Longitudinal patterns of concentration-discharge relationships in stream water draining the Hubbard Brook Experimental Forest, New Hampshire

Journal of Hydrology, 116 (1990) 147 165

147

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

13]

L O N G I T U D I N A L P A T T E R N S OF C O N C E N T R A T I O N - D I S C H A R G E R E L A T I O N S H I P S IN S T R E A M W A T E R D R A I N I N G THE H U B B A R D BROOK E X P E R I M E N T A L F O R E S T , N E W H A M P S H I R E

G.B. LAWRENCE' and C.T. DRISCOLL 2

1Department of Plant and Soil Sciences, Deering Hall, University of Maine, Orono, Maine 04469, (U.S.A.) 2Department of Civil and Environmental Engineering, Hinds Hall, Syracuse University, Syracuse, NY 13244-1190 (U.S.A.)

ABSTRACT Lawrence, G.B. and Driscoll, C.T., 1990. Longitudinal patterns of concentration~lischarge relationships in stream water draining the Hubbard Brook Experimental Forest, New Hampshire. J. Hydrol., 116: 147-165. Longitudinal variations of concentration~iischarge relationships and chemical fluxes were evaluated in two headwater streams at the Hubbard Brook Experimental Forest, New Hampshire. At high elevations changes in subsurface flow paths explained variations in H +, inorganic A1 and Si concentrations, whereas variations of DOC concentration were inconsistent with this mechanism. Flow responses of middle and low elevation subcatchments were influenced by variable contributions of hydrologic source areas and the elevational concentration gradient which exists in these catchments, but in most cases were not consistent with responses predicted by changes in flow paths. Spatial patterns of chemical fluxes indicate that, in general, catchment neutralization processes increased in effectiveness in the downslope direction. However, this pattern can be interrupted by secondary tributaries, both ephemeral and persistent, which originate in variable source areas that contribute acidic surface runoff during high flow conditions. Current models of catchment acidification need to incorporate spatial variations of biogeochemical processes and flow responses to improve predictions of short-term variations in surface water chemistry.

INTRODUCTION

Relationships b e t w e e n flow and the c o n c e n t r a t i o n of solutes in h e a d w a t e r streams h a v e been identified in n u m e r o u s studies over the past 20 years. E a r l y w o r k by J o h n s o n et al. (1969) on the base of five h e a d w a t e r c a t c h m e n t s at the H u b b a r d B r o o k E x p e r i m e n t a l F o r e s t (HBEF), New Hampshire, revealed t h a t decreases in Si and Na + c o n c e n t r a t i o n s , as well as increases in A1 and H ÷ c o n c e n t r a t i o n s , were r e l a t e d to increases in streamflow. It was h y p o t h e s i z e d t h a t d u r i n g low flow, s t r e a m w a t e r was derived from g r o u n d w a t e r e n r i c h e d in

0022-1694/90/$03.50

© 1990 Elsevier Science Publishers B.V.

148

G.B. LAWRENCE AND C.T. DRISCOLL

Si and Na÷; high flows resulted in dilution of Si and Na ÷ concentrations because of rain water entering the stream channel with little or no contact with the soil. A model simulated the observed variations in stream chemistry from flow measurements and a fitting parameter based on soil water residence time and field capacity. The fact that A1 and H ÷ concentrations increased with increases in flow was attributed to the mixing of soilwater from acidic upper horizons with that from lower horizons that are higher in concentrations of basic cations. More recent work has also related increases in flow to increases in runoff acidity owing to movement of water through upper soil horizons (Chen et al., 1984; Sullivan et al., 1986; Peters and Driscoll, 1987). Sullivan et al. (1986), however, observed that increases in flow could also be associated with decreases of inorganic A1 concentrations, and conditions of undersaturation with respect to A1 (OH)3 solubility. This response was attributed to water entering the streams too rapidly to achieve equilibrium with the soil. Lawrence et al. (1988) at the HBEF, comparing concentration~lischarge relationships at upper and lower elevations at Catchment 6, revealed that changes in soil water flow paths could only partially explain the variations observed in stream chemistry. The relationship of Johnson et al. (1969) between flow and concentrations of inorganic A1 and H ÷ at the base of the HBEF catchments was reversed at the upper elevations of Catchment 6. The flow path concept could explain the response at the high elevation site, but was inconsistent with the observed relationship at the low elevation site. Spatial trends in stream chemistry coupled with variations of flow contributions from the upper catchment appeared to override possible effects of changes in soil flow paths at the low elevation site. The variations of concentration-discharge relationships observed by Lawrence et al. (1988) indicate that the flow response of solute concentrations at the base of Catchment 6 is a function of flow responses which occur upslope. This study, however, considered only concentration~lischarge relationships at a single high and low elevation site, making it difficult to assess the influence of areal variations relative to changes in soil water flow paths. In an effort to provide a more complete understanding of the role of hydrology in expressing both vertical and areal variation within a catchment the current study addresses the following objectives: (1) to apply the model of Johnson et al. (1969) to concentration-discharge relationships along a longitudinal gradient to evaluate its predictive capabilities and theoretical basis; (2) to evaluate the role of subsurface flow paths and areal variations of concentration~iischarge relationships on a subcatchment basis to determine their importance in controlling temporal variations of stream chemistry; (3) to estimate chemical fluxes along an elevational gradient to establish relationships between intra-catchment variations and patterns of element transport.

PATTERNS OF CONCENTRATION DISCHARGE RELATIONSHIPS

149

EXPERIMENTAL METHODS

Study site This s t u d y was c o n d u c t e d in a d j a c e n t C a t c h m e n t s 5 (22.5 h e c t a r e s ) and 6 (13.1 h e c t a r e s ) of the H B E F , l o c a t e d in the W h i t e M o u n t a i n s of N e w H a m p s h i r e . B o t h c a t c h m e n t s are forested a n d lie on steep slopes (20-30%) w i t h a s o u t h e r l y aspect. V e g e t a t i o n is p r i m a r i l y s u g a r m a p l e (Acer saccharum), A m e r i c a n b e e c h (Fagus grandifolia) and yellow b i r c h (Betula alleghaniensis), w i t h red s p r u c e (Picea rubens) and b a l s a m fir (Abies balsamea) o c c u r r i n g on ridge-tops (Fig. 1). Soils, classified as spodosols ( p r i m a r i l y h a p l o r t h o d s ) , are well-drained w i t h a thick, p e r m e a b l e o r g a n i c l a y e r t h a t minimizes excess i n f i l t r a t i o n r u n o f f (Pierce, 1967). Soil d e p t h is e x t r e m e l y v a r i a b l e , b u t t e n d s to i n c r e a s e d o w n s l o p e ( L a w r e n c e et al., 1986). B e n e a t h the soils, a l a y e r of c o m p a c t e d glacial till o v e r l y i n g b e d r o c k forms an i m p e r m e a b l e b a r r i e r to deep

WATERSHE 6

732m

736m

70In'

'09m

663n

74m

602i

m

54'

Fig. 1. Stream sampling locations (circles) and stream gaging weirs (triangles) in Catchments 5 and 6. Subcatchment areas are defined by the topographic divides which correspond to each sampling site. Shading indicates the regions where coniferous vegetation (primarily red spruce and balsam fir) exceeds 10% of the total basal area, based on 25 m2 plots.

150

G.B. LAWRENCE AND C.T. DRISCOLL

seepage (Likens et al., 1977). Catchment 6 is drained by a perennial first-order stream that is characterized by a narrow stream channel and streambed composed of exposed bedrock and gravel. Catchment 5 is drained by a similar stream that includes a secondary tributary which contributes flow through much of the year (Fig. 1). Additional detail on these catchments is available elsewhere (Likens et al., 1977; Bormann and Likens, 1979; Lawrence et al., 1986).

Experimental approach and procedures Stream water was sampled monthly along an elevational gradient from June 1982 through May 1984 in Catchment 5, and from June 1982 through May 1985 in Catchment 6 (Fig. 1). Analyses were performed for all major solutes, including total monomeric A1 and organic (non-labile) monomeric A1, using procedures described previously (Lawrence et al., 1988). Inorganically complexed A1 was estimated as the difference between total monomeric A1 and organic monomeric A1 according to the method of Driscoll (1984). Acid neutralizing capacity (ANC) was also determined using the titration method of Gran (1952). Synthetic gibbsite saturation indices were computed by the chemical equilibrium model ALCHEMI (Schecher and Driscoll, 1987). Stream gaging weirs at the base of each catchment provided continuous measurements of streamflow throughout the study. Flow at sampling points upstream of the weirs was estimated from the weir measurement and the respective proportion of the basin drained by each upstream sampling site. Although this method can lead to errors from non-uniform patterns of rainfall distribution and intra-catchment variations of topography and soils, it was assumed that the small size and consistent slope of these catchments would limit these errors. This assumption is supported by Hooper (1986), who found that flow in Catchment 3 at the HBEF fit the relationship

Qi(t) = aQbw(t) where: Qw is flow measured at the weir at time t, Qi is flow at site i at time t (upstream of the weir), a is the proportion of the catchment drained by site i, and b is an exponential fitting parameter. The values of b ranged from 0.80 to 0.99 at three sites that drained between 5.6 and 63.2% of this catchment. Catchment 3 lies on the same south-facing ridge as Catchments 5 and 6 (~,- 2 km away) and is very similar in terms of slope, vegetation and soils (Likens et al., 1977; Hooper, 1986). To evaluate concentration-discharge relationships on a subcatchment basis, the successive areas drained by each sampling site were delineated on the basis of hillslope topography (Fig. 1). For each sampling date the influence of upslope contributions of stream water were then removed by subtracting the chemical flux (determined by measured concentrations and stream flow) entering, from the chemical flux leaving a particular subcatchment. The same was done with water flux, so that a net concentration, owing only to chemical

PATTERNS OF CONCENTRATION DISCHARGE RELATIONSHIPS

151

contributions from t ha t particular subcatchment, could be related to streamflow. Through this procedure, c o n c e n t r a t i o n ~ l i s c h a r g e relationships were evaluated in the absence of influences from areal variations upslope of each subcatchment. Net concentrations were not computed for Catchment 5 because only two years of data were available, whereas three years were available for Catchment 6. Estimates of chemical fluxes at each stream sampling point were calculated on a daily basis, then summed to provide annual values. Annual net flux values were calculated for each subcatchment by subtracting the upstream input in the same manner used to determine net concentrations. To facilitate comparison between subcatchments, net flux values were normalized on a per hectare basis and plotted as a function of cumulative drainage area. Average daily concentrations used to calculate fluxes were estimated by the model of J o h n s o n et al. (1969): C X

= --

xG+G

1 1 + {b}D

where: Ca is the concentration of rainwater or upper soil water, Cd is the c o n cen tr atio n of lower soil water - Ca, D is stream discharge (1 • hectare 1 • day -1), {b} is soil water residence time • field capacity 1, and C is resultant c o n cen tr atio n (predicted stream concentration). The model is based on the mixing of r ain wa t er (or upper soil water) and lower soil water, and assumes t h at the degree of mixing is proportional to streamflow. The theoretical basis of this model is evaluated below. For the purpose of flux calculations the model is considered to be empirical. To optimize the model parameters ({b}, Ca and Ca), non-linear regression was performed for each chemical constituent at each sampling site, using the TABLE

1

V a l u e s o f t h e J o h n s o n e t al. (1969) m o d e l p a r a m e t e r {b} ( e x p r e s s e d a s log10) a n d R ~ v a l u e o f {x~, r e g r e s s e d a g a i n s t m e a s u r e d s t r e a m c o n c e n t r a t i o n s i n C a t c h m e n t 6. R e g r e s s i o n s w h i c h e x h i b i t e d a n o n - s i g n i f i c a n t r e l a t i o n s h i p ( s i g n i f i c a n c e l e v e l > 0.001) a r e i n d i c a t e d b y * Site 1

Inorganic

A1

H'

DOC

log{b~ R2 log{b} R2

3.7 0.27 - 5.1 *

- 3.5 0.30 - 3.5 ,

3

log{b} R2

- 2.7 0.31

2.3 0.41

4

log{b} R2 log{b} R2

- 3.8 0.53 - 4.7 0.30

- 3.9 0.71 - 4.7 0.37

2

5

15.4 * 2.2 × 101~ , - 4.7 × 1014 * - 4.8 0.41 4.7 0.23

Si 3.8 0.66 -3.9 0.45 2.9 0.54 3.9 0.78 -4.0 0.76 -

152

G.B. LAWRENCE AND C.T. DRISCOLL

Measured [~organlc AI Ipbll

r Site

6-I

Net ]ne~amc AI [PMI Site 6 2

15

m mm

•

•

Sde 6 2 !

% •

;;2 °" °

•

Mean : t64

5~ Site 6 - 3

Site 6 - 3

•

II

¢...~

..

'Site6-4

~te

6 4

2O

_s" i

0

15.

~

~

20

'site6-51

2O

o

r

--

z!.:

Site 5-5 -

I

i

i i

I

I

I

o 2o

,o

~e

Fbw Imm/day)

~

~

~o

15

io

Flow

2o

25

3o

Ir~/dayl

Fig. 2. Relationships between measured and net concentrations of inorganic A1 and streamflow at Sites 1-5 in Catchment 6. Measured concentrations are plotted with the best-fit relationship of the Johnson et al. (1969) model. Net concentration values were determined by subtracting upstream contributions of chemical flux and streamflow to obtain concentrations that reflect each individual subcatchment. m e a s u r e d v a l u e s of c o n c e n t r a t i o n (pM) o b t a i n e d from the m o n t h l y s a m p l e s and t h e d i s c h a r g e of e a c h s a m p l i n g date (1 • day 1). Model-predicted c o n c e n t r a t i o n s for C a t c h m e n t 6 are c o m p a r e d w i t h m e a s u r e d c o n c e n t r a t i o n s in Figs. 2-5. C o n c e n t r a t i o n s of m o s t c o n s t i t u e n t s , at m o s t s a m p l i n g l o c a t i o n s , exhibited a s t a t i s t i c a l l y significant r e l a t i o n s h i p w i t h the discharge-based p a r a m e t e r x (Table 1). In cases w h e r e the r e l a t i o n s h i p w a s n o t significant, model-predicted c o n c e n t r a t i o n s a p p r o x i m a t e d the a r i t h m e t i c m e a n of m e a s u r e d c o n c e n t r a t i o n s (Figs. 2, 3 and 5), and therefore w e r e also used in flux c a l c u l a t i o n s . Error e s t i m a t e s of c o n c e n t r a t i o n s used to c o m p u t e flux v a l u e s w e r e d e v e l o p e d for all s a m p l i n g dates by c a l c u l a t i n g the a v e r a g e p e r c e n t a g e difference b e t w e e n m e a s u r e d c o n c e n t r a t i o n s and apparent c o n c e n t r a t i o n s

PATTERNS

OF CONCENTRATION

153

DISCHARGE RELATIONSHIPS

t2oMeasured H + C oncentratlons I~M) 1oo Bo

ii "~

60

•

•

......

i

Net H~Concentf'abon5 IpMJ Sure 5 2

Site 5 2

o;

$,'.

,.

blean = 385

2O "

SJte 5-3

Site 5-3

3O

20

40 50 8,0

15 ~0

do0 12o 140

Site6 4

S~t£ 6 4

,5, ll~li|,

•

,

. . . . .

L

-20 30

ol Site 6 5 tfi'

Site 6-5

•

30 20 I0 0 I0

ma

30 o

5

I0

15

F~w Immldayl

5

I0

15

2O

25

3~

4O

FlOW (mm/dayl

Fig. 3. Relationships between measured and net concentrations of H + and streamflow at Sites 1 5 in C a t c h m e n t 6. Measured c o n c e n t r a t i o n s are plotted with the best-fit relationship of the J o h n s o n et al. (1969) model. Net concentration values were determined by subtracting upstream contributions of chemical flux and streamflow to obtain c o n c e n t r a t i o n s t h a t reflect each individual subcatchment.

predicted by the model, with the exception of ANC. Error estimates fell within reasonable limits, averaging 19.4% with a range between 6 and 43% (Table 2). No statistically significant relationships occurred between ANC and flow. Therefore, ANC error was expressed as the standard deviation of the mean because this value closely approximated model-predicted concentrations and ANC values exhibited both positive and negative values, making the above error expression of limited value. RESULTS Measured concentrations of inorganic A1 and H + exhibited similar responses to variations in streamflow at each sampling location in Catchment

154

G.B. LAWRENCE

leo~leasu~edSL 19MI Site 6-t

t40*/ 120~p 100

40~

lel

C.T. DRISCOLL

q

m .

"i

?OOL

Net S~ ~M} Dte 5 2 I

Die 6

i

',oo!

a 60

AND

m•

~ .

~::¢ ~'. ,. , •

1oo

50 40 2O

- - ] 20 40 D!e

SLte 6-3 !

'~.

5-3 200 150 100

i

op

Site 6-4

Snte 5 4 i

150 tO0 1

:L

o! o

•

•

i Site 5-5

Site 6-5 5O

.

50

40 i

•

50

'[..Q..

~

m !

--i

• m.

100

• •

•

}m 5O

i. 5

5 1oFlow mm/dayl

29

3o~o

0

5

noHow Imm15/day}2o

3O

Fig. 4. Relationships between measured and net concentrations of Si and streamflow at Sites 1 5 in Catchment 6. Measured concentrations are plotted with the best-fit relationship of the Johnson et al. (1969) model. Net concentration values were determined by subtracting upstream contributions of chemical flux and streamflow to obtain concentrations that reflect each individual subcatchment.

6 (Figs. 2 and 3). At lower elevations (Sites 6-4 and 6-5), concentrations increased asymptotically with increases in flow. This response shifted to a hyperbolic decline in concentration with increases in flow at the highest site. Little or no relationship was observed between inorganic A1 and H ÷ concentrations and flow, at mid-elevation Sites 6-2 and 6-3 (Figs. 2 and 3). For concentrations of dissolved Si, however, a consistent hyperbolic response was observed with increases in streamflow at all sites in Catchment 6 (Fig. 4). DOC concentrations increased with increases in streamflow at the lowest two sites in Catchment 6, but did not exhibit a clear relationship at the other sampling locations (Fig. 5). The Johnson et al. (1969) model accurately described the relationships

PATTERNS OF CONCENTRATION

155

DISCHARGE RELATIONSHIPS

16ooMeasured gO£ l~Ml Site

6I

13oo

~ooo• •i • "•'... ""

•

Mean, ?B8 •

400

3o°oI i• 500-

Net OOC bMl ••

s,,e6-21

Site 6 2

• ~3oo

6oOr I i I

46O~I •

i

I

-

i "~

Mean: 410

ic

3O0

1:16oo

26O~ 100* SRe 5 3 4o0L

•m

r

•

2Oo~:~II" •

Mean : 252 "

&re 5 ] !

%. be

~

mi

I

200

•

400

" "

600 1oo~

-800 i-,ooo Site5-4

S,te 6 a

6o0

4oo~

- 4O0 3oo ~

m

mi ~ m

I

• "2OO io 1

I -2O0 i

Site5-5 !500

Site 5-5

2400

4-40°

"':~ ",5

I0 Flow rnrnlday

,5

2o

600 ]3o0

Flow In~/cIa~]I

Fig. 5. Relationships between measured and net concentrations of DOC and streamflow at Sites I 5 in Catchment 6. Measured concentrations are plotted with the best-fit relationship of the Johnson et al. (1969) model. Net concentration values were determined by subtracting upstream contributions of chemical flux and streamflow to obtain concentrations that reflect each individual subcatchment. between c o n c e n t r a t i o n and flow for inorganic A1, H +, Si and DOC (Figs. 2-5) in Catchment 6. The model parameter log{b} ranged approximately between - 3 and - 5, w h e n a clear relationship existed between c o n c e n t r a t i o n s and flow (Table 1; Figs. 2-5). Values of log{b} for inorganic A1 and H + exhibited a general decrease m o v i n g upslope in Catchment 6, but were very similar at all sites for Si, with the exception of Site 6-3. Values of log{b} for DOC were slightly < - 5 at Sites 6-4 and 6-5, but fell well outside the range of - 3 to 5 at upstream sites, where c o n c e n t r a t i o n s appeared to be independent o f streamflow (Table 1). With the exception of Site 6-4, no clear relationships between net concentration and streamflow were observed for inorganic A1, or H + in Catchment 6

156

G.B. LAWRENCEAND C.T. DRISCOLL

TABLE 2 A v e r a g e p e r c e n t a g e e r r o r of c o n c e n t r a t i o n s p r e d i c t e d by t h e mode l of J o h n s o n et al. (1969); d e t e r m i n e d by t h e p e r c e n t a g e d i f f e r e n c e b e t w e e n m e a s u r e d c o n c e n t r a t i o n s a n d a p p a r e n t modeld e r i v e d c o n c e n t r a t i o n s . E r r o r for A N C is e x p r e s s e d as t h e s t a n d a r d d e v i a t i o n of t h e mean, s i nc e both positive and negative values occur Site

I n o r g a n i c A1

H~

DOC

Si

ANC

Catchment 5 1 2 3 4 5 6

38 35 18 16 27 24

35 9.6 9.5 11 10 14

33 22 19 20 25 25

14 14 14 11 6.8 11

31.9 - 31.9 - 27.1 - 17.1 - 12.0 -8.95

+ + _+ + +_ _+

Catchment 6 1 2 3 4 5

43 25 23 17 29

9.3 12 20 11 13

22 20 19 26 23

23 21 16 11 10

66.5 -39.0 -23.9 15.3 10.9

+ 12.4 + 6.66 + 6.87 + 3.72 + 2.62

18.2 4.97 3.20 4.05 1.95 2.11

(Figs. 2 and 3). At Site 6-4, however, a strong asymptotic increase in the net concentrations of inorganic A1 and H ÷ was observed with increasing flow, similar to the responses of the measured concentrations for this site shown in Figs. 2 and 3. Net concentrations of inorganic A1 were greatest at Sites 6-2 and 6-3 and exhibited positive values under all flow conditions, indicating that these subcatchments were streamwater sources of inorganic A1 (Fig. 2). Net concentrations at Site 6-5 were less than zero on most sampling dates, indicating that this portion of the catchment was an effective sink for inorganic A1. Net concentrations of H ÷ were consistently greater than zero only at Site 6-2 (Fig. 3). In a similar way to measured concentrations, net concentrations of Si at Sites 6-4 and 6-5 decreased with increases in streamflow (Fig. 4). Net concentrations of Si at the higher elevation Sites (6-2 and 6-3), however, appeared to be independent of streamflow. All sites appeared to be sources of streamwater Si. The relationship between concentrations of DOC and streamflow, observed at Site 6-5, was reversed for net concentrations (Fig. 5). At Site 6-4, however, an increase in net DOC concent r a t i on was observed with increasing streamflow. In a similar way to measured concentrations, net DOC concentrations did not relate to streamflow at Sites 6-2 and 6-3. Site 6-3 was primarily a sink for DOC, Site 6-5 a source, and Sites 6-2 and 6-4 acted as both a source and a sink on various sampling days. Hillslope trends of annual flux values were quite similar among sampling years; however, notable differences occurred between catchments (Figs. 6 and 7). Inorganic A1 flux was greatest at the upper reaches of Catchment 5, whereas flux in Catchment 6 was most pronounced at the mid-elevation region.

PATTERNS

OF

CONCENTRATION-DISCHARGE

157

RELATIONSHIPS

WATFISHED5

)VATEFISHEO6

N/l~a

III

a d/da InorganicAI

:il

Inorganic At! m~

a

:l"

i

,

lm(L

-

~

~

~

J

Hydrogoo Ion

~If 0~

"

I

Hydrogen Ion

\'\\\,

~

~L 6~

•

[Dsso]vedOrg~ll]EDar~on

DissolvedOrganicCarH~

•

?!

• •

I

\

0=

if

i

\'~,

iii

o

- -

,

,

,

Dissolved Si

l)]ssolvedSi o

~ Neulrahz[ngCapac~ty 400

200

~,~ Neu~ratizmng CaaaCRy 1

eq/'na 5 Cu~iative

10

15

~ainage Area [ha~°

BOO

~

J 2

4

5

8

tO

CumulatH,e ~ainage/~ea (hal la

t4

Figs. 6 and 7. A n n u a l flux values for each individual subcatchment in Catchments 5 and 6. Upstream flux has been subtracted fromtotal flux estimated at each sampling location. Flux is expressed in M . hectare ° , w i t h the exception of DOC, expressed in k M c a r b o n , hectare ° and ANC, expressed in eq. hectare 0.

Minimum inorganic A1 flux in Catchment 5 occurred at the second lowest subcatchment (Site 5-5), with the lowest subcatchment (Site 5-6) exhibiting a substantial increase relative to this site. Catchment 6 flux values decreased consistently downslope of the mid-elevation peak, reaching negative values at the lowest sampling point. Trends of H ~ flux exhibited overall decreases from the upper elevations to the lowest regions of both Catchments 5 and 6 (Figs. 6 and 7). However, these trends were interrupted at the lowest subcatchment in Catchment 5 (Site 5-6) and second-lowest subcatchment in Catchment 6 (Site 6-4). In both catchments negative flux values were observed at lower elevations. Hillslope trends of DOC flux were virtually identical to trends of H + for their respective catchments

158

G.B. LAWRENCE AND C.T. DRISCOLL

(Figs. 6 and 7). Dissolved Si flux exhibited general increases moving downslope in both catchments (Figs. 6 and 7), with some variation between years. Like inorganic A1, longitudinal trends in the flux of ANC differed between catchments. In Catchment 5, negative values of ANC flux were observed at the upper three sites (Fig. 6), with a minimum value occurring at Site 5-2. The lowest three sites all exhibited positive ANC flux; however, values at Sites 5-4 and 5-6 were considerably less than at Site 5-5. In Catchment 6, ANC flux was negative at the upper two sites, but increased to positive values at the lower three sites (Figs. 6 and 7). Maximum ANC flux occurred at mid-elevation Site 6-3. DISCUSSION

Application of the Johnson et al. (1969) model The Johnson et al. model (1969) provides a useful tool for evaluating the hydrologic dependency of stream concentrations. Values of {b} observed at the base of Catchment 6 (Table 1) approximated those reported by Johnson et al. (1969), for A1 (log{b} = - 5 ) , H ÷ (log{b} = - 5 ) and Si (log{b} = - 4 ) . DOC relationships were not reported in the Johnson et al. (1969) study. Variation in the value of {b} among chemical constituents, however, limits the generality of this model. Johnson et al. (1969) interpreted {b} as a constant determined by the ratio of soil water residence time to field capacity, without chemical influences. Variations in the value of {b} for individual chemical constituents were also observed, moving upslope, where in some cases concentrations appeared to be unrelated to flow. In the case of inorganic A1 and H ÷, the value of {b} was greater at the uppermost sampling location than at the base of the catchment. Smaller rather than larger values, however, would be expected at upper elevations, where soils tend to be more shallow than those downslope (Lawrence et al., 1986), but have a greater concentration of organic matter (Huntington et al., 1988). These factors suggest a shorter soil water residence time and greater field capacity; both would contribute to a smaller value of (b}. Johnson et al. (1969) attributed variations of {b} to localized chemical effects related to vertical variation of soils and soil solution. This explanation requires that the model represents the mixing of upper and lower soil water, rather than rain and soil water. The theoretical development of the model, however, is based on the mixing of capillary water with gravitational water making it inappropriate to apply this model to the mixing of soil water from two different depths. Since the value of {b} varies for different chemical constituents, the model of Johnson et al. (1969) has been regarded as empirical (Hooper, 1986). This conclusion is supported by the results of the present study. However, at the uppermost site the value of logIb } appears to be converging on a value that ranges between - 3 . 5 and -3.8, with the exception of DOC (Table 1). The similarity of the concentration~lischarge relationships of inorganic A1, H ÷

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and Si at the top of the catchment, where much of the confounding influences of areal variation do not occur, suggests that the fit of the model may be more than mathematical coincidence. The rate of mixing between gravitational and capillary water may be one of a number of controlling factors of variations in stream chemistry.

The importance of subsurface flow paths A number of studies since the work of Johnson et al. (1969) have suggested the importance of subsurface flow paths as a control of stream chemistry (Chen et al., 1984; Peters and Driscoll, 1987; McDowell and Likens, 1988; Lawrence et al., 1988). This concept is based on vertical variations which typically occur in soils and soil solutions. Concentrations of H* and DOC tend to be highest in solutions draining the forest floor, whereas inorganic A1 and Si concentrations tend to be highest in lower mineral soil solutions (Lawrence et al., 1988). Solutions passing through glacial till tend to be further enriched with Si but exhibit extremely low concentrations of inorganic A1 and H +. The compacted structure of till in HBEF catchments, however, limits water movement through this layer. During periods of increased streamflow, the zone of soil saturation moves upward, resulting in lateral movement of water which can intercept the stream channel with little or no contact with the lower mineral soil. Under these conditions, streamwater concentrations of H + and DOC would be expected to increase, whereas inorganic A1 and Si concentrations would decrease. Observed variations of concentration-discharge relationships in Catchment 6, however, imply a more complicated system than the flow path concept described above. One explanation for this response is the areal variation of soil solution chemistry which occurs in Catchment 6 (Lawrence et al., 1988). Concentration-discharge relationships at any of the stream sampling locations selected for this study were influenced by upstream drainage areas as well as the subcatchment drained by that particular site. By relating net rather than measured concentrations to flow, upstream influences were removed, allowing the hydrologic controls of each subcatchment to be individually examined. Since the flow path concept relies only on vertical variations of soil solution, this mechanism can be tested by evaluating the observed relationships between net concentrations and streamflow. At Site 6-1, measured concentrations were assumed to be analogous to net concentrations since this site represented the highest sampling point in the catchment; concentrations of inorganic A1, H ÷ and Si all decreased with increases in streamflow (Figs. 2-4). These responses are consistent with the flow-path mechanism, with the apparent exception of H~. The soil flow-path mechanism however, may also be influencing H ÷ concentrations at Site 6-1. Previous research at the HBEF has indicated that, at upper elevations, increases in flow tend to increase pH through the dilution of both organic and inorganic strong acid anions (Lawrence et al., 1988).

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Although organic acidity is greater in forest soil water solutions, it tends to be diluted by increased flow, presumably owing to kinetic limitations on the mineralization of soil organic C (Lawrence et al., 1988). Concentrations of inorganic strong acid anions tend to be greater in the mineral soil, particularly in systems such as the HBEF impacted by acidic deposition (Driscoll et al., 1987). DOC concentrations do not appear to be related to streamflow at the uppermost site (Fig. 5). The rate-dependent mineralization of soil organic matter, as well as processes occurring within the stream channel may play a more important role in controlling variations of DOC concentration than changes in flow paths in this subcatchment. Relationships between net concentrations and streamflow at Sites 6-2 to 6-5 suggest that changes in flow paths had little or no influence on controlling stream chemistry at these sites. With the exceptions of inorganic A1 and H ÷ at Site 6-4 and Si at Sites 6-4 and 6-5, which may be explained by processes other than changes in flow paths, net concentrations appear to be independent of flow. Although influences upslope of subcatchment 6-4 were removed by computing net concentrations, the ephemeral tributary within subcatchment 6-4 may extend into an acidic, Al-rich hydrologic source area, similar to subcatchment 6-3, causing increases of A1 and H ÷ concentrations. This response results in undersaturation with respect to the solubility of synthetic gibbsite within this subcatchment, during high flow conditions (Fig. 8). During low flow conditions, with little or no contribution from the acidic source area, oversaturation with respect to synthetic gibbsite solubility results in precipitation of A1 within this region of the stream channel. Annual average solution concentrations of inorganic A1 approximate temperature-corrected concentrations expected from equilibrium with synthetic gibbsite in the lower reaches of this stream (Dahlgren et al., 1989). The flow response of net Si concentrations at Sites 6-4 and 6-5 can be attributed to greater variations of soil water residence time within the deeper mineral soil layer which occurs in the lower regions of the catchment, since establishment of equilibrium between silicate minerals and soil solution is time-dependent (Johnson et al., 1981; Dahlgren et al., 1989). Through these processes, changes in concentration could occur without lateral movement of water through upper soil horizons. Concentration-discharge relationships of the present study indicate that changes in flow paths provide only a limited explanation for temporal variations of stream chemistry. M t h o u g h changes in concentrations of inorganic A1, H* and Si at Site 6-1 could be explained by this theory, most concentration-discharge relationships downslope of Site 6-1 were inconsistent with the responses expected from the flow-path theory. Those which were not, could also be explained by other mechanisms. The inability of this theory to describe all of the observed variations of stream chemistry downstream of Site 6-1 is likely to be the result of a combination of factors which include: (1) areal variation in the depth of soil horizons, (2) modification of soil profiles in the

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Site 6-4

0.4 0

• 0

-0.41 -0.6 -0.8-0

I ~ -"

mm

• • T !

i

Flow

Ir~m/dayl

Fig. 8. Saturation indices of synthetic gibbsite as a function of streamflowat Site 4 in Catchment 6. vicinity of stream channels owing to lateral flow, and (3) macropore flow. These data indicate th a t additional processes need to be incorporated with the flowpath theory and that the importance given to changes in flow paths as a control of stream chemistry in current acidification models needs to be reconsidered.

The importance of areal variations of flow response Previous research in Catchment 6 identified distinctive elevational trends in stream chemistry which were associated with spatial variation of soils and vegetation, analogous to soil catenas (Lawrence et al., 1986). Superimposed upon these spatial variations are the hydrologic processes which generate streamflow. The interaction between these factors results in spatial variations of the response of stream chemistry to changes in streamflow. Flow response at the base of the catchment represents an integration of the flow responses which occur upstream, in the same manner that stream chemistry represents an integration of biogeochemical processes which occur upstream. The influence of longitudinal variations in flow response can be evaluated by successively comparing measured concentrations with net concentrations, moving from the top to the bottom of the catchment. In the case of inorganic A1 and H ÷, flow responses at the base of the catchment were largely determined by the influence of subcatchment 6-4, although they were modified somewhat by subcatchment 6-5 (Figs. 2 and 3). Like inorganic A1 and H ~ , net concentrations of Si did not exhibit a response to changes in flow at subcatchments 6-2 and 6-3, owing to strong biogeochemical buffering at levels between 35 and 40 ttM (Fig. 4). However, because of the lack of variation in net Si concentrations at these sites, the hyperbolic flow response observed at Site 6-1 was repeated at Sites 6-2 and 6-3 and extended downslope by the responses of net concentrations at Sites 6-4 and 6-5. Distinctive flow responses were observed for measured DOC concentrations

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at Sites 6-4 and 6-5, but not for net concentrations, suggesting upstream influences. During periods of increased runoff, surface flow extends further upslope in subcatchment 6-1. Concentrations of DOC in this ephemeral region of the stream channel (Site 6-0) average 1050 pM, compared with 790 pM at Site 6-1 and 190#M at Site 6-5. During low-flow conditions, upper regions of subcatchment 6-1 contribute little or no water to surface runoff, reducing DOC export to downslope regions of the catchment. Variable contributions of surface runoff from this source area can explain the flow response of measured concentrations at Sites 6-4 and 6-5, in the same manner that the variable source area in subcatchment 6-4 explains the flow responses of net inorganic A1 and H ~ concentrations at Site 6-4. This effect may not influence variations of DOC concentrations above Site 6-4 owing to the smaller difference that occurs between streamwater concentrations of the middle and upper elevations. Chemical transport in Catchments 5 and 6

Spatial variations in the biogeochemical processes that control stream chemistry in Catchments 5 and 6 are evident from the chemical flux values of each subcatchment (Figs. 6 and 7). Values of H ÷ flux decrease from upper to lower regions of the catchment according to a two-step neutralization process first described by Johnson et al. (1981). Subcatchments 5-1 and 6-1 contribute the greatest flux of H ~ within their respective catchments. The initial decline of H ~ flux moving downslope is associated with an increase in ANC flux resulting from the dissolution of solid phase A1 and a decrease in organic acidity suggested by the decrease of DOC flux. This reaction is particularly effective in subcatchment 6-3, where maximum ANC flux corresponds to maximum inorganic A1 flux and minimum H ÷ and DOC flux. At the lower regions of the catchments, neutralization shifts from A1 dissolution to the dissolution of silicate minerals. The ability of silicate minerals to neutralize drainage water acidity is limited, however, as indicated by the low, but positive ANC flux values at the lowest sites in each catchment. These patterns of drainage water neutralization are consistent with the concept of soil catenas proposed by Lawrence et al. (1986). Flux of H ÷ was greatest in coniferous, upper elevations where shallow soils enriched in organic matter limit mineral dissolution (Fig. 1). At mid-elevations, vegetation shifts to hardwood species and DOC flux decreases sharply. Neutralization of drainage waters in this region is accomplished through dissolution of labile soil A1. In lower regions, soils are deeper, providing the opportunity for neutralization through silicate weathering. Although neutralization processes tend to ocur along a hillslope gradient, the trend of H ÷ flux suggests that spatial patterns of soil catenas may be related to factors other than elevation. At Site 5-6, H ~, inorganic A1 and DOC flux all increased, relative to Site 5-5. This response is related to the tributary stream in subcatchment 5-6, which appears to extend into an acidic source area similar to upper elevation subcatchments (Fig. 1). Similarly, the ephemeral tributary

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in subcatchment 6-4 can explain the increase of H + and DOC flux in this subcatchment, relative to subcatchment 6-3. The positive values of H ÷ and inorganic A1 flux at Sites 5-6 and 6-4 indicate that these subcatchments were sources of acidity. Input of acidity from these subcatchments was not clear from ANC flux trends, however, owing to the buffering of A1. Hillslope trends of chemical flux in Catchments 5 and 6 indicate that the location of acidic source areas seem to be more related to the point where surface water originates, than either elevation or distance from the top of the catchment. Johnson et al. (1981) noted that the two stages of neutralization are usually accomplished within first- or second-order drainage basins. This was observed in Catchment 6, which had no persistent tributaries along the sampling reach. In headwater systems such as Catchment 5, however, acidic contributions from secondary tributaries can extend surface water acidification to reaches further downslope.

CONCLUSION

Evaluating spatial and temporal relationships of stream chemistry on a subcatchment basis has revealed the importance of intra-catchment variations in controlling stream chemistry at the base of a catchment. Areal variations in the biogeochemical properties of soils are reflected in streamwater neutralization and concentration~iischarge relationships which vary along the hillslope. In the shallow soils of uppermost elevations in Catchment 6, changes in subsurface flow paths explain the relationship between concentration and streamflow for inorganic A1, H ÷ and Si, but not DOC. Within subcatchments downslope, streamflow exhibits no direct influence on concentrations, with the exception of Site 6-4, where variable contributions from an acidic source area appear to control the flow response of stream concentrations, and Site 6-5 where Si concentrations may be controlled by soil water residence time. Since subcatchment 6-4 acts as both a sink and a source of inorganic A1, the stream bed within this reach may contribute A1 to stream water during high flow conditions that was previously precipitated during low flows. In-stream processes may also be important in subcatchments where flow does not appear to be related to stream concentrations. The interrelationship between areal variation of soils and flow responses of stream chemistry has direct relevance to current modeling efforts to predict future catchment acidification. Efforts to date have focused on intra-catchment variation primarily in terms of soil horizon development, with only the ILWAS model considering both areal and vertical soil variations within a catchment (Christophersen et al., 1982; Schnoor et al., 1984; Cosby et al., 1985; Gherini et al., 1985). ILWAS, however, assumes a constant hydrological response with respect to contributing area and stream channel development (Gherini et al., 1985). Topmodel, the hydrological component of the catchment acidification model Magic II (Cosby et al., 1985), simulates variable contributing areas of

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runoff generation, but does not model runoff routing so that soil and soil solution parameters must be lumped to represent the overall catchment. The observed gradient of drainage water neutralization within Catchments 5 and 6 suggests that further acidification would result in a greater flux of acidity from existing acidic regions within the catchment and/or enlargement of these acidic areas. In either case, it is likely that interactions between neutralization gradients and variable source areas would influence the magnitude and timing of downstream acidification. The most significant impact of increased catchment acidification on streamwater quality may be an increase in the frequency and/or severity of acidification episodes, rather than an increase in seasonal or annual averages of H ÷ and inorganic A1 concentrations which current acid rain models provide. To predict acidification episodes, intra-catchment variations need to be incorporated, both in terms of areal variations of biogeochemical processes and concentration-discharge relationships. To accomplish this goal, further research is needed to improve our understanding of the processes through which soil solution chemistry can be related to stream chemistry. ACKNOWLEDGMENTS

The authors gratefully acknowledge the field assistance of C. Wayne Martin and Donald Buso and the streamflow data provided by James Hornbeck of the Northeastern Forest Experiment Station, U.S.D.A., Forest Service, Broomall, Pennsylvania. This work was funded by the National Science Foundation (BSR-8406634) and is a contribution of the Hubbard Brook Ecosystem Study. REFERENCES Bormann, F.H. and Likens, G.E., 1979. P a t t e r n and Process in a Forested Ecosystem. Springer, New York, 253 pp. Chen, C.W., Gherini, S.A., Peters, N.E., Murdoch, P.S., Newton, R.M. and Goldstein, R.A., 1984. Hydrologic analysis of acidic and alkaline lakes. Water Resour. Res., 20: 187~1882. Christophersen, N., Seip, H.M. and Wright, R.F., 1982. A model for streamwater chemistry at Birkenes, Norway. Water Resour. Res., 18: 977-996. Cosby, B.J., Hornberger, G.M. and Galloway, J.N., 1985. Modeling the effects of acid deposition: assessment of a lumped parameter model of soil water and streamwater chemistry. Water Resour. Res., 21:51 433. Dahlgren, R.A., Driscoll, C.T. and McAvoy, D.C., 1989. Aluminum Precipitation and Dissolution Rates in Spodosol Bs horizons in the Northeastern USA. Soil. Sci. Soc. Am., 53:1045 1052. Driscoll, C.T., 1984. A procedure for the fractionation of aqueous aluminum in dilute acidic waters. Int. J. Environ. Anal. Chem., 16:267 284. Driscoll, C.T., Fuller, R.D., Santore, R.C. and Lawrence, G.B., 1987. The speciation of ions in acidic soil solutions. Proc. Int. Soil Sci. Cong., pp. 59432. Gherini, S., Mok, L., Hudson, R.J.M., Davis, G.F., Chen, C. and Goldstein, R., 1985. The ILWAS model: formulation and application. Water, Air Soil Pollut., 26: 95-113. Gran, G., 1952. Determination of the equivalence point in potentiometric titrations. Int. Congr. Anal. Chem., 77: 661~67. Hooper, R.P., 1986. The chemical response of an acid-sensitive headwater stream to snowmelt and storm events: a field study and simulation model. Ph.D. Thesis, Cornell University, Ithaca, NY.

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Huntington, T.G., Ryan, D.F. and Hamburg, S.P., 1988. Estimating soil nitrogen and carbon pools in a northern hardwood forest ecosystem. Soil Sci. Soc. Am. J., 52: 1162-1167. Johnson, N.M., Likens, G.E., Bormann, F.H., Fisher, D.W. and Pierce, R.S., 1969. A working model for the variation of stream water chemistry at the Hubbard Brook Experimental Forest, New Hampshire. Water Resour. Res., 5:1353 1363. Johnson, N.M., Driscoll, C.T., Eaton, J.S., Likens, G.E. and McDowell, W.H., 1981. Acid rain, dissolved aluminum and chemical weathering at the Hubbard Brook Experimental Forest, New Hampshire. Geochim. Cosmochim. Acta, 45:1421 1437. Lawrence, G.B., Fuller, R.D. and Driscoll, C.T., 1986. Spatial relationships of aluminum chemistry in the streams of the Hubbard Brook Experimental Forest, New Hampshire. Biogeochemistry, 2:115 135. Lawrence, G.B., Driscoll, C.T. and Fuller, R.D., 1988. Hydrologic control of aluminum chemistry in an acidic headwater stream. Water Resour. Res., 24: 659-669. Likens, G.E., Bormann, F.H., Pierce, R.S., Eaton, J.S. and Johnson, N.M., 1977. Biogeochemistry of a Forested Ecosystem. Springer, New York, 146 pp. McDowell, W.H. and Likens, G.E., 1988. Origin, composition and flux of dissolved organic carbon in the Hubbard Brook Valley. Ecol. Monogr., 58:177 195. Peters, N.E. and Driscoll, C.T., 1987. Hydrogeologic controls of surface water chemistry in the Adirondack region of New York State. Biogeochemistry, 3: 163-180. Pierce, R.S., 1967. Evidence of overland flow on forest watersheds. In: W.E. Sopper and H.W. Lull (Editors), International Symposium on Forest Hydrology. Pergamon, Elmsford, NY, pp. 247 253. Schecher, W.D. and Driscoll, C.T., 1987. An evaluation of uncertainty associated with aluminum equilibrium calculations. Water Resour. Res., 23: 52~534. Schnoor, J.L., Palmer, W.D. and Glass, G.E., 1984. Modelling impacts of acid precipitation for northeastern Minnesota. In: J.L. Schnoor (Editor), Modelling of Total Acid Precipitation Impacts. Butterworth, Toronto, pp. 15~173. Sullivan, T.J., Christophersen, N., Muniz, I.P., Seip, H.H. and Sullivan, P.D., 1986. Aqueous aluminum chemistry response to episodic increases in discharge. Nature, 323:3243 327.