Elements, Oxides, Silicates High Pressure Phases With Implications for the Earth's Interior

years o r decades . W h e n mode l s a r e calibrat­ ed , it genera l ly is a s s u m e d tha t t h e aqui fe r system is in a na tu ra l s teady state cond i t ion p r io r to h u m a n inf luence. P a p e r s p r e s e n t e d d u r i n g this sympos ium d e m o n s t r a t e t h e falla­ cy of this a s sumpt ion in m a n y s i tuat ions. Many of t h e talks s h o w e d tha t g r o u n d w a t e r flow systems can c h a n g e dynamical ly over geologic t ime in r e s p o n s e to c h a n g i n g b o u n d ­ ary condi t ions , hydrau l i c stresses, o r aqu i fe r p rope r t i e s . Such c h a n g e s can be caused by longte rm climatic changes , sea level changes , glacial load ing o r u n l o a d i n g , tectonic forces, mine ra l dissolut ion o r p rec ip i ta t ion , a n d d e ­ posit ion o r e ros ion of s ed imen t s . Each of these processes was a d d r e s s e d by o n e o r m o r e speakers . P e r h a p s t h e single mos t i m p o r t a n t a n d pe r ­ vasive t h e m e was tha t m o d e r n g r o u n d w a t e r systems may still be u n d e r g o i n g slow t r an ­ sient r e sponses to c h a n g e s in stresses tha t now a p p e a r cons tan t o r to processes tha t a r e n o longer active. T h i s m a y be mani fes ted by a n o m a l o u s hydrau l i c h e a d s o r by inconsis ten­ cies be tween m o d e r n hyd rau l i c g r ad i en t s a n d the d is t r ibut ions of solutes , i sotopes , o r t em­ p e r a t u r e . For e x a m p l e , several p a p e r s showed tha t t he p r e s e n t posi t ion of t h e fresh­ water-sal twater in ter face (or t rans i t ion zone) in coastal aquifers o n b o t h th% /3tlantic a n d Pacific coasts of the U n i t e d States a r e n o t now in equ i l ib r ium with sea level. F r e shwa te r ex tends f a r t he r of fshore t h a n can be ex­ pla ined by p re sen t -day sea level, a n d the in­ terface reflects a l ong t e rm ave rage sea level at abou t 1 5 3 0 m lower t h a n p r e sen t . Conse ­ quent ly , t he in ter face is m i g r a t i n g na tura l ly toward t he land . T h i s type of n a t u r a l t r an ­ sient c h a n g e m u s t be recognized a n d differ­ en t ia ted f rom artificially i n d u c e d effects if ac­ cura te p red ic t ions of fu tu r e c h a n g e s in g r o u n d w a t e r flow a n d quali ty a r e to be m a d e . Several p a p e r s focused o n the wes te rn Uni t ed States, w h e r e geologically r ecen t cli­ matic changes have a l t e red r e c h a r g e p a t t e r n s to aquifers . T h e c h a n g e in r e c h a r g e , in t u r n , causes changes in t he flow systems, which t hen affect wa te r qual i ty. O t h e r p a p e r s dis­ cussed l o n g e r t e r m geological processes relat­ ed to s ed imen ta t i on a n d e ros ion . Eros iona l u n l o a d i n g a n d t o p o g r a p h i c c h a n g e s can have s t rong influences o n g r o u n d w a t e r flow pat­ t e rns in low-permeabi l i ty e n v i r o n m e n t s . T h e r e , significant u n d e r p r e s s u r i n g is caused by ad jus tmen t t imes o n t h e o r d e r of several millions of years . Converse ly , h i g h depos i t ional ra tes a n d s e d i m e n t c o m p a c t i o n , such as occurs in t h e Gu l f Coas t basin , c an lead to significant o v e r p r e s s u r i n g . I n ce r ta in cases, it m igh t be possible for a h i g h t e m p e r a t u r e ba­ salt in t rus ion to i n d u c e p r e s s u r e anomal i e s in sed iments tha t a r e r e t a ined for t h o u s a n d s of years af ter t h e in t rus ion ' s h e a t has d iss ipa ted . Some of t h e o t h e r p a p e r s n o t e d t h e poten t ia l ­ ly i m p o r t a n t ro le of g r o u n d w a t e r flow in con­ trol l ing t h e mig ra t i on a n d a c c u m u l a t i o n of oil. U n d e r s t a n d i n g t h e evolu t ion of g r o u n d ­ water flow t h r o u g h p e t r o l e u m sou rce b e d s a n d t r aps can lead to i m p r o v e m e n t s in exp lo ­ ra t ion a n d d e v e l o p m e n t s t ra tegies . A c o m m o n fea tu re a m o n g mos t of t h e r e ­ p o r t e d s tudies was t h e use of n u m e r i c a l s imu­ lation mode l s to aid in analysis. G r o u n d w a t e r flow, so lu te t r anspor t , a n d h e a t t r a n s p o r t mode l s were consistently u s e d to s imula te processes over p e r i o d s of t h o u s a n d s of years to mill ions of years . In s u m m a r y , t h e p a p e r s d e m o n s t r a t e d tha t u n d e r s t a n d i n g l o n g t e r m climatic a n d geolog­ ic processes can aid in t h e analysis of g r o u n d ­ water flow systems a n d h e l p exp la in obse rved anomal ies in h e a d a n d wa te r qual i ty. C o n ­ versely, a k n o w l e d g e a n d u n d e r s t a n d i n g of t he dynamics a n d evolu t ion o f g r o u n d w a t e r flow systems m a y h e l p in exp l a in ing o r in te r ­ p r e t i n g cer ta in geologic f ea tu res a n d process­ es, such as d iagenes is , o r e depos i t ion , p e r m e ­ ability, a n d poros i ty e n h a n c e m e n t , a n d p e t r o ­ l e u m migra t ion . This report was contributed by Leonard F. Konikow, U.S. Geological Survey, Reston, Va. and Donald I. Siegel, Department of Geology, Syracuse University, Syracuse, N.Y.