Rock, soil and soil water chemistry in the Loch Dee catchment, S.W. Scotland : implications for the release of Al

Abstract

The solid geology of the Loch Dee catchment was characterised according to chemistry and mineralogy. The chemistry of weathered and unweathered rock samples were compared. Differences noted did not conform to the weathering scenarios of Goldich. The results, which were explained with reference to weathering rates, supported previous observations on ba.se flow water quality in the area. The drift geology of the area was characterised chemically and texturally and found to be of mixed local origin. Soils in the area were sampled and analysed. They exhibited some chemical differences due to soil profile type. The lack of soil chemical variation due to geology was explained by the mixed drift component of the soil parent material. Soil thin sections highlighted the heterogeneity of drift material in the soil and indicated drift cover was not uniform. Movement of organic material through the .soil profile was identified and considered important in the transport of Al within the catchment. Soil waters were sampled at monthly intervals over a 15 month period. Chemical analyses indicated a strong maritime influence that gave rise to seasonal variations. Other patterns in the soil water chemistry were explained by changing temperature and biological uptake throughout the year. Solutions extracted from soils overlying diorites and granodiorites were found to be relatively enriched in Al. Other chemical differences identified were thought to be related to .soil site distinctions. Soil waters extracted from soils overlying diorites had greater concentrations of Si than those extracted from soils over granodiorites. Concentrations of Al in these soil waters were related to soil water DOC concentration. This masked the role of Si in influencing Al form and further work is required to determine this relationship. DOC appeared to control the form of Al in solution – dialysed fractions of Al (potentially toxic) were less concentrated where solution DOC was greater. The presence of this and other ions in solution caused Al solubility to deviate from standard Al(OH)3 controls. Fractions of Al < 2.4 nm conformed to reversible non-ideal aluminosilicate mineral solubility controls. The possible presence of hydroxyaluminosilicate species in the soil waters was identified. These were disordered, probably in the form of proto-imogolite allophane. The presence of such species indicated an alternative mechanism for the movement of Al through the soil profile. XRD work on the soils in the area would be useful to positively identify the presence of imogolite or proto-imogolite.