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The Mountain Geologist v.3, 1966p l a n t s is f i r s t collected a n d allowed t o s e t t l e i na two-hundred-million-gallon w a s t e - s e t t l i n gb a s i n t h a t i s s e a l e d with a n a s p h a l t i c m e m b r a n e t o p r e v e n t s e e p a g e . It is t h e n f l o c c u l a t e da n d c l a r i f i e d . Next i t i s f i l t e r e d t o l e s s t h a n20 p a r t s p e r m i l l i o n of s u s p e n d e d s o l i d s l e s st h a n 5 m i c r o n s i n d i a m e t e r . It i s sterilizeda n d m o n i t e r e d f o r b a c t e r i a , t h e n pumped Intothe well. F o u r 130-horsepower posltivedisplacement electric pumps a r e available.N o r m a l l y , two o r t h r e e pumps a r e u s e d .The f i r s t contaminated waste was injectedi n t o t h e well d u r i n g M a r c h 1962, when 4 . 2m i l l i o n gallons of w a s t e w e r e i n j e c t e d i n t o t h ew e l l . T h e monthly v o l u m e of w a s t e i n j e c t e di n t o t h e w e l l is shown i n the l o w e r half off i g u r e 5 . D u r i n g the fir s t y e a r of o p e r a t i o n ,c o n s i d e r a b l e t r o u b l e w a s e x p e r i e n c e d with thef i l t e r plant w i t h the r e s u l t t h a t t h e i n j e c t i o nw e l l w a s often shut down f o r a few d a y s o rw e e k s a t a t i m e . F r o m M a r c h 1962 u n t i lS e p t e m b e r 1963 the m a x l r n u m i n j e c t i o n p r e s s u r ei s r e p o r t e d t o h a v e b e e n a b o u t 550 pounds, w i t ha f l u i d injection r a t e of 200 gallons p e r m i n u t e .At t h e end of S e p t e m b e r 1963 the i n j e c t i o nw e l l w a s s h u t down, and n o fluid w a s i n j e c t e duntil o p e r a t i o n s w e r e r e s u m e d 17 S e p t e m b e r1964. D u r i n g t h e shut-down p e r i o d , s u r f a c ee v a p o r a t i o n , f r o m the s e t t l i n g b a s i n , w a ss u f f i c i e n t t o handle the plant output.F r o m 17 S e p t e m b e r 1964 u n t i l t h e end ofM a r c h 1965, injection o p e r a t i o n s w e r e r e s u m e dby g r a v i t y d i s c h a r g e i n t o the w e l l . No w e l l head p r e s s u r e w a s n e c e s s a r y t o I n j e c t t h em a x i m u m of 2 . 4 million gallons of w a s t e p e rm o n t h i n t o t h e well.Beginning i n A p r i l 1965 l a r g e r q u a n t i t i e sof fluid w e r e injected. T h e f i l t e r plant o p e r a t e de f f i c i e n t l y , a n d fluid w a s u s u a l l y i n j e c t e d 16 t o24 h o u r s d a i l y . During A p r l l a n d May a m a x i m u m p u m p p r e s s u r e of 800 pounds w a s r e q u i r e d .F r o m June t o t h e end of S e p t e m b e r 1965 am a x i m u m p r e s s u r e of 1 , 0 5 0 pounds w a s r e q u i r e d t o i n j e c t 300 g a l l o n s p e r m i n u t e i n t o t h ewellT H E DENVER EARTHQUAKEST h e U. S. C o a s t a n d Geodetic S u r v e y r e p o r t s t h a t on 7 November 1882 a n e a r t h q u a k ew a s f e l t i n t h e Denver, L o u i s v i l l e , G e o r g e town, a n d S. E. Wyomlng a r e a (Wang, 1965).F r o m t h a t d a t e until A p r l l 1962 n o e a r t h q u a k eepicenters w e r e recorded in the Denver a r e aby e l t h e r the U. S. C o a s t and G e o d e t i c S u r v e yo r by t h e R e g i s C o l l e g e S e i s m o l o g i c a l O b s e r v a t o r y , l o c a t e d t e n m i l e s s o u t h e a s t of the RockyMountaln A r s e n a l w e l l ( J o s e p h V. Downey,p e r s o n a l c o m m u n i c a t i o n , 1965).D u r i n g tVle p e r i o d f r o m A p r i l 1962 t o t h eend of S e p t e m b e r 1965, 710 e a r t h q u a k e s w e r er e c o r d e d with e p i c e n t e r s i n t h e vicinity of t h eA r s e n a l a t the C e c i l H. G r e e n O b s e r v a t o r y ,B e r g e n P a r k , C o l o r a d o , o p e r a t e d by t h eC o l o r a d o School of M i n e s ( P a n , 1964, Wang,1965, J o n e s , 1965, M i n e s M a g a z i n e , 1965).T h e t o t a l n u m b e r of e a r t h q u a k e s r e p o r t e di n t h e D e n v e r a r e a is plotted i n the u p p e r halfof f i g u r e 5. T h e m a g n i t u d e of t h e e a r t h q u a k e sreported range f r o m 0 . 7 to 4 . 3 on the Richters c a l e . T a b l e 1 l i s t s the e a r t h q u a k e s i n C o l o r a d o of m a g n i t u d e 3 a n d l a r g e r , a c c o r d i n g t ot h e U. S. C o a s t a n d Geodetic S u r v e y r e p o r t s(Wang, 1965). Wang (1965) c a l c u l a t e d t h e e p i c e n t e r s a n d h y p o c e n t e r s of t h e 1963-65 D e n v e re a r t h q u a k e s , a n d t h e r e s u l t s of h i s c a l c u l a t i o n sa r e shown in figure 6.T h e m a j o r i t y of t h e e a r t h q u a k e e p i c e n t e r sa r e within a f i v e - m i l e r a d i u s of t h e w e l l . Allepicenters calculated f r o m four o r m o r e r e cording s t a t i o n s a r e w i t h l n s e v e n m i l e s of t h ewell.Wang (1965) c a l c u l a t e d t h e b e s t - f i t t i n gplane p a s s i n g t h r o u g h t h e zone of h y p o c e n t e r scalculated f r o m four o r m o r e recording stations.He concluded t h a t t h l s plane m i g h t b e a f a u l talong which movement was taking place. Theplane d i p s to t h e e a s t , a n d p a s s e s b e n e a t h t h ea r s e n a l w e l l a t a d e p t h of a b o u t six a n d o n e half m i l e s ( f i g . 6 ) .ROCK MOVEMENT AND EARTHQUAKESAn a t t e m p t h a s b e e n m a d e t o d e v e l o p am e t h o d of e s t i m a t i n g , d i r e c t l y f r o m s e i s m o g r a m s , wave e n e r g y r a d i a t e d d u r i n g a n e a r t h q u a k e . Using t h e f o r m u l a f a v o r e d by T o c h e r(1964) a n d R i c h t e r ( 1 9 5 8 ) , t h e e l a s t i c w a v ee n e r g y of a m a g n i t u d e 3 e a r t h q u a k e could b ep r o v i d e d by d r o p p i n g a 100 foot c u b e of r o c k ad i s t a n c e of a f e w f e e t .Admittedly, t h e f o r m u l a a p p l i e s t o d i s t a n te a r t h q u a k e s a n d is not r o u t i n e l y a p p l i c a b l e t ol a r g e n u m b e r of e a r t h q u a k e s , b u t i t d o e s s u g g e s tt h a t t h e D e n v e r e a r t h q u a k e s m a y b e c a u s e d byrelatively m m o r rock movements. 2005 The Rocky Mountain Association of Geologists

The Mountain Geologist v.3, 1966Table c.Dec.Jan.AprilApr lMayJuneJulyJulyJan.Feb.Feb.JulySept.Sept.Sept.18C O L O R A D O EARTHQUAKES, M A G N I T U D E 3 A N D L A R G E R( F r o m U. S. C o a s t a n d G e o d e t i c S u r v e y R e p o r t s )Greenwich Mean T i m eMm.Sec.Hour**Longitude(N)L a t t u d e Depth(km)(w)MagnitudeFelt AreaDenver, Louisville, Georgetown andS. E. W y o m l n gC i m a r r o n , Lake City, Montrose, Ophir,Ouray, Placerville, Powderhorn, Ridgew a y , TellurideF a r p l a y ,H a r t s e l , L e a d v i l l e , J e f f e r s o n ,Buena Vista, AlmaDenver, Derby, HendersonDenver, Derby, HendersonDenver, Jefferson, Adam sDenver, PuebloDenver, BoulderDenver, Derby, HendersonDenver, Derby, HendersonDenver, Derby, HendersonDenver, Derby, HendersonDenver, BoulderDenver, Derby, HendersonWestern Denver, LoulsvilleWestern Denver, LouisvilleWestern Denver, L o u s v r l l eWestern Denver, Lou svrlleWestern Denver, LouisvllleWestern Denver, LoulsvilleWestern Denver, Louisvllle745308242552285161631142929D e t e r m i n e d f r o m l o c a l seismogramdata.F o r Mountain Standard T i m e , s u b t r a c t 7 h o u r s .F o r Mountain Daylight Savlngs T i m e , s u b t r a c t 6 h o u r s .( W a n g , 1965, w i t h a d d t i o n s ) 2005 The Rocky Mountain Association of Geologists

The Mountain Geologist v.3, 1966EXPLANATIONEARTHQUAKE H'iPOCENlER39COUPUTEO FROMD- TARY\ . y. A--UP P E RA - APRECAMBRIAN METAMORPHIC--------.-I"ROCKROCK----M A N T L EE a r t h q u a k e h y p o c e n t e r s d u r i n g 1963-64 f r o m l o c a l s e i s m o l o g i c a l s t a t i o n s In t h e D e n v e rFigure 6.a r e a ( a f t e r Wang, 1965). A l l e p i c e n t e r s c a l c u l a t e d f r o m f o u r o r m o r e r e c o r d i n g s t a t i o n s a r e w i t h i ns e v e n m i l e s of the A r s e n a l w e l l . All h y p o c e n t e r s c a l c u l a t e d f r o m f o u r o r m o r e r e c o r d i n g s t a t i o n sa r e within a r e a indicated on s e c t i o n A - A . 2005 The Rocky Mountain Association of Geologists-soh-

The Mountain Geologist v.3, 1966P R E S S U R E INJECTION AND EARTHQUAKEFREQUENCYP r e s s u r e i n j e c t i o n began i n M a r c h 1962.T h e f i r s t t w o e a r t h q u a k e s w t he p i c e n t e r s int h e A r s e n a l a r e a w e r e r e c o r d e d d u r i n g April1962.T h e l o w e r half of f i g u r e 5 i s a g r a p h of them o n t h l y v o l u m e of w a s t e i n j e c t e d i n t o t h eA r s e n a l w e l l . T h e t o t a l n u m b e r of e a r t h q u a k e sr e c o r d e d i n t h e A r s e n a l a r e a i s plotted eachm o n t h i n t h e u p p e r half of t h e g r a p h .During the initial Injection perlod fromM a r c h 1962 t o t h e end of S e p t e m b e r 1963, thei n j e c t i o n p r o g r a m w a s often s h u t down f o r r e p a i r s t o t h e f i l t e r plant. D u r i n g t h s periodt h e r e d o e s not a p p e a r t o be a d i r e c t m o n t h bym o n t h c o r r e l a t i o n . H o w e v e r , t h e h i g h injectionm o n t h s of A p r i l , M a y a n d J u n e 1962 s e e m toc o r r e l a t e with t h e high e a r t h q u a k e f r e q u e n c ym o n t h s of J u n e , July and August. T h e highi n j e c t i o n m o n t h s of F e b r u a r y a n d M a r c h 1963m a y c o r r e l a t e w i t h the high e a r t h q u a k e monthof A p r i l .T h e p e r i o d of n o i n j e c t i o n f r o m S e p t e m b e r1 9 6 3 t o S e p t e m b e r 1964 c o i n c i d e s w i t h a periodof m i n i m u m e a r t h q u a k e f r e q u e n c y . T h e periodof l o w v o l u m e i n j e c t i o n by g r a v i t y f l o w , f r o mS e p t e m b e r 1964 t o A p r i l 1965. i s c h a r a c t e r i z e dby t w o m o n t h s ( O c t o b e r and F e b r u a r y ) ofg r e a t e r e a r t h q u a k e f r e q u e n c y than e x p e r i e n c e dduring the preceeding y e a r .T h e m o s t d i r e c t c o r r e l a t i o n of f l u i d injection w i t h e a r t h q u a k e f r e q u e n c y i s d u r i n g them o n t h s of J u n e t h r o u g h S e p t e m b e r 1965. T h i sp e r i o d w a s c h a r a c t e r i z e d by t h e pumping of300 g a l l o n s p e r m i n u t e , 16 t o 24 h o u r s a day,a t p r e s s u r e s , of f r o m 800 t o 1 , 0 5 0 pounds.A r e v i e w of t h e i n j e c t i o n p r o g r a m r e v e a l st h a t t h e r e h a v e b e e n f i v e characteristic p e r i o d sof i n j e c t i o n i n t o the w e l l (fig. 7 ) : I . A p r i l 1 9 6 2 - A p r i l 1963: High injectiona t medium p r e s s u r e .2 . May 1 9 6 3 - S e p t e m b e r 1963: Medium i n je'ction a t m e d i u m p r e s s u r e .3 . O c t o b e r 1 9 6 3 - S e p t e m b e r 1964: No i n jection.4 . S e p t e m b e r 1 9 6 4 - M a r c h 1965. Low injectlon a t z e r o p r e s s u r e ( g r a v i t y ) .5 . A p r l l 1 9 6 5 - S e p t e m b e r 1965. f i g h injection a t h i g h p r e s s u r e .T h e a v e r a g e n u m b e r s of e a r t h q u a k e s perm o n t h a r e plotted i n f i g u r e 7 a b o v e t h e a v e r a g ev o l u m e s of fluid i n j e c t e d p e r m o n t h f o r each oft h e s e five periods. The injection f o r March1962 is n o t u s e d i n t h e a v e r a g e b e c a u s e t h ee x a c t day when i n j e c t i o n w a s s t a r t e d is n o tknown.F i g u r e 7 indicates that t h e r e i s a d i r e c tcorrelation between a v e r a g e monthly injectlona n d e a r t h q u a k e f r e q u e n c y when a n i n j e c t i o np r o g r a m i s c a r r i e d out f o r a p e r i o d of f i v emonths.B a r d w e l l (1966) h a s p r e p a r e d a s t a t i s t i c a la n a l y s i s , p r e s e n t e d e l s e w h e r e in t h i s i s s u e ofT h e Mountain G e o l o g i s t , t h a t s u g g e s t s t h a t am a t h e m a t i c a l r e l a t i o n s h i p e x i s t s b e t w e e n theD e n v e r e a r t h q u a k e s a n d t h e v o l u m e of c o n t a m i n a t e d w a s t e i n j e c t e d i n t o the A r s e n a l well.E F F E C T O F EARTHQUAKES ON INJECTIONPRESSUREThe wellhead-pressure injection c h a r t sw e r e not a v a i l a b l e f o r t h e y e a r s 1962 a n d 1963.Only the e a r t h q u a k e s of m a g n i t u d e 3 o r l a r g e rw e r e checked against the p r e s s u r e injectlonc h a r t s f o r 1965. T h e s e e a r t h q u a k e s a r e l i s t e din table 1.No c h a r t s w e r e b e i n g r e c o r d e d a t the t i m eof t h e F e b r u a r y a n d July 1965 e a r t h q u a k e s .T h r e e p u m p s w e r e m a i n t a i n i n g a p r e s s u r e of7 2 5 pounds when the S e p t e m b e r 1 4 e a r t h q u a k eo c c u r r e d . T h e r e was no change in injection a tt h e t i m e of t h e e v e n t .Allowing f o r a f e w m i n u t e s t i m e d i s c r e p ancy between the c h a r t t i m e and recorded tlmeof t h e e a r t h q u a k e s , the two e a r t h q u a k e s of 29September m a y have affected the injectionp r e s s u r e . During the f i r s t earthquake, a t12:59 P . M . MDST, t h e p r e s s u r e r e c o r d i n gn e e d l e on t h e p r e s s u r e c h a r t jumped f r o m 970pounds t o 940 pounds and a l s o r e p e a t e d a tenm i n u t e t i m e i n t e r v a l on t h e c h a r t . D u r i n g t h es e c o n d e a r t h q u a k e , a t 1:20 P. M. MDST, thep r e s s u r e d r o p p e d f r o m 960 pounds to 780pounds. W h e t h e r t h i s 180-pound p r e s s u r e d r o pw a s d u e t o t h e e a r t h q u a k e o r t o t h e s l o w i n g upof one of t h e p u m p s is not known.F L U I D P R E S S U R E AND T H E ARSENALEARTHQUAKESThe evidencefrom drilling andt e s t i n g t h e A r s e n a l d i s p o s a l well i n d i c a t e s t h a tt h e P r e c a m b r i a n r e s e r v o i r is c o m p o s e d of ahighly f r a c t u r e d g r a n i t e g n e i s s which i s s u b stantially impermeable. The f r a c t u r e s a r ea l m o s t v e r t i c a l , a n d p o r o s l t y of t h e r e s e r v o i ris f o r m e d by t h e s e f r a c t u r e s . T h e e v i d e n c e 2005 The Rocky Mountain Association of Geologists

The Mountain Geologist v.3, 196660AVERAGENUMBEROFDENVEREARTHQUAKES1IAVERAGE GALLONS OF WASTE 1NJECTEDAXIMUM INJECT1Figure 7.Earthquake frequency-waste injection relationships during five c h a r a c t e r i s t i c periods.indicates that a s fluid was pumped out of ther e s e r v o i r the f r a c t u r e s closed, and a s fluidwas injected into the r e s e r v o i r the f r a c t u r e sopened. In other words, t h e pumping and injection t e s t s indicated that rock movemento c c u r r e d a s fluid was withdrawn o r injected a trelatively low p r e s s u r e s .The p r e s s u r e - d e p t h relations of the P r e c a m b r i a n r e s e r v o i r , showing hydrostatic andlithostatic p r e s s u r e variations with depth, a r eshown in figure 8. These data w e r e d e t e r mined f r o m the d r i l l s t e m t e s t . A s shown onthe c h a r t , the observed p r e s s u r e of the P r e c a m b r i a n r e s e r v o i r is a l m o s t 900 pounds l e s sthan hydrostatic p r e s s u r e .Hubbert and Rubey (1959) devised a s i m p l eand adequate m e a n s of reducing by the r e q u l r e damount the frictional r e s t s t a n c e to the slidingof l a r g e o v e r t h r u s t blocks down v e r y gentles l o p e s . This a r i s e s f r o m the c i r c u m s t a n c ethat the weight of s u c h a block i s jointly supported by solid s t r e s s and the p r e s s u r e of i n t e r s t i t i a l fluids. As the fluid p r e s s u r e a p p r o a c h e sthe lithostatic p r e s s u r e , corresponding t o aflotation of the overburden, the s h e e r s t r e s sr e q u i r e d to move the block a p p r o a c h e s z e r o .If high fluid p r e s s u r e s reduce frictionalr e s i s t a n c e and p e r m i t r o c k s to s l i d e down v e r ygentle slopes, i t follows that, a s fluid p r e s s u r ei s d e c r e a s e d , frictional r e s i s t a n c e betweenblocks of r o c k i s i n c r e a s e d , thus permittingthem to come to r e s t on increasingly s t e e pslopes. The s t e e p e r the slope upon whlch ablock of r o c k i s a t r e s t , the lower the r e q u i r e dr a i s e i n fluid p r e s s u r e n e c e s s a r y to producemovement.In the c a s e of the P r e c a m b r i a n r e s e r v o i rbeneath the A r s e n a l well, t h e s e r o c k s w e r e a t 2005 The Rocky Mountain Association of Geologists

The Mountain Geologist v.3, 1966FORMATION PRESSURE (lb/m2)OPEN FRACTURESF i g u r e 8 . P r e s s u r e - d e p t h relations, P r e c a m b r i a n r e s e r v o i r , Rocky Mountain A r s e n a lDisposal well.equilibrium on h l g h - a n g l e f r a c t u r e p l a n e s witha f l u l d p r e s s u r e of 900 pounds l e s s than hydrostatic p r e s s u r e b e f o r e injection began.A s f l u l d w a s I n j e c t e d into t h e P r e c a m b r l a nreservoir, the fluld p r e s s u r e a d j a c e n t t o thew e l l b o r e r o s e , and t h e frictional r e s i s t a n c ealong the f r a c t u r e planes was thereby reduced.When, f l n a l l y , enough f l u l d p r e s s u r c w a se x e r t c d o v e r a l a r g e enough a r e a m o v e m e n ttook p l a c e . T h e e l a s t l c wave e n e r g y r e l e a s e dwas recorded a s a n earthquake.S m c e the f o r m a t i o n fluid p r e s s u r e 1s 900pounds sub-hydrostatic, m e r e l y fllllng t h e holcwith c o n t a m l n a t e d w a s t c ( m o s t l y s a l t w a t e r )r a i s e s t h e f o r m a l l o n p r e s s u r e 900 pounds, ort o t h e equivalent of h y d r o s t a t i c p r e s s u r e . Anya p p l i e d m j e c t i o n p r e s s u r e a b o v c that of gravityflow b r i n g s a b

THE ROCKY MOUNTAIN ARSENAL DISPOSAL WELL DAVID M. EVANS: Consulting Geologist. Denver. Colorado ABSTKACT: Durlng 1961, a decp well was drilled at the Rocky Mountain Arsenal northeast of Denver, Colorado, to dispose of contaminated waste water. The well is bo