Evaluation of Pesticides in Ground Water - ACS Publications

7 Applications of Surface Geophysical Methods to Ground Water Pollution Investigations Nicholas De Rose

Downloaded by PURDUE UNIV on April 5, 2016 | http://pubs.acs.org Publication Date: July 17, 1986 | doi: 10.1021/bk-1986-0315.ch007

Roy F. Weston, Inc., Raritan Center, Edison, NJ 08837

Several surface geophysical survey methods are presented and the principles of operation for each technique discussed. These methods include seismic refraction, resistivity, electromagnetic conductivity, ground penetrating radar and magnetometry. Applications of these geophysical methods to groundwater contamination investigations include; determining lateral and vertical variations in soil, rock and groundwater characteristics; mapping the extent of groundwater contaminants present within aquifers; and locating buried objects. The suitability and potential application of surface geophysical surveys to monitor the migration of field-applied pesticides, and pesticides from waste disposal sites within the unsaturated and saturated zones is also discussed. An example to illustrate the use of geophysical techniques at a hypothetical pesticide field-application site is outlined. The example details the application of seismic refraction and electromagnetic conductivity techniques to define erratic subsurface conditions at a site where groundwater contamination is suspected. The site is underlain by an unconsolidated deposit of varying thickness and composition which is underlain by cavernous limestone. A groundwater monitoring system is designed based upon interpretation of the geophysical data obtained. By utilizing surface geophysical methods, extensive site coverage can be completed cost effectively and groundwater monitoring systems designed efficiently, thereby increasing the quality and success of site groundwater investigations. The s c i e n c e o f g e o p h y s i c s was o r i g i n a l l y d e v e l o p e d as t h e s t u d y o f the p h y s i c s o f t h e e a r t h s s t r u c t u r e and shape. G e o p h y s i c s i n c l u d e s measuring g r a v i t a t i o n a l , e l e c t r i c a l , e l e c t r o m a g n e t i c and magnetic f i e l d s , and r e c o r d i n g s e i s m i c v i b r a t i o n s t o i d e n t i f y c o m p o s i t i o n a l and s t r u c t u r a l f e a t u r e s o f t h e e a r t h . Developments from t h e 1 9 2 0 s 1

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0097-6156/ 86/0315-0118S06.75 / 0 © 1986 A m e r i c a n C h e m i c a l Society

Garner et al.; Evaluation of Pesticides in Ground Water ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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through lÇSO's r e s u l t e d i n the a p p l i c a t i o n o f g e o p h y s i c a l methods t o l o c a t e p o t e n t i a l m i n e r a l resource deposits i n c l u d i n g petroleumb e a r i n g g e o l o g i c s t r u c t u r e s and m e t a l l i c o r e b o d i e s . More r e c e n t advancements i n a p p l i e d g e o p h y s i c s have produced equipment and t e c h nology which a r e e a s i l y a c c e s s i b l e to p r o f e s s i o n a l c o n s u l t a n t s , i n d u s t r y and government through s e v e r a l r e l i a b l e m a n u f a c t u r e r s and s c i e n t i f i c equipment s u p p l i e r s . The f i e l d p o r t a b i l i t y , c o s t - e f f e c t i v e n e s s , and a b i l i t y t o c o l l e c t d a t a w h i c h does n o t r e q u i r e o b t a i n i n g a c t u a l samples o f s u b s u r f a c e material provides several considerations f o r incorporating surface g e o p h y s i c a l s u r v e y s i n t o groundwater contaminant i n v e s t i g a t i o n programs ( 1 ) . As p a r t o f a p r e l i m i n a r y s i t e i n v e s t i g a t i o n , s u r f a c e g e o p h y s i c a l s u r v e y s may be conducted t o i d e n t i f y p o t e n t i a l t r e n d s o r anomalous a r e a s p r i o r t o s e l e c t i n g t h e l o c a t i o n s o f s a m p l i n g p o i n t s f o r t h e m o n i t o r i n g system. U t i l i z i n g g e o p h y s i c a l s u r v e y s t o s c r e e n a r e a s o f i n v e s t i g a t i o n w i l l i n c r e a s e t h e e f f e c t i v e n e s s o f t h e monit o r i n g systems and s i t e e v a l u a t i o n by p r o v i d i n g a g e o p h y s i c a l - d a t a base f o r c o r r e l a t i o n w i t h samples o b t a i n e d i n t h e f i e l d and a n a l y t i c a l d a t a from t h e l a b o r a t o r y ( 2 ) . Surface Geophysical

Survey T e c h n i q u e s

T h i s paper d i s c u s s e s f i v e o f t h e most w i d e l y used t e c h n i q u e s f o r cond u c t i n g groundwater contaminant i n v e s t i g a t i o n s . They a r e c h a r a c t e r i z e d by r e l a t i v e l y s i m p l e o p e r a t i o n a l p r i n c i p l e s , f i e l d p r o c e d u r e s , and d a t a a n a l y s e s and i n t e r p r e t a t i o n . The f i v e s u r f a c e t e c h n i q u e s include: s e i s m i c r e f r a c t i o n , ground p e n e t r a t i n g r a d a r , e l e c t r i c a l r e s i s t i v i t y , e l e c t r o m a g n e t i c c o n d u c t i v i t y and magnetometry. Seismic r e f r a c t i o n and ground p e n e t r a t i n g r a d a r b o t h i d e n t i f y s u b s u r f a c e i n t e r f a c e s w h i c h a r e used t o d e t e r m i n e t h e t h i c k n e s s and d e p t h o f m a t e r i a l s and t o l o c a t e i s o l a t e d b u r i e d o b j e c t s . E l e c t r i c a l r e s i s t i v i t y , e l e c t r o m a g n e t i c c o n d u c t i v i t y , and magnetometry i d e n t i f y l a t e r a l and v e r t i c a l v a r i a t i o n s i n s u b s u r f a c e g e o l o g i c f o r m a t i o n s by l o c a t i n g l a t e r a l and v e r t i c a l v a r i a t i o n s i n b u l k e l e c t r i c a l , and magnetic p r o p e r t i e s o f the subsurface m a t e r i a l s . V a l i d i n t e r p r e t a t i o n s of geophysical data c o l l e c t e d i n the f i e l d r e q u i r e c o r r e l a t i o n w i t h i n f o r m a t i o n o b t a i n e d from b o t h c o n v e n t i o n a l sampling and a n a l y t i c a l programs. The t y p e s o f g e o p h y s i c a l and c o n v e n t i o n a l d a t a r e q u i r e d t o d e s c r i b e t h e s u b s u r f a c e environment w i l l v a r y , depending upon t h e p h y s i c a l , c h e m i c a l , and s t r u c t u r a l c o m p l e x i t y o f t h e s u b s u r f a c e , and t h e i n t e n d e d a p p l i c a t i o n o f t h e f i e l d study. S e i s m i c R e f r a c t i o n . S e i s m i c methods a r e u s e f u l t o o l s i n d e t e r m i n i n g the t h i c k n e s s and d e p t h o f g e o l o g i c u n i t s . I n a d d i t i o n , the v e l o c i t y w i t h w h i c h s e i s m i c waves a r e e i t h e r r e f l e c t e d o r r e f r a c t e d i s an i n d i c a t i o n of the p h y s i c a l p r o p e r t i e s of the subsurface m a t e r i a l s . S e i s m i c r e f r a c t i o n s u r v e y s c o n s i s t o f t r a n s m i t t i n g a wave i n t o t h e s u b s u r f a c e by means o f an a c o u s t i c s o u r c e . The wave t r a v e l s t h r o u g h the s u b s u r f a c e i n a l l d i r e c t i o n s and a t d i f f e r i n g v e l o c i t i e s u n t i l c o n t a c t i s made w i t h a g e o l o g i c i n t e r f a c e . The wave i s r e f r a c t e d to t h e s u r f a c e and» r e c e i v e d by an a r r a y o f geophones. The t r a v e l time o f t h e r e f r a c t e d wave i s r e c o r d e d on a s e i s m o g r a p h . F i g u r e 1 p r e s e n t s a s c h e m a t i c c r o s s s e c t i o n o f a l l t h e components used t o


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conduct a s e i s m i c r e f r a c t i o n survey t o determine the t h i c k n e s s of l a y e r V-j^ w h i c h o v e r l i e s l a y e r V . The seismograph p r o d u c e s a s e i s m i c r e c o r d w h i c h g r a p h i c a l l y d e p i c t s a l l t h e v i b r a t i o n s r e c e i v e d a t each geophone d u r i n g t h e s u r vey. The s e i s m i c r e c o r d i s i n t e r p r e t e d t o i d e n t i f y t h e " t r a v e l t i m e " of t h e s e i s m i c wave from t h e a c o u s t i c s o u r c e t o e a c h geophone. T r a v e l times o f r e f r a c t e d waves a r e i d e n t i f i e d on t h e s e i s m i c r e c o r d as " f i r s t a r r i v a l s " a t each geophone, and they a r e dependent upon t h e d e p t h t o w h i c h t h e r e f r a c t e d wave t r a v e l e d , and t h e s e i s m i c v e l o c i t y of t h e wave ( 3 ) . F i r s t a r r i v a l times a t each geophone a r e p l o t t e d on a r i t h m e t i c graph paper as time v s d i s t a n c e p l o t s f o r d a t a i n t e r p r e t a t i o n ( F i g u r e 2). The t r a v e l times and c o r r e s p o n d i n g d i s t a n c e s a t each geophone w i l l g e n e r a l l y p l o t a l o n g one o f s e v e r a l l i n e a r segments on t h e graph. The s l o p e o f each l i n e a r segment c o r r e s p o n d s t o t h e s e i s m i c v e l o c i t y o f each s e i s m i c l a y e r . The v e l o c i t y and e i t h e r t h e i n t e r c e p t time o r t h e c r i t i c a l d i s t a n c e a r e used t o c a l c u l a t e t h e d e p t h t o the i n t e r f a c e between t h e l a y e r s ( F i g u r e 2 ) . Seismic v e l o c i t i e s a r e d i r e c t l y r e l a t e d to m a t e r i a l d e n s i t y . B u l k d e n s i t i e s may v a r y a s a r e s u l t o f c o m p o s i t i o n , w a t e r c o n t e n t , age and d e p t h , w e a t h e r i n g , f r a c t u r i n g , and degree and type o f c o n solidation. D i f f e r e n t s o i l and r o c k types a r e c h a r a c t e r i z e d by c e r t a i n overlapping ranges i n s e i s m i c v e l o c i t y . By c o r r e l a t i n g o b s e r v e d d a t a w i t h i n f o r m a t i o n o b t a i n e d from c o n v e n t i o n a l s t u d i e s , the t y p e s and d i s t r i b u t i o n o f g e o l o g i c u n i t s p r e s e n t may be i n t e r p r e t e d ( 2 ) . The most common a p p l i c a t i o n o f s e i s m i c r e f r a c t i o n s u r v e y s i n groundwater c o n t a m i n a n t i n v e s t i g a t i o n s i s t o d e f i n e t h e t h i c k n e s s o f t h e o v e r b u r d e n ( s o i l ) and t o map the s t r a t i g r a p h y . L i m i t a t i o n s t o c o n s i d e r when e v a l u a t i n g t h e s u i t a b i l i t y o f t h e s e i s m i c r e f r a c t i o n method f o r a g i v e n s i t e i n c l u d e t h e f o l l o w i n g : (1) s u b s u r f a c e l a y e r s o f l i m i t e d t h i c k n e s s ( g e n e r a l l y l e s s t h a n f i v e f e e t ) a r e n o t d e t e c t e d ; (2) s u f f i c i e n t c o n t r a s t i n s e i s m i c v e l o c i t i e s ( b u l k d e n s i t y ) between s u b s u r f a c e l a y e r s must be p r e s e n t f o r t h e a c c u r a t e l o c a t i o n o f i n t e r f a c e s ; (3) b u l k d e n s i t i e s o f s u b s u r f a c e m a t e r i a l s must i n c r e a s e w i t h d e p t h .


2

Electrical Resistivity. Bulk e l e c t r i c a l r e s i s t i v i t i e s of subsurface m a t e r i a l s a r e d e t e r m i n e d by i n j e c t i n g an e l e c t r i c c u r r e n t i n t o t h e ground by a p a i r o f e l e c t r o d e s and m e a s u r i n g t h e r e s u l t i n g drop i n v o l t a g e t h r o u g h t h e ground by a second p a i r o f e l e c t r o d e s ( F i g u r e 3 ) . The magnitude o f t h e v o l t a g e drop depends upon t h e l i t h o l o g y , m o i s t u r e c o n t e n t and c o n c e n t r a t i o n s o f d i s s o l v e d s o l i d s i n p o r e water o f t h e s u b s u r f a c e m a t e r i a l s . By v a r y i n g t h e s p a c i n g and p o s i t i o n o f t h e e l e c t r o d e a r r a y , l a t e r a l and v e r t i c a l r e s i s t i v i t y t r e n d s c a n be d e t e r m i n e d . R e s u l t s from a r e s i s t i v i t y s u r v e y c a n be used t o (1) map l a t e r a l and v e r t i c a l v a r i a t i o n s i n t h e t h i c k n e s s e s o f s u b s u r f a c e m a t e r i a l s w i t h s i m i l a r r e s i s t i v i t i e s ; (2) d e l i n e a t e r e s i s t i v i t y b o u n d a r i e s a s s o c i a t e d w i t h v a r i a t i o n s i n groundwater q u a l i t y ; and (3) i d e n t i f y waste b u r i a l a r e a s o r l o c a t e b u r i e d t a n k s a s s o c i a t e d w i t h l o c a l r e s i s t i v i t y anomalies ( 2 ) . F i e l d s u r v e y s a r e c o n d u c t e d t o complete e i t h e r l a t e r a l p r o f i l e s or v e r t i c a l e l e c t r i c a l s o u n d i n g s (VES). R e s i s t i v i t y p r o f i l e s are completed by u t i l i z i n g a f i x e d e l e c t r o d e s p a c i n g and o b t a i n i n g an e l e c t r i c a l r e s i s t i v i t y v a l u e a t each s e l e c t e d s t a t i o n a l o n g t h e


D E ROSE



7.

F i g u r e 2. Time v s . d i s t a n c e p l o t o f " f i r s t a r r i v a l s " f o r s e i s m i c r e f r a c t e d waves a t c o r r e s p o n d i n g geophones.


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p r o f i l e traverse (Figure 4). A number o f p r o f i l e s may be completed t o c o n s t r u c t a r e s i s t i v i t y map of an e n t i r e s i t e or of s e l e c t e d features at a s i t e . L a t e r a l v a r i a t i o n s i n subsurface e l e c t r i c a l r e s i s t i v i t y may be used t o a s s e s s : (1) changes i n groundwater q u a l i t y ; (2) groundwater contaminant plumes and s a l t w a t e r i n t r u s i o n ; and (3) v a r i a t i o n s i n the c o m p o s i t i o n o f s u b s u r f a c e m a t e r i a l s ( 2 ) . V e r t i c a l e l e c t r i c a l s o u n d i n g s a r e completed by m e a s u r i n g a s e r i e s o f e l e c t r i c a l r e s i s t i v i t y v a l u e s a t one l o c a t i o n . Each measured v a l u e c o r r e s p o n d s t o one o f a s e r i e s of s e l e c t e d e l e c t r o d e spacings. The r e s u l t i n g s e t of e l e c t r i c a l r e s i s t i v i t y v a l u e s obt a i n e d a t each VES s t a t i o n a r e g e n e r a l l y p l o t t e d a g a i n s t e l e c t r o d e s p a c i n g to p r o d u c e sounding c u r v e s t h a t may be e m p i r i c a l l y e v a l u a t e d to i d e n t i f y trends (Figure 4 ) . The d a t a may a l s o be s o l v e d g r a p h i c a l l y by u s i n g master c u r v e s to d e t e r m i n e r e s i s t i v i t y l a y e r t h i c k n e s s e s , d e p t h s , and t r u e r e s i s t i v i t i e s . E l e c t r i c a l r e s i s t i v i t y s u r v e y s have been s u c c e s s f u l l y used to l o c a t e groundwater a q u i f e r s , d e l i n e a t e groundwater contaminant plumes, map s a l t water i n t r u s i o n , d e t e r m i n e s u b s u r f a c e s t r a t i g r a p h y , map t h i c k n e s s e s o f s u r f i c i a l d e p o s i t s , map f r a c t u r e zones i n b e d r o c k a r e a s , map k a r s t f e a t u r e s , l o c a t e b u r i e d man-made o b j e c t s and to implement i n - p l a c e e l e c t r i c a l l e a k d e t e c t i o n systems f o r s u r f a c e impoundments and underground s t o r a g e t a n k s (2,4,5,6,7). In a d d i t i o n r e s e a r c h i s o n - g o i n g t o e v a l u a t e the a b i l i t y o f r e s i s t i v i t y f o r d e t e c t i n g and mapping o r g a n i c plumes ( 8 ) . The f i e l d a p p l i c a t i o n o f e l e c t r i c a l r e s i s t i v i t y t e c h n i q u e s can be a f f e c t e d by the p r e s e n c e of nearby power l i n e s , f e n c e s , r a i l r o a d t r a c k s , and b u r i e d p i p e s and c a b l e s . These c u l t u r a l f e a t u r e s may c r e a t e e l e c t r i c a l i n t e r f e r e n c e o r a l t e r the s u b s u r f a c e p a t t e r n o f c u r r e n t f l o w d i s t r i b u t i o n . I n a d d i t i o n , i n o r d e r to complete e l e c t r i c a l r e s i s t i v i t y s u r v e y s you must be a b l e t o " s e a t " the e l e c t r o d e s i n the ground to e s t a b l i s h e l e c t r i c a l c o n t i n u i t y w i t h the s u b s u r f a c e m a t e r i a l s to be s t u d i e d . D a t a a n a l y s i s and i n t e r p r e t a t i o n of e l e c t r i c a l r e s i s t i v i t y d a t a may be l i m i t e d b e c a u s e : (1) r e s i s t i v i t y v a l u e s may be a s s o c i a t e d w i t h any one of s e v e r a l g e o l o g i c u n i t s ( i . e . a s i l t y sand u n i t may have s i m i l a r r e s i s t i v i t y v a l u e s as a sand u n i t s a t u r a t e d w i t h s a l t w a t e r ) ; (2) t h i n beds of lower r e s i s t i v i t y w i l l be masked when they a r e sandwiched between two l a y e r s o f h i g h e r r e s i s t i v i t y ; and (3) the i n t e r p r e t e d l a y e r t h i c k n e s s w i l l be g r e a t e r t h a n the a c t u a l t h i c k n e s s due to the a n i s o t r o p i c n a t u r e o f the i n d i v i d u a l l a y e r s , w h i c h a r e g e n e r a l l y c h a r a c t e r i z e d as h a v i n g g r e a t e r v e r t i c a l r e s i s t i v i t y v a l u e s than h o r i z o n t a l ( 9 ) . Electromagnetic Conductivity. The e l e c t r o m a g n e t i c conductivity survey i s s i m i l a r to the e l e c t r i c a l r e s i s t i v i t y s u r v e y i n the sense t h a t b o t h methods q u a n t i f y e l e c t r i c a l c h a r a c t e r i s t i c s o f s u b s u r f a c e materials. However, t h e r e a r e v a s t d i f f e r e n c e s between the f i e l d t e c h n i q u e s and the data and a n a l y s e s w h i c h c h a r a c t e r i z e the i n d i v i d u a l methods. The e l e c t r o m a g n e t i c c o n d u c t i v i t y s u r v e y c o n s i s t s of d i r e c t i n g an e l e c t r o m a g n e t i c f i e l d i n t o the ground from an above ground s o u r c e to c r e a t e a s e c o n d a r y e l e c t r o m a g n e t i c f i e l d t h a t i s measured by a r e c e i v e r . As a r e s u l t , d i r e c t measurements o f b u l k s u b s u r f a c e c o n d u c t i v i t i e s a r e o b t a i n e d (10) ( F i g u r e 5 ) .




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F i g u r e 4. R e s i s t i v i t y p l o t s f o r (A) l a t e r a l p r o f i l e (B) v e r t i c a l s o u n d i n g .

and f o r



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F i e l d equipment r e q u i r e d f o r a s u r v e y c o n s i s t s o f a low f r e q u e n cy t r a n s m i t t e r c o i l and a r e c e i v e r c o i l . The r e c e i v e r c o i l i n t e r c e p t s a p o r t i o n o f t h e s e c o n d a r y e l e c t r o m a g n e t i c f i e l d and p r o d u c e s an o u t p u t v o l t a g e w h i c h i s l i n e a r l y r e l a t e d t o s u b s u r f a c e conductivity (11). I n d u c i n g an e l e c t r o m a g n e t i c f i e l d e l i m i n a t e s t h e need f o r an e l e c t r o d e a r r a y and, t h e r e f o r e , t h e e l e c t r o m a g n e t i c f i e l d method i s a more r a p i d t o o l f o r s u r v e y i n g . The e l i m i n a t i o n o f e l e c t r o d e s t o measure e l e c t r i c a l p r o p e r t i e s o f s u b s u r f a c e m a t e r i a l s a l s o e n a b l e s e l e c t r o m a g n e t i c s u r v e y s t o be conducted i n many a r e a s where r e s i s t i v i t y s u r v e y s cannot be c o n s i d e r e d ( i . e . pavement a r e a s , v e r y d r y sandy s o i l s , f r o z e n ground, r a i l r o a d t r a c k s , e t c . ) . Electromagnetic c o n d u c t i v i t y s u r v e y s may a l s o be u s e d t o p r o d u c e r a p i d c o n t i n u o u s p r o f i l e s up t o depths o f 15 m e t e r s . E l e c t r o m a g n e t i c (EM) s u r v e y s a r e most w i d e l y used t o c r e a t e p r o f i l e s o r maps o f s u b s u r f a c e c o n d u c t i v i t y . L a t e r a l v a r i a t i o n s i n c o n d u c t i v i t y , a t a g i v e n d e p t h , may be i n t e r p r e t e d a s contaminant plumes, sand and g r a v e l d e p o s i t s , c l a y d e p o s i t s , k a r s t f e a t u r e s , s a l t water i n t r u s i o n o r b u r i e d o b j e c t s ( F i g u r e 6 ) . Because f i e l d s u r v e y s may be completed r a p i d l y , EM t e c h n i q u e s a r e o f t e n i m p l e mented t o map l a r g e s i t e s and t o p r o v i d e d e t a i l e d maps o f v a r i a b l e s i t e s u b s u r f a c e f e a t u r e s such a s i s o l a t e d k a r s t f e a t u r e s , and b u r i e d tanks o r 55 g a l l o n drums. Computer-data p r o c e s s i n g t e c h n i q u e s c a n be u t i l i z e d t o i n t e r p r e t and p r e s e n t r e s u l t s o b t a i n e d from s i t e s u r v e y s i n o r d e r t o f i l t e r o u t unwanted c u l t u r a l i n t e r f e r e n c e s , and t o e v a l u a t e plume c h a r a c t e r i s tics. C o r r e l a t i o n o f EM maps w i t h f i e l d - c o l l e c t e d g e o l o g i c i n f o r mation i s e s s e n t i a l f o r the f i n a l i n t e r p r e t a t i o n o f the survey d a t a . Two and t h r e e l a y e r c o n d u c t i v i t y models c a n be c o n s t r u c t e d u s i n g q u a n t i t a t i v e techniques. The r e s u l t i n g models a r e g e n e r a l l y l e s s d e t a i l e d i n the v e r t i c a l r e s o l u t i o n of the subsurface l a y e r s than t h o s e produced from v e r t i c a l e l e c t r i c a l s o u n d i n g s . E l e c t r o m a g n e t i c s u r v e y s may n o t be conducted i n a r e a s c h a r a c t e r i z e d by u n u s u a l l y h i g h o r low v a l u e s o f s u b s u r f a c e c o n d u c t i v i t y . I n a d d i t i o n , e l e c t r o m a g n e t i c methods a r e s u b j e c t t o i n t e r f e r e n c e from many c u l t u r a l f e a t u r e s and t h e p r e s e n c e o f n e a r b y e l e c t r i c a l fields. F i n a l l y , the a b i l i t y of electromagnetic c o n d u c t i v i t y surv e y s t o i d e n t i f y groundwater c o n t a m i n a n t plumes r e q u i r e s t h a t a s i g n i f i c a n t e l e c t r i c a l c o n d u c t i v i t y c o n t r a s t e x i s t s between contami n a t e d and n a t u r a l groundwater ( 2 ) . Ground P e n e t r a t i n g Radar. Ground p e n e t r a t i n g r a d a r (GPR) i s a n imp u l s e r a d a r system t h a t p r o v i d e s a c o n t i n u o u s p r o f i l e o f s u b s u r f a c e c o n d i t i o n s by r a d i a t i n g 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 t h e s u b s u r f a c e and d i s p l a y i n g t h e r e f l e c t i o n s from s u r f a c e and s u b s u r f a c e " i n t e r f a c e s " on a s t r i p c h a r t r e c o r d e r ( F i g u r e 7 ) . The term " i n t e r f a c e " i n t h i s g e o p h y s i c a l method r e f e r s t o any d i s c o n t i n u i t y i n e l e c t r i c a l p r o p e r t i e s such as s o i l o r g e o l o g i c b o u n d a r i e s o r imbedded o b j e c t s such a s drums, o r b o u l d e r s . The GPR system c o n s i s t s o f f i v e major components, these i n c l u d e : 1) power d i s t r i b u t i o n u n i t ; 2) r a d a r c o n t r o l u n i t ; 3) antenna t r a n s c e i v e r ; 4) g r a p h i c r e c o r d e r ; and 5) tape r e c o r d e r . The power d i s t r i b u t i o n u n i t p r o v i d e s p r o p e r AC/DC v o l t a g e s t o a l l t h e GPR e q u i p ment. The r a d a r c o n t r o l u n i t t r i g g e r s t h e antennae t r a n s c e i v e r t o



7.

D E ROSE


F i g u r e 5. C r o s s - s e c t i o n a l v i e w o f t r a n s m i t t e d e l e c t r o m a g n e t i c f i e l d and g e n e r a t e d s e c o n d a r y e l e c t r o m a g n e t i c f i e l d measured by a r e c e i v e r f o r c o m p l e t i n g e l e c t r o m a g n e t i c c o n d u c t i v i t y surveys.


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produce t h e e l e c t r o m a g n e t i c p u l s e . The r e f l e c t e d p o r t i o n s o f the t r a n s m i t t e d p u l s e a r e r e c e i v e d by t h e antennae t r a n s c e i v e r , c o n v e r t e d to the a u d i o f r e q u e n c y r a n g e and p r o c e s s e d by the r a d a r c o n t r o l u n i t . P r o c e s s i n g i s an o p e r a t o r - i n i t i a t e d p r o c e d u r e and a l l o w s f o r t h e s e l e c t i o n and enhancement o f t h a t p o r t i o n o f t h e d a t a w h i c h i s o f g r e a t e s t importance t o the o b j e c t i v e of the survey. The p r o c e s s e d d a t a i s sent t o t h e g r a p h i c r e c o r d e r w h i c h p r o d u c e s a permanent c h a r t p r o f i l e o f t h e s u b s u r f a c e i n t e r f a c e s . P r o c e s s e d d a t a may a l s o be s e n t t o t h e tape r e c o r d e r f o r s t o r a g e , r e p r o c e s s i n g , o r p r i n t i n g a t a l a t e r date. D a t a may be c o l l e c t e d a t a r a t e as much as 16 t i m e s f a s t e r than t h e g r a p h i c r e c o r d e r by tape r e c o r d i n g t h e d a t a ( 1 2 ) . F i e l d s u r v e y s may be completed on f o o t o r by u t i l i z i n g v e h i c l e s f o r mounting equipment and towing t h e antennae t r a n s c e i v e r . The r e s u l t i n g GPR p r o f i l e w h i c h i s produced by t h e g r a p h i c r e c o r d e r p r i n t s s t r o n g s i g n a l s as b l a c k and weak s i g n a l s as w h i t e ( F i g u r e 8 ) . The r e s u l t i s a d i s p l a y o f d a r k bands e x t e n d i n g a c r o s s t h e p r o f i l e a t v a r y i n g depths. These bands r e p r e s e n t t h e r e f l e c t i o n from an i n t e r face. The h o r i z o n t a l s c a l e o f t h e p r o f i l e i s dependent upon t h e t r a v e l time o f t h e antennae t r a n s c e i v e r . The v e r t i c a l s c a l e i s dependent upon t h e t r a v e l time o f t h e GPR p u l s e . The t r a v e l time o f the GPR p u l s e may be c o n v e r t e d t o d e p t h i f e i t h e r t h e d i e l e c t r i c c o n s t a n t o f t h e medium b e i n g p r o f i l e d o r t h e d e p t h t o a s p e c i f i c i n t e r f a c e i s known. T y p i c a l a p p l i c a t i o n s o f GPR s u r v e y s i n c l u d e , mapping d e p t h t o b e d r o c k , and mapping i n t e r f a c e s i n c l u d i n g changes i n s o i l t y p e , g e o l o g i c f o r m a t i o n s , and d e p t h t o water t a b l e . B u r i e d o b j e c t s and e x c a v a t i o n s may a l s o be l o c a t e d and i n some c a s e s i d e n t i f i e d , as w e l l as b u r i e d c u l t u r a l f e a t u r e s i n c l u d i n g p i p e s , c a b l e s , and c o n d u i t s (13). The e f f e c t i v e p e n e t r a t i o n d e p t h s o f the GPR system i s dependent upon t h e b u l k c o n d u c t i v i t y o f t h e s u b s u r f a c e m a t e r i a l s b e i n g p r o filed. The GPR s i g n a l i s r a p i d l y a t t e n u a t e d w i t h i n h i g h l y c o n d u c t i v e m a t e r i a l s ( i . e . c l a y s , h i g h l y i o n i c groundwater) w h i c h s e v e r e l y l i m i t s the p e n e t r a t i o n depths. O t h e r l i m i t a t i o n s o f GPR p r o f i l e s i n c l u d e : 1) masking o f a r e f l e c t i o n as a r e s u l t o f o v e r l a p p i n g i n t e r f a c e s , s u c h a s , where t h e s u r f a c e o f t h e water t a b l e i s l e v e l w i t h t h e t o p o f a b u r i e d drum; 2) v a r i a t i o n i n i d e n t i f y i n g r e f l e c t o r s depending upon t h e o r i e n t a t i o n of t h e t r a v e r s e t o t h e b u r i e d o b j e c t ; and 3) t h e p r e s e n c e o f v e g e t a t i v e c o v e r , s u r f i c i a l d e b r i s and i r r e g u l a r s u r f a c e topography w h i c h may l i m i t t h e p e n e t r a t i o n d e p t h o f t h e r e t u r n s i g n a l o r t h e a b i l i t y to t r a v e r s e t h e s i t e w i t h t h e GPR system. Magnetometry. Magnetometers measure w i t h a c c u r a c y and p r e c i s i o n , t h e i n t e n s i t y of t h e e a r t h ' s m a g n e t i c f i e l d . By mapping l a t e r a l and/or v e r t i c a l m a g n e t i c g r a d i e n t s , s u b s u r f a c e f e a t u r e s s u c h as g e o l o g i c o r e d e p o s i t s , b e d r o c k f e a t u r e s , b u r i e d drums and b u r i e d p i p e s may be l o c a t e d ( F i g u r e 9 ) . N o n f e r r o u s m e t a l s such as aluminum, c o p p e r , t i n and b r a s s cannot be d e t e c t e d by magnetometers ( 1 4 ) . S e v e r a l t y p e s o f magnetometers a r e p r e s e n t l y a v a i l a b l e , however, the two most w i d e l y r e f e r e n c e d i n e n g i n e e r i n g and groundwater publ i c a t i o n s a r e the p r o t o n and t h e f l u x g a t e magnetometers. Proton magnetometers measure t h e e a r t h ' s t o t a l m a g n e t i c f i e l d i n t e n s i t y by u t i l i z i n g t h e p r e c e s s i o n o f s p i n n i n g p r o t o n s i n a sample o f h y d r o -




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F i g u r e 8. Ground p e n e t r a t i n g r a d a r p r o f i l e r e c o r d . Hyperb o l i c r e f l e c t o r s (dark bands) r e p r e s e n t o u t l i n e o f b u r i e d drums.


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c a r b o n f l u i d t o g e n e r a t e a s m a l l s i g n a l whose f r e q u e n c y i s p r e c i s e l y p r o p o r t i o n a l t o the t o t a l m a g n e t i c f i e l d i n t e n s i t y . F l u x g a t e magnetometers measure v a r i a t i o n s i n the i n t e n s i t y o f the t o t a l e a r t h ' s m a g n e t i c f i e l d by m e a s u r i n g changes i n t h e m a g n e t i c s a t u r a t i o n l e v e l of an i r o n c o r e s e n s o r . U n l i k e the p r o t o n magnetometer, the s i g n a l o u t p u t o f a s i n g l e f l u x g a t e magnetometer i s e x t r e m e l y s e n s i t i v e t o orientation. To overcome t h i s s e n s i t i v i t y , the g r a d i e n t o f the e a r t h ' s m a g n e t i c f i e l d may be measured by mounting two f l u x g a t e s e n s o r s t o g e t h e r t o form a g r a d i o m e t e r . The g r a d i o m e t e r i s t h e most common method o f o b t a i n i n g measurements w i t h the f l u x g a t e magnetometer ( 1 4 ) . P r o t o n magnetometers a r e g e n e r a l l y more s e n s i t i v e than f l u x g a t e magnetometers, however, they a r e a l s o more s e n s i t i v e t o i n t e r f e r e n c e from unwanted m a g n e t i c o b j e c t s ( i . e . f e n c e s , d e b r i s and a u t o m o b i l e s ) . F l u x g a t e magnetometers a l s o p r o v i d e the advantage o f o b t a i n i n g cont i n u o u s measurements and d a t a r e a d o u t f o r c o n d u c t i n g t o t a l s i t e surveys. The magnitude o f an anomaly p r o d u c e d by a b u r i e d o b j e c t i s d i r e c t l y r e l a t e d t o the m a g n e t i c mass and m a g n e t i c i n t e n s i t y of the body, and i n v e r s e l y r e l a t e d t o the d e p t h o f the body. Magnetic a n o m a l i e s , s u c h as the c o n t a c t between a non-magnetic s e d i m e n t a r y d e p o s i t and a m a g n e t i c i g n e o u s r o c k body, t h a t a r e i d e n t i f i e d on magnetic p r o f i l e s o r c o n t o u r maps can be l o c a t e d u s i n g q u a l i t a t i v e t e c h n i q u e s ( F i g u r e 10). Computer d a t a p r o c e s s i n g and a n a l y t i c a l t e c h n i q u e s can d e t e r m i n e the d e p t h o f b u r i a l , the mass of the b u r i e d o b j e c t s , the g e o l o g i c s t r u c t u r e , and the d e p t h t o b e d r o c k . Magnetometer s u r v e y s a r e l i m i t e d t o i d e n t i f y i n g o r l o c a t i n g s u b s u r f a c e m a g n e t i c f e a t u r e s o r o b j e c t s . Magnetometer s u r v e y s may be h i n d e r e d by the p r e s e n c e o f unwanted l o c a l m a g n e t i c f i e l d s a s s o c i a t e d w i t h power l i n e s , r a i l r o a d t r a c k s , e t c . D i u r n a l v a r i a t i o n s , w h i c h a r e n a t u r a l changes i n the e a r t h ' s m a g n e t i c f i e l d over time, must be compensated f o r d u r i n g a magnetometer s u r v e y . During magnetic storms, w h i c h may o c c u r as o f t e n as s e v e r a l times a month, s i g n i f i c a n t v a r i a t i o n s i n the e a r t h ' s m a g n e t i c f i e l d w i l l r e s u l t , making completion of a magnetic survey i m p r a c t i c a l . Finally, identificat i o n of l o c a l a n o m a l i e s a r e a f f e c t e d by the p r e s e n c e o f many c u l t u r a l f e a t u r e s i n c l u d i n g s t e e l f e n c e s , v e h i c l e s , b u i l d i n g s and i r o n d e b r i s . The s e a r c h f o r b u r i e d o b j e c t s may a l s o be l i m i t e d by the p r e s e n c e of n a t u r a l i r o n o r e d e p o s i t s and bog i r o n s , o r by o t h e r v a r i a t i o n s i n subsurface geology. Groundwater A p p l i c a t i o n s C u r r e n t l y , t h e r e a r e s e v e r a l p u b l i c a t i o n s w h i c h document a p p l y i n g s u r f a c e g e o p h y s i c a l t e c h n i q u e s t o groundwater contaminant i n v e s t i g a t i o n s (2,8,15,16,17). Two c o n c l u s i o n s drawn i n a l l of the above referenced p u b l i c a t i o n s are: (1) g e o p h y s i c a l s u r v e y s p r o v i d e s e v e r a l u s e f u l n o n d e s t r u c t i v e t e s t methods f o r e v a l u a t i n g s u b s u r f a c e g e o h y d r o l o g i c c o n d i t i o n s ; and (2) t h e s e methods may o f t e n improve the q u a n t i t y and q u a l i t y of the d a t a base o b t a i n e d from c o n v e n t i o n a l d e s t r u c t i v e t e s t methods, w h i l e r e d u c i n g o v e r a l l i n v e s t i g a t i o n and monitoring costs. Advantages of g e o p h y s i c a l s u r v e y s i n c l u d e the c o s t e f f e c t i v e b e n e f i t s p r o v i d e d f o r by t h e r a p i d a c q u i s i t i o n of d a t a . Geophysical



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F i g u r e 9. C r o s s s e c t i o n a l v i e w o f ground s u r f a c e t r a v e r s e , X-X , and the d i s t o r t i o n i n t h e m a g n e t i c f i e l d i n t e n s i t y c r e a t e d by s e v e r a l b u r i e d drums. 1

F i g u r e 10. M a g n e t i c p r o f i l e a l o n g ground s u r f a c e t r a v e r s e , X-X , and t h e l o c a l anomaly c r e a t e d by b u r i e d drums. 1



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s u r v e y t e c h n i q u e s can be implemented d u r i n g p r e l i m i n a r y p r o j e c t s t a g e s i n o r d e r t o e s t a b l i s h a s i t e d a t a base, and t h e r e b y i d e n t i f y a n o m a l i e s which r e q u i r e more d e t a i l e d e v a l u a t i o n s . I n a d d i t i o n , c o n t i n u o u s d a t a c o l l e c t i o n t e c h n i q u e s may be u t i l i z e d f o r mapping f e a t u r e s t o a g r e a t e r degree o f r e s o l u t i o n n o t g e n e r a l l y a t t a i n a b l e by u s i n g c o n v e n t i o n a l sampling t e c h n i q u e s . A t h a z a r d o u s waste s i t e s the use o f g e o p h y s i c a l s u r v e y t e c h n i q u e s would be advantageous p r i o r t o c o m p l e t i n g o n - s i t e t e s t d r i l l ing or e x c a v a t i o n a c t i v i t i e s (1,13). The a p p l i c a t i o n of g e o p h y s i c a l t e c h n i q u e s i n l o c a t i n g s u s p e c t e d c o n t a i n e r b u r i a l a r e a s would r e d u c e the l i k e l i h o o d of unexpected c o n t a c t by p e r s o n n e l or equipment w i t h b u r i e d hazardous, t o x i c or r a d i o a c t i v e m a t e r i a l s . S i t e c h a r a c t e r i z a t i o n with surface geophysical techniques a l s o helps i n i d e n t i f y i n g the p r o p e r p r o c e d u r e s and p r e c a u t i o n s t h a t s h o u l d be implemented d u r i n g t h e c o n v e n t i o n a l phases o f an i n v e s t i g a t i o n , i n o r d e r t o e l i m i n a t e the p o s s i b l e spread or r e l e a s e of t o x i c or r a d i o a c t i v e materials. S e l e c t e d I l l u s t r a t i v e Models. Three i l l u s t r a t i v e models o f geop h y s i c a l s u r v e y s a r e b r i e f l y p r e s e n t e d t o p r o v i d e examples o f the r e s u l t s t h a t can be o b t a i n e d by c o m p l e t i n g s u r f a c e g e o p h y s i c a l surveys. The models i n c l u d e : 1) a v e r t i c a l e l e c t r i c a l r e s i s t i v i t y sounding program (VES) t h a t e v a l u a t e s s a l t water i n t r u s i o n ; 2) an e l e c t r o m a g n e t i c c o n d u c t i v i t y s u r v e y t o i d e n t i f y groundwater c o n t a m i nant plumes and contaminant s o u r c e s ; and 3) an e v a l u a t i o n of l i m e s t o n e t e r r a i n w i t h s e i s m i c r e f r a c t i o n and e l e c t r o m a g n e t i c conduct i v i t y surveys. V e r t i c a l E l e c t r i c a l R e s i s t i v i t y Sounding. The f i r s t example i l l u s t r a t e s the a b i l i t y o f VES t e c h n i q u e s t o c h a r a c t e r i z e r e g i o n a l subs u r f a c e g e o l o g y i n t o r e s i s t i v i t y l a y e r s t h a t have s i m i l a r r e s i s t i v i t y v a l u e s a t s i m i l a r depths. Sediment l i t h o l o g i e s and groundwater q u a l i t y c h a r a c t e r i s t i c s were d e t e r m i n e d f o r each u n i t based on observed r e s i s t i v i t y v a l u e s . The s u r v e y was conducted i n Northampton County, V i r g i n i a w h i c h i s l o c a t e d a t the s o u t h end of t h e Del-Mar-Va P e n i n s u l a (9) ( F i g u r e 1 1 ) . Data was c o l l e c t e d a t s i x VES s i t e s s e l e c t e d a t t h e s t u d y a r e a . The VES d a t a were a n a l y z e d u s i n g computer p r o c e s s i n g t e c h n i q u e s t o c r e a t e 5 and 10 l a y e r s u b s u r f a c e models (18,19). F i g u r e 12 p r e s e n t s a t h r e e - d i m e n s i o n a l i s o m e t r i c p r o j e c t i o n p r e p a r e d from the computerg e n e r a t e d , 5 - l a y e r VES c u r v e s . Three g e n e r a l l i t h o l o g i e s d e s c r i b e the s u b s u r f a c e c o n d i t i o n s . The upper l a y e r , w h i c h p r o d u c e s h i g h r e s i s t i v i t y v a l u e s , c o n s i s t s m o s t l y of u n s a t u r a t e d sand t h a t ranges i n d e p t h from about 0 t o 150 feet. The second r e s i s t i v i t y l a y e r , w h i c h i s c h a r a c t e r i z e d by modera t e l y low r e s i s t i v i t y v a l u e s , o c c u r s a t depths g e n e r a l l y r a n g i n g between 150 t o 160 f e e t and c o n s i s t s of f r e s h w a t e r - b e a r i n g sands. The Manokin and Pocomoke a c q u i f e r s a r e p r e s e n t w i t h i n t h i s l a y e r . In a d d i t i o n , l e n s e s o f c l a y and/or b r a c k i s h water o c c u r w i t h i n t h i s layer. V a l u e s of r e s i s t i v i t y w i t h i n the second l a y e r d e c r e a s e e a s t ward, i n d i c a t i n g an i n c r e a s e i n the t h i c k n e s s and/or f r e q u e n c y of the c l a y l e n s e s and/or b r a c k i s h water zones.



D E ROSE


VES Profile Location

F i g u r e 11. VES s i t e a r e a l o c a t i o n i n Northampton County, Virginia. Reproduced w i t h p e r m i s s i o n from Réf. 9.



F i g u r e 12. VES f i v e l a y e r , t h r e e - d i m e n s i o n a l i s o m e t r i c projection. Reproduced w i t h p e r m i s s i o n from Ref. 9.


7.

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V e r y low r e s i s t i v i t y v a l u e s c h a r a c t e r i z e t h e d e e p e s t l a y e r , i n d i c a t i n g the presence of e i t h e r b r a c k i s h , or s a l i n e water-bearing sediments. The v e r t i c a l e x t e n t o f t h i s l a y e r i s w e l l d e f i n e d by sudden l a r g e s c a l e d e c r e a s e s i n r e s i s t i v i t y , however, t h e l a t e r a l extent of the l a y e r i s not w e l l d e f i n e d . E l e c t r o m a g n e t i c C o n d u c t i v i t y Survey. The r e s u l t s o f an e l e c t r o m a g n e t i c c o n d u c t i v i t y s u r v e y conducted a t a uranium m i l l t a i l i n g s s i t e a r e shown i n F i g u r e s 13 and 14. The p u r p o s e o f t h i s s u r v e y was t o i d e n t i f y s u b s u r f a c e c o n d u c t i v i t y a n o m a l i e s t h a t c o u l d be a s s o c i a t e d w i t h v a r i a t i o n s i n groundwater q u a l i t y . Once t h e a n o m a l i e s were i d e n t i f i e d , t e s t b o r i n g s and m o n i t o r i n g w e l l s were completed t o determine t h e i r s i g n i f i c a n c e . A s u r v e y g r i d was e s t a b l i s h e d a t t h e s i t e by m e a s u r i n g 150 and 300 f o o t i n t e r v a l s a c r o s s a 6,000 f o o t by 3,900 f o o t a r e a . The s u r v e y a r e a i s p r e d o m i n a n t l y f l a t , w i t h t h e major t o p o g r a p h i c f e a t u r e s b e i n g t a i l i n g s p i l e s and l e v e e s around t h e e v a p o r a t i o n ponds. The s u b s u r f a c e s o i l s a r e composed o f s i l t y c l a y and g r a v e l deposits. The d e p t h t o groundwater i s g e n e r a l l y l e s s t h a n 5 f e e t below l a n d s u r f a c e . E l e c t r o m a g n e t i c c o n d u c t i v i t y measurements were made a t 316 survey s t a t i o n s . Two s e t s o f s u b s u r f a c e c o n d u c t i v i t y v a l u e s were o b t a i n e d a t each s u r v e y s t a t i o n . These c o n s i s t e d o f one s e t o f r e a d i n g s t o an e f f e c t i v e measurement d e p t h o f 25 f e e t w i t h a l a r g e p o r t i o n o f t h e t o t a l r e a d i n g c o n t r i b u t e d by near s u r f a c e m a t e r i a l s ( F i g u r e 13), and one s e t o f r e a d i n g s t o an e f f e c t i v e measurement depth o f 50 f e e t w i t h a s m a l l p o r t i o n o f t h e t o t a l r e a d i n g c o n t r i b u t e d by near s u r f a c e m a t e r i a l s ( F i g u r e 1 4 ) . C o n d u c t i v i t y c o n t o u r maps were computer g e n e r a t e d u s i n g each d a t a s e t . The c o n t o u r maps were t h e n used t o i d e n t i f y anomalous a r e a s o f s u b s u r f a c e c o n d u c t i v i t y ( F i g u r e s 13 and 1 4 ) . A comparison o f t h e r e s u l t i n g two c o n d u c t i v i t y c o n t o u r maps shows a n o m a l o u s l y h i g h c o n d u c t i v i t y v a l u e s i n an a r e a e x t e n d i n g from t h e s i t e c o o r d i n a t e s N8800, E4700 t o N6700, E3800. In addition, h i g h e r v a l u e s were c o n s i s t e n t l y r e c o r d e d a t t h e g r e a t e r e f f e c t i v e measurement depth i n comparison t o t h o s e v a l u e s o b t a i n e d a t t h e s h a l l o w e r e f f e c t i v e measurement d e p t h . T h i s i n d i c a t e s that subsurf a c e m a t e r i a l s w i t h i n t h e anomalous a r e a have h i g h e r c o n d u c t i v i t y v a l u e s a t depth r a t h e r t h a n near t h e s u r f a c e . Those a n o m a l i e s t h a t a r e shown i n F i g u r e 13, b u t a r e n o t e v i d e n t i n F i g u r e 14, a r e r e l a t e d t o near s u r f a c e v a r i a t i o n s i n s o i l t y p e and/or t h e a c c u m u l a t i o n o f e v a p o r i t e d e p o s i t s . The i s o l a t e d anomaly h i g h l i g h t e d on F i g u r e 14, i s p r o b a b l y r e l a t e d t o one o f s e v e r a l geothermal s p r i n g s . E l e c t r i c a l r e s i s t i v i t y measurements were a l s o t a k e n a t 28 l o c a t i o n s s e l e c t e d a c r o s s t h e s i t e t o v e r i f y t h e d a t a c o l l e c t e d by e l e c t r o m a g n e t i c methods. The d a t a o b t a i n e d was i n agreement w i t h the r e s u l t s o f t h e e l e c t r o m a g n e t i c c o n d u c t i v i t y s u r v e y . The e n t i r e g e o p h y s i c a l s u r v e y o f t h e f i e l d s i t e was completed i n f o u r days. L a t e r i n v e s t i g a t i o n s , i n c l u d i n g t h e i n s t a l l a t i o n and sampling o f m o n i t o r i n g w e l l s and a n a l y s i s o f groundwater samples from t h o s e w e l l s , c o n f i r m e d t h a t t h e anomaly o f h i g h c o n d u c t i v i t y was t h e r e s u l t o f groundwater c o n t a m i n a t i o n o c c u r r i n g downgradient from t h e tailings piles.




134


135



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°°2500

3000

4doo

5000

5500

6000 ο 6500 i 70.00 ζ

F i g u r e 14. C o n d u c t i v i t y c o n t o u r map r e s u l t i n g from e l e c t r o m a g n e t i c c o n d u c t i v i t y s u r v e y completed a t an e f f e c t i v e measurement d e p t h o f 50 f e e t .



136


Geophysical I n v e s t i g a t i o n at a P o t e n t i a l F i e l d - A p p l i c a t i o n S i t e . Based upon s e v e r a l s u c c e s s f u l s i t e e v a l u a t i o n s t h a t employed geop h y s i c a l methods combined w i t h c o n v e n t i o n a l e x p l o r a t o r y t e c h n i q u e s i n i n v e s t i g a t i n g s i t e s u n d e r l a i n by l i m e s t o n e , t h e f o l l o w i n g p o t e n t i a l s i t e e v a l u a t i o n was d e v e l o p e d . The i n t e n t o f t h i s h y p o t h e t i c a l s i t u a t i o n was t o i l l u s t r a t e the c o n c e p t of an i n t e g r a t e d s i t e e v a l u a t i o n i n which g e o p h y s i c a l techniques are u t i l i z e d to i n c r e a s e the e f f e c t i v e n e s s o f the groundwater m o n i t o r i n g system. The a r e a of the proposed s i t e f o r the a p p l i c a t i o n o f p e s t i c i d e s i s about t w e n t y - f i v e a c r e s and i s u n d e r l a i n by r e s i d u a l s o i l of v a r i a b l e l i t h o l o g y which i s u n d e r l a i n by t h e L e i t h s v i l l e F o r m a t i o n and the T r i a s s i c B o r d e r c o n g l o m e r a t e . The L e i t h s v i l l e F o r m a t i o n i s a l i m e s t o n e o f Cambrian age. The s i t e i s a l s o c h a r a c t e r i z e d by a r e l a t i v e l y s h a l l o w water t a b l e ( F i g u r e 1 5 ) . The i n i t i a l s i t e i n v e s t i g a t i o n t o d e f i n e s u b s u r f a c e c o n d i t i o n s i n c l u d e d s e i s m i c r e f r a c t i o n p r o f i l e s and a l i m i t e d number o f t e s t borings. I n t e r p r e t a t i o n and c o r r e l a t i o n of s e i s m i c d a t a w i t h r e s u l t s o b t a i n e d from the t e s t b o r i n g s i d e n t i f i e d a r e a s u n d e r l a i n by l i m e s t o n e a t s h a l l o w depths ( l e s s t h a n 10 f e e t ) , by l i m e s t o n e and a t moderate depths (15 t o 25 f e e t ) , and a r e a s where d e p t h t o l i m e s t o n e i s i r r e g u l a r ( F i g u r e 16). W i t h i n the a r e a c h a r a c t e r i z e d by moderate d e p t h t o l i m e s t o n e , based upon comparison w i t h the r e s u l t s of the s e i s m i c r e f r a c t i o n p r o f i l e s , f o u r t e s t b o r i n g s encountered limestone at g r e a t depths a s s o c i a t e d w i t h l o c a l i z e d k a r s t f e a t u r e s . These d a t a p o i n t s were t h e r e f o r e i d e n t i f i e d as anomalous v a l u e s . In a d d i t i o n , conglomerate was e n c o u n t e r e d u n d e r l y i n g a p o r t i o n o f the s i t e a r e a at d e p t h s o f 15 t o 25 f e e t . Based upon t e s t b o r i n g s , the s i t e was u n d e r l a i n a t s h a l l o w depths by c o a r s e , l i g h t g r a y l i m e s t o n e and sand, w i t h minor amounts of s i l t and c l a y . The s i t e a r e a c h a r a c t e r i z e d by moderate d e p t h to l i m e s t o n e was u n d e r l a i n by f i n e , r e d d o l o m i t i c l i m e s t o n e and s i l t y c l a y , w i t h minor amounts o f sand. The r e m a i n i n g s i t e a r e a was u n d e r l a i n by c o a r s e sandstone and conglomerate and s i l t and c l a y w i t h moderate amounts o f sand. An e l e c t r o m a g n e t i c s u r v e y was r u n a t the s i t e a t s t a t i o n s p l a c e d a l o n g a 50 f o o t g r i d . The r e s u l t s o f the s u r v e y c o r r e l a t e d f a v o r a b l y w i t h the r e s u l t s o b t a i n e d from the i n i t i a l i n v e s t i g a t i o n . A Phase I G e o l o g i c Map was p r e p a r e d f o r t h e s i t e based upon t h e r e s u l t s o f the s e i s m i c r e f r a c t i o n p r o f i l e s , t e s t b o r i n g s and e l e c t r o m a g n e t i c c o n d u c t i v i t y survey ( F i g u r e 17). H i g h a r e a s of c o n d u c t i v i t y were g e n e r a l l y a s s o c i a t e d w i t h t h e c e n t r a l p a r t o f the s i t e w h i c h i s u n d e r l a i n by m o d e r a t e l y deep l i m e s t o n e and s i l t and c l a y . Low cond u c t i v i t y v a l u e s were g e n e r a l l y a s s o c i a t e d w i t h t h o s e a r e a s o f t h e s i t e u n d e r l a i n by l i m e s t o n e a t s h a l l o w d e p t h s . I n a d d i t i o n , two l i n e a r t r e n d i n g a r e a s of v a r i a b l e c o n d u c t i v i t y were mapped i n the a r e a between the s h a l l o w and m o d e r a t e l y deep l i m e s t o n e a r e a s . F o l l o w - u p t e s t b o r i n g s were completed f o r t h e f i n a l s i t e c h a r a c t e r i z a t i o n and i m p l e m e n t a t i o n of a groundwater m o n i t o r i n g system. Based upon e v a l u a t i o n of the t e s t b o r i n g d a t a and the g e o p h y s i c a l d a t a , t h e s i t e was found t o be u n d e r l a i n by t h r e e d i f f e r e n t l i m e s t o n e members ( 2 0 ) . The two c o n t a c t zones w h i c h o c c u r between the t h r e e members were a s s o c i a t e d w i t h the two l i n e a r t r e n d i n g a r e a s o f v a r i a ble conductivity. These zones were c h a r a c t e r i z e d by the development of deep k a r s t zone t h a t r e s u l t e d from the d i s s o l u t i o n of the l i m e -


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€1

F i g u r e 15.

G e o l o g i c map o f p r o p o s e d f i e l d - a p p l i c a t i o n

site.

Legend 28

Depth (ft) to Bedrock Based on Test Borings

KN3

Seismic Refraction - Irregular Bedrock Depth

I

I Seismic Refraction - Shallow Bedrock Depth

I

I Seismic Refraction - Moderate Bedrock Depth Anomdous Depth to Bedrock (Based on Comparison of Seismic Refraction and Test Boring)

F i g u r e 16. G e o l o g i c map o f p r o p o s e d f i e l d - a p p l i c a t i o n s i t e summarizing b e d r o c k depths d e t e r m i n e d from s e i s m i c r e f r a c t i o n s u r v e y s and t e s t b o r i n g d a t a .



138

s t o n e by groundwater f l o w i n g a l o n g t h e c o n t a c t zone ( F i g u r e 1 7 ) . K a r s t f e a t u r e s i n c l u d i n g p i n n a c l e s , c a v i t i e s , and v o i d s were found e x t e n d i n g t o d e p t h s g r e a t e r t h a n 100 f e e t a l o n g t h e c o n t a c t z o n e s . I n o r d e r t o map the e x t e n t of the minor s o l u t i o n c a v i t i e s found i n o t h e r a r e a s of t h e s i t e , d e t a i l e d e l e c t r o m a g n e t i c c o n d u c t i v i t y s u r v e y s were completed on a g r i d s p a c i n g of twenty f e e t . M o n i t o r i n g w e l l l o c a t i o n s were chosen based on r e s u l t s from t h e g e o p h y s i c a l s u r v e y s and the t e s t d r i l l i n g w h i c h c h a r a c t e r i z e d the s u b s u r f a c e c o n d i t i o n s u n d e r l y i n g the s i t e ( F i g u r e 1 8 ) . Without u t i l i z a t i o n o f g e o p h y s i c a l techniques, 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 would have been i m p r a c t i c a l w i t h o u t i n c u r r i n g e x c e s s i v e d r i l l i n g costs.


Summary and

P o t e n t i a l A p p l i c a t i o n s to the P e s t i c i d e I n d u s t r y

F i v e c o s t - e f f e c t i v e s u r f a c e g e o p h y s i c a l s u r v e y t e c h n i q u e s have been d e s c r i b e d w h i c h a r e w i d e l y u s e d i n groundwater s i t e i n v e s t i g a t i o n s . By employing g e o p h y s i c a l t e c h n i q u e s t o complement c o n v e n t i o n a l methods of s u b s u r f a c e e x p o r a t i o n , a m o n i t o r i n g network can be des i g n e d t h a t has an o v e r a l l i n c r e a s e i n e f f i c i e n c y and i s c o s t e f f e c t i v e (2,6). The a p p l i c a t i o n of t h e s e s u r f a c e g e o p h y s i c a l t e c h n i q u e s t o groundwater c o n t a m i n a n t i n v e s t i g a t i o n s i n c l u d e the f o l l o w i n g : 1) r e g i o n a l and l o c a l e v a l u a t i o n s o f s u b s u r f a c e g e o h y d r o l o g i c c o n d i t i o n s ; 2) f e a s i b i l i t y s t u d i e s o f p r o p o s e d f i e l d a p p l i c a t i o n s i t e s and m a n u f a c t u r i n g , s t o r a g e and d i s p o s a l f a c i l i t i e s ; 3) d e s i g n o f m o n i t o r i n g systems; 4) c h a r a c t e r i z a t i o n o f e x i s t i n g f i e l d a p p l i c a t i o n s i t e s and m a n u f a c t u r i n g , s t o r a g e and d i s p o s a l f a c i l i t i e s ; and 5) d e s i g n o f l e a k d e t e c t i o n systems u t i l i z i n g e l e c t r i c a l r e s i s t i v i t y techniques. The d e t e c t i o n of p e s t i c i d e c o n t a m i n a n t plumes, o r i g i n a t i n g from g e n e r a l l y n o n - i o n i c o r g a n i c c h e m i c a l s , p r e s e n t s a d i f f i c u l t s i t u a t i o n t o e f f e c t i v e l y m o n i t o r by u t i l i z i n g a p p l i c a b l e g e o p h y s i c a l t e c h n i q u e s s u c h as e l e c t r i c a l r e s i s t i v i t y and e l e c t r o m a g n e t i c c o n ductivity. However, t h i s r e p o r t e d l y has been s u c c e s s f u l l y accomp l i s h e d where contaminant c o n c e n t r a t i o n s i n groundwater a r e g r e a t e r t h a n 10 mg/1. A t h i g h contaminant c o n c e n t r a t i o n s the d e t e c t i o n o f o r g a n i c plumes may be p o s s i b l e as a r e s u l t o f the f o r m a t i o n of a c o n d u c t i v i t y low w h i c h i s a s s o c i a t e d w i t h the n o n - i o n i c c o n t a m i n a n t s i n c o n t r a s t to the s u r r o u n d i n g h i g h l y i o n i c groundwater. Hydraulic p r o c e s s e s o c c u r r i n g i n the u n s a t u r a t e d zone of f i n e g r a i n e d s o i l s may a l s o r e s u l t i n the a c c u m u l a t i o n o f o r g a n i c compounds above the water t a b l e f o r m i n g a low c o n d u c t i v i t y anomaly o v e r l y i n g the o r g a n i c contaminant plume. I t i s a l s o p o s s i b l e that degradation by-products o f p e s t i c i d e s may c r e a t e a mappable i o n i c contaminant plume. S e v e r a l r e p o r t s have been p u b l i s h e d w h i c h d e s c r i b e the d e s i g n and i m p l e m e n t a t i o n o f e l e c t r i c a l and e l e c t r o m a g n e t i c e a r l y w a r n i n g and/or l e a k d e t e c t i o n systems ( 7 ) . The g e n e r a l p r e m i s e by w h i c h t h e systems o p e r a t e i s t h a t by e s t a b l i s h i n g an e l e c t r i c a l r e s i s t i v i t y or c o n d u c t i v i t y map o f a s i t e o r f a c i l i t y i n i t s n a t u r a l s t a t e t h e e f f e c t s of p o t e n t i a l s i t e d i s c h a r g e s t o groundwater can be m o n i t o r e d by c o n t i n u e d s i t e s u r v e y i n g . I t i s also p o s s i b l e that f o r c e r t a i n p e s t i c i d e f i e l d a p p l i c a t i o n s i t e s the e s t a b l i s h m e n t of a b a s e l i n e c o n d u c t i v i t y map a t s e l e c t e d d e p t h s , f o r example 1.0 m e t e r s , 3.0



7.

D E ROSE


Legend Triassic M Border Conglomerate Cambrian Leithsville Formation

I : ; y. /M Walkill Member I

I Hamburg Member

|Ξ~.>3 Califon Member Karst Contact Zone (Pinnacles)

F i g u r e 17. D e t a i l e d g e o l o g i c map o f proposed f i e l d - a p p l i c a t i o n s i t e based upon r e s u l t s o f s e i s m i c r e f r a c t i o n and e l e c t r o magnetic c o n d u c t i v i t y s u r v e y s and t e s t b o r i n g d a t a .

F i g u r e 18. G e o l o g i c map o f f i e l d a p p l i c a t i o n s i t e showing k a r s t f e a t u r e s and proposed m o n i t o r i n g w e l l l o c a t i o n s .


139

140


m e t e r s , and 7.5 m e t e r s , may p r o v e t o be u s e f u l i n d e t e r m i n i n g a r e a s where s u b s u r f a c e m i g r a t i o n o f p e s t i c i d e s o r t h e i r d e g r a d a t i o n p r o d u c t s i s o c c u r r i n g . Such t e c h n i q u e s may be a p p l i c a b l e t o u n s a t u r a t e d and s a t u r a t e d zone m o n i t o r i n g programs.


Literature Cited

1. Lord, A. E., Jr.; Koerner, R. M.; Freestone, F. J. J. of Hazardous Materials 1982, 5, 221-233. 2. Benson, R.C.;Glaccum, R. Α.; Noel, M. R. "Geophysical Techniques for Sensing Buried Wastes and Waste Mibratum"; U. S. Environmental Protection Agency: Las Vegas, Nev., 1984; p. 236. 3. Griffiths, P. H.; King, R. F. "Applied Geophysics for Engineers and Geologists"; Pergammon Press: Elmsford, Ν. Y., 1975; p. 223. 4. Kelly, W. E.; Frohlich, R. K. Groundwater 1985, 23, 182-189. 5. Alao, D. A. Bull, of the Assoc. of Eng. Geologists 1985, 22, 95-100. 6. Schwartz, F. W.; McClymont, G. L. Groundwater 1977, 15, 197-202. 7. Schultz, D. W.; Duff, Β. M.; Peters, W. R. Proc. Intl. Conf. on Geomembranes, 1984, p. 445-449. 8. Evans, R. B.; Schweitzer, G. E. Environ. Sci. Technol. 1984, 18, 330A-339A. 9. Cross, J. W.; Baine, E. H.; Baine, P. M.; Carroll, R. I.; DeRose, N.; Foster, M. A. "Industrialization of Virginia's Eastern Shore: An Environmental Impact Study in Northampton County" Old Dominion Univ.: Norfolk, Va.; 1977, p. 271. 10. McNeill, J. D. Technical Note TN-5 Geonics Ltd.: Ontario, Canada, 1980, p. 22. 11. Technical Note TN-6 Geonics Ltd.: Ontario, Canada, 1980,p.15. 12. "Ground Penetrating Radar - Summary of Capabilities", Roy F. Weston, Inc., West Chester, PA.; 1984. 13. Koerner, R. M.; Lord, Α. Ε., Jr. Proc. 5th Natl. Conf. on Magmt. of Uncont. Haz. Waste Sites, 1984. 14. Breiner, S. "Applications Manual for Portable Magnetometers"; Geometries: Sonnyvale, Calif., 1973; p. 58. 15. Sendlein, L. V. Α.; Yazicigil, H. Groundwater Monitor Rev. Fall, 1981. 16. Yazicigil, H.; Sendlein, L. V. A. Groundwater Monitor Rev. Winter, 1982. 17. Kaufmann, R. F.; Gleason, Α.; Ellwood, R. B.; Lindsey, G. P. Groundwater Monitor Rev. Fall, 1981. 18. Zohdy, A. A. R. Computer Program, Ntl. Tech. Inf. Ser., No. PB-232703, 1973. 19. Zohdy, A. A. R. Computer Program, Ntl. Tech. Inf. Ser., No. PB-232056, 1974. 20. Markewicz, S. J., et al. Pennsylvania Geologists Field Confer ence Annual Meeting, Guide Book 42, 1977, p. 117. RECEIVED April 7, 1986


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