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## Tr e n d s i n N i t r a t e a n d D i s s o l v e d - S o l i d s C o n c e n t r a t i o n s i n G ro u n d Wa t e r , C a rs o n V a l l e y , D o u g l a s C o u n t y , N e v a d a , 1 9 8 5 - 2 0 0 1 W

a t e r - R e s o u rc e s I n v e s t i g a t i o n s R e p o r t 0 3 - 4 1 5 2 N e v a d a B a s i n a n d R a n g e S t u d y U n i t N a t i o n a l Wa t e r - Q u a l i t y A s s e s s m e n t P ro g r a m 

### By M i c h a e l R
. R o s e n A B S T RAC T A n a l y s i s o f tr e n d s i n n i tr at e a n d t o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n s o v e r ti m e i n C ar s o n V a l l e y , N ev a d a , i n d i c at e s th at 5 6 p e r c e n t o f 2 7 m o n i t o ri n g w e l l s th at h av e l o n g - t e rm r e c o r d s o f n i tr at e c o n c e n tr ati o n s s h o w i n c r e a s i n g tr e n d s , 1 1 p e r c e n t s h o w d e c r e a s i n g tr e n d s , a n d 3 3 p e r c e n t h av e n o t c h a n g e d
. T o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n s h av e i n c r e a s e d i n 5 2 p e r c e n t o f th e s e w e l l s a n d ar e s t ab l e i n 4 8 p e r c e n t . N o n e o f th e s e w e l l s s h o w d e c r e a s i n g tr e n d s i n t o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n s . T h e w e l l s s h o w i n g i n c r e a s i n g tr e n d s i n n i tr at e a n d t o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n s w e r e a l w a y s i n ar e a s th at u s e s e p ti c w a s t e - d i s p o s a l s y s t e m s . T h e r e f o r e , th e p ri m ary c au s e o f th e s e i n c r e a s e s i s l i ke l y th e i n c r e a s e i n s e p ti c - t a n k u s a g e o v e r th e p a s t 4 0 y e ar s . I N T R O D U C T I O N D o u g l a s C o u n ty , i n w e s t e rn N ev a d a ( f i g . 1 ) , i s a r ap i d l y d ev e l o p i n g ar e a th at d e p e n d s o n g r o u n d w at e r f o r d o m e s ti c a n d p u b l i c w at e r s u p p l y . T h e r e s u l t o f th i s d ev e l o p m e n t i s d e g r a d e d g r o u n d

- w at e r q u a l i ty i n s o m e ar e a s . F o r e x a mp l e , h i g h n i tr at e c o n c e n tr ati o n s h av e b e e n f o u n d i n th e I n d i a n H i l l s ar e a o f n o rth e rn D o u g l a s C o u n ty i n C ar s o n V a l l e y ( S e i l e r a n d o th e r s , 1 9 9 9 ; T h o d a l , 1 9 9 6 )
. D e g r a d ati o n o f g r o u n d

- w at e r q u a l i ty i s p o s s i b l e o v e r a w i d e r ar e a o f C ar s o n V a l l e y a s d ev e l o p m e n t c o n ti n u e s . O n c e d e g r a d e d , i t c a n t ake m a ny y e ar s f o r g r o u n d

- w at e r q u a l i ty t o i mp r o v e , w h i c h c a n l i m i t f u tu r e g r o w th
. W at e r q u a l i ty a n d g r o u n d

- w at e r l ev e l s h av e b e e n m o n i t o r e d s i n c e 1 9 8 5 i n D o u g l a s C o u n ty t o h e l p e n s u r e a r e l i ab l e w at e r s u p p l y . T h i s o n g o i n g m o n i t o ri n g i s b e i n g d o n e i n c o o p e r ati o n w i th th e C ar s o n W at e r S u b c o n s e r v a n c y D i s tri c t a n d D o u g l a s C o u n ty . A l th o u g h 1 6 y e ar s ( 1 9 8 5

- 2 0 0 1 ) o f d at a e x i s t , d at a h av e b e e n a n a l y z e d o n l y f o r c h a n g e s i n g r o u n d

- w at e r q u a l i ty u p t o 1 9 8 8 ( T h o d a l , 1 9 9 6 )
. T h i s r e p o rt s u m m ari z e s a f u rth e r a n a l y s i s o f th e s e d at a , i n c l u d i n g d at a c o l l e c t e d u p t o 2 0 0 1 , t o d e t e rm i n e th e c au s e s o f c h a n g e s i n n i tr at e a n d d i s s o l v e d

- s o l i d s c o n c e n tr ati o n s th r o u g h ti m e . W H Y A R E N I T RA T E A N D D I S S O L V E D - S O L I D S C O N C E N T RA T I O N S I M P O RT A N T ? T h e r e c o g n i ti o n th at h i g h c o n c e n tr ati o n s o f n i tr o g e n , s p e c i f i c a l l y n i tr at e , i n dri n ki n g - w at e r s u p p l i e s c o u l d c au s e h e a l th p r o b l e m s w a s d e t e rm i n e d o v e r 5 0 y e ar s a g o b y C o m l y ( 1 9 4 5 )
. M e th e m o g l o b i n , a f o rm o f h e m o g l o b i n i n th e b l o o d th at i s tr a n s f o rm e d s o th at i t i s u n ab l e t o d e l i v e r o x y g e n , r e s u l t s i n th e d i s e a s e c a l l e d m e th e m o g l o b i n e m i a w h e n

b o u n d a r i e s o f n i t r a t e c o n c e n t ra t i o n s . F i g u r e 1 . G e n e ra l i z e d n i t ra t e c o n c e n t ra t i o n s ( a s n i t r o g e n ) i n g r o u n d w a t e r f o r t h e C a r s o n V a l l e y , 2 0 0 0

. N i t ra t e c o n c e n t ra t i o n s f r o m 3 4 w e l l s ( n o t a l l o f w h i c h w e r e t r e n d s i t e l o c a t i o n s ) w e r e u s e d t o c o n s t r u c t t h e c o n t o u r s . C o n t o u r s r e p r e s e n t a p p r ox i m a t e



### U

. S

. D e p a r t m e n t o f t h e I n t e r i o r J u l y 2 0 0 3 U

. S

. G e o l o g i c a l S u r v e yamounts become high enough to cause a bluish discoloration of the skin and can be f atal. Inf ants under the age of 6 months are more susceptible to this disease because the y lack the appropriate enzyme that con v erts methemoglobin back to hemoglobin (A v ery , 1999). Comly's research became widely accepted when subsequent i n v esti g ations indicated a consistent pattern of high-nitrate drinking w ater in in f antile methemoglobinemia cases. In 1975, the U.S. E n vironmental Protection Agen c y (USE P A) established a maximum contaminant l ev el (MCL) for nitrate in drinking w ater of 10 mg/L (milligrams per liter) as nitrogen. High nitrate concentrations also h av e been lin ked to h ypertension (Malberg and others, 1978), central nerv ous system birth defects (Dorsch and others, 1984), certain cancers (Hill and others, 1973), non-Hodgkin's l ymphoma ( W ard and others, 1996; W eisen b u rge r, 1991), and diabetes (P arslo w and others, 1997). Ho wev er, def initi v e relations are lacking and more research is needed to conf irm the links (Spalding and Exner, 1993). A

v ery (1999) suggested that the correlation between high nitrate concentrations and reported cases of methemoglobinemia is not related to nitrate specif ically , b ut to associated bacterial contamination that occurs with high nitrate concentrations in rural areas such as from septic tanks and f arm animal w aste. Re g ardless of the human health concerns, it has been widely recognized that high concentrations of nitrogen in ground w ater can lead to increased plant and alg al gro wth of lakes and streams that hav e groundw ater inputs (Spalding and Exner, 1993). Nitrate concentrations in ground w ater that could hav e serious en vironmental ef fects may be much lo wer than the maximum USEP A drinking-w ater guideline of 10 mg/L because gro wth in some plant communities may be limited by nitrogen. This means that additional nitrogen caused by human acti vities around lakes and ri v ers may cause alg al blooms and change the ecology of the surfacew ater system. Data collected by the Nev ada Di vision of En vironmental Protection on the Carson Ri v er sho w that at certain times, the Carson Ri v er has limited nitrogen av ailable for plant gro wth (P ahl, 2002). This means additional nitrogen from ground w ater could cause detrimental ef

fects to the ri v er ecology
.

Vie w of a se w ered housing dev elopment in Douglas County , Nev ada, looking south to Jobs P eak (left) in the Carson Range

. Dissolv ed solids are deri v ed mainly from minerals in rocks near the land surf ace and in the aquifer. Small amounts of contaminants added to the ground w ater generally do not signif icantly increase the dissolv edsolids concentration of the ground w ater. Ho wev er, changes in dissolv edsolids concentrations o v er time may indicate altered ground-w ater circulation in aquifers that are stressed by large withdraw als or are recei ving additional recharge from w ater that is high in dissolv ed solids. In Carson V alle y , ground-w ater recharge sources that are potentially high in dissolv ed solids are lawn irrig ation, agricultural runof f, and se w age ef

fluent.

A secondary preferred drinking-w ater standard for total dissolv ed solids of 500 mg/L for public w ater supplies has been adopted for Nev ada. If w ater supplies that meet the preferred standard are not av ailable, the MCL of 1,000 mg/L is used, and this MCL is enforceable by the State (Nev ada Bureau of Consumer Health Protection Services, 1980). WHA T ARE THE SOURCES OF NITR OGEN IN CARSON V

ALLEY? A primary focus of this study is to identify sources of nitrate in ground w ater in Carson V alle y . Sev eral potential sources of nitrate e xist that are deri v ed from human acti vities in Carson V alle y . These include nitrate from agriculture (urine and feces from li v estock, fertilizer application to crops, etc.), irrig ation using treated ef fluent that contains elev ated nitrate concentrations, septic systems, and domestic application of fertilizer. Natural sources of nitrogen in the ground w ater are limited in Carson V alle y , and nitrate concentrations in wells that hav e not been impacted by human nitrogen sources generally are less than 1 mg/L as nitrogen.

Rapid g ro wth in Douglas County , Nev ada, using septic-tank systems w as impor tant to consider when ev aluating nitrate and total dissolv ed-solids concentrations in g round w ater

. Carson V alle y has e xperienced rapid gro wth in areas outside of the e xisting public w ater and se w age systems of Minden and Gardnerville. This gro wth has led to the installation of septic systems at a rate of o v er 1,000 ev ery 10 years (table 1).
T ab le 1. Increase of per mitted septic systems in Carson V alle y o v er time Date range Number of parcels with septic systems Increase 1851-1970 1971-1980 1981-1990 1991-2001 446 1,591 2,760 4,064 1,145 1,169 1,304 WHA T W AS MEASURED T O DETERMINE NITR OGEN SOURCES? Potential nitrogen sources in Carson V alle y were identif ied with data from the Douglas County Multi-Agenc y Geographic Information Center
. These data included locations of septic systems on a parcel basis (including when the y were permitted), locations of w aste w ater irrig ation systems, reserv oir locations, and agricultural land uses. Potential nitrogen sources were compared annually to nitrate concentrations measured between 1985 and 2001. Nitrate concentrations in ground w ater generally e xceeded 2 mg/L in the proximity of septic tanks (f ig. 1). T otal dissolv edsolids concentrations e xhibited a similar distrib ution to nitrate concentrations. F or this study , more than 800 nitrate and total dissolv edsolid concentrations analyses were e xamined, plotted, and analyzed statistically
. 

### 20 2 4 6 8 10 12 14 16 18 20

200 300 400 500

0 2 4 6 8 10 12 14 16 18 20

1985 1988 1991 1994 1997 2000 1985 1988 1991 1994 1997 2000 300 350 400 450 500

0 2 4 6 8 10 12 14 16 18 20

0 2 4 6 8 10 12 14 16 18 20 **Well 4**

**Well 5 B** TOTAL-DISSOLVED SOLIDS, IN MILLIGRAMS PER LITERTOTAL-DISSOLVED SOLIDS, IN MILLIGRAMS PER LITER **Well 1**

**Well 2 Well 3**  **A**

**C Well 6** 

**Well 7 D**  **Well 8**

**Well 9**PRECIPITATION, IN INCHESPRECIPITATION, IN INCHES
140 160 180 200 220 240

50 100 150 200 **Monthly precipitation** 

Figu re 3. T rends in total dissol v ed-solids concentrations for selected wells in Carson V all e y . Precipitation data from Minden is plotted for reference. W ell locations are sh own in figure 5. 0 2 4 6 8 10 12 14 16 18 20

0 2 4 6 8 10 12 14 16 18 20 0 1 2 3 4 5

0 2 4 6 8 10 12 14 16 18 20

0 1 2 3 4

0 2 4 6 8 10 12 14 16 18 20 0 1 2 3 4 5

0 2 4 6 8 10 12 14 16 18 20

1985 1988 1991 1994 1997 2000 1985 1988 1991 1994 1997 2000 **Well 6** Environmental Protection Agency maximum contaminant level **D C**  **B**NITRATE, IN MILLIGRAMS PER LITER AS NITROGENNITRATE, IN MILLIGRAMS PER LITER AS NITROGEN **Well 1**

**Well 2**

**Well 3 A**  **Monthly precipitation**

**Well 7**

**Well 8**

**Well 9 Well 4**  **Well 5**PRECIPITATION, IN INCHESPRECIPITATION, IN INCHES

Figu re 2. T rends in nitrate concentrations for selected wells in Carson V all e y . Precipitation data from Minden is plotted for reference. U.S. E n vironmental Protection Agen cy maximum contaminant l e v el for nitrate (as nitrogen) of 10 mg/L is sh own in figure 2C. W ell locations are sh o wn in figure 4.



### 3H O W W E R E T R E N D S D E T E R M I N E D ? C h a n g e s i n n i tr at e a n d t o t a l d i s s o l v e d

- s o l i d c o n c e n tr ati o n s o v e r ti m e w e r e a n a l y z e d b y s tu d y i n g g r ap h i c a l r e p r e s e n t ati o n o f th e d at a a n d b y c o mp ari n g v i s u a l e s ti m at e s o f tr e n d s ( f i g s . 2 a n d 3 ) t o a n o n p ar a m e tri c s t ati s ti c a l t e s t c a l l e d K e n d a l l ' s t au . K e n d a l l ' s t au p r o v i d e s a c o n s i s t e n t , u n b i a s e d q u a n ti t ati v e t e s t o f w h e th e r th e v i s u a l tr e n d s d e t e c t e d ar e r e a l
. T h e s t ati s ti c a l t e s t w a s c o n d u c t e d at a 9 5 p e r c e n t c o n f i d e n c e l i m i t t o r e d u c e th e p r o b ab i l i ty o f d e t e c ti n g a tr e n d w h e n n o n e i s p r e s e n t . T h e p r o c e d u r e a d o p t e d i n th i s s tu d y w a s s i m i l ar t o th e a n a l y s i s u s e d b y T h o d a l ( 1 9 9 6 )
. R e s u l t s o f th e tr e n d a n a l y s i s f o r e a c h w e l l u s i n g th e K

e n d a l l ' s t au t e s t ar e s h o w n i n f i g u r e s 4 a n d 5
. W H A T W E R E T H E R E S U L T S O F T H E S T U D Y ? F i g u r e 1 s h o w s a c o n t o u r m ap o f n i tr at e c o n c e n tr ati o n s i n 2 0 0 0
. T h e h i g h e s t n i tr at e c o n c e n tr ati o n s o c c u rr e d n e ar o r d i r e c t l y u n d e r ar e a s w h e r e th e r e ar e h i g h c o n c e n tr ati o n s o f p ar c e l s w i th s e p ti c t a n k s . A g ri c u l tu r a l

F i g u r e 4 . T r e n d s i n n i t r a t e c o n c e n t r a t i o n s p l o t t e d b y w e l l l o c a t i o n . I n d i v i d u a l t r e n d s i n n i t ra t e c o n c e n t r a t i o n s o f w e l l s 1
- 9 a r e p l o t t e d i n f i g u r e 2

. ar e a s a n d ar e a s w h e r e w a s t e w at e r i s i rri g at e d o n t o f i e l d s g e n e r a l l y d o n o t ap p e ar t o h av e e l ev at e d n i tr at e c o n c e n tr ati o n s . T h i s i n d i c at e s th at th e s o u r c e o f n i tr o g e n i s p r o b ab l y e i th e r s e p ti c - t a n k l e a c h f i e l d s o r f e rti l i z e r ap p l i e d t o d o m e s ti c l aw n s . A l th o u g h i t i s n o t p o s s i b l e t o d i s ti n g u i s h b e t w e e n th e t w o s o u r c e s d i r e c t l y , i t w o u l d b e e x p e c t e d th at i f i rri g ati o n o f f e rti l i z e d l aw n s w e r e th e m a i n c au s e o f e l ev at e d n i tr at e s , th at a s e a s o n a l p att e rn o f h i g h n i tr at e c o n c e n tr ati o n s m i g h t o c c u r i n s h a l l o w w e l l s th at h av e w at e r l ev e l s c l o s e t o th e l a n d s u r f a c e . S e a s o n a l p att e rn s o f n i tr at e a n d t o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n s h av e n o t b e e n o b s e r v e d i n a ny o f th e l o n g - t e rm m o n i t o ri n g w e l l s w h e n q u art e r l y n i tr at e s a mp l i n g w a s p art o f th e m o n i t o ri n g p r o g r a m a n d n o c o rr e l ati o n w i th m o n th l y p r e c i p i t ati o n w a s o b s e r v e d ( f i g s . 2 a n d 3 )
. T h i s i n d i c at e s th at n i tr o g e n i n th e g r o u n d w at e r i s d e ri v e d f r o m a r e l ati v e l y c o n s t a n t s o u r c e , s u c h a s l e a c h at e f r o m s e p ti c t a n k s . A n a l y s e s o f n i tr at e a n d d i s s o l v e d

- s o l i d s c o n c e n tr ati o n s w e r e d o n e o n w at e r s a mp l e s f r o m 2 7 m o n i t o ri n g w e l l s i n th e C ar s o n V a l l e y th at h av e l o n g - t e rm r e c o r d s ( g r e at e r th a n 5 y e ar s o f d at a f r o m 1 9 8 5 th r o u g h 2 0 0 1 )
. N i tr at e c o n c e n tr ati o n d at a i n d i c at e th at 5 6 p e r c e n t o f th e s e w e l l s ( 1 5 w e l l s ) s h o w i n c r e a s i n g tr e n d s o v e r ti m e , 1 1 p e r c e n t ( 3 w e l l s ) s h o w d e c r e a s i n g tr e n d s , a n d 3 3 p e r c e n t ( 9 w e l l s ) d i d n o t c h a n g e d u ri n g th e s a mp l i n g p e ri o d ( f i g . 4 )
. T o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n s i n c r e a s e d i n 5 2 p e r c e n t o f th e s e w e l l s ( 1 4 w e l l s ) a n d w e r e s t ab l e i n 4 8 p e r c e n t ( 1 3 w e l l s )
. N o n e o f th e s e w e l l s s h o w d e c r e a s i n g tr e n d s i n t o t a l d i s s o l v e d

s o l i d s c o n c e n tr ati o n s ( f i g . 5 )
. I n m a ny c a s e s , th e i n c r e a s e s i n n i tr at e c o n c e n tr ati o n s w e r e n o t l arg e , a n d i n a l l b u t t w o w e l l s n i tr at e c o n c e n tr ati o n s w e r e b e l o w th e U S E P A M C L f o r n i tr at e o f 1 0 m g /L a s n i tr o g e n . H o w ev e r, i n s o m e l o c ati o n s th e c o n s i s t e n t i n c r e a s e i n n i tr at e c o n c e n tr ati o n s i s ap p ar e n t i n m o r e th a n 5 0 p e r c e n t o f th e l o n g - t e rm m o n i t o ri n g w e l l s , a n d s o m e i n c r e a s e s w e r e o n th e o r d e r o f f i v e ti m e s th e i n i ti a l c o n c e n tr ati o n r e c o r d e d at th e b e g i n n i n g o f th e m o n i t o ri n g p r o g r a m . T h e s e i n c r e a s e s i n d i c at e th at f u rth e r i n c r e a s e s ar e l i ke l y a n d m a y b e c o m e m o r e w i d e s p r e a d i n th e v a l l e y . T h e a g e o f th e g r o u n d w at e r i n th e C ar s o n V a l l e y a q u i f e r s y s t e m i s n o t p r e c i s e l y kn o w n , b u t s o m e a g e d ati n g b y T h o m a s a n d o th e r s ( 1 9 9 9 ) i n d i c at e d i t t ake s ab o u t 3 0

- 4 0 y e ar s f o r r e c h arg e w at e r t o r e a c h w e l l s th at ar e ab o u t 2 0 0 f t d e e p . T h i s i n d i c at e s th at c o n t a m i n ati o n c o u l d p e r s i s t f o r a l o n g ti m e i n t o th e f u tu r e ev e n i f a l l h u m a n i n p u t s o f n i tr at e ar e s t o p p e d
. F i g u r e 1 a l s o s h o w s th at s o m e o f th e h i g h e s t n i tr at e c o n c e n tr ati o n s ar e n e ar th e C ar s o n R i v e r, i n d i c ati n g th at n u tri e n t s m a y a f f e c t th e e c o l o g y o f th e ri v e r i n s o m e p l a c e s . M o s t o f th e w e l l s s h o w i n g i n c r e a s i n g tr e n d s ar e n e ar th e m arg i n s o f th e v a l l e y , w h e r e s e p ti c t a n k s ar e u s e d i n h o u s i n g d ev e l o p m e n t s . T h e v a l l e y m arg i n s ar e n o t h e av i l y u s e d f o r a g ri c u l tu r a l p u rp o s e s , s o i t i s n o t l i ke l y th e s e i n c r e a s e s ar e c au s e d b y a g ri c u l tu r a l l a n d u s e s . T h r e e o f th e 2 7 w e l l s ( w e l l s A , B , a n d C ; f i g . 4 ) s h o w d e c r e a s i n g tr e n d s i n n i tr at e c o n c e n tr ati o n s . T h e d e c r e a s i n g tr e n d s i n t w o o f th e th r e e w e l l s ( w e l l s B a n d C ) ar e s m a l l a n d r e p r e s e n t d e c r e a s e s o f ap p r o x i m at e l y 1 m g /L o v e r 1 6 y e ar s . C o n c e n tr ati o n s i n th e th i r d w e l l ( w e l l A ) d e c r e a s e d b y 6 m g /L i n th e e ar l y 1 9 9 0 ' s b u t h av e d e c r e a s e d at a mu c h s l o w e r r at e s i n c e th at ti m e . A l th o u g h s t ati s ti c a l l y s i g n i f i c a n t , th e s e d e c r e a s i n g tr e n d s ar e n o t c h ar a c t e ri s ti c o f th e m o n i t o ri n g n e t w o rk a s a w h o l e a n d l i ke l y r e p r e s e n t l o c a l c h a n g e s i n n i tr o g e n s o u r c e s . F i g u r e 2 s h o w s i n c r e a s i n g tr e n d s i n n i n e w e l l s th r o u g h o u t C ar s o n V

a l l e y . I n c r e a s e s b e g a n at d i f f e r e n t ti m e s i n e a c h w e l l , w h i c h r e f l e c t s d i f

f e r e n t n i tr o g e n s o u r c e s a n d tr av e l ti m e s o f g r o u n d w at e r i n th e a q u i f e r. F i g u r e 2 D s h o w s a s h arp s t e p i n c r e a s e i n w e l l 8 , w h i c h i n d i c at e s n i tr o g e n arri v e d at th e w e l l s u d d e n l y . T h i s m a y h av e b e e n c au s e d b y th e i n tr o d u c ti o n o f n e w s e p ti c t a n k s o r d o m e s ti c w e l l s n e arb y o r b y c h a n g e s i n p u mp i n g r at e s i n e x i s ti n g n e arb y w e l l s . O th e r tr e n d s ap p e ar t o b e m o r e g r a d u a l a n d c o n s i s t e n t . S e p ti c - t a n k u s e h a s o c c u rr e d i n m o s t p art s o f th e C ar s o n V a l l e y s i n c e 1 9 7 0
. Fr o m 1 9 8 0 t o th e p r e s e n t ( 2 0 0 1 ) , f e w n e w ar e a s h av e b e e n d ev e l o p e d , b u t p ar c e l s i z e h a s d e c r e a s e d o v e r ti m e a n d s e p ti c - t a n k d e n s i ty ap p e ar s t o h av e i n c r e a s e d i n m a ny ar e a s o f th e v a l l e y . T h e o b s e r v ati o n th at n i tr at e c o n c e n tr ati o n s ar e n o t i n c r e a s i n g i n a l l w e l l s n e ar ar e a s th at u s e s e p ti c t a n k s i n d i c at e s th at m a ny f a c t o r s c a n d e l a y o r p r ev e n t c o n c e n tr ati o n i n c r e a s e s . I mp o rt a n t f a c t o r s i n c l u d e ( 1 ) th e l e n g th o f ti m e th e s e p ti c t a n k s h av e b e e n i n u s e , ( 2 ) th e e f f i c i e n c y o f th e


### 4F i g u r e 5 . T r e n d s i n t o t a l d i s s o l v e d
- s o l i d s c o n c e n t ra t i o n s p l o t t e d b y w e l l l o c a t i o n . I n d i v i d u a l t r e n d s i n t o t a l d i s s o l v e d
- s o l i d s c o n c e n t ra t i o n s o f w e l l s 1
- 9 a r e p l o t t e d i n f i g u r e 3

. l e a c h

- f i e l d g r a s s e s i n u s i n g n i tr o g e n a s a n u tri e n t s o u r c e , ( 3 ) th e p r e s e n c e o f c l a y l a y e r s ab o v e th e p o i n t w h e r e g r o u n d w at e r e n t e r s th e w e l l , a n d ( 4 ) th e l o c ati o n o f th e w e l l r e l ati v e t o s e p ti c t a n k s . E a c h o f th e s e f a c t o r s i s b ri e f l y d i s c u s s e d b e l o w . T h e ti m e i t t ake s f o r w at e r t o g o f r o m th e l a n d s u r f a c e t o th e w at e r t ab l e m a y b e m a ny y e ar s . T h e r e f o r e , i n c r e a s e s i n n i tr at e c o n c e n tr ati o n s i n w e l l s m a y n o t o c c u r u n ti l m a ny y e ar s a f t e r a s e p ti c t a n k i s i n s t a l l e d
. S e p ti c - t a n k l e a c h f i e l d s ar e d e s i g n e d t o p r o v i d e n u tri e n t s t o th e s o i l ab o v e th e f i e l d
. I f th e p l a n t s g r o w i n g o n th e l e a c h f i e l d ar e e f f i c i e n t at t aki n g u p th e n i tr o g e n , i n c r e a s e i n n i tr at e c o n c e n tr ati o n s m a y b e s m a l l
. L o w p e rm e ab i l i ty c l a y l a y e r s ab o v e th e i n t ake f o r a w e l l m a y p r ev e n t n i tr at e f r o m th e l a n d s u r f a c e f r o m g e tti n g i n t o th e w e l l
. I n C ar s o n V a l l e y , c l a y l a y e r s c o m e f r o m o l d f i n e - g r a i n e d l ake o r ri v e r d e p o s i t s a n d m a y b e l o c a l o r s o m e w h at e x t e n s i v e . I f a w e l l i s f ar e n o u g h aw a y f r o m a s e p ti c - t a n k l e a c h f i e l d a n d i s u p g r a d i e n t f r o m th e f i e l d , i t i s l i ke l y th at th e w e l l w i l l n o t b e i n f l u e n c e d b y n i tr at e f r o m th i s s e p ti c s y s t e m . H o w ev e r, s e p ti c t a n k s f r o m n e i g h b o ri n g p r o p e rti e s c o u l d i n f l u e n c e d o w n g r a d i e n t w e l l s . A s i mp l e r e l ati o n b e t w e e n n i tr at e c o n c e n tr ati o n s i n g r o u n d w at e r a n d d e p th t o w at e r d o e s n o t e x i s t ( f i g . 6 )
. N i tr at e c o n c e n tr ati o n s g e n e r a l l y d e c r e a s e w i th d e p th
. T h e n i tr at e c o n c e n tr ati o n at 2 5 0 f t b e l o w l a n d s u r f a c e i s d o u b l e th e m e d i a n c o n c e n tr ati o n o f a l l 2 7 w e l l s s a mp l e d , w h i c h h av e a m e d i a n d e p th t o w at e r o f 1 6 0 f t b e l o w l a n d s u r f a c e . N i tr at e c o n c e n tr ati o n s i n e x c e s s o f 2 m g /L o c c u r d i s ti n c t l y at ap p r o x i m at e d e p th s o f 2 0 a n d 1 0 0 f t b e l o w l a n d s u r f a c e ( f i g . 6 )
. W at e r f r o m s u p p l y w e l l s th at ar e o p e n t o th e s e i n t e r v a l s m a y h av e h i g h e r th a n av e r a g e n i tr at e c o n c e n tr ati o n s . T

o t a l d i s s o l v e d

- s o l i d c o n c e n tr ati o n s a l s o w e r e f o u n d t o b e i n c r e a s i n g i n ab o u t 5 0 p e r c e n t o f th e w e l l s s a mp l e d
. H o w ev e r, th e s a m e w e l l s th at h a d i n c r e a s i n g n i tr at e c o n c e n tr ati o n s w e r e n o t a l w a y s a s s o c i at e d w i th s t ati s ti c a l l y s i g n i f i c a n t t o t a l d i s s o l v e d

- s o l i d s i n c r e a s e s ( f i g s . 2 a n d 3 )
. T h i s m a y b e d u e t o s ev e r a l r e a s o n s ( 1 ) th e a m o u n t o f n i tr at e i n c r e a s e m a y b e s m a l l c o mp ar e d t o th e t o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n i n th e w at e r ; th e r e f o r e , a n i n c r e a s e i n n i tr at e m a y n o t b e r e f l e c t e d i n th e t o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n o f th e w at e r i n th e w e l l , ( 2 ) n i tr at e i n th e s e p ti c - t a n k l e a c h at e m a y b e e f f e c ti v e l y t ake n u p b y r o o t s , p l a n t m at e ri a l , o r b a c t e ri a i n th e s o i l , b u t o th e r c h e m i c a l s s u c h a s c h l o ri d e a n d s o d i u m th at ar e a s s o c i at e d w i th h u m a n w a s t e w at e r m a y p a s s th r o u g h t o th e a q u i f e r a n d i n c r e a s e th e t o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n , ( 3 ) th e n i tr at e a n d d i s s o l v e d s o l i d s m a y n o t b e f r o m th e s a m e s o u r c e i n s o m e ar e a s , a n d ( 4 ) s h a l l o w w e l l s m a y b e a f f e c t e d b y p u mp i n g , r e c h arg e , a n d c l i m ati c c h a n g e s th at m a y n o t a f f e c t n i tr at e c o n c e n tr ati o n s . N o n e th e l e s s , t o t a l d i s s o l v e d

- s o l i d s c o n c e n tr ati o n s g e n e r a l l y ar e h i g h e r i n th e ar e a s w h e r e th e r e ar e h i g h e r c o n c e n tr ati o n s o f s e p ti c t a n k s . M E D I A N N I T RA T E C O N C E N T RA T I O N S , I N M I L L I G RA M S P E R L I T E R A S N I T R O G E N 0 5 1 0 1 5 2 0 0

5 0

1 0 0

1 5 0

2 0 0

2 5 0
F i g u r e 6 . M e d i a n n i t ra t e c o n c e n t ra t i o n s v e r s u s d e p t h t o w a t e r o f t h e 2 7 l o n g - t e r m m o n i t o r i n g w e l l s . E r r o r b a r s a r e o n e s t a n d a r d d ev i a t i o n .
W H A T F U RT H E R W O R K I S N E E D E D ? F u rth e r w o rk i s n e e d e d i n a l l p art s o f C ar s o n V a l l e y t o r e f i n e e s ti m at e s o f th e l e n g th o f ti m e i t t ake s f o r w at e r t o tr av e l f r o m th e ar e a s o f r e c h arg e t o w e l l s th at ar e u s e d f o r e i th e r p u b l i c o r d o m e s ti c w at e r s u p p l y . B y u s i n g g r o u n d

- w at e r a g e - d ati n g t e c h n i q u e s , a n e s ti m at e o f h o w l o n g th e i n c r e a s i n g tr e n d s m i g h t p e r s i s t a n d w h at th e e x t e n t o f th e p r o b l e m m i g h t b e f o r f u tu r e g e n e r ati o n s c o u l d b e a n s w e r e d
. D i f f e r e n ti ati n g b e t w e e n f e rti l i z e r ap p l i c ati o n s a n d s e w a g e s o u r c e s o f n i tr at e c a n b e u n d e rt ake n b y u s i n g s t ab l e i s o t o p e s o f n i tr o g e n . F e rti l i z e r a n d s e w a g e s o u r c e s g e n e r a l l y h av e d i s ti n c t i s o t o p i c s i g n atu r e s , a n d th e s e s i g n atu r e s c o u l d b e d i s ti n g u i s h e d w i th s t ati s ti c a l l y s i g n i f i c a n t s a mp l i n g . A d e t a i l e d a n a l y s i s o f th e a m o u n t o f n i tr o g e n b e i n g a d d e d t o th e g r o u n d w at e r ( a m a s s - b a l a n c e c a l c u l ati o n ) b y h u m a n a c ti v i ti e s i s n e e d e d t o a d dr e s s h o w h i g h th e n i tr at e c o n c e n tr ati o n s m a y ri s e o v e r ti m e . B e c au s e n i tr at e i s f o rm e d b y th e c o mb i n ati o n o f n i tr o g e n a n d o x y g e n , m o n i t o ri n g o f d i s s o l v e d

- o x y g e n c o n c e n tr ati o n s i n g r o u n d

- w at e r s a mp l e s w o u l d b e h e l p f u l f o r d e t e rm i n i n g th e a m o u n t o f n i tr at e th at c o u l d b e e x p e c t e d at e a c h s a mp l i n g s i t e . F i n a l l y , th e n u mb e r o f w e l l s i n th e m o n i t o ri n gDEPTHTOWATER,INFEETBELOWLANDSURFACE



### 5netw ork is adequate to gi v e only a broad o v ervie w of where nitrate and total dissolv ed solids are a problem in the v alle y . The addition of wells to the netw ork w ould pro vide a more complete areal co v erage of the v alle y , especially in areas of increasing urbanization, thus pro viding decisionmakers with a better understanding of the problem. SUMMAR Y The results of this study indicate that nitrate and total dissolv edsolids concentrations are increasing in o v er 50 percent of the wells sampled o v er a 16-year period in Carson V alle y . Ho wev er, nitrate and total dissolv ed-solids increases are not al w ays in the same wells. Mapping of land use in Carson V alle y indicates that the likely cause of these increases is related to the increase in septic-tank usage o v er the past 40 years. Nitrate and total dissolv ed-solids concentrations in 2001 were mostly belo w the USEP A MCL for nitrate and the Nev ada State secondary MCL for total dissolv ed solids; ho wev er, increased output from septic tanks as population gro ws in the v alle y may continue the upw ard trend in these constituents. Ev en without further population gro wth, the residence time of w ater in the aquifer indicates that high nitrate and total dissolv ed-solids concentrations will continue in parts of the v alle y for possibly the ne xt 30 to 40 years until some equilibrium is reached. Suf f icient data are not av ailable to determine whether this equilibrium will be abo v e or belo w the USEP A MCL for nitrate and the State MCL for total dissolv ed solids. This study was done in cooper ation with the Car son W ater Subconservancy District and is part of the Ne vada Basin and Rang e

Study Unit component of the National W ater Quality Assessment (N

A WQA) pr o gr am of the U .S. Geolo gical Surve y . Ref erences Cited A

v ery , A.A., 1999, Inf antile methemoglobinemia—Ree xamining the role of drinking w ater nitrates: En vironmental Health Perspecti v es, v . 107, no. 7, p. 583-586. Comly , H.H., 1945, Cyanosis in inf ants caused by nitrates in well w ater: Journal of the American Medical Association, v . 29, p. 112-116. Dorsch, M.M., Scragg, R.K.R., McMichael, A.J., Baghurst, P .A., and Dyer, K.F., 1984, Congenital malformations and maternal drinking w ater supply in rural South Australia—A case-control study: American Journal of Epidemiology , v . 119, p. 474-486. Hill, M.J., Hawksw orth, G., and T attersall, G., 1973, Bacteria, nitrosamines and cancer of the stomach: British Journal of Cancer, v . 28, p. 562-567. Malberg, J.W ., Sav age, E.P ., and Osteryoung, J., 1978, Nitrates in drinking w ater and the early onset of h ypertension: En vironmental Pollution, v . 15, p. 155-160. Nev ada Bureau of Consumer Health Protection Services, 1980, W ater supply re gulations, P art 1. W ater quality standards--Monitoring, record keeping, and reporting: Nev ada Bureau of Consumer Health Protection Services report, 17 p. P ahl, R., 2002, Carson Ri v er special monitoring project—2001: Department of Conserv ation and Natural Resource, Nev ada Di vision of En

vironmental Protection, Bureau of W ater Quality Planning, 16 p. P arslo w , R.C., McKinne y , P .A., Law , G.R., Staines, A., W illiams, R., and Bodansk y , H.J., 1997, Incidence of childhood diabetes mellitus in Y

orkshire, northern England, is associated with nitrate in drinking w ater—An ecological analysis: Diabetologia, v . 40, p. 550-556. Seiler, R.L., Zaugg, S.D., Thomas, J.M., and Ho wcroft, D.L., 1999, Caf feine and pharmaceuticals as indicators of w aste w ater contamination in wells: Ground W ater, v . 37, no. 3, p. 405-410. Spalding, R.F., and Exner, M.E., 1993, Occurrence of nitrate in groundw ater - A revie w: Journal of En vironmental Quality , v . 22, p. 392-402. Thodal, C.E., 1996, T rends in chloride, dissolv ed-solids, and nitrate concentrations in ground w ater, Carson V alle y and T opaz Lake areas, Douglas County , Nev ada, 1959-88: U.S. Geological Surv e y W ater
Resources In v estig ations Report 95-4254, 32 p. Thomas, J.M., Thodal, C.E., and Seiler, R.L., 1999, Identif ication of nitrate sources contrib uting to ground w ater in the Indian Hills area of Douglas County , Nev ada: U.S. Geological Surv e y W ater -Resources In v estig ations Report 99-4042, 22 p. W

ard, M.H., Mark, S.D., Cantor, K.P ., W eisenb urger, D.D., Correa-V illasenor, A., and Zahm, S.H., 1996, Drinking w ater nitrate and the risk of nonHodgkin's lymphoma: Epidemiology , v . 7, no. 5, p. 465-471. W

eisenb urger, D.D., 1991, Potential health consequences of ground-w ater

contamination by nitrate in Nebraska, in Bog ardi, I., and K uzelka, R.D., eds., Nitrate contamination—Exposure, consequence, and control: N A

T O ASI Serial G, Ecological Sciences 30, Berlin, Springer -V erlag, p. 309- 315.

Irr igation using treated se w age effluent w as considered in ev aluating the concentrations of nitrate and total dissolv ed solids in g round w ater

.
For more inf ormation: F or information on h ydrologic data and reports relating to the N A

WQA Program in Nev ada: Nev ada Basin and Range N A

WQA Study Unit Project Chief U.S. Geological Surv e y 333 W . Nye Lane, Rm 203 Carson City , NV 89706-0866 Additional information for N A

WQA can be found by accessing the N A

WQA "homepage" on the W orld W ide W eb at: < http://wate r .usgs.g o v/n a wqa > F or information on w ater resources in Nev ada and other USGS Programs: Public Information Assistant U.S. Geological Surv e y 333 W est Nye Lane, Room 203 Carson City , NV 89706-0866 T

el: (775) 887-7649 F ax (775) 887-7629 Email: GS- W -NVpu b lic-in f o@usgs.g o v URL: http://n e v ada.usgs.g o v Copies of this report can be purchased from: U.S. Geological Surv e y Branch of Information Services Box 25286, MS 517 Den v er, CO 80225-0046



### 6 J ul y 2003�

� � � � � � USGS W ater -Resources In vestigations Repor t 03-4152