Evaluation of Pesticides in Ground Water - American Chemical Society

8 DRASTIC: A System to Evaluate the Pollution Potential of Hydrogeologic Settings by Pesticides Linda Aller, Truman Bennett, Jay H . Lehr, and Rebecca Petty

Downloaded by PURDUE UNIV on September 26, 2017 | http://pubs.acs.org Publication Date: July 17, 1986 | doi: 10.1021/bk-1986-0315.ch008

National Water Well Association, Worthington, O H 43085

A methodology is described which allows the pollution potential of any area to be systematically evaluated anywhere in the United States. The system, which optimizes the use of existing data, has two major portions: the designation of mappable units, termed hydrogeologic settings, and the superposition of a relative ranking system called DRASTIC. Hydrogeologic settings incorporate the major hydrogeologic factors which are used to infer the potential for pesticides to enter ground water. These factors, which form the acronym DRASTIC, include depth to water, net recharge, aquifer media, soil media, topography, impact of the vadose zone and hydraulic conductivity of the aquifer. The relative ranking scheme uses a combination of weights and ratings to produce a numerical value, called the DRASTIC Index, which helps prioritize areas with respect to pollution potential. O n l y i n t h e past few y e a r s has t h e n a t i o n become aware o f t h e dangers o f ground-water c o n t a m i n a t i o n and o f the many ways i n which ground water can become contaminated. 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 one such p r a c t i c e which may r e s u l t i n ground-water c o n t a m i n a t i o n . The p o t e n t i a l f o r c o n t a m i n a t i o n t o occur i s a f f e c t e d by the p h y s i c a l c h a r a c t e r i s t i c s o f t h e a r e a , t h e c h e m i c a l n a t u r e o f t h e p e s t i c i d e and t h e r a t e , frequency and method o f a p p l i c a t i o n . Ground-water c o n t a m i n a t i o n can be minimized by c o n t r o l l i n g t h e p e s t i c i d e a p p l i c a t i o n r a t e and f r e q u e n c y , by t h e c h o i c e o f a p p r o p r i a t e p e s t i c i d e s and by e v a l u a t i n g and managing p e s t i c i d e a p p l i c a t i o n i n v u l n e r a b l e areas. T h i s paper p r e s e n t s a s t a n d a r d i z e d system which can be used t o e v a l u a t e the ground-water p o l l u t i o n p o t e n t i a l o f any h y d r o g e o l o g i c s e t t i n g i n t h e U n i t e d S t a t e s . The system has been designed t o use i n f o r m a t i o n which i s a v a i l a b l e through a v a r i e t y 0097-6156/ 86/ 0315-0141 $06.00/ 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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of s o u r c e s . I n f o r m a t i o n on t h e parameters i n c l u d i n g the depth t o water i n an a r e a , net r e c h a r g e , a q u i f e r media, s o i l media, g e n e r a l topography o r s l o p e , vadose zone media and h y d r a u l i c c o n d u c t i v i t y of the a q u i f e r i s n e c e s s a r y t o e v a l u a t e the groundwater p o l l u t i o n p o t e n t i a l o f any a r e a u s i n g h y d r o g e o l o g i c s e t t i n g s . T h i s system has been p r e p a r e d t o a s s i s t p l a n n e r s , managers and a d m i n i s t r a t o r s i n the t a s k of e v a l u a t i n g the r e l a t i v e v u l n e r a b i l i t y of a r e a s t o ground-water c o n t a m i n a t i o n by p e s t i c i d e s . I t has been assumed t h a t the r e a d e r has o n l y a b a s i c knowledge of h y d r o g e o l o g y and the p r o c e s s e s which g o v e r n ground-water c o n t a m i n a t i o n . T h i s methodology i s n e i t h e r d e s i g n e d nor i n t e n d e d to r e p l a c e o n - s i t e i n s p e c t i o n s . R a t h e r , i t i s i n t e n d e d t o p r o v i d e a b a s i s f o r c o m p a r a t i v e e v a l u a t i o n of a r e a s w i t h r e s p e c t t o p o t e n t i a l f o r p o l l u t i o n of ground water. C l a s s i f i c a t i o n Systems One of the fundamental needs o f any n a t u r a l s c i e n c e i s the development of an e f f e c t i v e system t o group s i m i l a r e n t i t i e s i n t o c a t e g o r i e s . W e l l - e s t a b l i s h e d systems e x i s t i n the f i e l d s of botany, g e o l o g y , and many o t h e r s c i e n c e s (1). These systems p e r m i t an a p p r o p r i a t e l y t r a i n e d person t o g a i n c e r t a i n i n s i g h t about an e n t i t y s i m p l y by knowing the a p p r o p r i a t e c a t e g o r y i n which i t i s grouped. T h i s s y s t e m a t i c and l o g i c a l way of i m p o s i n g an a r t i f i c i a l system on n a t u r a l e n t i t i e s has l o n g been used i n the f i e l d o f g e o l o g y a l s o . For example, r o c k s have been c l a s s i f i e d a c c o r d i n g to o r i g i n and minerals grouped a c c o r d i n g t o c r y s t a l systems. However, as a s c i e n c e expands and changes, so must the t y p e s of systems used t o d e s c r i b e t h o s e c h a r a c t e r i s t i c s which need t o be s t u d i e d . The f i e l d of h y d r o g e o l o g y i s one a r e a of g e o l o g y which has o n l y been o v e r t l y r e c o g n i z e d s i n c e t h e term was c o i n e d by Lucas i n 1879 (2). S i n c e t h a t time h y d r o g e o l o g y has expanded, from a d i s c i p l i n e d e v o t e d t o water o c c u r r e n c e and a v a i l a b i l i t y , to i n c l u d e t h e broad a s p e c t of water q u a l i t y and s o l u t e c h e m i s t r y . D e f i n i t i o n o f water q u a l i t y i s fundamental t o the p r o t e c t i o n o f the ground-water r e s o u r c e from p o l l u t i o n . The i d e a of an o r g a n i z e d way to d e s c r i b e ground water systems i s not new. H e i n z e r (3) prepared a s m a l l - s c a l e map of the U n i t e d S t a t e s showing g e n e r a l ground-water p r o v i n c e s . Thomas (4) and Heath (5) prepared s i m i l a r but more d e t a i l e d maps and d e s c r i p t i o n s which grouped a q u i f e r s m a i n l y on t h e i r w a t e r - b e a r i n g c h a r a c t e r i s t i c s w i t h i n c e r t a i n g e o g r a p h i c a r e a s . B l a n k and Schroeder (6) attempted t o c l a s s i f y a q u i f e r s based on the p r o p e r t i e s of r o c k s which a f f e c t ground water. Of a l l t h e s e systems, g e o g r a p h i c ones have been more w i d e l y accepted as ways t o d e s c r i b e the q u a n t i t y o f water which i s a v a i l a b l e i n v a r i o u s r e g i o n s . Some E x i s t i n g Systems Which E v a l u a t e Ground-Water P o l l u t i o n Potential W i t h i n the l a s t 20 y e a r s the need t o expand t h e s e systems o r t o c r e a t e a new system t o a d d r e s s ground-water q u a l i t y has become e v i d e n t . Many d i f f e r e n t systems have been d e v e l o p e d t o a d d r e s s s i t e s e l e c t i o n f o r waste d i s p o s a l f a c i l i t i e s such as s a n i t a r y



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l a n d f i l l s or l i q u i d waste ponds. Among t h e s e , the LeGrand System (7) and the m o d i f i e d v e r s i o n used by the U.S. EPA i n the S u r f a c e Impoundment Assessment (SIA) a r e p r o b a b l y the most w e l l known. The LeGrand system uses n u m e r i c a l w e i g h t i n g t o e v a l u a t e groundwater p o l l u t i o n p o t e n t i a l from a g i v e n waste d i s p o s a l s i t e . By e v a l u a t i n g the s i t e t h r o u g h a s e r i e s of f o u r s t a g e s , a d e s c r i p t i o n of the h y d r o g e o l o g y of the s i t e , the r e l a t i v e a q u i f e r s e n s i t i v i t y combined w i t h the contaminant s e v e r i t y , the n a t u r a l p o l l u t i o n p o t e n t i a l p r e s e n t e d a t t h a t s i t e , and the e n g i n e e r i n g m o d i f i c a t i o n s which might change t h a t p o t e n t i a l a r e a l l e v a l u a t e d . The LeGrand system presupposes o n l y a l i m i t e d t e c h n i c a l knowledge but encourages t h e user t o become f a m i l i a r w i t h the c o n c e p t s p r e s e n t e d i n the manual so t h a t s k i l l e d judgements can be made i n the s u b j e c t i v e p o r t i o n of the system. The s i m i l a r i t i e s between s i t e s a r e emphasized and the uniqueness of each s i t e i s downplayed. The U.S. EPA methodology (8) uses the b a s i c LeGrand System to d e f i n e the h y d r o g e o l o g i c framework, but m o d i f i e s the system t o p l a c e emphasis on e s t a b l i s h i n g a m o n i t o r i n g p r i o r i t y f o r the f a c i l i t y . Once the h y d r o g e o l o g i c c h a r a c t e r i s t i c s have been r a t e d , a t a b l e i s used t o d e f i n e the m o n i t o r i n g p r i o r i t y . T h i s p r i o r i t y may be a d j u s t e d by the r a t e r u s i n g p r e s c r i b e d t e c h n i q u e s . Once a g a i n , o n l y a l i m i t e d t e c h n i c a l knowledge i s presupposed. Other systems have been designed t o t a i l o r t h e r e s u l t s t o more s p e c i f i c purposes. T h o r n t h w a i t e and Mather (9) and Fenn e t a l . (10) d e v e l o p e d w a t e r - b a l a n c e methods t o p r e d i c t the l e a c h a t e g e n e r a t i o n a t s o l i d waste d i s p o s a l s i t e s . T h i s approach i s based on the premise t h a t by knowing the amount of i n f i l t r a t i o n i n t o the l a n d f i l l and the d e s i g n of the c e l l , t h e l e a c h a t e q u a n t i t y f o r the l a n d f i l l can be determined. The system i s i n t e n d e d t o be used a s a t o o l by e n g i n e e r s i n the e a r l y d e s i g n phase of a f a c i l i t y . Gibb e t a l . (11) d e v i s e d a r a t i n g scheme t o e s t a b l i s h p r i o r i t i e s f o r e x i s t i n g waste d i s p o s a l s i t e s w i t h r e s p e c t t o t h e i r t h r e a t t o human h e a l t h by ground water. V i a r a n k i n g the s i t e through f o u r f a c t o r s , (1) h e a l t h r i s k o f the waste and h a n d l i n g mode, (2) p o p u l a t i o n a t r i s k , (3) p r o x i m i t y t o w e l l s o r a q u i f e r s , and (4) s u s c e p t i b i l i t y of a q u i f e r s , a number t h a t ranges from 0100 was used t o d i s p l a y the r e l a t i v e r i s k . The system was used i n a s p e c i f i c 2-county assessment by t e c h n i c a l l y q u a l i f i e d i n d i v i d u a l s . Another r a t i n g scheme, d e v e l o p e d by the M i c h i g a n Department o f N a t u r a l Resources (12), i s d e s i g n e d t o rank l a r g e r numbers o f s i t e s i n terms of r i s k of e n v i r o n m e n t a l c o n t a m i n a t i o n . By e v a l u a t i n g the f i v e c a t e g o r i e s : (1) r e l e a s e p o t e n t i a l , (2) e n v i r o n m e n t a l exposure, (3) t a r g e t s , (4) c h e m i c a l h a z a r d , and (5) e x i s t i n g exposure, the u s e r o b t a i n s a number r a n g i n g from 0 t o 2000 p o i n t s which e v a l u a t e s the r e l a t i v e hazard o f t h a t s i t e w i t h respect t o other s i t e s i n Michigan. S e l l e r and Canter (13) e v a l u t e d s e v e n e m p i r i c a l methods t o determine t h e i r u s e f u l n e s s i n p r e d i c t i n g t h e ground-water p o l l u t i o n e f f e c t s of a waste d i s p o s a l f a c i l i t y a t a p a r t i c u l a r s i t e . The methods they r e v i e w e d i n c l u d e d r a t i n g schemes, a d e c i s i o n t r e e approach, a m a t r i x and a c r i t e r i a - l i s t i n g method. They determined t h a t each method took i n t o account the n a t u r a l c o n d i t i o n s and f a c i l i t y d e s i g n and c o n s t r u c t i o n , but t h a t each method was b e s t a p p l i e d t o the s p e c i f i c s i t u a t i o n f o r which i t was designed.


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T h i s b r i e f r e v i e w of s e l e c t e d e x i s t i n g systems r e v e a l s t h a t t h e r e a r e a number o f methods t h a t can be a p p l i e d t o s i t e - s p e c i f i c s i t u a t i o n s or t o e v a l u a t i o n of the p o l l u t i o n p o t e n t i a l of e x i s t i n g s i t e s . However, a p l a n n i n g t o o l i s needed f o r use b e f o r e t h e s i t e - s p e c i f i c methods a r e employed. The system must (1) f u n c t i o n as a management t o o l , (2) be s i m p l e and easy-to-use, (3) u t i l i z e a v a i l a b l e i n f o r m a t i o n , and (4) be a b l e t o be used by i n d i v i d u a l s w i t h d i v e r s e backgrounds and l e v e l s of e x p e r t i s e . T h i s document c o n t a i n s a system which attempts t o meet these needs and t o p r o v i d e the p l a n n i n g t o o l n e c e s s a r y b e f o r e s i t e - s p e c i f i c e v a l u a t i o n s .


The DRASTIC System The system presented h e r e i n i s p a r t of a more c o m p l e t e system d e v e l o p e d f o r the U n i t e d S t a t e s E n v o r i n m e n t a l P r o t e c t i o n Agency. A c o m p l e t e d e s c r i p t i o n i s c o n t a i n e d i n the d r a f t document ΕΡΛ if 570/9-84-002 (14). The methodology has two major p o r t i o n s : the d e s i g n a t i o n of mappable u n i t s , termed h y d r o g e o l o g i c s e t t i n g s ; and the a p p l i c a t i o n of a scheme f o r r e l a t i v e r a n k i n g of h y d r o g e o l o g i c parameters, c a l l e d DRASTIC, which h e l p s the u s e r e v a l u a t e the r e l a t i v e ground-water p o l l u t i o n p o t e n t i a l of any h y d r o g e o l o g i c s e t t i n g s . A l t h o u g h the two p a r t s of the system a r e i n t e r r e l a t e d , they a r e d i s c u s s e d s e p a r a t e l y i n a l o g i c a l progression. Hydrogeologic

Settings

T h i s document has been prepared u s i n g the concept of h y d r o g e o l o g i c s e t t i n g s . A h y d r o g e o l o g i c s e t t i n g i s a composite d e s c r i p t i o n of a l l the major g e o l o g i c and h y d r o l o g i e f a c t o r s which a f f e c t and c o n t r o l ground-water movement i n t o , through and out of an a r e a . I t i s d e f i n e d as a mappable u n i t w i t h common h y d r o g e o l o g i c c h a r a c t e r i s t i c s , and as a consequence, common v u l n e r a b i l i t y t o c o n t a m i n a t i o n by i n t r o d u c e d p o l l u t a n t s . From these f a c t o r s i t i s p o s s i b l e t o make g e n e r a l i z a t i o n s about both ground-water a v a i l a b i l i t y and ground-water p o l l u t i o n p o t e n t i a l . I n o r d e r t o a s s i s t u s e r s who may have a l i m i t e d knowledge of h y d r o g e o l o g y , the e n t i r e s t a n d a r d i z e d system f o r e v a l u a t i n g ground water p o l l u t i o n p o t e n t i a l has been d e v e l o p e d w i t h i n the framework of an e x i s t i n g c l a s s i f i c a t i o n system of ground-water r e g i o n s of the U n i t e d S t a t e s . Heath (5}_ d i v i d e d t h e U n i t e d S t a t e s i n t o 15 ground-water r e g i o n s based on the f e a t u r e s i n a ground-water system which a f f e c t the o c c u r r e n c e and a v a i l a b i l i t y of ground water ( F i g u r e 1). These r e g i o n s i n c l u d e : 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Western Mountain Ranges A l l u v i a l Basins Columbia Lava P l a t e a u C o l o r a d o P l a t e a u and Wyoming B a s i n High P l a i n s N o n g l a c i a t e d C e n t r a l Region G l a c i a t e d C e n t r a l Region Piedmont and B l u e Ridge N o r t h e a s t and S u p e r i o r U p l a n d s A t l a n t i c and G u l f C o a s t a l P l a i n


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11. Southeast C o a s t a l P l a i n 12. A l l u v i a l V a l l e y s 13. Hawaiian I s l a n d s 14. Alaska 15. P u e r t o R i c o and V i r g i n I s l a n d s For t h e purposes o f t h e p r e s e n t system, Region 12 ( A l l u v i a l V a l l e y s ) has been r e i n c o r p o r a t e d i n t o each o f t h e o t h e r r e g i o n s and Region 15 ( P u e r t o R i c o and V i r g i n I s l a n d s ) has been o m i t t e d . S i n c e t h e f a c t o r s t h a t i n f l u e n c e ground-water o c c u r r e n c e and a v a i l a b i l i t y a l s o i n f l u e n c e t h e p o l l u t i o n p o t e n t i a l o f an a r e a , t h i s r e g i o n a l framework i s used t o h e l p f a m i l i a r i z e t h e user w i t h the b a s i c h y d r o g e o l o g i c f e a t u r e s o f t h e r e g i o n . Because p o l l u t i o n p o t e n t i a l cannot be determined on a r e g i o n a l s c a l e , s m a l l e r " h y d r o g e o l o g i c s e t t i n g s " were d e v e l o p e d w i t h i n each of t h e r e g i o n s d e s c r i b e d by Heath (5). These h y d r o g e o l o g i c s e t t i n g s c r e a t e u n i t s which a r e mappable and, a t t h e same time, permit f u r t h e r d e l i n e a t i o n o f t h e f a c t o r s which a f f e c t p o l l u t i o n potential. Each h y d r o g e o l o g i c s e t t i n g i s d e s c r i b e d i n a w r i t t e n n a r r a t i v e s e c t i o n and i l l u s t r a t e d i n a b l o c k diagram. F i g u r e 2 shows t h e format which i s used. The d e s c r i p t i o n s a r e used t o h e l p o r i e n t the user t o t y p i c a l g e o l o g i c and h y d r o l o g i e c o n f i g u r a t i o n s which a r e found i n each r e g i o n and t o h e l p f o c u s a t t e n t i o n on s i g n i f i c a n t parameters which a r e i m p o r t a n t i n p o l l u t i o n p o t e n t i a l assessment. The b l o c k diagram e n a b l e s t h e user t o v i s u a l i z e t h e d e s c r i b e d s e t t i n g by i n d i c a t i n g i t s g e o l o g y , geomorphology and hydrogeology. A s e t o f h y d r o g e o l o g i c s e t t i n g s has been d e v e l o p e d f o r each ground-water r e g i o n . To d a t e , h y d r o g e o l o g i c s e t t i n g s have been i d e n t i f i e d and d e s c r i b e d i n the U n i t e d S t a t e s . A l t h o u g h s i m i l a r h y d r o g e o l o g i c s e t t i n g s may appear i n more than one ground-water r e g i o n , t h e document i s designed s o t h a t once t h e broad geographic a r e a i s l o c a t e d the user does not have t o r e f e r t o o t h e r h y d r o g e o l o g i c s e t t i n g s i n o t h e r r e g i o n s . T h i s means t h a t a l t h o u g h s i m i l a r h y d r o g e o l o g i c s e t t i n g s may appear more than once i n t h e document, they have been t a i l o r e d t o r e f l e c t t h e t y p i c a l h y d r o g e o l o g i c c o n d i t i o n s w i t h i n each i n d i v i d u a l r e g i o n . Factors Affecting P o l l u t i o n P o t e n t i a l I n h e r e n t i n each h y d r o g e o l o g i c s e t t i n g a r e t h e p h y s i c a l c h a r a c t e r i s t i c s which a f f e c t t h e ground-water p o l l u t i o n p o t e n t i a l . Many d i f f e r e n t b i o l o g i c a l , p h y s i c a l and c h e m i c a l mechanisms may a c t i v e l y a f f e c t t h e a t t e n u a t i o n o f a contaminant and, t h u s , t h e p o l l u t i o n p o t e n t i a l o f t h a t system. Because i t i s n e i t h e r p r a c t i c a l nor f e a s i b l e t o o b t a i n q u a n t i t a t i v e e v a l u a t i o n s of i n t r i n s i c mechanisms from a r e g i o n a l p e r s p e c t i v e , i t i s necessary t o l o o k a t the broader parameters which i n c o r p o r a t e t h e many processes. A f t e r a c o m p l e t e e v a l u a t i o n o f many c h a r a c t e r i s t i c s and t h e m a p p a b i l i t y o f the d a t a , t h e most i m p o r t a n t mappable f a c t o r s t h a t c o n t r o l t h e ground-water p o l l u t i o n p o t e n t i a l were determined t o be: D - Depth t o Water R - (Net) Recharge



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F i g u r e 2. Format o f h y d r o g e o l o g i c s e t t i n g . This setting i s i d e n t i c a l t o 10Ba R i v e r A l l u v i u m w i t h Overbank e x c e p t t h a t no s i g n i f i c a n t f i n e - g r a i n e d f l o o d p l a i n d e p o s i t s occupy the stream valley. T h i s r e s u l t s i n s i g n i f i c a n t l y h i g h e r r e c h a r g e and sandy s o i l s a t the s u r f a c e . Water l e v e l s a r e t y p i c a l l y c l o s e r t o the s u r f a c e because banks o f f i n e - g r a i n e d d e p o s i t s a r e not p r e s e n t . Throughout much o f t h i s r e g i o n , t h e r e i s an abundance o f c o u r s e g r a i n e d m a t e r i a l , which l i m i t s t h i s s e t t i n g f o r water s u p p l y . These m a t e r i a l s however, p r o v i d e a good s o u r c e o f r e c h a r g e to the u n d e r l y i n g c o n s o l i d a t e d and s e m i c o n s o l i d a t e d b e d r o c k . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 14.


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- A q u i f e r Media - S o i l Media - Topography ( S l o p e ) - Impact o f the Vadose Zone - ( H y d r a u l i c ) C o n d u c t i v i t y o f the A q u i f e r

These f a c t o r s have been a r r a n g e d t o form t h e acronym, DRASTIC f o r ease o f r e f e r e n c e . W h i l e t h i s l i s t i s not a l l i n c l u s i v e , t h e s e f a c t o r s , i n c o m b i n a t i o n , were determined t o i n c l u d e t h e b a s i c r e q u i r e m e n t s needed t o a s s e s s the g e n e r a l p o l l u t i o n p o t e n t i a l of each h y d r o g e o l o g i c s e t t i n g . The DRASTIC f a c t o r s r e p r e s e n t measurable parameters f o r which d a t a a r e g e n e r a l l y a v a i l a b l e from a v a r i e t y of sources without d e t a i l e d reconnaissance. I t i s r e c o g n i z e d t h a t many o f the f a c t o r s may be c o n s i d e r e d t o be o v e r l a p p i n g . However, g r e a t c a r e has been t a k e n t o t r y t o s e p a r a t e the f a c t o r s f o r purposes o f the d e v e l o p e d system. A c o m p l e t e d e s c r i p t i o n o f the i m p o r t a n t mechanisms c o n s i d e r e d w i t h i n each f a c t o r and a d e s c r i p t i o n o f the s i g n i f i c a n c e o f the f a c t o r s f o l l o w s . A D e s c r i p t i o n o f the DRASTIC F a c t o r s Depth t o Water. The water t a b l e i s the e x p r e s s i o n o f the s u r f a c e below the ground l e v e l where a l l the pore spaces a r e f i l l e d w i t h water. Above the water t a b l e , the pore spaces a r e f i l l e d w i t h water and a i r . The water t a b l e may be p r e s e n t i n any t y p e o f media and may be e i t h e r permanent o r s e a s o n a l . Depth t o water r e f e r s t o the depth t o the water s u r f a c e i n an u n c o n f i n e d a q u i f e r . Depth t o water does not i n c l u d e s a t u r a t e d zones which have i n s u f f i c i e n t p e r m e a b i l i t y t o y i e l d s i g n i f i c a n t enough q u a n t i t i e s of water t o be c o n s i d e r e d an a q u i f e r . T h i s document a l s o c a n be used t o e v a l u a t e u n c o n f i n e d a q u i f e r s . For purposes o f t h i s document, depth t o water r e f e r s t o the top o f the a q u i f e r where the a q u i f e r i s c o n f i n e d . I n t h i s case, depth t o water may i n c l u d e s a t u r a t e d zones above t h e top o f the a q u i f e r . The depth t o water i s i m p o r t a n t p r i m a r i l y because i t d e t e r m i n e s t h e depth o f m a t e r i a l through which a contaminant must t r a v e l b e f o r e r e a c h i n g the a q u i f e r , and i t may h e l p t o determine the amount o f time d u r i n g which c o n t a c t w i t h the s u r r o u n d i n g media i s m a i n t a i n e d . The depth t o water i s a l s o i m p o r t a n t because i t p r o v i d e s t h e maximum o p p o r t u n i t y f o r o x i d a t i o n by a t m o s p h e r i c oxygen. I n g e n e r a l , t h e r e i s a g r e a t e r chance f o r a t t e n u a t i o n t o o c c u r a s t h e depth t o water i n c r e a s e s because deeper water l e v e l s i n f e r longer t r a v e l times. Net Recharge. The p r i m a r y s o u r c e o f ground water i s p r e c i p i t a t i o n which i n f i l t r a t e s t h r o u g h the s u r f a c e o f t h e ground and p e r c o l a t e s to the water t a b l e . Net r e c h a r g e i n d i c a t e s the amount o f v/ater per u n i t a r e a o f l a n d which p e n e t r a t e s t h e ground s u r f a c e and r e a c h e s the water t a b l e . A l t h o u g h t h i s amount o f water may v a r y s e a s o n a l l y or a n n u a l l y , a water b a l a n c e i s u s u a l l y a c h i e v e d o r l o n g - t e r m t r e n d s can be e s t a b l i s h e d . T h i s r e c h a r g e water i s thus a v a i l a b l e t o t r a n s p o r t a contaminant v e r t i c a l l y t o t h e v/ater t a b l e and h o r i z o n t a l l y w i t h i n t h e a q u i f e r . I n a d d i t i o n , the q u a n t i t y t o water a v a i l a b l e f o r d i s p e r s i o n and d i l u t i o n i n the vadose zone and i n t h e s a t u r a t e d zone is c o n t r o l l e d by. this,*parameter. I n a r e a s

American unemicarlSociety Library 1155 18th St., N.W. Garner et al.; Evaluation of Pesticides Ground Water Washington, O.C. in20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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EVALUATION OF PESTICIDES IN G R O U N D WATER

where the a q u i f e r i s u n c o n f i n e d , recharge t o the a q u i f e r u s u a l l y o c c u r s more r e a d i l y and the p o l l u t i o n p o t e n t i a l i s g e n e r a l l y g r e a t e r than i n a r e a s w i t h c o n f i n e d a q u i f e r s . C o n f i n e d a q u i f e r s a r e p a r t i a l l y p r o t e c t e d from c o n t a m i n a n t s i n t r o d u c e d a t the s u r f a c e by l a y e r s of low p e r m e a b i l i t y media ( a q u i t a r d s ) which r e t a r d water movement t o the c o n f i n e d a q u i f e r . In p a r t s of some c o n f i n e d a q u i f e r s , head d i s t r i b u t i o n i s such t h a t movement of v/ater i s through the c o n f i n i n g bed from the c o n f i n e d a q u i f e r i n t o the u n c o n f i n e d a q u i f e r . In t h i s s i t u a t i o n , t h e r e i s l i t t l e o p p o r t u n i t y f o r l o c a l c o n t a m i n a t i o n of the c o n f i n e d a q u i f e r . The p r i n c i p a l recharge a r e a f o r the c o n f i n e d a q u i f e r i s o f t e n many m i l e s away. Many c o n f i n e d a q u i f e r s a r e not t r u l y c o n f i n e d and a r e p a r t i a l l y recharged by m i g r a t i o n of water through the c o n f i n i n g l a y e r s . The more water t h a t l e a k s through, the g r e a t e r the p o t e n t i a l f o r recharge t o c a r r y p o l l u t i o n i n t o the a q u i f e r . Recharge v/ater, then, i s a p r i n c i p a l v e h i c l e f o r l e a c h i n g and t r a n s p o r t i n g s o l i d or l i q u i d contaminants t o the water t a b l e . T h e r e f o r e , the g r e a t e r the recharge, the g r e a t e r the p o t e n t i a l p o l l u t i o n . T h i s i s t r u e u n t i l the amount of recharge i s g r e a t enough t o cause d i l u t i o n of any contaminant which would be i n t r o d u c e d i n t o the system. At t h i s p o i n t , the p o l l u t i o n p o t e n t i a l v/ould cease t o i n c r e a s e and might a c t u a l l y decrease. For purposes of t h i s document, t h i s phenomenon has been acknowledged but the system does not r e f l e c t the d i l u t i o n f a c t o r . One a d d i t i o n a l f a c t o r w h i c h must be c o n s i d e r e d i s augmentat i o n of n a t u r a l recharge through a r t i f i c i a l recharge or by irrigation. When a range f o r net recharge i s a s s i g n e d , these a d d i t i o n a l sources of water must be c o n s i d e r e d . A q u i f e r Media. A q u i f e r media r e f e r s t o the c o n s o l i d a t e d or u n c o n s o l i d a t e d medium which s e r v e s as an a q u f i e r (such as sand and g r a v e l or l i m e s t o n e . An a q u i f e r i s d e f i n e d as a w a t e r - b e a r i n g rock f o r m a t i o n which w i l l y i e l d s u f f i c i e n t q u a n t i t i e s of v/ater f o r use. Water i s h e l d by a q u i f e r s i n the pore spaces of g r a n u l a r and c l a s t i c rock and i n the f r a c t u r e s and s o l u t i o n openings of n o n - c l a s t i c and n o n - g r a n u l a r rock. Rocks which y i e l d v/ater from pore spaces have primary p o r o s i t y ; r o c k s where the water i s h e l d i n openings such as f r a c t u r e s and s o l u l t i o n openings which were c r e a t e d a f t e r the r o c k was formed have secondary p o r o s i t y . The a q u i f e r medium e x e r t s the major c o n t r o l o v e r the r o u t e and path l e n g t h which a contaminant must f o l l o w . The path l e n g t h i s an i m p o r t a n t c o n t r o l ( a l o n g w i t h h y d r a u l i c c o n d u c t i v i t y and g r a d i e n t ) i n d e t e r m i n i n g the time a v a i l a b l e f o r a t t e n u a t i o n p r o c e s s e s such as s o r p t i o n , r e a c t i v i t y and d i s p e r s i o n and a l s o the amount of e f f e c t i v e s u r f a c e a r e a of m a t e r i a l s c o n t a c t e d i n the a q u i f e r . The r o u t e which a contaminant w i l l take can be s t r o n g l y i n f l u e n c e d by f r a c t u r i n g or by any o t h e r f e a t u r e such as i n t e r c o n n e c t e d s e r i e s of s o l u t i o n openings which may p r o v i d e pathways f o r e a s i e r f l o w . In g e n e r a l , the l a r g e r the g r a i n s i z e and the more f r a c t u r e s or openings w i t h i n the a q u i f e r , the h i g h e r the p e r m e a b i l i t y and the l o w e r the a t t e n u a t i o n c a p a c i t y ; c o n s e q u e n t l y the g r e a t e r the p o l l u t i o n p o t e n t i a l . S o i l Media. S o i l media r e f e r s t o t h a t uppermost p o r t i o n of the vadose zone c h a r a c t e r i z e d by s i g n i f i c a n t b i o l o g i c a l a c t i v i t y . For



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purposes o f t h i s document, s o i l i s commonly c o n s i d e r e d t h e upper weathered zone o f t h e e a r t h which a v e r a g e s s i x f e e t o r l e s s . Soil has a s i g n i f i c a n t impact o f t h e amount o f r e c h a r g e which can i n f i l t r a t e i n t o t h e ground and, hence, on t h e a b i l i t y o f a contaminant t o move v e r t i c a l l y i n t o t h e vadose zone. M o r e o v e r , the a t t e n u a t i o n p r o c e s s e s o f f i l t r a t i o n , biodégradation, s o r p t i o n and v o l a t i l i z a t i o n may be q u i t e s i g n i f i c a n t i n t h i s zone. Thus, f o r c e r t a i n o n - l a n d s u r f a c e p r a c t i c e s such a s a p p l i c a t i o n s o f p e s t i c i d e s , s o i l can be a p r i m a r y i n f l u e n c e on p o l l u t i o n p o t e n t i a l . In g e n e r a l , t h e p o l l u t i o n p o t e n t i a l o f a s o i l i s l a r g e l y a f f e c t e d by t h e type o f c l a y p r e s e n t , t h e s h r i n k / s w e l l p o t e n t i a l of t h a t c l a y and t h e s o i l t e x t u r e . The q u a n t i t y o f o r g a n i c m a t e r i a l p r e s e n t i n t h e s o i l may a l s o be a major f a c t o r . I n g e n e r a l , t h e l e s s t h e c l a y s h r i n k s and s w e l l s and t h e f i n e r t h e t e x t u r e , and t h e g r e a t e r the o r g a n i c c o n t e n t , t h e l e s s t h e p o l l u t i o n p o t e n t i a l . S o i l media a r e b e s t d e s c r i b e d by r e f e r r i n g t o t h e b a s i c s o i l types a s c l a s s i f i e d by t h e S o i l C o n s e r v a t i o n S e r v i c e . Topography. As used here, topography r e f e r s t o t h e s l o p e and s l o p e v a r i a b i l i t y o f t h e l a n d s u r f a c e . B a s i c a l l y , topography helps c o n t r o l the l i k e l i h o o d that a p o l l u t a n t w i l l run o f f or remain on t h e s u r f a c e i n one a r e a l o n g enough t o i n f i l t r a t e . T h e r e f o r e , t h e g r e a t e r t h e chance of i n f i l t r a t i o n , t h e h i g h e r t h e p o l l u t i o n p o t e n t i a l a s s o c i a t e d w i t h t h e s l o p e . Topography i n f l u e n c e s s o i l development and t h e r e f o r e has an e f f e c t on a t t e n u a t i o n . Topography i s a l s o s i g n i f i c a n t from t h e s t a n d p o i n t t h a t t h e g r a d i e n t and d i r e c t i o n o f f l o w o f t e n can be i n f e r r e d by water t a b l e c o n d i t i o n s from t h e g e n e r a l s l o p e o f t h e l a n d . T y p i c a l l y , s t e e p e r s l o p e s s i g n i f y h i g h e r ground-water v e l o c i t y . Impact o f Vadose Zone. The vadose zone i s d e f i n e d a s t h a t zone above t h e water t a b l e and below t h e s o i l media which i s u n s a t u r a t e d . For purposes o f t h i s document, t h i s s t r i c t d e f i n i t i o n can be a p p l i e d t o a l l water t a b l e a q u i f e r s . However, when e v a l u a t i n g a c o n f i n e d a q u i f e r , t h e "impact" o f t h e vadose zone i s expanded t o i n c l u d e both t h e vadose zone and a n y s a t u r a t e d zones which o v e r l i e the a q u i f e r . The s i g n i f i c a n t l y r e s t r i c t i v e zone above t h e a q u i f e r which forms t h e c o n f i n i n g l a y e r i s used a s t h e type o f medium which has t h e most s i g n i f i c a n t impact. The type o f vadose zone media d e t e r m i n e s t h e a t t e n u a t i o n c h a r a c t e r i s t i c s o f t h e m a t e r i a l below t h e t y p i c a l s o i l h o r i z o n and above t h e water t a b l e . Biodégradation, n e u t r a l i z a t i o n , m e c h a n i c a l f i l t r a t i o n , c h e m i c a l r e a c t i o n and d i s p e r s i o n a r e a l l p r o c e s s e s which may o c c u r w i t h i n t h e vadose zone w i t h a g e n e r a l l e s s e n i n g o f biodégradation w i t h depth. The media a l s o c o n t r o l t h e path l e n g t h and r o u t i n g , thus a f f e c t i n g time a v a i l a b l e f o r a t t e n u a t i o n and t h e q u a n t i t y o f m a t e r i a l encountered. The r o u t i n g i s s t r o n g l y i n f l u e n c e d by any f r a c t u r i n g present. The m a t e r i a l s a t t h e t o p o f t h e vadose zone a l s o e x e r t an i n f l u e n c e on s o i l development. Hydraulic Conductivity of the Aquifer. Hydraulic c o n d u c t i v i t y r e f e r s t o t h a a b i l i t y o f t h e a q u i f e r m a t e r i a l s t o t r a n s m i t water, which i n t u r n , c o n t r o l s t h e r a t e a t which ground water w i l l f l o w under a g i v e n h y d r a u l i c g r a d i e n t . The r a t e a t which t h e ground w a t e r f l o w s a l s o c o n t r o l s t h e r a t e a t w h i c h a contaminant w i l l be


150


moved away from t h e p o i n t a t which i t e n t e r s t h e a q u i f e r . H y d r a u l i c c o n d u c t i v i t y i s c o n t r o l l e d by t h e amount o f i n t e r c o n n e c t i o n o f v o i d spaces w i t h i n t h e a q u i f e r which may occur a s a consequence o f i n t e r g r a n u l a r p o r o s i t y , f r a c t u r i n g , bedding p l a n e s , e t c . F o r purposes o f t h i s document, h i g h h y d r a u l i c c o n d u c t i v i t i e s a r e a s s o c i a t e d w i t h h i g h e r p o l l u t i o n p o t e n t i a l . T h i s i s because t h e p o l l u t a n t has t h e p o t e n t i a l f o r moving q u i c k l y away from t h e p o i n t i n t h e a q u i f e r where i t i s i n t r o d u c e d . O b v i o u s l y , a wide range o f h y d r a u l i c c o n d u c t i v i t i e s may be p r e s e n t i n a l l a r e a s .


DRASTIC A n u m e r i c a l r a n k i n g system t o a s s e s s ground-water p o l l u t i o n p o t e n t i a l i n h y d r o g e o l o g i c s e t t i n g s has been d e v i s e d u s i n g t h e DRASTIC f a c t o r s . The system c o n t a i n s t h r e e s i g n i f i c a n t p a r t s : w e i g h t s , ranges and r a t i n g s . A d e s c r i p t i o n of t h e t e c h n i q u e used f o r w e i g h t s and r a t i n g s can be found i n Dee e t a l . (15). Weights. Each DRASTIC f a c t o r has been e v a l u a t e d w i t h r e s p e c t t o the o t h e r t o determine t h e r e l a t i v e importance o f each f a c t o r . Each DRASTIC f a c t o r has been a s s i g n e d a r e l a t i v e weight r a n g i n g from 1 t o 5 ( T a b l e I ) . The most s i g n i f i c a n t f a c t o r s have w e i g h t s of 5; t h e l e a s t s i g n i f i c a n t , a weight o f 1. T h i s e x e r c i s e was a c c o m p l i s h e d by an a d v i s o r y committee u s i n g a D e l p h i (consensus) approach. These w e i g h t s a r e a c o n s t a n t and may n o t be changed when u s i n g t h e system.

T a b l e I . A s s i g n e d Weights f o r DRASTIC F e a t u r e s "Reproduced from Ref. 14" Feature

A g r i c u l t u r a l Weight

Depth t o Water T a b l e Net Recharge A q u i f e r Media S o i l Media Topography Impact o f t h e Vadose Zone Hydraulic C o n d u c t i v i t y of the Aquifer Source:

5 4 3 5 3 4 2

Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 14.

Ranges. Each DRASTIC f a c t o r i s d i v i d e d i n t o e i t h e r ranges o r s i g n i f i c a n t media t y p e s which have an impact on p o l l u t i o n p o t e n t i a l ( T a b l e s I I - V I I I ) . The media t y p e s have been a s s i g n e d d e s c r i p t i v e names t o a s s i s t t h e user.


8.

ALLER ET AL.

Table

DRASTIC:

II.


Ranges and R a t i n g s

Depth

t o Water

f o r Depth t o Water

(feet)

Range

Rating


0-5 5-15 15-30 30-50 50-75 75-100 100+ Source: Table

10 9 7 5 3 2 1 Reproduced w i t h p e r m i s s i o n I I I . Ranges and R a t i n g s

Net

Recharge

from r e f e r e n c e 1 4 .

f o r Met Recharge

(inches)

Range

Rating

0-2 2-4 4-7 7-10 10+

1 3 6 8 9

Source:

Table

Reproduced w i t h p e r m i s s i o n


I V . Ranges and R a t i n g s f o r A q u i f e r M e d i a

A q u i f e r Media Range

Rating

Massive Shale Metamorphic/Igneous Weathered Metamorphic/Igneous T h i n Bedded Sandstone, Limestone, S h a l e M a s s i v e Sandstone M a s s i v e Limestone Sand and G r a v e l Basalt K a r s t Limestone Source:

Sequences

Reproduced w i t h p e r m i s s i o n

Typical

Ratin

1- 3 2- 5 3- 5

2 3 4

5-9 4- 9 4-9 4-9 2-10 9-10

6 6 6 8 9 10



152


T a b l e V.

Ranges and R a t i n g s f o r S o i l

Media

S o i l Media


Range

Rating

T h i n o r Absent Gravel Sand Peat S h r i n k i n g and/or Aggregated C l a y Sandy Loam Loam S i l t y Loam C l a y Loam Muck N o n s h r i n k i n g and Nonaggregated C l a y Source:

10 10 9 8 7 6 5 4 3 2 1


Table

V I . Ranges and R a t i n g s f o r Topography

Topography ( p e r c e n t

slope)

Range

Rating

0-2 2-6 6-12 12-18 18+

10 9 5 3 1

Source: T a b l e VIÏ.


Ranges and R a t i n g s f o r Impace o f Vadose Zone Media

Impact o f Vadose Zone Media Range Silt/Clay Shale Limestone Sandstone Bedded Limestone, Sandstone, S h a l e Sand and G r a v e l w i t h s i g n i f i c a n t S i l t and C l a y Metamorphic/Igneous Sand and G r a v e l Basalt K a r s t Limestone Source:

Rating

Typical

Rating

1-2 2-5 2-7 4-8 4-8

1 3 6 6 6

4-8 2-8 6-9 2-10 8-10

6 4 S 9 !P_



8.

ALLER ET AL.

Table VIII.

DR/iSTIC:


153

Ranges and R a t i n g s f o r H y d r a u l i c C o n d u c t i v i t y


H y d r a u l i c C o n d u c t i v i t y (gpd/ft ) Range

Rating

1-100 100-300 300-700 700-1000 1000-2000 2000+

1 2 4 6 8 10

Source:


R a t i n g s . Each range f o r each DRASTIC f a c t o r has been e v a l u a t e d w i t h r e s p e c t t o t h e o t h e r s t o determine t h e r e l a t i v e s i g n i f i c a n c e of each range w i t h r e s p e c t t o p o l l u t i o n p o t e n t i a l . Based on graphs, t h e range f o r each DRASTIC f a c t o r has been a s s i g n e d a r a t i n g which v a r i e s between 1 and 10 ( T a b l e s I I - V I I I ) . The f a c t o r s o f D, R, S, T, and C h a v e been a s s i g n e d one v a l u e p e r r a n g e . A and I have been a s s i g n e d a " t y p i c a l " r a t i n g and a v a r i a b l e r a t i n g . The v a r i a b l e r a t i n g a l l o w s t h e user t o choose e i t h e r a t y p i c a l v a l u e o r t o a d j u s t t h e v a l u e based on more s p e c i f i c knowledge. T h i s system a l l o w s t h e user t o determine a n u m e r i c a l v a l u e f o r any h y d r o g e o l o g i c s e t t i n g by u s i n g an a d d i t i v e model. The e q u a t i o n f o r d e t e r m i n i n g t h e DRASTIC Index i s : DD +RR + A A + S S + T T + 1 1 + C C = Pol lution Potentia1 R V R W RW R W R W R V RW

Where : R = rating W = weight Once a DRASTIC Index has been computed, i t i s p o s s i b l e t o i d e n t i f y a r e a s which a r e l i k e l y t o be s u s c e p t i b l e t o ground v/ater c o n t a m i n a t i o n r e l a t i v e t o one another. The h i g h e r t h e DRASTIC Index, t h e g r e a t e r t h e ground-water p o l l u t i o n p o t e n t i a l . The DRASTIC Index p r o v i d e s o n l y a r e l a t i v e e v a l u a t i o n t o o l and i s n o t designed t o p r o v i d e a b s o l u t e answers. T a b l e IX shows a t y p i c a l index computed f o r t h e h y d r o g e o l o g i c s e t t i n g , lOBa R i v e r A l l u v i u m w i t h o u t Overbank, which i s d e s c r i b e d i n F i g u r e 2. I n c o n t r a s t , T a b l e X and F i g u r e 3 i l l u s t r a t e a v e r y s i m i l a r hydrogeologic s e t t i n g with a p o l l u t i o n p o t e n t i a l that i s s i g n i f i c a n t l y lower. These numbers, a l t h o u g h not unique v a l u e s , can be e v a l u a t e d w i t h r e s p e c t t o one another by knowing t h a t f o r a] 1 h y d r o g e o l o g i c s e t t i n g s e v a l u a t e d i n t h e U n i t e d S t a t e s , DRASTIC I n d i c e s ranged from 53 t o 224. T h i s r e l a t i v e comparison h e l p s t h e user e v a l u a t e p o l l u t i o n p o t e n t i a l w i t h r e s p e c t t c any o t h e r a r e a . In a r e a s o f w i d e l y v a r i a b l e h y d r o g e o l o g y , t h e p o l l u t i o n p o t e n t i a l


154


T a b l e I X — C o m p u t a t i o n o f the DRASTIC Index f o r S e t t i n g 10 B b - R i v e r A l l u v i u m W i t h o u t Overbank D e p o s i t SETTING

10 Bb R i v e r A l l u v i u m Without Overbank D e p o s i t

FEATURE

Net

table

Recharge


A q u i f e r Media Soil

RATING

WEIGHT

RANGE

Depth t o water

AGRICULTURAL

NUMBER

5-15

5

9

45

10+

4

9

36

3

8

24

Sand and G r a v e l

Media

Sand

5

9

45

Topography

0-2$

3

10

30

4

6

24

2

8

16

Impact Vadose zone

S & G w/sig.

Hydraulic Conductivity

Silt

1000-2000

Agricultural D r a s t i c Index Source:


T a b l e X — C o m p u t a t i o n o f t h e DRASTIC Index f o r S e t t i n g A l l u v i u m w i t h Overbank D e p o s i t SETTING

Recharge

A q u i f e r Media

NUMBER

15-30

5

7

35

7-10

4

8

32

3

8

24

5

4

20

3

10

30

Silt/Clay

4

1

4

700-1000

2

6

12

Silty

Topography Impact

RATING

Sand and G r a v e l

Media

Loam

0-2?

Vadose

Hydraulic Conductivity

AGRICULTURAL

WEIGHT

RANGE

Depth t o Water T a b l e

Soil

10Ba-River

10 Ba R i v e r A l l u v i u m With Overbank D e p o s i t

FEATURE

Net

220

Agricultural D r a s t i c Index 157 SOURCE:




8.

A L L E R ET AL.

DRASTIC:


155

F i g u r e 3. Sample h y d r o g e o l o g i c s e t t i n g i n A t l a n t i c and G u l f Coastal Plain. T h i s h y d r o g e o l o g i c s e t t i n g i s c h a r a c t e r i z e d by low topography and t h i n t o m o d e r a t e l y t h i c k d e p o s i t s o f a l l u v i u m along parts o f r i v e r v a l l e y s . The a l l u v i u m i s u n d e r l a i n by c o n s o l i d a t e d and s e m i c o n s o l i d a t e d s e d i m e n t a r y r o c k s . Water i s o b t a i n e d from sand and g r a v e l l a y e r s which a r e i n t e r b e d d e d w i t h finer-grained a l l u v i a l deposits. The f l o o d p l a i n i s c o v e r e d by varying thicknesses o f fine-grained, s i l t y deposits called overbank d e p o s i t s . The overbank t h i c k n e s s i s u s u a l l y g r e a t e r a l o n g major streams (as much as 40 f e e t ) and t h i n n e r a l o n g minor streams. P r e c i p i t a t i o n i n t h e r e g i o n i s abundant, b u t r e c h a r g e i s reduced because o f t h e s i l t y s o i l s which t y p i c a l l y c o v e r t h e surface. Water l e v e l s a r e t y p i c a l l y m o d e r a t e l y s h a l l o w . The a l l u v i u m may s e r v e as a s i g n i f i c a n t s o u r c e o f water and may be i n d i r e c t h y d r a u l i c connection with the u n d e r l y i n g sedimentary r o c k s . The a l l u v i u m may a l s o s e r v e as a s o u r c e o f r e c h a r g e t o t h e u n d e r l y i n g bedrock. Many streams i n t h i s s e t t i n g p r o v i d e o n l y f i n e - g r a i n e d d e p o s i t s ( s i l t s and c l a y s ) and as such do n o t form good a q u i f e r s . They s t i l l , however, p r o v i d e a good s o u r c e o f r e c h a r g e . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 14.


156


may a l s o v a r y w i d e l y w i t h an a s s o c i a t e d spread o f DRASTIC I n d i c e s . I n a r e a s v/ith more s u b t l e changes i n h y d r o g e o l o g y , t h e DRASTIC I n d i c e s would r e f l e c t more s u b t l e changes. The system does n o t attempt t o d e f i n e "good" o r "bad" a r e a s , but s i m p l y o f f e r s t h e user a t o o l t o e v a l u a t e t h e r e l a t i v e p o l l u t i o n p o t e n t i a l o f whatever a r e a s a r e d e s i r e d . The user may wish t o then c o n s i d e r a d d i t i o n a l f a c t o r s such as importance o f the a q u i f e r p o p u l a t i o n o r o t h e r f a c t o r s i n f u l l y a s s e s s i n g t h e importance o f p o l l u t i o n p o t e n t i a l i n any a r e a .


S t a t u s o f the P r o j e c t T h i s paper summarizes t h e e f f o r t s t o produce a s t a n d a r d i z e d system to e v a l u a t e ground water p o l l u t i o n p o t e n t i a l . The p r o j e c t was designed t o span two years. During t h e f i r s t year, t h e methodology was d e v e l o p e d w i t h t h e h e l p o f an a d v i s o r y committee. The second phase o f t h e p r o j e c t comprises a year o f t e s t i n g o f the system through a s e r i e s o f demonstration mapping p r o j e c t s conducted i n t e n c o u n t i e s throughout t h e u n i t e d S t a t e s . These c o u n t i e s a r e being chosen f o r t h e i r wide v a r i a b i l i t y i n h y d r o g e o l o g y and t h e i r p o s i t i o n i n d i f f e r e n t ground-water r e g i o n s . During t h i s v e r i f i c a t i o n s t a g e , t h e system w i l l be expanded and changed t o more f u l l y i n c o r p o r a t e t h e e l e m e n t s which w i l l make t h e DRASTIC methodology most u s e f u l t o u s e r s . Conculsions I t i s e v i d e n t t h a t a l l o f t h e DRASTIC parameters a r e i n t e r a c t i n g , dependent v a r i a b l e s . T h e i r s e l e c t i o n i s based on a v a i l a b l e data q u a n t i t a t i v e l y d e v e l o p e d and r i g o r o u s l y a p p l i e d , but on a s u b j e c t i v e understanding of " r e a l world" c o n d i t i o n s a t a g i v e n a r e a . The v a l u e o f t h e DRASTIC parameters i s i n t h e f a c t t h a t they a r e based on i n f o r m a t i o n t h a t i s r e a d i l y a v a i l a b l e f o r most p o r t i o n s of t h e U n i t e d S t a t e s , and which can be o b t a i n e d and m e a n i n g f u l l y mapped i n a minimum o f time and a t minimum c o s t . The DRASTIC r a n k i n g scheme can then be a p p l i e d by e n l i g h t e n e d laymen for v a l i d comparative e v a l u a t i o n s with acceptable r e s u l t s . Acknowledgments The work was funded by t h e U n i t e d S t a t e s E n v i r o n m e n t a l P r o t e c t i o n Agency through t h e Robert S. K e r r Research L a b o r a t o r y , Ada, Oklahoma. The a u t h o r s hereby extend t h e i r thanks t o Jack K e e l e y and J e r r y T h o r n h i l l o f t h e U.S. EPA f o r t h e i r a s s i s t a n c e d u r i n g the p r o j e c t . G r a t e f u l acknowledgment o f t h e c o n t r i b u t i o n s o f a v e r y a b l e a d v i s o r y committee i s a l s o made: M i c h a e l Apgar, De lav/are Department o f N a t u r a l Resources J i m Bachmaier, U.S. EPA, O f f i c e o f S o l i d Waste W i l l i a m Back, USGS Harvey Banks, C o n s u l t i n g E n g i n e e r , I n c . Truman Bennett, Bennett & W i l l i a m s , I n c . Robert Ξ. Bergstrom, I l l i n o i s S t a t e G e o l o g i c a l Survey Stephen Born, U n i v e r s i t y o f W i s c o n s i n Keros C a r t w r i g h t , I l l i n o i s S t a t e G e o l o g i c a l S u r v e y



8.

ALLER ET AL.

DRASTIC:


S t u a r t Cohen, U.S. EPA, O f f i c e o f P e s t i c i d e Programs S t e v e CorcHe, U.S. EPA, O f f i c e o f Research & Development George H. D a v i s , USGS, r e t i r e d Stan D a v i s , U n i v e r s i t y o f A r i z o n a A r t Day, U.S. EPA, Land D i s p o s a l Branch, O f f i c e o f S o l i d Waste N o r b e r t Dee, U.S. EPA, O f f i c e o f Ground Water P r o t e c t i o n Donald A. Duncan, South C a r o l i n a Department o f H e a l t h and Environmental Control C a t h e r i n e E i d e n , U.S. EPA, O f f i c e o f P e s t i c i d e Programs G r o v e r Emrich, SMC M a r t i n , I n c . G l e n G a l e n , U.S. EPA, Land D i s p o s a l Branch, O f f i c e o f S o l i d Waste P h y l l i s Carman, C o n s u l t a n t , Tennessee J i m Gibb, I l l i n o i s S t a t e Water S u r v e y Todd G i d d i n g s , Todd G i d d i n g s & A s s o c i a t e s , I n c . R a l p h Heath, USGS, r e t i r e d Ron H o f f e r , U.S. EPA, O f f i c e o f Ground Water P r o t e c t i o n George Hughes, O n t a r i o M i n i s t r y o f t h e Environment Jack K e e l e y , U.S. EPA, K e r r E n v i r o n m e n t a l Research Laboratory J e r r y Kotas, U.S. EPA, O f f i c e o f Waste Programs Enforcement Harry LeGrand, C o n s u l t a n t , North C a r o l i n a Fred L i n d s e y , U.S. EPA, Waste Managment and Economics Division M a r t i n M i f f l i n , U n i v e r s i t y o f Nevada P a u l a Mugnuson, Geraghty & M i l l e r , I n c . W a l t e r M u l i c a , IEP, I n c John Osgood, P e n n s y l v a n i a Bureau o f Water Q u a l i t y Wayne P e t t y j o h n , Oklahoma S t a t e U n i v e r s i t y Paul Roberts, Stanford U n i v e r s i t y J a c k Robertson, Weston D e s i g n e r s & C o n s u l t a n t s Dave S e v e r n , U.S. EPA, Hazard E v a l u a t i o n D i v i s i o n J e r r y T h o r n h i l l , U.S. EPA, K e r r Research Center Frank T r a i n e r , USGS, r e t i r e d Warren Wood, USGS

Literature Cited

1. Joel, A. H. In "Soil Classification"; C. W. Finkl, Jr., Ed.; Hutchinson Ross Publishing Co.: Stroudsburg, Pennsylvania, 19S2, pp. 52-59. 2. Davis, S. N.; DeWiest, R. J. "Kydrogeology"; John Wiley & Sons: New York, 1966, pp. 1-463. 3. Meinzer, O. E. "Outline of Ground-Water Hydrology"; USGS WATER SUPPLY PAPER No. 494, United States Geological Survey: Washington, D.C., 1923; pp. 1-71. 4. Thomas, H. E. "Ground Water Regions of the United States - Their Storage Facilities"; Interior and Insular Affairs Committee, U.S. House of Representatives: Washington, D.C., 1952; pp. 1-76. 5. Heath, R. C. "Ground Water Regions of the United States"; USGS WATER SUPPLY PAPER No. 2242, United States Geological Survey: Washington, D.C., 1984; pp. 1-78. 6. Blank, H. R.; Schroeder, M.C. Ground Water 1973, 11, 2, 3-5. Garner et al.; Evaluation of Pesticides in Ground Water ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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7. LeGrand, H. E., 1983. "A Standardized System for Evaluating Waste-Disposal Sites"; National Water Well Association: Worthington, Ohio, 1983; pp. 1-49. 8. "Surface Impoundment Assessment National Report," United States Environmental Protection Agency, 1983, No. 570/984/002. 9. Thornthwaite, S. W.; Mather, J. R. "Instructions and Tables for Computing Potential Evapotranspiration and the Water Balance"; Drexel Institute of Technology: Centerton, New Jersey, 1957; Vol. 10, No. 3, pp. 1-311. 10. Fenn, D. G.; Hanley, K. J.; DeGeare, T. V. "Use of the Water Balance Method for Predicting Leachate Generation from Solid Waste Disposal Sites"; U.S. EPA Report No. 168, United States Protection Agency: Cincinnati, Ohio, 1975, pp. 1-40. 11. Gibb, J. P.; Barcelona, M. J.; Schock, S.C.;Hampton, M. W. "Hazardous Waste in Ogle and Winnebago Counties: Potential Risk Via Ground Water Due to Past and Present Activities"; Illinois Department of Energy and Natural Resources Document No. 83/26, State of Illinois: Champaign, Illinois, 1983, pp. 1-66. 12. "Site Assessment System (SAS) for the Michigan Priority Ranking System Under the Michigan Environmental Response Act," Michigan Department of Natural Resources, 1983. 13. Seller, L. E.; Canter, L. W. "Summary of Selected Ground Water Quality Impact Assessment Methods"; NCGWR Report No. 80-3, National Center for Ground Water Research: Norman, Oklahoma, 1980, pp. 1-142. 14. Aller, L.; Bennett, T.; Lehr, J. H.; Petty, R. J. "DRASTIC: A Standardized System for Evaluating Ground V/ater Pollution Potential Using Hydrogeologic Settings"; USEPA No. 600/2-85/ 018, United States Environmental Protection Agency: Ada, Oklahoma, 1985; pp. 1-163. 15. Dee, N.; Baker, J.; Drobny, N.; Duke, K.; Whitman, I.; Fahringer, D. Water Resources Research 1973, 9, 3, 523535. RECEIVED April 7, 1986


Evaluation of Pesticides in Ground Water - American Chemical Society

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