John McAlister

Observations on the factors influencing stability of building stones following contour scaling: A case study of oolitic limestones from Budapest, Hungary

Abstract Salt-induced decay of porous, granular building stones is often manifested initially by contour scaling, followed by rapid surface retreat through granular disaggregation and/or multiple flaking. It has been hypothesised that rapid retreat in polluted environments could be avoided if newly scaled surfaces can be stabilised by, for example, the re-growth of gypsum crusts. In this paper, we investigate the conditions required to ‘switch on and off’ rapid decay through a case study of oolitic limestones in a city that continues to experience high levels of atmospheric pollution. In a locally humid location adjacent to the River Danube, positive feedbacks are seen to occur following scaling that favour continuous rapid retreat linked to the synergistic salt weathering effects of halite and gypsum. In contrast, the Castle Hill overlooking the Danube is less prone to winter and autumn fogs and there is evidence that stone retreat here may be more sporadic and that episodes of contour scaling can be interspersed with periods of stability marked by the re-growth of gypsum crusts. On individual stone blocks it is therefore possible to see fragmentary evidence of several crusts. The negative feedbacks that operate to stabilise newly scaled surfaces are thought to be associated with low chloride concentrations, and a relatively dry microclimate at the block surface that inhibits the frequency and depth of surface wetting. This is linked in turn to the continentality of the meteorological climate of Hungary. It is possible, however, that particular significance attaches to the rapid deposition and effective retention of particulate pollutants rich in water-soluble sulphate and coincident biological colonisation of scaled surfaces that aids the adherence of precipitated dust.

Modelling the rapid retreat of building sandstones: a case study from a polluted maritime environment

Abstract Sandstones are widely used as building stones throughout NW Europe. Unlike limestone, sandstones tend to experience episodic and sometimes rapid surface retreat associated with the action of salts and often leading to the development of hollows/caverns in the stone. The unpredictability of these decay dynamics can present significant problems when planning conservation strategies. Consequently, successful conservation requires a better understanding of the factors that trigger decay and determine the subsequent decay pathway. An overview of results from previous studies provided the basis for simulation experiments aimed at identifying the factors that (a) initiate decay and (b) permit the continuance of salt weathering despite rapid loss of surface material. These simulation studies involve investigation of changes in micro-environmental conditions as surface hollows develop and examination of salt weathering dynamics within such hollows. These data combined with knowledge gained from previous work have allowed the refinement of a conceptual model of rapid sandstone retreat. In this model decay is linked to the establishment of positive feedback conditions through interactions between factors such as porosity, permeability, mineralogy and their effect on salt penetration.

The use of sequential extraction to examine iron and trace metal mobilisation and the case-hardening of building sandstone: a preliminary investigation

Abstract Mobilisation of iron in building stone due to acid rainfall and chemical cleaning may cause its uneven precipitation, discoloration of the stone and the formation of an indurated iron-cemented surface crust. Breaching of the crust can result in accelerated internal excavation, by, for example, salt weathering. Therefore, to understand the decay of iron-rich sandstone, the conditions necessary for iron mobilisation and precipitation and how amorphous iron oxides bind trace elements within the stone must be known. This preliminary study moves away from traditional total-element analysis procedures that provide limited information on internal elemental distributions and mobility. A selective extraction protocol, developed to study element speciation in soils and palaeosols, was modified to extract metals from operationally defined phases within a weathered, iron-rich sandstone from a ca. 150-year-old building in Belfast, N. Ireland. These phases include amorphous Fe and Mn oxides that are known to be effective scavengers of trace metal ions and constitute the principal components of surface stains and cement. Knowledge of the relationships between Fe and Mn and associated trace metal suites may eventually allow the latter to be used as tracers for Fe/Mn movement in stone.

Aeolian abrasion of quartz particles and the production of silt-size fragments: preliminary results

Summary Experimental abrasion of quartz sand in an air stream shows that particles with initially high angularity are progressively rounded by the chipping of edges and corners. Grains closely resemble dune sand grains from deserts with respect to rounding and microscopic surface textures after 48 hours of abrasion. Results show an initially high weight loss from the original grains (primarily by edge chipping), compared with subsequent abrasion. High percentages of coarse and medium silt are produced but there is a gradual increase in the percentage of fine silt as rounding increases and the quantity of fines generated declines. Silica extraction from the fines using water shows concentrations of several hundred ppm. Implications for loess production and silcrete formation are suggested.

Concentration and Bioavailability of Heavy Metals in Sediments from Niterói Harbour (Guanabara Bay/S.E. Brazil)

Abstract The coastal area of Niterói is marked by intensive naval activities and holds one of the countries main naval estates. The harbour and dockyards have been sited in the international literature as potential sources and sinks for accumulation of heavy metals and hence contaminated sediments. The aim of this paper is to assess the concentrations of heavy metals in Niterói Harbour and verify their bioavailability by determining the catalytic effects of the bacteria using esterase activity (EST) and electron transport system activity (ETSA). Samples were analysed for Ni, Zn, Pb, Cr and Cu and normalisation procedures were used to assess whether their concentrations represent background or contamination of the sediment. Heavy metal concentrations, especially for Cu, Zn and Pb were found to be much higher than natural background levels and the index of “geoaccumulation” shows moderate to extreme contamination. However, the absence of the inhibition of dehydrogenase activity indicated that the analysed heavy metals are not bioavailable in the EC50 values.

Rock weathering in blockfields: some preliminary data from mountain plateaus in North Norway

Abstract The formation of blockfields is a process usually attributed to weathering. In mountain areas this is generally assumed to be mechanical weathering (frost shattering). Evidence from two high plateaus [900 and 1350 m above sea level (a.s.l.)] in North Norway (c. 70°N) suggests that chemical action is at least as important as mechanical activity in blockfield formation. The bedrock in both areas consists of complex banded gabbros. Blockfields circumscribe ice masses and are generally > 1 m thick. They contain high percentages of material in the silt and clay sized fractions, including a variety of clay minerals: gibbsite, chlorite, vermiculite and kaolinite, as well as magnetite/maghemite. The blockfield thickness and presence of these weathering products suggests both a considerable (pre-Pleistocene) length of time required for development as well as warmer conditions than are found now (mean annual air temperature c. 0°C) or in the period since deglaciation. It is suggested that these blockfields represent a preglacial palaeosurface which formed initially under warmer conditions and has survived, largely intact, beneath all the Pleistocene ice sheets.

Mineralogy, chemistry and palaeoenvironmental significance of an Early Tertiary Terra Rossa from Northern Ireland: A preliminary review

Abstract Early Tertiary basalts of the Antrim Plateau have preserved palaeosols developed on late Cretaceous limestone. These palaeosols include dark red clays and clays with flints that are rich in silica and contain a complex clay mineral assemblage of 2:1 lattice clays, kaolinite and sepiolite. They occur in infilled solution hollows and joints, can be associated with a rudimentary tower karst and are similar in appearance and composition to present-day Mediterranean terra rossa soils. Elsewhere, grey flint-rich clay-sized material dominated by silica and kaolinite developed on an undulating landscape, sometimes now capped by a lignite horizon. Deposits and the palaeolandscape suggest a humid tropical to sub-tropical environment contrary to previous published inferences of arid conditions, but corresponding to humid tropical conditions proposed for the formation of overlying inter-basaltic soils.

Geochronology and heavy metal flux to Guanabara bay, Rio de Janeiro state: a preliminary study

Guanabara Bay, located in Rio de Janeiro state. It is surrounded by the second most important metropolitan area of the country. Over recent decades, land disturbance and urbanization in the surrounding area has significantly increased sediment input to the bay and had a negative effect on its overall environmental. This is especially related to high volumes of untreated sewage and industrial effluents. This study evaluates the history of this human impact through detailed examination of a sediment core taken from the northern portion of Guanabara Bay. A geochronology is established using ²¹⁰Pb dating and related to organic carbon and heavy metal fluxes to the sediments. This gave a calculated net average sedimentation rate for the core of 0.67 cm.year⁻¹. The organic carbon and heavy metals flux started to increase at the beginning of the last century and the highest values was observed in the top of the cores.

The presence of calcium oxalate dihydrate (weddellite) in street sediments from Niterói, Brazil and its health implications.

Established research in first world urban environments has shown street sediments to be effective sinks for heavy metals and emphasises the health risks to children by its inhalation or ingestion. In third world countries with fewer pollution controls, contamination may be augmented by additional pollutants such as sewage discharge into urban streets and/or periodic inundation with sewage-laden run-off. This pollutant not only contains heavy metals and organic matter, but calcium oxalates that may derive directly from the sewage itself. To study this premise street sediments were collected from sites of varying industrialisation and sewage contamination within the city of Niteroi, Brazil. Calcium oxalate dihydrate (weddellite) was identified by X-ray diffraction, fourier transform infra red and scanning electron microscopy techniques and oxalate concentrations in < 63 μm and colloidal (clay and organic matter) fractions, were determined by ion chromatography. Oxalate in colloidal fractions averaged 43601 mg kg−1 in sewage contaminated industrialised urban zones, 4519 mg kg−1 in suburban areas and 17477 mg kg−1 in suburbs with favelas (shantytowns-of Niterói). Oxalate concentrations coincide with high levels of metal ions (Fe, Mn, Zn, Pb, Ni, Cr) that stabilise calcium oxalate dihydrate and prevent its dehydration to the monohydrate (whewellite). Lower levels of oxalate were found in lichen-covered roof sediment, plaster and mortar samples. Oxalate concentration coincides with sewage contamination and street sediments appear to act as a sink for calcium oxalate dihydrate derived from sewage. Previous studies have shown that low levels of exposure can damage kidneys and the nervous system, while prolonged exposure can cause severe corneal, retinal and skin damage, cyanosis and possibly gangrene. Contact with blood serum may result in hypocalcemia, muscular stimulation, convulsions and finally collapse. The conditions described are common throughout urban areas of the developing world and may suggest a particular environmental hazard in these regions.

Study of heavy metal concentration and partitioning in the Estrela River: implications for the pollution in Guanabara Bay - SE Brazil

In this study, the geochemical analysis of ten sediment samples collected along the fluvial system of the Estrela River, which flows into the northern portion of Guanabara Bay, shows the presence of anthropogenic impacts in this area. Concentrations of Fe, Mn, Zn, Cu, Pb, Cr and Ni obtained were slightly higher, when compared with values found in natural environments. The particle size and organic matter content in most of the analyzed stations showed features not conducive to the accumulation of pollutants due to the low organic matter content and the strong presence of sand fraction. There was also the fractionation of heavy metals in sediments and it was found the prominence of residual and reducible phase, besides the significant occurrence of organic fractions in some analyzed stations. These factors, thus, highlight the potential risks of contamination, where the metals associated with the organic phase can become bioavailable in processes of dissolution, provided by physico-chemical changes that can occur in this aquatic environment.