The Salem limestone and Shelburne marble samples used in this project were exposed at the five National Acid Precipitation Assessment Program (NAPAP) research sites. Weathering experiments performed at these sites, including the collection and analysis of rainfall samples, attempted to quantify acid-rain damage to marble and limestone building stone.
Michael M. Reddy, from the Branch of Water Resources Division, United States Geological Survey (USGS).
Michael is a hydrologist and is currently the Research Advisor for Surface Water Chemistry in the Water Resources Division of the USGS, stationed in Boulder, Colorado. He attended the State University of New York at Buffalo, where he received his B.S. degree in 1966 and his Ph.D. in 1970, both in chemistry. He worked as an instructor at SUNY Buffalo for several years, before becoming a research scientist at the New York State Department of Health; he joined the United States Geological Survey in 1980. Throughout his career, he has been studying the physical chemistry of calcification and has produced a number of insightful papers on the inhibition of crystallization, including the effects of soluble organic compounds.
Michael Reddy started this research in 1983 and finished the work in 1995.
The National Acid Precipitation Assessment Program (NAPAP) coordinated the efforts of both the National Park Service (NPS) and the U. S. Geological Survey (USGS) in their NPS/USGS Materials Research Program (MRP) which began in the early 1980s. The main interest of the MRP focused on the chemical changes and erosion of limestone and marble stone surfaces observed during a 10-year stone exposure program.
In 1984, the MRP established four initial field sites in order to observe stone deterioration effects under a range of environmental conditions: Washington, DC; Raleigh (Research Triangle Park), NC; Newcomb, NY and Chester, NJ. A fifth site was added two years later in Steubenville, Ohio. The exposure sites were equipped with instruments to record a wide range of environmental parameters such as: particulate material, air quality, chemical precipitation of rainfall and the meteorological conditions. The investigators applied many techniques at the field sites to determine the extent of stone deterioration; these included: gravimetric (weight loss) experiments, surface chemistry, rain-runoff chemistry, determination of the volume and pH of precipitation, surface metrology and the determination of stone mineralogy.
The Environmental Protection Agency, in cooperation with the Bureau of Mines, provided Aerometric data. Since 1990, the University of Illinois provided the Aerometric monitoring at the field sites where a weekly exchange of filter packs enabled the evaluation of air-borne particles and gas concentrations. The University of Illinois researchers checked every month the meteorological instruments and recording devices, and the field rainwater collected was chemically analyzed by the USGS - Water Resources Division in Denver. From 1986-88, Paul Hess at ANL was responsible for managing the Aerometric database.
In an uncontaminated environment, the deterioration of marble and limestone by weathering is a slow process and measurable changes occur through nature over hundreds of years. However, in industrialized societies, the observable deterioration may be accelerated because of the increased presence of acidic contaminants in both the rainfall and air.
The purpose of this research was to attempt to quantify the dissolution of carbonate building stones as a function of rain volume, acidity and intensity or more simply put, to investigate the effects of acid rain on carbonate stone resources. Two types of stone were studied, Salem limestone (from Indiana) and Shelburne marble (from Vermont) as these are used in buildings and historical monuments. Four field research sites were chosen for this study, located in: Raleigh (Research Triangle Park), North Carolina; Washington, D.C.; Chester, New Jersey; and Newcomb, New York; and in 1986, a new site was established at Steubenville, Ohio to replace the discontinued NJ site.
It was known that environmental variables, such as hydrogen ions and sulfur dioxide (H+ and SO2), were present in the rainfall-runoff from slabs of fresh and weathered Salem limestone and Shelburne marble exposed at the stone field sites, and it was important to correlate this information with the amount of stone erosion observed. The methodology involved in trying to do this meant using sophisticated kinetic and equilibrium models of solution-solid interactions. These models were developed at the USGS in conjunction with Donald Langmuir at Colorado School of Mines in Golden, Colorado and were used to describe the mechanisms and rates of carbonate stone dissolution in terms of open-system thermodynamics.
This project was composed of the following:
Each site contained four stone-exposure racks, fabricated of polypropylene and holding two stones each, which were inclined at 30° to the horizon and positioned facing south. The rainfall run-off was directed from the base of the rack, through a plastic tube containing a glass-wool plug, into a covered plastic, 1-liter, collection bottle.
Salem limestone exhibits a wide range of pore sizes, high capillarity and low permeability to water. The amount of sulfur accumulation in the stone thus depends on the mechanism of sulfate partitioning between the aqueous and solid phases. Any physical characterization of limestone which provides a damage model for acidic precipitation could then be generalized and applied to other systems of interest, such as acidic damage to porous sandstone or concrete. This would be a huge boost towards understanding the effects of acid rain precipitation in both North America and other parts of the world.
Reddy's research investigated the physical nature of the limestone sulfonation process, in which wetting and drying of the stone drive sulfur from the stone surface (as aqueous sulfate through the pore network by diffusion and convection). He could then formulate damage mechanisms based upon this physical process. He had to comprehensively investigate the characterization of the limestone in terms of porosity, permeability and capillary action. This is very important research because the successful preservation of porous and permeable building stone, exposed to acidic atmospheric environments, hinges on the understanding of acidic precipitation damage mechanisms.
This paper attempts to quantify acid-rain damage to marble and limestone building stone: it takes into account the results of weathering experiments to determine proposed chemical models of the damage.
The procedures applied in this work may be used more generally, to develop stone-deterioration relations to environmental contaminants.
This interesting report clearly states how the rainfall samples were collected and analyzed as regards to the effects of acid rain on carbonate-rock building materials. For example, each field site had four racks, which all sat on their own base (a plywood box) containing the collection/sample bottles.
The racks were designed with two 12² by 24² slots for stone slabs, glass sheets, or blanks, providing a maximum of eight slots per site. To avoid contamination between slots, a plexiglass splash guard was placed between the slots.
The ability to provide representative uncontaminated samples, by taking great care during the collection, processing and shipping of the samples, was vital for the evaluation of the results.
This work used a couple of measurement methods to assess stone material loss due to acid rain. These methods included measuring and analyzing rain-runoff (the carbonate stone surface loss is calculated from the amount of calcium ion added to rainfall as it reacts and flows from the surface of the stone) and the use of an inorganic gravimetric (weigiht loss) method. All measurement methods yielded similar resullts in terms of the rate of surface recession. In the first two years of rainfall exposure in North Carolina, it was found that the rate of marble surface recession was near 15 um and values of this order were apparent for the NJ, NY and DC sites. It was found to be more difficult to assess the rate of surface recession for limestone but arate of loss similar to marble was inferred.
The above report was published on a CD-ROM called "Explore the Research Program - Acid Rain and Beyond". The CD-ROM was developed with funding from the National Park Service and the National Center for Preservation Technology and Training.
