Geochemical modelling of fluoride concentration changes during Aquifer Storage and Recovery (ASR) in the Chalk aquifer in Wessex, England

Abstract

During ASR-cycle testing at a site in the confined Chalk near Lytchett Minster in Dorset, the high concentration of fluoride in the recovered water posed severe limitations on the success of the scheme. Based on physical modelling, the dual porosity character of the Chalk combined with high fluoride concentrations in the native water were identified as the key factors controlling the measured concentrations. However, mixing of water between the matrix pores and fractures was not sufficient to explain the fluoride concentration and it was concluded that there was an additional release of fluoride from aquifer interaction. This led to an additional increase in the fluoride concentration in the recovered water. In order to investigate this hypothesis, a geochemical model incorporating reactions between the injected water, the native groundwater and the aquifer minerals was developed. The geochemical model PHREEQC-2 was set up so that it was capable of modelling ASR-cycles (including radial flow and diffusive mixing as a consequence of dual porosity). The physical aspects of the model were calibrated using a 3-D dual porosity transport model (SWIFT). Different geochemical processes (e.g. limited mineral availability, reaction kinetics) causing fluoride concentrations above those expected from dual porosity mixing were investigated. Comparing the modelled results with the observations from the test site suggested that slow dissolution of fluoride minerals (fluorite) was likely to be responsible for the additional increase in fluoride concentration in the recovered water

Fluoride and environmental health: a review

“…With the exception of villiaumite, the common fluoride-bearing minerals are only sparingly soluble and release fluoride to water slowly (Nordstrom and Jenne 1977;Cronin et al 2000;Gaus et al 2002;Saxena and Ahmed 2003). Therefore, in the absence of villiaumite, it might be expected that fluoride concentrations in groundwater will be relatively low unless the percentage of fluoride-bearing minerals is high, but there are many exceptions to this rule.…”

“…This is an especially important control on fluoride concentrations because the dissolution rates of most fluoride-bearing minerals are slow (Nordstrom and Jenne 1977;Gaus et al 2002) and the maximum concentrations allowed by fluorite solubility are not always attained in groundwater (Robertson 1986). A number of field studies have demonstrated the influence that residence time can have on dissolved fluoride levels, especially in crystalline rocks where groundwater moves primarily through fractures (e.g., Bardsen et al 1996;Conrad et al 1999;Gosselin et al 1999;Frengstad et al 2001;Saxena and Ahmed 2003;Kim and Jeong 2005;Chae et al 2006a;Chae et al 2007).…”

Assessment of Managed Aquifer Recharge through Modeling—A Review

“…Pyrite oxidation is one of the key geochemical reactions identified [107][108][109][110][111][112] which often leads to mobilization of trace metals or metalloids [107][108][109][110][111][112][113][114][115]. Other geochemical reactions that could be identified by modeling are the dissolution of fluoride minerals [116] and the acidification in the aquifer [114,115]. Furthermore, scenario simulations were done to test how the dissolution of metals can be controlled [114].…”

Integrated project management of sustainable water storage and seawater intrusion prevention in a costal city

“…Aquifer storage and recovery can provide an effective tool for sustainable development and management of available water resources within a watershed to balance the needs of competing water demands [7][8]. Drivers for aquifer storage and recovery are also expanding beyond seasonal storage to include acting as a salinity barrier, aquifer replenishment, and for water quality improvement [9]. This paper addressed the issue of sustainable water storage and seawater intrusion prevention in a costal city, Haihou City in China.…”