis aquifer with the static water level approximately 1.5 mbgl. Due to the facies changes, the aquifer is characterised as semi-unconfined or unconfined with delayed yield due to either the calcrete lenses or finer grained sands in the upper layers, respectively (Tredoux and Cavé, 2002). Due the heterogeneity of the aquifer the calculated transmissivity values range between 50 to 1 300 m2/day (Fleisher, 1990 cited in Tredoux and Cavé, 2002). Under natural conditions groundwater flows in a westerly to south-westerly direction towards the coast discharging into the Atlantic Ocean.
Iron Clogging in Production Boreholes in the Atlantis Water Supply Scheme Loss in borehole production capacity within the two well fields of the AWSS was first identified in the early1990’s and is due to elevated iron and sulphate in the abstracted groundwater and red-brown iron encrustations coating the borehole pumps, with the root cause of initiating clogging due to over-pumping of the boreholes (Tredoux and Cavé, 2002). The iron encrustation was initially suggested to be microbiologically-mediated (More Water, 2001) but Smith (2006) found no bacterial presence in the analyses of the iron oxyhydroxide encrustation sampled from Witzand production boreholes that coated the pumps and flow meters. The iron encrustations were extremely hydrous (up to 80 % water mass). They were also sodium and calcium-rich that consisted of aggregates of tiny particles
