Aluminium precipitates formed downstream of springs in a mountainous environment

Areas a few square metres in size, devoid of higher vegetation other than moss, have been mapped at 600–700 m above sea level in the mountains of the western part of central Norway. The moss is covered by a grey precipitate during dry summer periods. The precipitate has been identified by ICP-AES analysis of HNO₃-extract, X-ray diffraction (XRD) and by scanning electron microscopy (SEM) as an aluminium(Al)-hydroxide, probably amorphous Al-hydroxide and diaspore (Al(OH)₃), containing 21–25% Al by weight. In comparison, the underlying moraine deposits contain 1.5–3.5% Al by weight. A small spring, where groundwater discharges, is located uphill of each contaminated area. The Al content of the water which emerges from these springs decreases downhill away from the spring and is inversely proportional to the pH. The runoff waters originating at the springs have been modelled using the computer-codes MINTEQA2 and ALCHEMI and are found to be saturated with respect to amorphous Al(OH)₃. This study raises the very important question as to what extent a lower pH in the influent meltwater may leach out more aluminium and possibly lead to the formation of larger amounts of precipitate.