Currently Available Geophysical Methods for Use ... - ACS Publications

8 Currently Available Geophysical Methods for Use in Hazardous Waste Site Investigations ROY B. EVANS

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: October 20, 1982 | doi: 10.1021/bk-1982-0204.ch008

U.S. Environmental Protection Agency, Environmental Monitoring Systems Laboratory, Las Vegas, NV 89114 Monitoring to estimate exposure is e s s e n t i a l to risk assessment near sources of exposure. Cost- e f f e c t i v e hazardous waste s i t e assessments i n c l u d e three phases: (1) p r e l i m i n a r y site assessment, utilizing aerial photography and site i n s p e c t i o n s ; (2) geophysical surveys to p i n p o i n t buried wastes and to help define plumes of conductive contaminants in groundwater; and (3) c o n f i r m a t i o n of groundwater contamination through monitoring w e l l networks designed on the b a s i s of the geo p h y s i c a l survey. C u r r e n t l y a v a i l a b l e g e o p h y s i c a l methods most a p p l i c a b l e in hazardous waste site investigations i n c l u d e metal detectors and magnetometers ( u s e f u l in l o c a t i n g buried wastes); groundpenetrating radar ( u s e f u l in d e f i n i n g trench boundaries); electromagnetic i n d u c t i o n or EM (useful in surveys of shallow plumes of conductive groundwater contaminants); resistivity (useful i n surveys of site s t r a t i g r a p h y and deep groundwater contaminant plumes); and seismic methods (most u s e f u l in surveying geologic s t r a t i g r a p h y ) . M o n i t o r i n g i s u s u a l l y an e s s e n t i a l p a r t of r i s k assessment; e n v i r o n m e n t a l samples a r e c o l l e c t e d i n v a r i o u s media n e a r s o u r c e s o f p o t e n t i a l e x p o s u r e , and t h e s e s a m p l e s a r e a n a l y z e d f o r t o x i c substances suspected of being present. In the past, i n v e s t i g a t i o n o f h a z a r d o u s waste s i t e s h a s commonly depended upon d r i l l i n g t o o b t a i n i n f o r m a t i o n o n t h e g e o l o g i c s e t t i n g , upon m o n i t o r i n g w e l l s f o r s a m p l e s o f g r o u n d w a t e r , and upon l a b o r a t o r y a n a l y s i s o f s o i l and w a s t e s a m p l e s . D u r i n g t h e p a s t d e c a d e , e x t e n s i v e d e v e l o p m e n t i n remote s e n s i n g g e o p h y s i c a l e q u i p m e n t , f i e l d methods, a n a l y t i c a l t e c h n i q u e s , and a s s o c i a t e d computer p r o c e s s i n g h a s g r e a t l y i m p r o v e d o u r a b i l i t y t o c h a r a c t e r i z e hazardous waste s i t e s . Some g e o p h y s i c a l methods o f f e r a d i r e c t means o f d e t e c t i n g c o n t a m i n a n t p l u m e s , f l o w 0097-6156/82/0204-0093$06.25/0 © 1982 American Chemical Society

Long and Schweitzer; Risk Assessment at Hazardous Waste Sites ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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d i r e c t i o n , and b u r i e d drums. Some a r e a p p l i c a b l e t o measurements o f c o n t a m i n a n t s and d i r e c t i o n o f f l o w w i t h i n t h e v a d o s e z o n e ; o t h e r s o f f e r a way t o o b t a i n d e t a i l e d i n f o r m a t i o n a b o u t s u b s u r f a c e g e o l o g y . The c a p a b i l i t y t o c h a r a c t e r i z e t h e subsurface r a p i d l y without d i s t u r b i n g the s i t e o f f e r s b e n e f i t s i n t e r m s o f l e s s c o s t , l e s s r i s k , and b e t t e r u n d e r s t a n d i n g o f site conditions. C o s t - e f f e c t i v e d e s i g n o f h a z a r d o u s waste s i t e g r o u n d w a t e r i n v e s t i g a t i o n s i n v o l v e s an i n t e g r a t e d , t h r e e - p h a s e d a p p r o a c h : (1) p r e l i m i n a r y s i t e assessment, i n v o l v i n g t h e use o f a e r i a l p h o t o g r a p h y , o n - s i t e i n s p e c t i o n s , and r e a d i l y a v a i l a b l e i n f o r m a t i o n t o a p p r o x i m a t e s i t e b o u n d a r i e s and l o c a t i o n s o f w a s t e c o n c e n t r a t i o n s , a s w e l l as p r o b a b l e s i t e g e o l o g y ; ( 2 ) g e o p h y s i c a l s u r v e y s t o p i n p o i n t b u r i e d w a s t e s and e s t i m a t e q u a n t i t i e s , and t o d e l i n e a t e plumes o f c o n d u c t i v e c o n t a m i n a n t s i n g r o u n d w a t e r ; and ( 3 ) c o n f i r m a t i o n o f g r o u n d w a t e r c o n t a m i n a t i o n t h r o u g h m o n i t o r i n g w e l l n e t w o r k s d e s i g n e d on t h e b a s i s o f plumes and s u b s u r f a c e s t r a t i g r a p h y d e f i n e d by t h e geophysical surveys. The s p a t i a l c h a r a c t e r i z a t i o n o f t h e s i t e by g e o p h y s i c a l means c a n make p o s s i b l e t h e e f f i c i e n t l o c a t i o n o f m o n i t o r i n g w e l l s and t h e r e d u c t i o n o f r i s k s i n v o l v e d i n exploratory d r i l l i n g . Field

Problems

The t h r e e g e n e r a l o b j e c t i v e s u s u a l l y i n v o l v e d i n s u b s u r f a c e investigations are: • Location of buried m a t e r i a l s , • D e t e r m i n a t i o n o f t h e p r e s e n c e o f plumes and t h e d i r e c t i o n , r a t e o f movement, and d i s t r i b u t i o n o f contaminants; • C h a r a c t e r i z a t i o n of the geohydrologic conditions, n a t u r a l and manmade. L o c a t i o n o f b u r i e d m a t e r i a l s a t a hazardous waste s i t e i s u s u a l l y f o r the purpose of r e m e d i a l a c t i o n ; i . e . , e x c a v a t i n g t h e s e m a t e r i a l s and u l t i m a t e l y d i s p o s i n g o f them. The k e y unknowns a r e t y p e ( b u l k - d u m p e d o r p a c k a g e d i n drums o r o t h e r c o n t a i n e r s ) , q u a n t i t y (volume o f w a s t e ; number o f d r u m s ) , and l o c a t i o n , p a r t i c u l a r l y depth of b u r i a l . The c o n c e r n s a r e f o r s a f e e x c a v a t i o n w i t h o u t p u n c t u r i n g c o n t a i n e r s o r b r e a c h i n g any e x i s t i n g t r e n c h l i n e r s and t h u s a g g r a v a t i n g t h e c l e a n u p problems. D e t e r m i n a t i o n o f t h e p r e s e n c e o f c o n t a m i n a n t plumes and t h e i r f l o w d i r e c t i o n and movement r a t e i s commonly r e q u i r e d a t hazardous waste s i t e s . The f i r s t d e t e r m i n a t i o n i s w h e t h e r l e a k a g e from t h e hazardous waste s i t e i s o c c u r r i n g . I f the e x i s t e n c e o f a plume i s c o n f i r m e d , i t s d i r e c t i o n and e x t e n t s h o u l d be e s t a b l i s h e d and i d e n t i f i e d . A preliminary geophysical s u r v e y c a n a i d i n b e t t e r d e f i n i n g t h e c o n t a m i n a n t plume, l e a d i n g t o more e f f e c t i v e m o n i t o r i n g w i t h a s m a l l e r number o f m o n i t o r i n g wells.


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C h a r a c t e r i z a t i o n o f the n a t u r a l s e t t i n g i s u s u a l l y a major p o r t i o n o f the f i e l d i n v e s t i g a t i o n . A t most s i t e s , p e r m e a b i l i t y o f t h e l o c a l s o i l and r o c k t y p e s , t h e d e p t h o f t h e w a t e r t a b l e , and t h e d i r e c t i o n o f g r o u n d w a t e r f l o w w i l l s t r o n g l y i n f l u e n c e movement o f c o n t a m i n a n t s f r o m t h e p o i n t o f d i s p o s a l . The anomalies which occur n a t u r a l l y w i t h i n the g e o h y d r o l o g i c s e c t i o n must be t a k e n i n t o c o n s i d e r a t i o n . S u r f a c e d r a i n a g e , sewers, and b u r i e d u t i l i t i e s c a n a f f e c t s u r f a c e and g r o u n d w a t e r f l o w a r o u n d a hazardous waste s i t e . Many o f t h e s e p r o b l e m s c a n o f t e n be a v o i d e d by t h e u s e o f an i n t e g r a t e d a p p r o a c h c o m b i n i n g c o n t e m p o r a r y g e o p h y s i c a l methods t o s u p p o r t t r a d i t i o n a l d r i l l i n g p r o c e d u r e s .


Available Geophysical

Methodology

T h i s paper d i s c u s s e s s i x c u r r e n t l y a v a i l a b l e g e o p h y s i c a l s e n s i n g methods: g r o u n d p e n e t r a t i n g r a d a r ( G P R ) , e l e c t r o m a g n e t i c i n d u c t i o n (EM), r e s i s t i v i t y , s e i s m i c r e f r a c t i o n , m e t a l d e t e c t i o n , and m a g n e t o m e t r y . O t h e r g e o p h y s i c a l methods a r e a v a i l a b l e and a r e p r o v i n g t o be e f f e c t i v e ; s t i l l o t h e r s ( f o r example, complex r e s i s t i v i t y ) a r e emerging. However, t h i s d i s c u s s i o n has been l i m i t e d t o t h e s i x methods i n v o l v e d i n t h e l a r g e s t number o f a c t u a l s i t e i n v e s t i g a t i o n s t o d a t e . T a b l e I shows t h e p o s s i b l e r o l e s o f e a c h o f t h e s e methods i n h a z a r d o u s waste s i t e assessments. These methods s h o u l d be r e g a r d e d a s c o m p l e m e n t i n g one a n o t h e r . No one method i s u s e d f o r a l l a p p l i c a t i o n s , a n d some methods a r e u s e f u l i n more t h a n one application. I n g e n e r a l , m e t a l d e t e c t o r s and m a g n e t o m e t e r s a r e most u s e f u l i n l o c a t i n g b u r i e d w a s t e s ; g r o u n d p e n e t r a t i n g r a d a r i s the technique o f c h o i c e f o r d e f i n i n g the boundary o f b u r i e d t r e n c h e s ; e l e c t r o m a g n e t i c i n d u c t i o n (EM) a n d r e s i s t i v i t y a r e t h e most u s e f u l i n d e f i n i n g plumes o f c o n d u c t i v e c o n t a m i n a n t s i n g r o u n d w a t e r ; a n d r e s i s t i v i t y and s e i s m i c t e c h n i q u e s a r e most useful i n determining geologic stratigraphy. M e t a l D e t e c t o r s . M e t a l d e t e c t o r s r e s p o n d t o changes i n e l e c t r i c a l c o n d u c t i v i t y c a u s e d by t h e p r e s e n c e o f m e t a l l i c o b j e c t s , b o t h f e r r o u s and n o n - f e r r o u s . A t t h e same t i m e , m e t a l d e t e c t o r s a r e r e l a t i v e l y i n s e n s i t i v e t o changes i n s o i l m o i s t u r e or groundwater c o n d u c t i v i t y . The m a g n i t u d e o f r e s p o n s e f r o m a metal detector i s a function of several v a r i a b l e s . 1) Target t o sensor d i s t a n c e (response f a l l s o f as the s i x t h power o f t h e d i s t a n c e ) 2) Target s i z e 3) Target o r i e n t a t i o n 4) Target geometry 5) Type o f t a r g e t m e t a l 6) M e c h a n i c a l / e l e c t r i c a l i n t e g r i t y o f the t a r g e t 7) Search c o i l s i z e The o p e r a t i o n o f s i m p l e m e t a l d e t e c t o r s i s diagrammed s c h e m a t i c a l l y i n F i g u r e 1, w h i c h shows a n i n d u c t i o n b a l a n c e



X - PRIMARY METHOD ( X ) - SECONDARY METHOD

LOCATION AND DEFINITION OF BURIED METALLIC OBJECTS (E.G., DRUMS, ORDINANCE)

LOCATION OF BOUNDARY DEFINITION OF BURIED TRENCHES

MAPPING OF GEOHYDROLOGIC FEATURES

MAPPING OF CONDUCTIVE LEACHATES AND CONTAMINANT PLUMES (E.G., LANDFILLS, ACIDS, BASES)

(X)

(X)

X

X

X

X

X

(X)

ELECTROMAGNETIC INDUCTION

(X)

X

X

RESISTIVITY

(X)

X

SEISMIC

X

(X)

METAL DETECTOR

POTENTIAL APPLICATIONS OF GEOPHYSICAL METHODS

GROUND PENETRATING RADAR

TABLE I .


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Figure 1. Simplified diagram of metal detector system.



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device. Such a d e v i c e c o n s i s t s o f a t r a n s m i t t e r l o o p c o i l and a r e c e i v e r l o o p c o i l g e o m e t r i c a l l y a r r a n g e d i n s u c h a way t h a t a n u l l or z e r o - i n d u c t i o n c o n d i t i o n i s a c h i e v e d i n the absence of n e a r b y m e t a l o b j e c t s . When t h e c o i l s a r e p r o p e r l y a d j u s t e d , t h e r e c e i v e r i s i n s e n s i t i v e to the e l e c t r o m a g n e t i c f i e l d of the transmitter. However, a s i g n a l w i l l be d e t e c t e d i f t h e e l e c t r o m a g n e t i c f i e l d a t t h e r e c e i v e r c o i l i s a l t e r e d by t h e s u p e r p o s i t i o n of a secondary e l e c t r o m a g n e t i c f i e l d r e s u l t i n g from c u r r e n t s induced i n a nearby m e t a l l i c o b j e c t or conductive mass by t h e t r a n s m i t t e r c o i l . D e v i a t i o n s from the balance c o n d i t i o n a r e d e t e c t e d as e l e c t r i c a l s i g n a l s i n t h e r e c e i v e r and a r e e l e c t r o n i c a l l y c o n v e r t e d t o a m e t e r d e f l e c t i o n as w e l l as t o an a u d i b l e t o n e . M e t a l d e t e c t o r s t e n d t o be i n s e n s i t i v e t o b u r i e d o b j e c t s o f s m a l l c r o s s - s e c t i o n , s u c h as m e t a l r o d s . (_1) F i g u r e 2 shows p r o f i l e s t a k e n w i t h a m e t a l d e t e c t o r a t a r u r a l s i t e where i t was s u s p e c t e d t h a t drums c o n t a i n i n g d i o x i n had been dumped i n t o a t r e n c h and b u r i e d . M u l t i p l e p a r a l l e l p a s s e s were made p e r p e n d i c u l a r t o t h e s u s p e c t e d t r e n c h as p a r t o f an e f f o r t t o d e t e r m i n e t h e l o c a t i o n and q u a n t i t y o f drums i n the t r e n c h . (2) M a g n e t o m e t e r s . M a g n e t o m e t e r s c a n be u s e d t o d e t e c t p e r t u r b a t i o n s i n t h e g e o m a g n e t i c f i e l d c r e a t e d by b u r i e d f e r r o m a g n e t i c o b j e c t s s u c h as s t e e l c o n t a i n e r s o r drums, t o o l s , or scrap metal. An i n d u c e d m a g n e t i z a t i o n i s p r o d u c e d i n any m a g n e t i c m a t e r i a l w i t h i n t h e e a r t h ' s m a g n e t i c f i e l d , and t h i s i n d u c e d f i e l d i s s u p e r i m p o s e d on t h e g e o m a g n e t i c f i e l d . If s t r o n g enough, t h i s i n d u c e d f i e l d p r o d u c e s a l o c a l i z e d anomaly i n the geomagnetic f i e l d . F i g u r e 3 i s a schematic of a simple magnetometer. The i n d u c e d m a g n e t i c f i e l d o f a b u r i e d o b j e c t depends on several variables: 1) T a r g e t mass; 2) Target to sensor d i s t a n c e ; 3) T a r g e t m a t e r i a l and i t s i n t e g r i t y ; 4) Target geometry ( p r i m a r i l y l e n g t h to diameter r a t i o ) 5) Target o r i e n t a t i o n ; 6) M a g n i t u d e and d i r e c t i o n o f permanent magnetism i n t h e target A v a r i e t y of magnetometers are c u r r e n t l y a v a i l a b l e f o r d i f f e r e n t types of s u r v e y s . B o t h t o t a l f i e l d and g r a d i o m e t e r s e a r c h magnetometers are used. P r o t o n p r e c e s s i o n and c e s i u m s y s t e m s a r e u s e d f o r measurements o f t h e t o t a l m a g n e t i c f i e l d . F l u x g a t e and c e s i u m g r a d i o r a e t e r s c a n be u s e d f o r s e a r c h work. A g r a d i o m e t e r i s a d i f f e r e n t i a l magnetometer w h i c h m e a s u r e s d i f f e r e n c e s i n t h e m a g n i t u d e and d i r e c t i o n o f t h e a m b i e n t f i e l d over a f i x e d d i s t a n c e ; t h i s f i x e d d i s t a n c e i s u s u a l l y small w i t h r e s p e c t to the d i s t a n c e to the o b j e c t c r e a t i n g the magnetic a n o m a l y . An a d v a n t a g e o f some g r a d i o m e t e r s y s t e m s i s t h e i r a b i l i t y to sense v e r t i c a l f i e l d g r a d i e n t s w h i l e r e m a i n i n g i n s e n s i t i v e t o h o r i z o n t a l g r a d i e n t components. T h i s f e a t u r e



Figure 2. Metal detector data over trench with buried drums.


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Figure 3.

Simplified diagram of magnetometer system.


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a l l o w s gradioraeters t o sense the v e r t i c a l f i e l d o f s u b s u r f a c e t a r g e t s i n the presence o f h o r i z o n t a l i n t e r f e r e n c e t a r g e t s such as s t e e l f e n c e s . ( 3 ) F i g u r e 4 shows m u l t i p l e p a r a l l e l p r o f i l e s t a k e n w i t h a continuous r e a d i n g gradiometer over the d r u m - f i l l e d b u r i a l t r e n c h ( t h e same t r e n c h f r o m w h i c h F i g u r e 2 was t a k e n ) . These p r o f i l e s , w h i c h add c o n f i r m a t i o n o f t h e p r e s e n c e o f drums i n t h e t r e n c h , were t a k e n i n s i d e a h i g h s t e e l f e n c e e r e c t e d a r o u n d t h e suspected b u r i a l s i t e . ( 2 ) G r o u n d - P e n e t r a t i n g R a d a r . The g r o u n d - p e n e t r a t i n g radar (GPR) s y s t e m most f r e q u e n t l y u s e d i n h a z a r d o u s w a s t e s i t e i n v e s t i g a t i o n s i s an impulse system w h i c h r a d i a t e s s h o r t - d u r a t i o n e l e c t r o m a g n e t i c p u l s e s i n t o the ground from a n a n t e n n a n e a r t h e s u r f a c e . These p u l s e s a r e r e f l e c t e d f r o m v a r i o u s i n t e r f a c e s w i t h i n t h e e a r t h and a r e p i c k e d up by t h e r e c e i v e r s e c t i o n o f t h e a n t e n n a and r e t u r n e d t o t h e c o n t r o l u n i t f o r p r o c e s s i n g a n d d i s p l a y . These r e f l e c t i o n s o c c u r a t different s o i l horizons, soil/rock interfaces, rock/air i n t e r f a c e s ( v o i d s ) , manmade o b j e c t s , o r a t a n y i n t e r f a c e w h i c h c r e a t e s a c o n t r a s t i n complex d i e l e c t r i c p r o p e r t i e s . F o r e x a m p l e , d i g g i n g a t r e n c h and f i l l i n g i t a g a i n c a n c r e a t e a d i f f e r e n c e between t h e d i e l e c t r i c p r o p e r t i e s o f t h e d i s t u r b e d e a r t h and t h o s e o f t h e u n d i s t u r b e d m a t e r i a l w h i c h c a n be s e n s e d by t h e GPR. F i g u r e 5 i s a schematic showing the v a r i o u s components o f a GPR s y s t e m . F o r p r e s e n t a t i o n o f d a t a , GPR s i g n a l s a r e p r o c e s s e d and d i s p l a y e d by a g r a p h i c r e c o r d e r . As t h e a n t e n n a i s moved a l o n g the s u r f a c e , the g r a p h i c d i s p l a y r e s u l t s i n a p i c t u r e - l i k e r e c o r d showing a c o n t i n u o u s p r o f i l e a l o n g a t r a v e r s e , v e r y s i m i l a r t o a g e o l o g i c c r o s s - s e c t i o n found a t a roadcut. U n f o r t u n a t e l y , the depth o f radar p e n e t r a t i o n i s very s i t e specific. D e p t h s o f 3 t o 10 m e t e r s a r e commonly a t t a i n e d t h r o u g h o u t t h e c o u n t r y ; 20 m e t e r p e n e t r a t i o n s have been a c h i e v e d u n d e r i d e a l c o n d i t i o n s a t some s i t e s . This depth i s reduced i f ground water i n c r e a s e s i n e l e c t r i c a l c o n d u c t i v i t y , o r i f t h e r e are s u f f i c i e n t l y high concentrations of f i n e grained m a t e r i a l s ( s i l t s or c l a y s ) present. For example, h i g h c o n c e n t r a t i o n s o f s a l t s , m o n t m o r i l l o n i t e c l a y o r l o s s e s are h i g h l y a t t e n u a t i v e of t h e r a d a r p u l s e and p e n e t r a t i o n may n o t e x c e e d one m e t e r . ( 4 ) F i g u r e 6 shows t h e c r o s s - s e c t i o n r e s u l t i n g f r o m a s i n g l e GPR t r a v e r s e a c r o s s t h e b a r r e l - f i l l e d t r e n c h f r o m w h i c h F i g u r e s 2 and 4 were t a k e n . The images p r o d u c e d by t h r e e 5 5 - g a l l o n b a r r e l s a r e i n d i c a t e d on t h e f i g u r e . Figure 7 i s a composite, c o m p a r i n g t h e t r a c e s o f F i g u r e s 2, 4, a n d 6. ( 5 ) Resistivity. The r e s i s t i v i t y method m e a s u r e s t h e e l e c t r i c a l r e s i s t i v i t y o f the g e o h y d r o l o g i c s e c t i o n , which i n c l u d e s s o i l , r o c k , and g r o u n d w a t e r . I n t e r p r e t a t i o n of these measurements p r o v i d e i n f o r m a t i o n on l a y e r i n g a n d d e p t h s o f s u b s u r f a c e h o r i z o n s a s w e l l a s l a t e r a l changes i n t h e



102

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Currently Available Geophysical Methods for Use ... - ACS Publications

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