Cultural Resource Monitoring: Concurrent Aerometric and Materials

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Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 25, 2018 | https://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch018

Cultural Resource Monitoring: Concurrent Aerometric and Materials Deterioration Studies at Mesa Verde National Park 1

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D. A. Dolske and W. T. Petuskey 1

State Water Survey Division, Illinois Department of Energy and Natural Resources, 2204 Griffith Drive, Champaign, IL 61820 Department of Chemistry, Arizona State University, Tempe, AZ 85287

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The effects of acid deposition on the sandstone masonry of Anasazi cliff-dwelling ruins at Mesa Verde National Park, Colorado, are being investigated. Potential correlations are sought between surface erosion rates and continuously monitored aerometric parameters. A mechanism of deterioration by acceleration of natural erosion processes is hypothesized. Gaseous and particulate pollutant concentrations and meteorological variables are measured at Spruce Tree House ruin. Two test walls, closely resembling Anasazi structures, have been constructed of sandstone specimens typical of masonry units found at Mesa Verde. Deterioration of the stone occurs by a combination of chemical and mechanical processes which weaken the material which bonds the quartz grains. The surface recession rate is periodically recorded using photogrammetry, and microstructural and microchemical alterations in the bonding phase are observed using optical and electron probing techniques. Backscattered-imaging electron microscopy is sensitive to the chemistry of constituent phases, with spatial resolution equivalent to secondary electron imaging. The monitoring scheme is discussed in the context of ongoing research. The f o u r c o r n e r s r e g i o n of the U n i t e d S t a t e s c o n t a i n s some of t h i s c o u n t r y ' s t r e a s u r e d c u l t u r a l r e s o u r c e s i n the form of d w e l l i n g r u i n s of p r e h i s t o r i c A n a s a z i c i v i l i z a t i o n . The r u i n s have been exposed f o r a thousand y e a r s t o a n a t u r a l environment w h i c h was l a r g e l y u n a f f e c t e d by human a c t i v i t y . However, i n v e r y r e c e n t y e a r s r a p i d l y i n c r e a s i n g v i s i t a t i o n of c u l t u r a l s i t e s i n the a r e a by t o u r i s t s , c o n s t r u c t i o n and o p e r a t i o n of l a r g e man-made i n s t a l l a t i o n s such as power p l a n t s which burn f o s s i l f u e l , and r a p i d r e g i o n a l p o p u l a t i o n growth have changed t h i s s i t u a t i o n . L o c a l a i r c h e m i s t r y and c l i m a t i c c o n d i t i o n s may be s u f f i c i e n t l y a l t e r e d as t o a f f e c t the mode of d e t e r i o r a t i o n of the masonry o f the r u i n s . A system has been 0097-6156/86/0318-025°$06.00/ 0 © 1986 American Chemical Society

Baboian; Materials Degradation Caused by Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

M A T E R I A L S D E G R A D A T I O N C A U S E D BY A C I D R A I N

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developed f o r m o n i t o r i n g the e f f e c t s o f the d e p o s i t i o n of a c i d i f y i n g s u b s t a n c e s on c e r t a i n b u i l d i n g m a t e r i a l s found i n these c u l t u r a l resources. A p i l o t t e s t system has been e r e c t e d near Spruce Tree House r u i n a t Mesa Verde N a t i o n a l P a r k , C o l o r a d o . The o b j e c t i v e s of the p r o j e c t a r e t o d e v i s e and implement systems f o r s i m u l t a n e o u s m o n i t o r i n g of a e r o m e t r i c and m a t e r i a l s - d e t e r i o r a t i o n parameters. The t a r g e t m a t e r i a l i s the sandstone masonry of the r u i n s . The system w i l l p r o v i d e d a t a a l l o w i n g c o r r e l a t i o n s t o be s t u d i e d c o n c e r n i n g t h e i n f l u e n c e of l o c a l a i r c h e m i s t r y on the r a t e of d e t e r i o r a t i o n . Two s p e c i f i c hypotheses form the b a s i s f o r t h e p i l o t m o n i t o r i n g system d e s i g n : (1) C o r r e l a t i o n s between the r a t e o f d e t e r i o r a t i o n of the sandstone masonry and f l u c t u a t i o n s i n the c o n c e n t r a t i o n s o f a c i d i c components d e l i v e r e d by the l o c a l atmosphere can be detected. (2) I n p u t s of a c i d i c components a c c e l e r a t e d e t e r i o r a t i o n by n a t u r a l p r o c e s s e s by i n c r e a s i n g the r a t e of d i s s o l u t i o n o f the bonding phases i n the sandstone m i c r o s t r u c t u r e . Methods Simultaneous m a t e r i a l s - d e t e r i o r a t i o n and a e r o m e t r i c measurements are made a t one l o c a t i o n . C o r r e l a t i o n s o f d e t e r i o r a t i o n parameters w i t h v a r y i n g a t m o s p h e r i c p h y s i c a l and c h e m i c a l parameters w i l l be examined u s i n g m u l t i v a r i a t e s t a t i s t i c a l methods when s u f f i c i e n t d a t a are accumulated. D e t e r i o r a t i o n parameters b e i n g measured i n c l u d e r a t e s of s u r f a c e e r o s i o n , s u r f a c e c h e m i c a l changes, and m i c r o s t r u c t u r a l a l t e r a t i o n s . Atmospheric monitoring i n c l u d e s p a r t i c u l a t e s u l f a t e , n i t r a t e , and s u l f u r d i o x i d e c o n c e n t r a t i o n s , as w e l l as t e m p e r a t u r e , r e l a t i v e h u m i d i t y , and parameters of wind and water e r o s i o n . Materials

Monitoring

The o b j e c t i v e s o f the m a t e r i a l s m o n i t o r i n g p o r t i o n o f t h i s p r o j e c t a r e t o ( 1 ) p r o v i d e a means f o r p e r i o d i c a l l y a s s e s s i n g any changes i n the c h e m i s t r y o f the sandstone masonry used i n A n a s a z i I n d i a n r u i n s , and ( 2 ) p r o v i d e a means of a s s e s s i n g t h e r a t e o f t h e i r d e t e r i o r a t i o n . Towards t h i s end, a m o n i t o r i n g system has been d e v i s e d based on the photography o f stone s u r f a c e s and the c h e m i c a l a n a l y s e s of specimens t a k e n a t r e g u l a r i n t e r v a l s . The i n f o r m a t i o n o b t a i n e d i s t o be compared t o timewise changes i n the a i r c h e m i s t r y and m e t e o r o l o g i c a l c o n d i t i o n s t o determine the e x i s t e n c e of any s t a t i s t i c a l l y significant correlations. The sandstone masonry used by the A n a s a z i i s g e n e r a l l y thought t o o r i g i n a t e from nearby c l i f f s from w h i c h huge p o r t i o n s s p a l l e d o f f due to the e x p a n s i o n - c r a c k i n g f o r c e s a s s o c i a t e d w i t h s a l t s w h i c h p e r c o l a t e from behind the s u r f a c e . These were f a s h i o n e d , d r e s s e d , and used i n the d w e l l i n g s f o r w h i c h these a n c i e n t people a r e known. M o n i t o r i n g c o n s i s t s of p e r i o d i c e x a m i n a t i o n o f two t e s t w a l l s c o n s t r u c t e d of a c t u a l r u i n s s t o n e s . These were s e l e c t e d i n


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DOLSKE A N D PETUSKEY

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c o n s u l t a t i o n w i t h Park p e r s o n n e l and were t a k e n from r u b b l e p i l e s l e f t by crews who r e c o n s t r u c t e d and s t a b i l i z e d the e x i s t i n g r u i n s . G e n e r a l l y , the s t o n e s have l o s t t h e i r c o n t e x t w i t h r e s p e c t t o any e x i s t i n g r u i n s and t h e r e f o r e are of s m a l l a r c h a e o l o g i c a l v a l u e . The s e l e c t i o n c r i t e r i o n was t h a t each stone have an exposed and w e l l - w e a t h e r e d s u r f a c e which was r e a s o n a b l y f l a t f o r p h o t o g r a p h i c inspection. A s e r i e s of f i v e s t o n e s were s e l e c t e d f o r each t e s t w a l l . One w a l l i s l o c a t e d on a r o c k ledge f u l l y exposed t o the elements and i s near the a e r o m e t r i c s t a t i o n . T w e n t y - f i v e meters away, the second s i t e i s l o c a t e d at the extreme down-canyon s i d e of Spruce Tree House R u i n s which i s under the p r o t e c t i o n of r o c k overhang. At each s i t e i s an a r r a y of sandstone specimens c o r e d out of a s i n g l e sandstone b l o c k . Four c o r e s each are mounted i n f u s e d q u a r t z h o l d e r s w h i c h themselves are h e l d i n an a c r y l i c r a c k . A s i n g l e h o l d e r w i t h f o u r c o r e s i s removed each month and s u b j e c t e d t o a v a r i e t y of c h e m i c a l analyses. In d e v i s i n g a scheme f o r m o n i t o r i n g the r a t e of d e t e r i o r a t i o n of the s a n d s t o n e , the assumption was made t h a t any e f f e c t t h a t a t m o s p h e r i c p o l l u t a n t s may have on the s t o n e s ' i n t e g r i t y w i l l appear at or near the s u r f a c e . I t i s not a common o c c u r r a n c e t h a t masonry i s s a t u r a t e d throughout by w a t e r . C o n s e q u e n t l y , an adequate measure of e r o s i o n i s the r a t e of r e c e s s i o n of the s u r f a c e . H e r e , the bonding m a t e r i a l i s d i s s o l v e d or d i s r u p t e d by r e p e a t e d exposures t o m o i s t u r e which may c o n t a i n c h e m i c a l s which a c c e l e r a t e the p r o c e s s . The r a t e of r e c e s s i o n i s c h a r a c t e r i z e d by the r a t e at which the loosened q u a r t z g r a i n s are s u b s e q u e n t l y removed. The r a t e of r e c e s s i o n i s measured by p e r i o d i c a l l y examining s e l e c t e d sandstone s u r f a c e s . Photographs are made a t m a g n i f i c a t i o n s of 8X and 32X. S a p p h i r e and p o l y c r y s t a l l i n e a l u m i n a r o d s , about 1-mm d i a m e t e r , are embedded i n the sandstone s u r f a c e s t o p r o v i d e p o s i t i o n a l r e f e r e n c e s . By comparing photographs t a k e n a t monthly i n t e r v a l s , i t i s p o s s i b l e t o detemine when each s u r f a c e g r a i n d i s a p p e a r s from the s u r f a c e . T h i s a l l o w s an average r e c e s s i o n r a t e to be c a l c u l a t e d . Under some c i r c u m s t a n c e s , agglomerates of s e v e r a l g r a i n s w i l l d i s l o d g e from the s u r f a c e at once. S i n c e i t i s d i f f i c u l t to determine the p r e c i s e number of g r a i n s i n v o l v e d , stereophotogrammetry i s used t o determine the depth of the v o i d s ( o r peaks i f e x i s t e n t p r i o r to r e m o v a l ) . C u r r e n t l y , i t appears t h a t monthly i n t e r v a l s between photographs are s u f f i c i e n t t o r e s o l v e such events i n a t i m e w i s e manner. C h e m i c a l changes i n the sandstone are m o n i t o r e d by a c o m b i n a t i o n of schemes. One of the most i m p o r t a n t t o o l s i s the use of b a c k s c a t t e r e d e l e c t r o n imaging microscopy (BSEI) of p r e p a r e d c r o s s s e c t i o n s of sandstone specimens. U n l i k e s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM), which produces a t o p o g r a p h i c a l image of the s u r f a c e of m a t e r i a l s , BSEI produces an image of f l a t - p o l i s h e d s u r f a c e s w h i c h i s s e n s i t i v e t o the c o m p o s i t i o n of the m a t e r i a l . E x a m i n a t i o n of a sandstone specimen can r e v e a l s e v e r a l d i f f e r e n t phases w h i c h are d i s t i n g u i s h e d by t h e i r image b r i g h t n e s s . B a s i c a l l y , the b r i g h t n e s s i s a f u n c t i o n of the e f f i c i e n c y at which the p r i m a r y e l e c t r o n s are s c a t t e r e d by each compound. T h i s i s a f u n c t i o n of the d e n s i t y of


MATERIALS DEGRADATION

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e l e c t r o n s i n the s t r u c t u r e and t h e r e f o r e depends on the average atomic number. Compounds c o n s i s t i n g of h i g h l e v e l s of heavy elements are e f f i c i e n t s c a t t e r e r s and appear l i g h t e r i n tone than compounds of l i g h t e r elements which appear t o be d a r k e r . T h i s s e n s i t i v i t y t o the average atomic number, or Z - c o n t r a s t , w i l l make i t p o s s i b l e t o d e t e c t phase t r a n s f o r m a t i o n s t h a t might o c c u r . F o r i n s t a n c e , i t would be p o s s i b l e to d e t e c t the s o l i d s t a t e t r a n s f o r m a t i o n of c a l c i u m c a r b o n a t e to a s u l f a t e phase i n the presence of h i g h s u l f a t e concentrations. I t i s a l s o i m p o r t a n t t o a s s e s s what a t m o s p h e r i c contaminants are d e p o s i t e d on the s u r f a c e . C o n s e q u e n t l y , two e x a m i n a t i o n schemes have been adopted f o r t h i s purpose. I n g e n e r a l , the contaminants of g r e a t e s t concern are those w h i c h are water s o l u b l e . T h e r e f o r e , sandstone c o r e s are p u l v e r i z e d and d i s p e r s e d i n d i s t i l l e d w a t e r . A f t e r c e n t r i f u g a t i o n and f i l t e r i n g , the decanted l i q u i d i s a n a l y z e d f o r s u l p h a t e s , n i t r a t e s and c h l o r i d e s u s i n g i o n chromatography. F i n a l l y , the s u r f a c e s of the c o r e s are examined both o p t i c a l l y and by secondary e l e c t r o n microscopy t o determine the e x t e n t of m i c r o s t r u c t u r a l changes t h a t are o c c u r i n g due t o a t m o s p h e r i c exposure. Energy d i s p e r s i v e a n a l y s i s of x - r a y s (EDAX) a l s o s e r v e s t o d e t e c t a t m o s p h e r i c p a r t i c l e s w h i c h have d e p o s i t e d onto the c o r e s u r f a c e . A l l t h i s i n f o r m a t i o n can then be used t o a t l e a s t q u a l i t a t i v e l y i d e n t i f y d e t e r i o r a t i o n p r o c e s s e s t h a t may be o c c u r i n g . Aerometric Monitoring The a e r o m e t r i c m o n i t o r i n g system was d e s i g n e d t o p r o v i d e adequate t e m p o r a l r e s o l u t i o n f o r the o b s e r v a t i o n of f e a t u r e s i n the a t m o s p h e r i c s i g n a l t h a t may d r i v e the h y p o t h e s i z e d mechanisms of a c c e l e r a t e d sandstone d e t e r i o r a t i o n . M e t e o r o l o g i c a l parameters b e i n g measured a t Spruce Tree House i n c l u d e t e m p e r a t u r e , r e l a t i v e h u m i d i t y , h o r i z o n t a l and v e r t i c a l wind v e l o c i t y , wind d i r e c t i o n , i n c i d e n t s o l a r r a d i a t i o n , and r a i n f a l l r a t e . Atmospheric c h e m i s t r y i s m o n i t o r e d i n s e v e r a l ways. The c h e m i c a l c o m p o s i t i o n of p r e c i p i t a t i o n i s o b t a i n e d from the Mesa Verde N a t i o n a l Atmospheric D e p o s i t i o n Program r e g i o n a l r a i n s a m p l i n g s t a t i o n , which i s 1.6 km n o r t h of the Spruce Tree House s i t e . A t the s i t e of the t e s t w a l l s , a s e r i e s - f i l t r a t i o n system c o l l e c t s p a r t i c u l a t e and gaseous p o l l u t a n t s . P a s s i v e gas a b s o r p t i o n samplers are mounted w i t h i n 4 m of the w a l l s . A l l m e t e o r o l o g i c a l parameters are d i g i t a l l y r e c o r d e d as h o u r l y averages. I n a d d i t i o n , temperature and h u m i d i t y a r e i n d e p e n d e n t l y measured and c o n t i n o u s l y r e c o r d e d on a s t r i p c h a r t . A t o t a l i z i n g anemometer g i v e s weekly wind r u n . P r e c i p i t a t i o n and a c t i v e l y sampled a i r c h e m i s t r y are c o l l e c t e d w e e k l y , w h i l e the p a s s i v e gas m o n i t o r s are exposed f o r minimum one-month i n t e r v a l s . The c o n t i n u o u s s t r i p c h a r t n o t i n g the temperature and h u m i d i t y , m e c h a n i c a l anemometer, and p a s s i v e gas samplers are c o l l e c t i v e l y r e f e r r e d t o as the n o n - i n t e n s i v e method. R e s u l t s of the n o n - i n t e n s i v e approach w i l l be compared w i t h the more d e t a i l e d d a t a from the a c t i v e a i r s a m p l i n g and d i g i t a l l y r e c o r d e d m e t e o r o l o g i c a l m o n i t o r i n g . I f the comparison i s f a v o r a b l e , i t i s u l t i m a t e l y planned t o s t u d y m i c r o c l i m a t e v a r i a b i l i t y on a r u i n - s p e c i f i c b a s i s w i t h a network of s e v e r a l n o n - i n t e n s i v e s i t e s .


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I t i s i m p o r t a n t t o note t h a t w h i l e the sandstone d e t e r i o r a t i o n measurements a r e done monthly, t h e a e r o m e t r i c d a t a a r e of much f i n e r t e m p o r a l r e s o l u t i o n . T h i s i s done t o a l l o w the i n v e s t i g a t i o n o f v a r i a t i o n s i n d e t e r i o r a t i o n r a t e s w i t h r e s p e c t t o not o n l y averaged a e r o m e t r i c d a t a , but a l s o extremes and the range of d i u r n a l v a r i a t i o n s w i t h i n those averages. The m e t e o r o l o g i c a l s e n s o r s a r e mounted a t the t o p of a 7 m tower, l o c a t e d i n a c l e a r i n g 40 m s o u t h of Spruce Tree House. The s e n s o r s are thus about 5 m above t h e e l e v a t i o n o f the t e s t w a l l s , and j u s t above the f o r e s t canopy w i t h i n the canyon. The exposure of the i n s t r u m e n t s was chosen t o r e f l e c t a c c u r a t e l y the s p e c i f i c environment e x p e r i e n c e d by the t e s t w a l l s and Spruce Tree House. The n o n - i n t e n s i v e experiments a r e l o c a t e d i n t h e same c l e a r i n g , on a s e p a r a t e mast about 2 . 5 m t a l l . I n a d d i t i o n , temperature t r a n s d u c e r s and s u r f a c e time-of-wetness s e n s o r s have been mounted on the t e s t w a l l s . These parameters a r e a l s o r e c o r d e d on d i g i t a l t a p e . The a c t i v e a i r s a m p l i n g system i s l o c a t e d on the m e t e o r o l o g i c a l tower. A c o n s t a n t f l o w pump draws a i r t h r o u g h a f o u r - s t a g e s e r i e s f i l t e r pack at 10.0 L/min. The f i l t e r h o l d e r i s suspended face-down beneath a 40 cm diameter p o l y e t h y l e n e f u n n e l , 2.5 m above ground. Four 47 mm diameter f i l t e r s a r e mounted i n a p o l y c a r b o n a t e h o l d e r . The f i r s t f i l t e r i s 8.0 jxm pore diameter N u c l e p o r e , to c o l l e c t l a r g e p a r t i c l e s ( d i a m e t e r > 2.5^111) ( C a h i l l , e t a l , 1977). The second f i l t e r i s 1.0 p\m T e f l o n , used t o c o l l e c t the r e m a i n i n g s m a l l p a r t i c l e s . T h i r d , a 1.0 yum N y l o n f i l t e r s e l e c t i v e l y adsorbs n i t r i c a c i d vapor ( G o l d a n , e t a l , 1983). The sum o f p a r t i c u l a t e n i t r a t e from the f i r s t two f i l t e r s and n i t r i c a c i d from the t h i r d f i l t e r s h o u l d r e p r e s e n t w e l l the t o t a l ambient n i t r a t e c o n c e n t r a t i o n . The f o u r t h and l a s t f i l t e r i s a double l a y e r of c e l l u l o s e f i b e r paper w h i c h has been doped w i t h g l y c e r o l and p o t a s s i u m c a r b o n a t e , t o absorb s u l f u r d i o x i d e (Johnson and A t k i n s , 1975). L o a d i n g o f c l e a n f i l t e r s and the e x t r a c t i o n o f exposed ones a r e done a t t h e l a b o r a t o r y a t t h e I l l i n o i s S t a t e Water Survey. The f i l t e r p a c k s a r e s e a l e d i n s e v e r a l l a y e r s of p o l y e t h y l e n e f o r shipments between the f i e l d s i t e and l a b o r a t o r y . A n a l y s i s of the e x t r a c t s i s done by i o n chromatography. About twenty p e r c e n t o f t h i s p r o j e c t ' s t o t a l sample l o a d i s r e s e r v e d f o r q u a l i t y assurance p u r p o s e s , i n c l u d i n g p r o c e d u r a l , r e a g e n t , and f i e l d blanks. Discussion A l t h o u g h t h e r e i s some v a r i a b i l i t y , the sandstones found i n Mesa Verde N a t i o n a l Park a r e r e l a t i v e l y u n i f o r m i n terms of g e n e r a l c o m p o s i t i o n and m i c r o s t r u c t u r e when compared t o t h e broad spectrum o f sandstone t y p e s . P r i n c i p a l l y , they c o n s i s t of a narrow s i z e d i s t r i b u t i o n o f q u a r t z g r a i n s (100-150 m i c r o n s ) t h a t account f o r about 80 t o 90% of the m i n e r a l c o n t e n t . G e o l o g i s t s c l a s s i f y t h i s type as a q u a r t z a r e n i t e where the g r a i n shapes a r e subrounded t o s u b - a n g u l a r . The a n g u l a r i t y i s caused by a u t h i g e n i c q u a r t z overgrowths on i n i t i a l l y rounded g r a i n s . The remainder of the m i n e r a l c o n t e n t c o n s i s t s o f 5 t o 10% c l a y m i n e r a l s , 3 t o 8% p o t a s s i u m f e l d s p a r , a combined 5% of h e m a t i t e ^ 6 3 0 4 ) and c a l c i t e (CaCX^), and a combined 1% of such a c c e s s o r y phases as z i r c o n ( Z r S i O ^ ) , t o u r m a l i n e , a p a t i t e and b a r i t e ( B a C O o ) .



264

M A T E R I A L S D E G R A D A T I O N C A U S E D BY A C I D R A I N

A major component of the sandstone i s i t s p o r o s i t y w h i c h ranges between 21 t o 36 volume p e r c e n t . Because of t h i s , i t s s t r u c t u r a l i n t e g r i t y owes much t o the i n t e r l o c k i n g p a c k i n g of the g r a i n s . The dominant cementing m a t e r i a l i s the a u t h i g e n i c q u a r t z . Although n o r m a l l y a v e r y s t r o n g and c h e m i c a l l y s t a b l e bonding m a t e r i a l , i t does not p r o v i d e an e x t e n s i v e m a t r i x o f bonding m a t e r i a l and t h e r e b y l e a v e s the stones i n a f r i a b l e s t a t e . Other cementing m a t e r i a l s , of secondary importance on a volume percentage b a s i s , may be i m p o r t a n t w i t h r e s p e c t t o the r e s i s t a n c e of the stone t o w e a t h e r i n g p r o c e s s . C l a y m i n e r a l bonding o c c u r s t o a much l e s s e r e x t e n t and g e n e r a l l y appears i n i n t e r s t i t i a l p a t c h e s . N o r m a l l y , c a l c i t e and h e m a t i t e phases appear as d i s c r e t e g r a i n s , however, some stones have been found t o c o n t a i n a p p r e c i a b l e amounts as a cement. The v a r i a b i l i t y o f the sandstones found i n t h e Park seems t o be a f u n c t i o n of the r e l a t i v e p r o p o r t i o n s o f the secondary bonding m a t e r i a l . T h i s has an apparent i n f l u e n c e on the i n t e g r i t y o f r u i n s s t o n e s exposed t o n a t u r a l w e a t h e r i n g p r o c e s s e s . Frequent o b s e r v a t i o n s have been made where i n d i v i d u a l masonry u n i t s i n a r u i n s w a l l d e t e r i o r a t e s r a p i d l y whereas the n e i g h b o r i n g b l o c k s a r e r e l a t i v e l y unaffected. The e r o s i o n mechanism of g r e a t e s t concern t o t h i s p r o j e c t i s t h e c h e m i c a l a l t e r a t i o n o f the bonding m a t r i x of the s a n d s t o n e . I f cementing m a t e r i a l i s d i s s o l v e d , the i n d i v i d u a l q u a r t z g r a i n s on the s u r f a c e a r e l a i d bare t o m e c h a n i c a l elements of the e r o s i o n p r o c e s s , namely wind and water a c t i o n . The time dependent element i n t h i s p r o c e s s i s c u r r e n t l y u n c l e a r a l t h o u g h one would presume t h a t t h e d i s s o l u t i o n of the bonding m a t e r i a l i s s u f f i c i e n t l y slow t o dominate. By i t s e l f , t h i s i s a f e a s i b l e mechanism o f n a t u r a l e r o s i o n . The i m p o r t a n t q u e s t i o n i s whether i t i s a c c e l e r a t e d by c h e m i c a l p o l l u t a n t s . Dry d e p o s i t i o n o f n i t r a t e s , s u l p h a t e s and c h l o r i d e s w h i c h s u b s e q u e n t l y a r e dampened by dew and r a i n can y i e l d s o l u t i o n s of h i g h a c i d i t y on a l o c a l i z e d s c a l e . T h i s would a c t p a r t i c u l a r l y s t r o n g l y on such compounds as the c a r b o n a t e s and have d e c r e a s i n g l y l e s s e f f e c t on f e l d s p a r s , c l a y s , f e r r o u s compounds and t h e a c c e s s o r y phases. L e a s t a f f e c t e d would be q u a r t z . At f i r s t g l a n c e , i t would seem t h a t a c i d i c environments s h o u l d not be p a r t i c u l a r l y i m p o r t a n t c o n s i d e r i n g t h e preponderence o f l o o s e q u a r t z c e m e n t a t i o n . However, t h e r e i s some evidence t h a t these sandstones a r e s u s c e p t i b l e a t l e a s t t o some degree. Previous p r e l i m i n a r y i n v e s t i g a t i o n s have i n d i c a t e d t h a t t h e c o h e s i v e s t r e n g t h of sandstone cores soaked i n m i l d a c i d s o l u t i o n s i s 10 t o 90% lower than those soaked i n n e u t r a l s o l u t i o n s . T h i s would i n d i c a t e t h a t bonding m a t e r i a l i s weakened p a r t i c u l a r l y i n those cases where s i g n i f i c a n t q u a n t i t i e s of c a r b o n a t e s a r e p r e s e n t . However, i t i s i n d e e d d i f f i c u l t t o a p p l y such o b s e r v a t i o n s t o t h e c u r r e n t s i t u a t i o n s i n c e the manner of d e l i v e r y o f a c i d i c components and degree o f u n i f o r m exposure i s q u i t e u n c e r t a i n . The p r o j e c t i s p r e s e n t l y i n the f i r s t f u l l y e a r o f d a t a c o l l e c t i o n a t t h e Mesa Verde N a t i o n a l Park f i e l d s i t e . The c o n s t r a i n t s of m u l t i v a r i a t e s t a t i s t i c a l a n a l y s i s r e q u i r e t h a t a t


18.



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l e a s t s e v e r a l y e a r s of s a m p l i n g be completed b e f o r e any i n t e r p r e t a t i o n can be s t a t e d r e g a r d i n g p o s s i b l e d i r e c t e f f e c t s o f a c i d d e p o s i t i o n on the sandstone masonry.


Literature

Cited

Cahill, T.A., Ashbaugh, L.L., Barone, J.B., Eldred, R.A., Feeney, P.J., Flocchini, R.G., Goodart, C., Shadoan, D.J., and Wolfe, G.W., 1977: Analysis of respirable fractions in atmospheric particulates via sequential filtration. J. Air Poll. Control Assoc., 27, p. 675. Goldan, P.D., Kuster, W.C., Albritton, D.L., Fehsenfeld, F.C., Connell, P.S., Norton, R.B., and Huebert, B.J., 1983: Calibration and tests of the filter-collection method for measuring clean-air ambient HNO . Atmos. Environ., 17, p. 1355. 3

Johnson, D.A. and Atkins, D.H.F., 1975: An airborne system for the sampling and analysis of sulphur dioxide and atmospheric aerosols. Atmos. Environ., 9, p. 825. RECEIVED January 2, 1986


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