Mining unique soft old water within the Manson Impact Structure, Iowa (USA)

Schilling, Keith E.; Anderson, Raymond R.; Peate, David W.; Dorale, Jeffrey A.; Alexander, E. C., Jr

Mining unique soft old water within the Manson Impact Structure, Iowa (USA) | Exploitation minière de l’unique eau douce ancienne au sein de la structure d’impact de Manson en Iowa, Etats Unis d’Amérique La minería de única agua blanda antigua dentro de Manson Impact Structure, Iowa (EEUU) (美国)爱荷华州曼森冲击构造内开采独特的古老软地下水 Mineração de água branda antiga dentro da Estrutura de Impacto Manson, Iowa (EUA)

2015

Schilling, Keith E. | Anderson, Raymond R. | Peate, David W. | Dorale, Jeffrey A. | Alexander, E. C., Jr

The City of Manson, Iowa (USA), lies near the center of the Manson Impact Structure (MIS), a 37-km diameter impact crater that formed about 74 million years ago. The MIS is present at the bedrock surface, but it is completely buried by 20–90 m (70–300 ft) of glacial till and displays no surface expression. Groundwater samples were collected from Manson’s municipal well and nearby domestic wells to assess the sustainability of unusually soft water that typifies the central peak of the Manson Impact Structure. Groundwater quality indices from the various aquifer sources were distinctly different, with groundwater from the central peak of the crater exhibiting low Ca (<22 mg/L) and alkalinity (<127 mg/L as CaCO₃), and high concentrations of fluoride (>3.7 mg/L; max = 10.0 mg/L) and certain trace elements (e.g. Li, Mo, W). In contrast, groundwater collected from wells in overlying Quaternary aquifers was very hard (high Ca and Mg), with high alkalinity (>261 mg/L as CaCO₃), sulfate (58–485 mg/L) and occasionally nitrate-N (up to 6 mg/L). Age-dating techniques using³H,¹⁴C, and³⁶Cl suggested water older than 35,000 years to possibly as old as 1,000,000 years within the central peak aquifer, but indicated recent water in overlying aquifers. Pumping of the Manson municipal wells appears to be mining old water of the central peak aquifer and drawing modern water containing elevated hardness and nitrate-N down into the aquifer. The Manson example illustrates a source-water challenge of balancing aquifer quality with sustainability.

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