Ion and Acid Budgets for a Forested Atlantic Coastal Plain Watershed and Their Implications for the Impacts of Acid Deposition

1985; and t h e pH o f b u l k p r e c i p i t a t i o n and s t ream w a t e r f r om 1975 th rough 1984. We c a l c u l a t e d cha rge ba lances f o r p r e c i p i t a t i o n and s t ream w a t e r and used t h e mass ba lances between i n p u t s and o u t p u t s t o c o n s t r u c t an average annua l a c i d budget f o r t h e watershed and t o draw i n f e r e n c e s about t h e f a t e and impacts o f a c i d d e p o s i t i o n . The mean pH o f b u l k p r e c i p i t a t i o n even ts was 4.14 ( range 3.41-6.11) f o r 1981-1984 w h i l e t h e mean pH o f week ly s t ream samples was 5.17 ( range 4.156.37). A t t h e 90% c o n f i d e n c e l e v e l , t h e average annual pH o f b o t h p r e c i p i t a t i o n and s t ream w a t e r d e c l i n e d s i g n i f i c a n t l y between 1975 and 1984 and t h e pH o f s t ream w a t e r was s i g n i f i c a n t l y c o r r e l a t e d w i t h p r e c i p i t a t i o n pH ( r 2 = 0.44, P = 0.06). Bo th p r e c i p i t a t i o n and s t ream w a t e r had a s l i g h t excess o f an ions ove r c a t i o n s , b u t t h e d i sc repancy was o n l y 4.3% f o r p r e c i p i t a t i o n and 1.5% f o r s t ream wa te r . B u l k p r e c i p i t a t i o n was p r i m a r i l y a d i l u t e s o l u t i o n o f n i t r i c and s u l f u r i c a c i d s w i t h s m a l l e r amounts o f ammonium, c h l o r i d e , and m e t a l l i c c a t i o n s , w h i l e s t ream w a t e r was p r i m a r i l y a s o l u t i o n o f m e t a l l i c s u l f a t e s and c h l o r i d e s . The d i f f e r e n c e s r e f l e c t t h e n e a r l y comple te r e t e n t i o n o f ammonium, hydron ium, and n i t r a t e i o n s ; t h e p a r t i a l r e t e n t i o n o f s u l f a t e ; and a n e t l o s s o f m e t a l l i c c a t i o n s f r o m t h e watershed. -1 -1 An average o f 1102 eq ha y r of a c i d was n e u t r a l i z e d i n t h e watershed. Of t h i s , 76% e n t e r e d as hydron ium i o n s i n p r e c i p i t a t i o n , 21% was genera ted by ecosystem processes t h a t r e t a i n e d t h e ammonium i o n s i n p r e c i p i t a t i o n , and 3% came f r o m d i s s o c i a t i o n o f c a r b o n i c a c i d w i t h i n t h e ecosystem. T o t a l a c i d neut r a l i z a t i o n ev idenced by t h e mass ba lances o f o t h e r i o n s was 1048 eq h a I 1 y r , o f wh ich 42% was f rom r e l e a s e o f m e t a l l i c c a t i o n s , 38% f r o m r e t e n t i o n o f n i t r a t e , 20% f rom r e t e n t i o n o f s u l f a t e , and <1% f rom r e t e n t i o n o f phosphate. The watershed ecosystem n e u t r a l i z e d 98% o f t h e a c i d i t y i n b u l k p r e c i p i t a t i o n , b u t t h e remainder was s u f f i c i e n t t o a c i d i f y t h e e f f l u e n t stream. Because o f n i t r a t e r e t e n t i o n w i t h i n t h e ecosystem, t h e n i t r i c a c i d i n p r e c i p i t a t i o n was e f f e c t i v e l y s e l f n e n t r a l i z i n g and d i d n o t a f f e c t s t ream chemis t r y . S u l f u r i c a c i d was t h e a c i d i c p o l l u t a n t r e s p o n s i b l e f o r c a t i o n l e a c h i n g and s t ream a c i d i f i c a t i o n . N a t u r a l a c i d sources ( c a r b o n i c and o r g a n i c a c i d d i s s o c i a t i o n ) c o n t r i b u t e d l e s s t h a n 3% o f t h e t o t a l a c i d budget . The t o t a l l o s s r a t e of a l l m e t a l l i c c a t i o n s was n o t a l a r m i n g c o n s i d e r i n g t h e s i z e o f s o i l p o o l s and rep1 e n i shment f r o m wea the r i ng ; however, c a l c i um i s p r e s e n t i n t h e s o i l o n l y i n t r a c e amounts. I f p r e s e n t l e a c h i n g r a t e s c o n t i n u e w i t h no r e p lacement , 36% o f t h e s o i l c a l c i u m poo l wou ld be exhausted i n 70 y e a r s . Obse rved l e v e l s o f a c i d and d i s s o l v e d aluminum i n t h e e f f l u e n t s t ream a r e w i t h i n t h e ranges t h a t cause adverse changes i n a q u a t i c communit ies. E x i s t i n g maps o f p o t e n t i a l a c i d d e p o s i t i o n impacts do n o t c o r r e c t l y i d e n t i f y o u r s i t e as a c i d s u s c e p t i b l e . Such maps shou ld be i n t e r p r e t e d w i t h c a r e u n t i 1 more a c c u r a t e p r e d i c t i o n s a r e a v a i l a b l e . INTRODUCTION Much r e c e n t resea rch has been d i r e c t e d toward e v a l u a t i n g t h e impac t o f a tmosphe r i c d e p o s i t i o n o f n i t r i c and s u l f u r i c a c i d s on t e r r e s t r i a l and a q u a t i c systems (see rev iews i n ERL, 1983; OTA, 1985). A c i d d e p o s i t i o n ( " a c i d r a i n " ) can be h a r m f u l , u n i m p o r t a n t , o r even b e n e f i c i a l t o ecosytems ( L i k e n s e t a l . , 1977; Johnson e t a1 ., 1982; L i n t h u r s t , 1984; P a e r l , 1985) . F o r a t e r r e s t r i a l ecosystem and i t s r e c e i v i n g wa te rs , t h e impact of a c i d d e p o s i t i o n depends on t h e geo logy, s o i l s , and eco logy o f t h e system; t h e r e f o r e , assessments must be made f o r i n d i v i d u a l systems on a t o t a l ecosystem b a s i s (Johnson e t a l . , 1982; OTA, 1985). We a r e s t u d y i n g t h e e f f e c t s of a c i d d e p o s i t i o n a t t h e Smi thson ian I n s t i t u t i o n ' s Rhode R i v e r s tudy s i t e on t h e wes te rn sho re o f t h e Chesapeake Bay, where t h e s o i l s , v e g e t a t i o n , and l a n d uses a r e t y p i c a l f o r t h e c o a s t a l p l a i n o f sou the rn Mary land. The impac ts o f a c i d d e p o s i t i o n on c o a s t a l f o r e s t s a r e i m p o r t a n t because t h e f o r e s t s a r e i n h e r e n t l y v a l u a b l e and because t e r r e s t r i a l systems c o n t r o l t h e impact of a c i d d e p o s i t i o n on e c o l o g i c a l l y and economi c a l l y i m p o r t a n t r e c e i v i n g wa te rs , such as t h e Chesapeake Bay and i t s t r i b u t a r i e s . We a p p l y t h e sma l l watershed t e c h n i q u e ( L i k e n s aJ., 1977) t o i n f e r t h e mechanisms o f n e u t r a l i z a t i o n o f a c i d d e p o s i t i o n by comparing t h e i o n i c i n p u t s i n p r e c i p i t a t i o n t o t h e i o n i c o u t p u t s i n t h e s t ream d i s c h a r g e from a sma l l m o n i t o r e d watershed. A comple te a c c o u n t i n g o f a l l i o n s i s needed because hydron ium i o n i s i n v o l v e d i n most b i o l o g i c a l and g e o l o g i c a l r e a c t i o n s t h a t p ro duce o r consume o t h e r i o n s ( D r i s c o l l and L i k e n s , 1982). We c o n s t r u c t an a c i d budget t o e v a l u a t e t h e r e l a t i v e impor tance o f d i f f e r e n t pathways o f a c i d neut r a l i z a t i o n , and c o n s i d e r t h e i m p l i c a t i o n s o f t h e budget f o r t h e impacts o f a c i d d e p o s i t i o n on t h e f o r e s t and on a q u a t i c communi t ies i n t h e r e c e i v i n g wat e r s . METHODS Study s i t e The Rhode R i v e r d r a i n a g e b a s i n i s p a r t o f t h e i n n e r m i d A t l a n t i c c o a s t a l p l a i n and i s l o c a t e d on t h e w e s t e r n s h o r e o f Chesapeake Bay a t 3B053'N, 76'35'W (20 km sou th o f Annapo l i s , Mary land ) . The mean annual r a i n f a l l from a 160-year weather r e c o r d i s 108 cm, and t h i s r a i n f a l l i s d i s t r i b u t e d e v e n l y t h roughou t t h e y e a r (Higman and C o r r e l l , 1982) . The mean January and J u l y t empera tu res a r e 1.6 and 25.2OC, r e s p e c t i v e l y (Higman and C o r r e l l , 1982). We mon i to red t h e i n p u t s and o u t p u t s f r om a 6.3 ha subwatershed r e f e r r e d t o as watershed 110. The average s l o p e i s 8.3% and t h e s o i l s a r e sandy loams f r o m t h e Eocene Nanjemoy f o r m a t i o n ( C o r r e l l e t a1 ., 1984) . Lower e l e v a t i o n s o i l s a r e f r om t h e Keypo r t s e r i e s ; i n t e r m e d i a t e e l e v a t i o n s o i l s a r e from t h e Howel 1 and Don lon ton s e r i e s ; and upper e l e v a t i o n s o i 1s a r e f r o m t h e Monmouth, Ade lph ia , and C o l l i n g t o n s e r i e s ( K i r b y and Matthews, 1973) . I n t h e modern s o i l c l a s s i f i c a t i o n , a l l t h e s e s e r i e s a r e u l t i s o l s ; and t h e Keypo r t , Don lon ton , and Ade lph ia s e r i e s a r e a q u i c h a p l u d u l t s w h i l e t h e Howel l , Monmouth, and C o l l i n g t o n s e r i e s a r e t y p i c h a p l u d u l t s ( K i r b y and Matthews, 1973) . The average p a r t i c l e c o m p o s i t i o n o f t h e watershed s o i l s i s 46% sand, 39% s i l t , and 1 5 W l a y ( P i e r c e , 1982) . F o r 60 cm s o i l co res c o l l e c t e d i n a l l seasons f r o m d i f f e r e n t s i t e s on watershed 110, t h e mean pH was 4.9 and s o i l o r g a n i c m a t t e r c o n t e n t was 4.2% ( C o r r e l l , 1982). The watershed i s u n d e r l a i n by an imperv ious c l a y l a y e r ( M a r l b o r o C l a y ) wh i ch fo rms an e f f e c t i v e a q u i c l u d e and a l l o w s w a t e r d i s c h a r g e t o be measured by a V-notch w i e r on t h e ephemeral s t ream d r a i n i n g t h e b a s i n ( C o r r e l l , 1977; C h i r l i n and S c h a f f n e r , 1977). Bedrock i s l o c a t e d hundreds o f meters below t h e a q u i c l u d e (O t ton , 1955; C o r r e l l , 1977) and does n o t a f f e c t t h e c h e m i s t r y o f s u r f a c e wa te r . Whigham (1984) d e s c r i b e d t h e v e g e t a t i o n as a mixed spec ies , b road lea f dec iduous f o r e s t dominated by w h i t e , sou the rn red, and b l a c k oaks (Quercus a lba , Q. f a l c a t a , Q. v e l u t i n a ) ; p i g n u t and mockernut h i c k o r i e s (Carya g l a b r a , C. tomentosa) ; t u l i p p o p l a r ( L i r i o d e n d r o n t u l i p i f e r a ) ; and sweetgum ( L i q u i d ambar s t y r a c i f l u a ) . The w e l l deve loped u n d e r s t o r y i s dominated by dogwood