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Sources and transport of organic carbon in an Arizona river-reservoir system
Pergamon H I : 0043-1354(96100404-6
Wat. Res. Vol. 31, No. 7, pp. 1751-1759, 1997 © 1997ElsevierScienceLtd. All rights reserved Printed in Great Britain 0043-1354/97$17.00+ 0.00
SOURCES A N D TRANSPORT OF ORGANIC CARBON IN A N ARIZONA RIVER-RESERVOIR SYSTEM STUART J. PARKS and LAWRENCE A. BAKER* Department cf Civil and Environmental Engineering, Arizona State University, Tempe, AZ 85287-5306, U.S.A. (Received May 1996; accepted in revised form December 1996)
A~traet--Sources and transport of organic carbon were studied in two large, Sonoran desert watersheds (Salt and Verde rivers) and a two-reservoir system on the Verde River. Total organic carbon (TOC) concentrations in the unregulated rivers above the reservoirs did not follow a simple relationship with flow, TOC concentrations usually declined during spring runoff and reached low concentrations (1-3 mg L-~) by early summer. In most years, distinct peaks (10-30 mg L-~) occurred in late summer, coincident with small but abrupt increases in flow associated with the first monsoon rains. A two-reservoir mass balance showed that 72% of the particulate organic carbon (POC) input was retained, probably due largely to simple sedimentation. Production of dissolved organic carbon (DOC) within the reservoir was 41% of the inflow loading, even though the reservoirs accounted for only 0.14% of the watershed area. Reservoir DOC production comprised a large fraction of total watershed production because upstream DOC production was extremely low (0.2 g C m -2 yr-~), the reservoirs were moderately productive, and water residence times were fairly long during most of the year. We postulate that reservoirs are major contributors to total watershed DOC production in arid regions. © 1997 Elsevier Science Ltd Key words-~'lissolved organic carbon, total organic carbon, natural organic matter, trihalomethanes, reservoirs, watersheds
INTRODUCTION
Natural organic matter (NOM), measured as dissolved and particudate organic carbon (DOC and POC), is a key constituent in lakes and reservoirs. Aquatic DOC plays a major role in the transport and bioavailability of metals through complexation reactions, participates in important redox reactions, and is an important pH buffer in low-alkalinity systems. Organic carbon adsorbed on suspended particles increases their sorption capacity for hydrophobic organic chemicals. The role of NOM in aquatic systems is reviewed by Thurman (1985). In the past few years there has been a heightened interest in the role of NOM as a precursor in the formation of trihalomethanes (THMs) and other disinfection by-products (DBPs) that form during the disinfection of municipal water. The conventional approach for reducing DBP levels in drinking water treatment plants is to modify the treatment process. Modifications of in-plant treatment processes are expensive and will become more so as standards for DBPs become tighter. The limitations to in-plant DBP control have led several investigators to examine the sources of DBP precursors in surface waters. For example, Palmstrom et al. (1988) *Author to whom all correspondence should be addressed. [Fax: + 1 602 965 0557].
reported that 30% of the THM precursors in an Ohio municipal water system were formed in an upstream storage reservoir. Amy et al. 0990) found that irrigation return flows were a significant source of THM precursors to the California State Project water system. The transport of NOM has been intensively studied in forested watersheds and the processes involved are reasonably well understood (reviewed by Thurman, 1985). In contrast, NOM transport in arid and semi-arid watersheds is not well understood. The goal of this study is to identify sources of organic carbon in the watershed and reservoirs of the Verde River in Arizona and to determine factors controlling temporal trends of total organic carbon (TOC) in both the Salt and Verde rivers.
STUDY AREA
The study area included the unregulated portions of the watersheds of the Salt and Verde rivers in Arizona and a two-reservoir system on the lower Verde River (Fig. I). These two rivers provide a major fraction of the municipal water supply to the Phoenix metropolitan area. Both watersheds are large and originate at elevations above 2500 m. Rainfall exceeds 80 cm yr -~ at the highest elevations and decreases with elevation to around 20 cm yr-~ in the
1751
1752
S.J. Parks and L.A. Baker
~e 9508500 orseshoe servoir
AreaInDetail
""
salRit ver .9~%~Sites""'"'"" ~
N
A Fig. 1. The Salt and Verde river watersheds in Arizona. The inset shows the location of Horseshoe and Bartlett reservoirs in greater detail.
vicinity of the reservoirs. Snowpack generally occurs above 2200 m and normally accounts for 50-80% of runoff (D. Reigel, SRP, pers. commun). At lower elevations, rainfall occurs primarily during winter frontal storms and short-lived late summer (JulySeptember) monsoons. Annual runoff is very low (Table 1), occurring primarily during the spring runoff associated with snowmelt. As is typical of desert rivers, flow varied enormously within a given year and among years for both rivers. The ratio of historical peak daily flow:mean daily flow was 131:1 for the Salt River and 171:1 for the Verde River. During the period of T O C observations (1976-1984 for the Salt River and 1981-1984 for the Verde River) the ratio of peak daily flow:average flow was around 20:1 for both rivers (Parks, 1995).
The two reservoirs located at the lower end of the Verde River serve to control flooding and provide water for electrical power generation, irrigation, minicipal use, and recreation. Their physical and hydrological characteristics are shown in Table 2. The upper reservoir, Horseshoe, is nearly completely drawn down during the summer. Bartlett Reservoir, located about 10 km downstream, is maintained at a more constant stage and serves as an important recreation site.
METHODS
Our study included an examination of historical water quality records at two USGS water quality monitoring stations on the Verde and Salt rivers and a TOC/DOC mass
Table I. Physical and hydrologicalcharacteristics of the Salt and Verde river watersheds above their reservoirs. The period of record is 50 yr for the Verde River (USGS station 09508500) and 82 yr for the Salt River (USGS station 09498500). Source: Boner e ! a l . (1990) Salt River above Verde River above Lake Roosevelt HorseshoeReservoir Watershed area (km 2) 11,152 15,172 Maximum elevation (m) 3870 2680 Minimum elevation (m) 664 618 Average flow (m3 s ~) 25.1 15.6 Average runoff (cm yr t) 7.1 3.2 Minimum daily flow (m3 s ~) 1.6 1.4 Maximum daily flow (m~ s -~) 3295 2670 Historical daily peak flow:averagedaily flow 131:1 171:1
Sources of organic carbon Table 2. Physical characteristics of Horseshoe and Bartlett reservoirs Horseshoe Bartlett Volume at full capacity (m~) 1.6 x l0s 2.2 x los Area at full capacity (ha) 1133 1488 Overall hydraulic residence time (yr~) 0.05 0.15 Median hydraulic residence time (yrb) 0.07 0.67 Maximum depth (m) 47 54 Length (km) 8 19 *The overall hydraulic residence time was computed as total outflow/mean volume for the period June 17, 1994 to August 2. 1995. bMedian hydraulic residence time was computed from daily hydraulic residence tinaes (daily outflow/volume).
balance for the Horseshoe-Bartlett reservoir system on the Verde River. Temporal variability in TOC was examined for two U.S. Geological Survey (USGS) water quality stations: "Salt River above Roosevelt" (09498500), which was sampled at approximately monthly intervals from March, 1976 to September, 1984, and "Verde above Horseshoe below Tangle Creek" (09508500), which was sampled from October, 1980 to September, 1984. These stations are located immediately above the first water storage reservoir on each river. The Salt and Verde rivers are unregulated above these stations, although irrigation withdrawal and return flows alter the natural pattern of flow in the Verde River. Interpretation of TOC data was facilitated by continaous flow measurements at these stations. A water budget for the Horseshoe-Bartlett reservoir system was developed using data from the USGS and the Salt River Project (SRP), the utility which operates the reservoirs. Continuou,; flow records at the USGS gauging stations above Horseshoe Reservoir and below Bartlett Reservoir were used to compute river inflows and outflows. SRP provided daily data on reservoir releases, evaporation rates, and reservoir volumes for both reservoirs. Precipitation directly to the surface of the reservoirs was estimated from a regional precipitation map. "Seepage", which was computed as the residual from other measured terms, included real seepage plus aggregated errors, including inflows from the ungauged portion of the watershed immediately surrounding the reservoirs (4.9% of the total watershed). All water budget components were computed on a daily time step and aggregated over the course of the study, just over 1 yr. The USGS assisted us by collecting TOC and DOC samples from the upstream Verde River site (09508500). A helicopter was used to provide access to this remote site. We collected samples :Lmmediately below Bartlett Reservoir whenever possible. Because dam modifications were underway during the study, downstream access was not always possible. On these occasions we integrated TOC and DOC profiles from !:he hypolimnion to estimate outlet concentrations. A comparison of average hypolimnetic concentrations and outlet concentrations on four occasions when both measurements were made showed that the two values were nearly identical. Our sampling was informally stratified to represent varying hydrologic conditions. Because the riverine water input was 97.5% of the total water budget, we assumed that TOC and DOC inputs from precipitation and the non-gauged portion of the watershed were negligible. Bartlett Reservoir was sampled more intensively to learn more about spatial and temporal variations in chemistry. Three sites along the midline of the reservoir (Fig. 1) were sampled throughout the year. At each site, samples were cc,llected at three to six depths using a Kemmerer sampler. Temperature, dissolved oxygen, pH, and Secchi disk were measured in the field (Parks, 1995).
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All TOC/DOC samples were collected in pre-ashed, amber glass bottles. DOC samples were filtered through Whatman GF/C filters that had been pre-ashed, cleaned by filtering 75 mL distilled, deionized water through them, and oven dried at 105°C. TOC and DOC samples were acidified with H2504 to pH < 2 for preservation. Samples collected by the USGS were stored on ice for up to 36 h prior to filtration and preservation in our lab. TOC and DOC were analyzed using one of two Rosemount DC-180 carbon analyzers. During the first half of the study, analyses were conducted at the City of Phoenix's Deer Valley Water Treatment Plant. Analyses in the second half of the study were conducted in our laboratory using a newly purchased Rosemount analyzer. An interlaboratory check for 18 samples revealed a bias of 0.6% for TOC and 5.1% for DOC. Coefficients of variation for TOC and DOC were 7 and 14%, respectively, in our lab, and 6% and 7% in the City lab. Sample preparation and analyses of nutrients (total phosphorus, ammonium, soluble reactive phosphorus), chlorophyll a, conductivity, and suspended solids generally followed APHA (1989); details are described in Parks (1995). TOC and DOC mass balances were computed in two ways. Both took advantage of the available continuous flow record. In the first method (a "simple" method; Dolan et al., 1981), loading within a time interval was computed from the total flow within the time interval multiplied by the concentration at the midpoint of the interval. Total loading was computed as the sum of loadings for all time intervals. We also computed loadings using the Beale's stratified ratio estimator (Beale, 1962 in Dolan et al., 1981; Young et al., 1988), in which the loading is calculated as follows: m,
( + ls~., \ 1
(l)
nm~m,.~ 1S~
where py = estimated load, /~ = mean daily flow for the year, m: = mean daily loading for the days on which concentrations were determined, rn~ = mean daily flow for the days on which concentrations were determined, n = number of days on which concentrations were determined, l
n
S~.,. - (n -
1) ~ x,y~ - nm~my "i=l
2_
1
S" - (n - 1)
z,'~x,:
'
- n m;
x, -- individual measured flow, y~ = daily loading for each day on which concentration was determined. The estimate is derived from a ratio, m, lm,., which is the ratio of the mean of measured loads to the mean of flows when loads were measured. This ratio is used with the overall mean flow,/~, to obtain the load. This method takes advantage of situations like ours in which loading is calculated with "sparse" concentration data but an abundance of flow data. The method also reduces the bias introduced when high flows are undersampled (nearly always). The bias in the estimate is removed by the term in brackets in equation (1). As n, the number of days concentration measurements are available, increases, the influence of the bias correction term decreases (Dolan et al., 1981). A third potential method for estimating loadings, regression analysis, was not used because TOC and DOC could not be predicted from flow (see Discussion below; also see Baker, 1996, for comparison of loading estimate methods).
S. J. Parks and L.A. Baker
1754 50 m 40 "T -J ¢rj
Salt River
30--
ee
E
(5
20 m lO-
• 0
~=lO
|0
' I'
I'
30
60
ee • •
I'l' 90
•
•
120
I'
I'
150
180
I'l 210
240
Flow, m 3 sec -'1
40
L
30
E
20
O~
•
Verde River
(5 10 . ~ t
@
•
00
o
I'
I'
0
30
•
I' 60
•
l' 90
I ' I 'I' 120
Flow,
150
180
I' 210
l 240
m 3 sec -1
Fig. 2. Flow versus TOC in the Salt River (top), and Verde River (bottom). TOC data are from USGS files.
RESULTS
T O C above the reservoirs
relationships that follow the pattern that we observed. Flow is clearly a factor in determining TOC concentration, but the relationship is not a simple stochastic relationship. In contrast, we previously reported a highly predictive log-log relationship between flow and arsenic at both of these sites (Baker et al., 1994). Close inspection of temporal trends in TOC and flow suggest that two hydrologic events explain much of the variation in TOC levels. First, in most years TOC levels are fairly high (5-10 mg L -~) in the springtime and decline in an exponential fashion during spring runoff (February-June; Fig. 3). Because this period is represented by only 3-5 TOC data points in a given year, only a few of the trends are statistically significant (P < 0.05 for 2yr on the Salt River and l y r on the Verde River). However the consistency of the trend in most years suggests that the phenomenon is "real". This phenomenon is probably caused by a buildup of DOC in the soil (vadose zone) during the fall and winter at higher elevations. As snowmelt starts, DOC is washed out of the vadose zone. The first flush, which occurs well before peak flow, is high in DOC; DOC concentrations then decline as the DOC pool becomes depleted. This phenomenon is common in forested watersheds (reviewed by Thurman, 1985) and has recently been modeled by Hornberger et al. (1994) for the Snake River in Colorado.
TOC in both the Salt and Verde rivers varied with flow in an unusual way (Fig. 2). For both rivers, TOC was fairly constant and nearly always less than 10 mg L -~ when flows were greater than 30 m 3 s-'. When flows were less than 30 m 3 s -1 TOC concentrations varied enormously, from approximately 1 mg L-' to greater than 30 mg L -t. Regression anaylses using common transformations did not yield meaningful equations that could be used to predict TOC from flow (Table 3). Although a few had slopes significantly different from 0, they all had very low r2 values and poorly distributed residuals. Several investigators report a lack of relationship between TOC and flow (e.g. Jones et al., in press; Hornberger et al., 1994), but we are not aware of other TOC-flow Table 3. Statistical analyses of flow versus TOC in the unregulated portion of the Verde and Salt rivers. The Salt River is represented by USGS Station 09498500 and the Verde River is represented by USGS Station 09508500 (also see Fig. 2)
Relationship
River
r2
Significance level for F-test
Flowrate-TOC Flowrate-log TOC Log flowrate-TOC Log flowrate-log TOC FIowrate-1/(TOC) FIowrate-TOC FIowrate-log TOC Log flowrate-TOC Log flowrate-log TOC Flowrate-1/(TOC)
Salt Salt Salt Salt Salt Verde Verde Verde Verde Verde
0.062 0.062 0.036 0.068 0.065 0.012 0.090 0.022 0.163 0.095
0.0l 3 0.013 0.059 0.009 0.011 0.477 0.046 0.334 0.006 0.040
10-•1' • • •
"7 -J
E
[] O A
1976 1978 1979 1980 1981 1982 1983 1984
• •
1982 1983
•
d
O
I--
"'l'"l'"l'"l'"l"'l Jan Feb Mar Apr May Jun
Jul
10-
L E " I--
1
,,
rill
Jan Feb Mar Apr May Jun
Jul
Fig. 3. TOC concentrations (log scale) during the snowmclt period for the Salt River (top) and Verde River (bottom). Downward
trends arc significant at the P = 0.05 level f o r
1976 and 1983 on the Salt River and for 1983 on the Verde
River. Other trends, though not statistically significant, are consistently negative.
Sources of organic carbon 4o ,50
3o
~o 14.
10 0
~o
E
1o
0
I--
0
Jun
Jul
Aug
Sep
Oct
Nov
Fig. 4. TOC concentrations and mean daily flows in the Salt
River during the summer and fall of 1977. Even with sparse data collection, late summer peaks were seen in sevenof nine years of the USGS record.
Flows and TOC concentrations declined to base levels by June. Early summer TOC concentrations generally remained at around 1-3 mg L -t until the monsoon rains started in late July or August. In most years, the first monsoon rain, indicated by a small but sudden increase in flow, was accompanied by an abrupt pulse in TOC, usually to concentrations well above 10 mg L -t. Figure 4 shows a typical trend for the Salt River. By early in the summer of 1977 TOC levels had declined to ~ 2 mg L -t. The first major monsoon storm event in late July (peak flow of 25 m 3 s -~) was accompanied by a TOC level of 33mg L -~. Eleven out of 12 of the TOC values > 10 mg L -~ in the USGS record for the Salt River occurred between July 25 and September 30. Late summer TOC ]pulses were observed for seven of the nine years of record. The phenomenon was less distinct in the Verde River, probably because upstream irrigation diversions alter natural patterns and the data record is shorter. These TOC pulses probably represent a "first flush" phenomenon. We postulate that organic matter builds up on the soil surface of desert watersheds because the annual vegetation which grows in the spring dies and starts to decay during the dry summer. After several months with no precipitation, the first monsoon rains flush this accumulated material from the watershed. Because the volume of this first flush is low, TOC concentrations are very high. Water balance for the Horseshoe-Bartlett reservoir system The water budget for the study period shows that the Verde River accounts for 97.5% of the inflow to the two-reservoir system (Table 4). The flow of the
1755
Verde River exiting Bartlett Reservoir is a bit larger than the Horseshoe Reservoir inflow because the reservoirs were drawn down during the course of the study. About 3.1% of the water entering the reservoirs leaves by evaporation. Seepage from Horseshoe Reservoir (16.6% of gauged outflow) enters Bartlett Reservoir, but net seepage for the two-reservoir system is only 1.7% of river inflow. Precipitation to the surface of the two reservoirs is very small, less than 1% of total inflow. The expectedly low seepage estimate suggests that there are no major errors in the hydrologic budget. Nearly all of the inflow to the two-reservoir system occurred during spring runoff. At the end of a peak storm flow (February 15-16, 1995) the outflow-based hydraulic retention time for the two-reservoir system was only 2.1 days. In contrast, there was no outflow from Bartlett Reservoir in the period March 23-30 (hydraulic retention time = oo). Rapid flushing during springtime is reflected by very low overall residence times for these reservoirs: 0.05yr for Horseshoe Reservoir and 0.15 yr for Bartlett Reservoir. The median daily hydraulic residence time for Bartlett Reservoir (0.67 yr) is considerably longer than the overall hydraulic residence time, reflecting the fact that water stored in this reservoir is released slowly after filling. Limnological characteristics of Bartlett Reservoir Bartlett Reservoir exhibits a monomictic stratification pattern, with stratification occuring in the summer. During spring runoff (represented by the March 16, 1995 sampling date), the reservoir was highly turbid along its entire length (Secchi disk < 20 cm) and had lower conductivity than at other times of the year. The reservoir was not stratified at this time. By June, distinct stratification was evident and hypolimnetic oxygen concentrations began to decline. By late summer the entire hypolimnion was anoxic (Fig. 5a). Chlorophyll a concentrations in the epilimnion averaged 4.5 keg L -~ and Secchi disk transparency averaged 2 m, placing the reservoir roughly in the "mesotrophic" category (Reckhow and Chapra, 1983). Nearly all of the TOC in the reservoir was DOC, a situation common for most lakes. This is true even for the highly turbid spring inflow period. At any given site and time TOC and DOC profiles were nearly uniform with depth (Fig. 5b). The lack of distinct spatial patterns is not unusual, because most
Table 4. Water budget from June 17, 1994to August02, 1995for Horseshoeand Bartlett reservoirs Two-reservoir Horseshoe Bartlett system Reservoir Reservoir AV/At (ra3day-I) -209,529 -79,150 -130,379 Verde River inflow(Qm, m3 day-') 2.279,098 2.279,098 2,008,122 Bartlett outflow(Qo=,m3day-t) 2,475,826 2,008,122 2,475,826 Evaporalion from reservoirsurface (m3day-') 71.812 26,490 45,322 Precipitationto reservoirsurface(m3 day-~) 18,247 9288 8959 Net seepage (m3 day-~) 40,764 -332,924 373,688 Seepage [% of Q=) 1.7 -14.5 16.5
S. J. Parks and L. A. Baker
1756
o
Table 5. Organic carbon mass balances for the Verde River two-reservoir system from June 17, 1994 to August 02, 1995. The Beale's stratified ratio estimator method was used to calculate mass loading rates
lO
20
A. Oxygen
3O
~
6/17194
+
7/21~
40
~
8/17/94
0
+
E ~.~
a
9~/~
O
12/18/94
Change in reservoir storage (kg day -~) Mass loading above Horseshoe Reservoir (kg day -~) Mass loading below Bartlett Reservoir (kg day ~) Net source (kg day -~) Net source (as % input)
TOC
DOC
POC
149
56
93
8077
6831
1246
9831
9570
261
1903 24%
2795 41%
-892 -72%
3/16/95
,
0
0
4
;
8
,
12
16
Dissolved Oxygen (mg L-1) B. T O C
_
- 4 " - 7/21/94 + --B0 -e-
10-
E .="
20-
8/17/94 12/18/94 3116/95 8•02•95
30a
4050 0
-'
I 4
'
' 8
I
12
TOC (mg L-1) Fig. 5. Chemical profiles for (a) oxygen and (b) T O C in
methods, our simple method calculations yielded similar results. Using this method, autochthonous DOC production was 62% of input DOC and POC retention was 65% of inflow POC. The similarity in results between the two methods of calculation provides support that the computed sources and sinks are "real". The fact that TOC concentrations in the outlet of Bartlett Lake were always higher than TOC concentrations in the inlet to Horseshoe Reservoir (Fig. 6) provides additional confidence in our conclusion that the reservoirs are a net source of TOC. Unmeasured inputs are unlikely to be of sufficient importance to alter our conclusions. Although precipitation TOC and DOC inputs were not measured, these inputs are unlikely to account for more than t % of the total input of either constituent. Runoff from the ungauged portion of the watershed (as noted, 4.9% of total watershed input) may possibly contribute significant amounts of POC, but are unlikely to be a major contributor of DOC (see 'Discussion'). If ungauged washes contributed significant POC, the effect would be a larger calculated POC Sink.
Bartlett Reservoir near the dam. DISCUSSION
of the DOC in lakes is relatively recalcitrant (Wetzel, 1983). We did not incur major algae blooms on any of our sampling trips, so we would have missed any sudden peaks of easily degradable DOC that might have accompanied algae blooms. Higher TOC and DOC concentrations were observed during the spring flooding period (Fig. 5b). This is the only time we observed differences in TOC and DOC levels along the main axis of the reservoir. Organic carbon mass balance
Mass balance calculations using the stratified ratio estimator showed that DOC comprised 85% of the TOC loading in the Verde River inflow (Table 5). The mass of DOC leaving the reservoir was 41% higher than the mass of DOC entering the reservoir. In contrast, 72% of the inflow particulate organic carbon (POC, computed as TOC-DOC) was retained within the two-reservoir system. Although the Beale's ratio method is considered to be a more accurate method of computing chemical loadings than simple
Watershed export of TOC from the Verde River (0.2 g C m -2 yr -~) is similar to the 5 yr average export from nearby Sycamore Creek (0.fg C m -2 yr-'; Jones et al., in press), but considerably lower than S-1
"7
6-
.-I O}
4-
--4k-- Above Horseshoe J i --Q-- BelowBartlett ~
E
o
ii,l,l,l,i,[,i,l,l,l,l,l,l,lCl
Fig. 6. T O C in the inlet to Horseshoe Reservoir and in the outlet from Bartlett Reservoir.
Sources of organic carbon TOC export from watersheds in more verdant regions. Brinson (1976, in Thurman, 1985) showed that annual TOC export from watersheds (g m-2 yr-~) is directly proportional to annual runoff (cm yr-~). The low watershed q?OC export rate observed in this study is consistent with Brinson's export-runoff relationship. The observed flow-TOC concentration relationship for both rivers was unusual when compared to typical flow-concentration relationships. Most of the high TOC values ( > 10 mg L -~) that occur during slightly elevated flows represent wash-off from the first monsoon rains. Most of the TOC entering Horseshoe Reservoir was in the form of DOC. POC concentrations were low, even during the flooding period when suspended solids concentrations exceeded 100mg L -~. In contrast, Cole et al. (1990) estimated that particulate organic matter was ~ 50% of total organic matter (TOM) for four perennial rivers and 93% of TOM for 14 intermittent streams in New Mexico. Fisher and Grimm (1985) found that particulate organic matter averaged 28% of TOM in four successive cloudburst storms in a small So~aoran catchment in Arizona. One reason for the relatively low contribution of POC to TOC loading in the Verde River is that the watershed is quite large (15,171 km2). Because of this, much of the suspended matter initially washed from the surface probably is trapped in the watershed. The ratio of sediment generated to sediment delivered, the sediment delivery ratio, can be less than 10% for watersheds larger l:han 1000km 2 (Walling, 1983). Sediment trapping would reduce the POC and POC:DOC ratio as watershed size increases. Net retention of POC in lakes and reservoirs has been observed in many other studies. Groeger and Kimmel (1984) showed that POC retention can be predicted from hydraulic residence times. The 72% retention of POC observed in our study is very close to the value predicl:ed by the Groeger and Kimmel regression (approximately 60%, based on a tworeservoir retention time of 0.26 yr). The fact that net POC retention is :~imilar to net suspended solids retention (78%; Baker et al., 1994) suggests that simple sedimentation is an important mechanism for POC removal. Net production of DOC is the difference between in-lake sources and sinks. Extracellular release of DOC from algae and macrophytes is well documented (reviewed in Wetzel, 1983; Thurman, 1985). The fraction of net primary productivity that is exuded as DOC is probably < 20% (Wetzel, 1983).
1757
Decomposition processes also release DOC, sometimes leading to a buildup of DOC in the hypolimnion of lakes. Several processes contribute to the loss of DOC within lakes and reservoirs. Photodegradation rates of DOC are significant. Valentine and Zepp (1993) estimated that production of carbon monoxide from degradation of DOC was 0.1q).5% day -~ for 15 high-DOC waters. In laboratory experiments using lakewater from the Superior National Forest in Minnesota, Engstrom (1987) determined DOC loss rates of 0.46yr -t under a "warm/light" condition (artificial light; 25°C) and 0.16yr -: under a "cold/dark" condition (no light; 4°C). Biological degradation of DOC is also important, particularly for fresh DOC released during decomposition or exudation of algae and macrophytes. Precipitation of DOC can be a significant sink in acidic waters with high concentrations of aluminium, iron, or calcium. Very few studies have quantified autochthonous TOC and DOC production simultaneously or compared autochthonous production with allochthonous production. Among the few lakes for which we do have this information, results are quite variable (Table 6). Broberg and Persson (1984) observed a net sink of DOC in acidic Lake Gardsjon, an oligotrophic, acidic (pH = 4.6) lake in Sweden. DOC inputs nearly equaled DOC outputs for Mirror Lake, an oligotrophic lake in New Hamshire. In contrast, DOC output exceeded input by 70% in Lawrence Lake, Michigan. Although both lakes are oligotrophic, littoral productivity was more important in Lawrence Lake, perhaps accounting for the difference. On an areal basis, production of DOC in the Horseshoe-Bartlett system (38 g m -2 yr t) is higher than reported for the lakes in Table 6, probably because algal productivity is higher. The relative importance of allochthonous versus autochthonous DOC production probably depends on at least three factors. The first is watershed production of DOC. As noted earlier watershed TOC export rates are greater in wetter regions than in drier regions. Since most TOC is DOC, the same relationship should occur. The second factor is water residence time. The relative contribution of autochthonous DOC production is probably low in very rapidly flushed lakes, simply because the water does not reside in the lake long enough for DOC to accumulate. At the other extreme, for lakes with very long residence times (e.g. many seepage lakes), most DOC probably results from autochthonous
Table 6. Autochthonous production of TOC and D O C in comparison with watershed inputs (allochthonous production) in several studies. Net production is indicated by positive ( + ) values; net losses are indicated by negative ( - ) values Lake and reference Lawrence Lake, MI (Wetzel et al., 1972, cited in Wetzel, 1983) Mirror Lake, NH (Jorden et al., 1982, cited in Wetzel, 1983) Gardsjon (Broberg and iPersson, 1984) Horseshoe and Bartlett reservoirs (this study)
POC (g m -2 yr -~) (% of input) -1.3 -0.37 --12.4
-32 -32 --72
DOC (g m--' y r - ' ) (% of input) 14.8 0.4 - 14.8 38
70 3.5 - 55 41
1758
S.J. Parks and L.A. Baker
production. The third factor is primary productivity. Although there is no demonstrated relationship between net areal DOC production and primary productivity for lakes, it is reasonable to postulate that such a relationship exists. These factors may explain the rather high proportion of DOC that is produced within the Horseshoe-Bartlett reservoir system. First, runoff in the unregulated portion of Verde River watershed is very low (3.2 cm yr-~), so watershed export of DOC is very low, around 0.2 g C m 2 yr-~. Thus, DOC production from the reservoirs is relatively important simply because export from the upstream watershed is so low. Second, although the nominal hydraulic residence time of the two-reservoir system is only 0.26 yr, these reservoirs are rapidly flushed during the flooding period and have relatively long residence times during the summer and fall (up to several hundred days). This gives refractory DOC time to accumulate in the water column. Finally, the net autochthonous DOC production rate of 38 g m -2 yr -~ is high, at least compared to the few lakes for which comparable data are available. Taken together, these environmental factors result in net autochthonous DOC production that is approximately half of aUochthonous input, even though the two reservoirs comprise only 0.14% of the watershed area. This study has shown that reservoir DOC production comprises a major fraction of total DOC export from the Verde River and suggests that reservoirs may generally be an important source of DOC in arid climates. Several important questions need to be addressed. First, how does the nature of the DOC change between the inlet and outlet of the reservoirs? Of particular interest in the context of drinking water supply reservoirs is the change in reactivity of DOC with respect to the formation of disinfection by-products. Second, would reductions in nutrient supply reduce DOC levels? If algae are responsible for autochthonous DOC production, then reductions in nutrient inputs, which are well known to reduce algae abundance, should also reduce DOC production in lakes and reservoirs. Finally, how does the relative importance of autochthonous and allochthonous DOC production change with respect to varying hydrologic conditions? We postulate that DOC export from both the watershed and the reservoirs changes dramatically from year-to-year, reflecting the extreme interannual variations in precipitation and runoff that characterize this region. CONCLUSIONS 1. Two distinct temporal trends in TOC concentrations in the unregulated portions of the Salt and Verde rivers were observed: (a) dilution during the smowmelt period, and (b) an abrupt pulse to very high concentrations during the first monsoon rains.
2. The two-reservoir system at the lower end of the Verde River retained 72% of input POC and produced 41% as much DOC as the entire upstream watershed. 3. On an areal basis, the reservoirs produced around 20 times more DOC than the watershed. 4. Although the Verde River reservoirs accounted for only 0.14% of the watershed, they produced nearly one-third (29%) of the total DOC exported from the system. Acknowledgements--This project was supported by the
Arizona Water Resources Research Center (P403983) and ASU's Faculty Grant-in-Aid Program (F-019-94). We would like to thank Leslie Farnsworth-Lee for her assistance with sampling and analysis; the City of Phoenix Water Services Department for running TOC and DOC analyses in the early part of the project; Gregg Elliott, Slavco Jovanovic, and Dallas Reigel from the Salt River Project for providing water budget data, maps, and insights; and Henry Sanger and Larry Young from the USGS for assistance in collecting samples from the upstream Verde River site.
REFERENCES
APHA (American Public Health Association), (1989) Standard Methods for the Examination of Water and Wastewater, 19th edition. Donelly & Sons, New York.
Amy G. L., Thompson J. M., Tan L., Davis M. K., and Krasner S. W. (1990). Evaluation of THM percursor contributions from agricultual drains. J. Am. Wat. Works Assoc. Jan., 57-64. Baker L. A. (1996) Lakes and Reservoirs. In Water Resources Handbook, (Edited by L. Mays), Chapter 9. McGraw-Hill Publishers, New York. Baker L. A,, Qureshi T. and Farnsworth L. (1994) Sources of arsenic in the Verde and Salt rivers, Arizona. Proc. Wat. Environment Federation Conf., Sept., 1994, Chicago. Boner F. C., Konieczki A. D. and Davis R. G. (1990) Water Resources Data, Arizona: Water Year 1990. U.S. Geological Survey Water-Data Report AZ-90-1, USGS, Tuscon, AZ. Broberg O. and Persson G. (1984) External budgets for phosphorus, nitrogen, and dissolved organic carbon for the acidified Lake Gardsjon. Arch. Hydrobiol. 99, 160-175. Cole R. A., Ward T. J. and Bolton S. M. (1990) Estimating intermittent runoff concentrations of organic matter and the allochthonous organic loading of New Mexico reservoirs. Lake and Reservoir Mgmt 6, 187-196. Dolan D. M, Yui A. K. and Geist R. D. (1981) Evaluation of river load estimation methods for total phosphorus. J. Great Lakes Res. 7, 207-214. Engstrom D. R. (1987) Influence of vegitation and hydrology on humus budgets of Labrador Lakes. Can. J. Fish. Aquat. Sci. 44, 1306--1314. Fisher S. G. and Grimm N. B. (1985) Hydrolic and material budgets for a small Sonoran Desert watershed during three consecutive cloudburst floods. J. Arid Environs. 9, 105--118. Groeger A. W. and Kimmel B. L. (1984) Organic matter supply and processing in lakes and reservoirs. Lake and Reservoir Mgmt, EPA 440/5-84-001,Officeof Water, U.S. EPA, Washington DC. Hornberger G. M., Bencala K. E. and McKnight D. M. (1994) Hydrological Controls on Dissolved Organic Carbon During Smowmelt in the Snake River Near Montezuma, Colorado. Biogeochemistry, 25, 147-165.
Sources of organic carbon Jones J. B., Fisher S. G. and Grimm N. B. Biogeochemistry of dissolved organic carbon in Sycamore Creek, Arizona. Hydrobiologia, in press. Palmstrom N. S., Carlson R. E. and Cooke D. G. (1988) Potential links between eutrophication and the formation of carcinogens in drinking water. Lake and Reservoir Mgmt. 4, 1-15. Parks S. J. (1995) Sources of organic carbon in arid watersheds. M.S. ~Ehesis, Department of Civil and Environmental Engi:aeering, Arizona State University. Reckhow K. H. and Chapra S. C. (1983) Engineering Approaches for Lake Management (2 volumes) Butterworth Publishers. Thurman E. M. (1985) Organic Geochemistry of Organic
1759
Waters. Martinus Nijhoff/Dr. W. Junk Publishers, Dordrecht, The Netherlands. Valentine R. L. and Zepp R. G. (1993) Formation of carbon monoxide from the photodegradation of terrestrial dissolved organic carbon in natural waters. Environ. Sci. Technol. 27, 409-412. Walling D. E. (1983) The sediment delivery problem. J. Hydrol. 65, 209-237. Wetzel R. G. (1983) Limnology. Saunders College Publishing, Orlando. Young T. C., DePinto J. V and Heidtke T. M. (1988) Factors influencing the efficiency of some estimators of fluvial total phosphorus loading. Wat. Resour. Res. 24, 1535-1540.
Wat. Res. Vol. 31, No. 7, pp. 1751-1759, 1997 © 1997ElsevierScienceLtd. All rights reserved Printed in Great Britain 0043-1354/97$17.00+ 0.00
SOURCES A N D TRANSPORT OF ORGANIC CARBON IN A N ARIZONA RIVER-RESERVOIR SYSTEM STUART J. PARKS and LAWRENCE A. BAKER* Department cf Civil and Environmental Engineering, Arizona State University, Tempe, AZ 85287-5306, U.S.A. (Received May 1996; accepted in revised form December 1996)
A~traet--Sources and transport of organic carbon were studied in two large, Sonoran desert watersheds (Salt and Verde rivers) and a two-reservoir system on the Verde River. Total organic carbon (TOC) concentrations in the unregulated rivers above the reservoirs did not follow a simple relationship with flow, TOC concentrations usually declined during spring runoff and reached low concentrations (1-3 mg L-~) by early summer. In most years, distinct peaks (10-30 mg L-~) occurred in late summer, coincident with small but abrupt increases in flow associated with the first monsoon rains. A two-reservoir mass balance showed that 72% of the particulate organic carbon (POC) input was retained, probably due largely to simple sedimentation. Production of dissolved organic carbon (DOC) within the reservoir was 41% of the inflow loading, even though the reservoirs accounted for only 0.14% of the watershed area. Reservoir DOC production comprised a large fraction of total watershed production because upstream DOC production was extremely low (0.2 g C m -2 yr-~), the reservoirs were moderately productive, and water residence times were fairly long during most of the year. We postulate that reservoirs are major contributors to total watershed DOC production in arid regions. © 1997 Elsevier Science Ltd Key words-~'lissolved organic carbon, total organic carbon, natural organic matter, trihalomethanes, reservoirs, watersheds
INTRODUCTION
Natural organic matter (NOM), measured as dissolved and particudate organic carbon (DOC and POC), is a key constituent in lakes and reservoirs. Aquatic DOC plays a major role in the transport and bioavailability of metals through complexation reactions, participates in important redox reactions, and is an important pH buffer in low-alkalinity systems. Organic carbon adsorbed on suspended particles increases their sorption capacity for hydrophobic organic chemicals. The role of NOM in aquatic systems is reviewed by Thurman (1985). In the past few years there has been a heightened interest in the role of NOM as a precursor in the formation of trihalomethanes (THMs) and other disinfection by-products (DBPs) that form during the disinfection of municipal water. The conventional approach for reducing DBP levels in drinking water treatment plants is to modify the treatment process. Modifications of in-plant treatment processes are expensive and will become more so as standards for DBPs become tighter. The limitations to in-plant DBP control have led several investigators to examine the sources of DBP precursors in surface waters. For example, Palmstrom et al. (1988) *Author to whom all correspondence should be addressed. [Fax: + 1 602 965 0557].
reported that 30% of the THM precursors in an Ohio municipal water system were formed in an upstream storage reservoir. Amy et al. 0990) found that irrigation return flows were a significant source of THM precursors to the California State Project water system. The transport of NOM has been intensively studied in forested watersheds and the processes involved are reasonably well understood (reviewed by Thurman, 1985). In contrast, NOM transport in arid and semi-arid watersheds is not well understood. The goal of this study is to identify sources of organic carbon in the watershed and reservoirs of the Verde River in Arizona and to determine factors controlling temporal trends of total organic carbon (TOC) in both the Salt and Verde rivers.
STUDY AREA
The study area included the unregulated portions of the watersheds of the Salt and Verde rivers in Arizona and a two-reservoir system on the lower Verde River (Fig. I). These two rivers provide a major fraction of the municipal water supply to the Phoenix metropolitan area. Both watersheds are large and originate at elevations above 2500 m. Rainfall exceeds 80 cm yr -~ at the highest elevations and decreases with elevation to around 20 cm yr-~ in the
1751
1752
S.J. Parks and L.A. Baker
~e 9508500 orseshoe servoir
AreaInDetail
""
salRit ver .9~%~Sites""'"'"" ~
N
A Fig. 1. The Salt and Verde river watersheds in Arizona. The inset shows the location of Horseshoe and Bartlett reservoirs in greater detail.
vicinity of the reservoirs. Snowpack generally occurs above 2200 m and normally accounts for 50-80% of runoff (D. Reigel, SRP, pers. commun). At lower elevations, rainfall occurs primarily during winter frontal storms and short-lived late summer (JulySeptember) monsoons. Annual runoff is very low (Table 1), occurring primarily during the spring runoff associated with snowmelt. As is typical of desert rivers, flow varied enormously within a given year and among years for both rivers. The ratio of historical peak daily flow:mean daily flow was 131:1 for the Salt River and 171:1 for the Verde River. During the period of T O C observations (1976-1984 for the Salt River and 1981-1984 for the Verde River) the ratio of peak daily flow:average flow was around 20:1 for both rivers (Parks, 1995).
The two reservoirs located at the lower end of the Verde River serve to control flooding and provide water for electrical power generation, irrigation, minicipal use, and recreation. Their physical and hydrological characteristics are shown in Table 2. The upper reservoir, Horseshoe, is nearly completely drawn down during the summer. Bartlett Reservoir, located about 10 km downstream, is maintained at a more constant stage and serves as an important recreation site.
METHODS
Our study included an examination of historical water quality records at two USGS water quality monitoring stations on the Verde and Salt rivers and a TOC/DOC mass
Table I. Physical and hydrologicalcharacteristics of the Salt and Verde river watersheds above their reservoirs. The period of record is 50 yr for the Verde River (USGS station 09508500) and 82 yr for the Salt River (USGS station 09498500). Source: Boner e ! a l . (1990) Salt River above Verde River above Lake Roosevelt HorseshoeReservoir Watershed area (km 2) 11,152 15,172 Maximum elevation (m) 3870 2680 Minimum elevation (m) 664 618 Average flow (m3 s ~) 25.1 15.6 Average runoff (cm yr t) 7.1 3.2 Minimum daily flow (m3 s ~) 1.6 1.4 Maximum daily flow (m~ s -~) 3295 2670 Historical daily peak flow:averagedaily flow 131:1 171:1
Sources of organic carbon Table 2. Physical characteristics of Horseshoe and Bartlett reservoirs Horseshoe Bartlett Volume at full capacity (m~) 1.6 x l0s 2.2 x los Area at full capacity (ha) 1133 1488 Overall hydraulic residence time (yr~) 0.05 0.15 Median hydraulic residence time (yrb) 0.07 0.67 Maximum depth (m) 47 54 Length (km) 8 19 *The overall hydraulic residence time was computed as total outflow/mean volume for the period June 17, 1994 to August 2. 1995. bMedian hydraulic residence time was computed from daily hydraulic residence tinaes (daily outflow/volume).
balance for the Horseshoe-Bartlett reservoir system on the Verde River. Temporal variability in TOC was examined for two U.S. Geological Survey (USGS) water quality stations: "Salt River above Roosevelt" (09498500), which was sampled at approximately monthly intervals from March, 1976 to September, 1984, and "Verde above Horseshoe below Tangle Creek" (09508500), which was sampled from October, 1980 to September, 1984. These stations are located immediately above the first water storage reservoir on each river. The Salt and Verde rivers are unregulated above these stations, although irrigation withdrawal and return flows alter the natural pattern of flow in the Verde River. Interpretation of TOC data was facilitated by continaous flow measurements at these stations. A water budget for the Horseshoe-Bartlett reservoir system was developed using data from the USGS and the Salt River Project (SRP), the utility which operates the reservoirs. Continuou,; flow records at the USGS gauging stations above Horseshoe Reservoir and below Bartlett Reservoir were used to compute river inflows and outflows. SRP provided daily data on reservoir releases, evaporation rates, and reservoir volumes for both reservoirs. Precipitation directly to the surface of the reservoirs was estimated from a regional precipitation map. "Seepage", which was computed as the residual from other measured terms, included real seepage plus aggregated errors, including inflows from the ungauged portion of the watershed immediately surrounding the reservoirs (4.9% of the total watershed). All water budget components were computed on a daily time step and aggregated over the course of the study, just over 1 yr. The USGS assisted us by collecting TOC and DOC samples from the upstream Verde River site (09508500). A helicopter was used to provide access to this remote site. We collected samples :Lmmediately below Bartlett Reservoir whenever possible. Because dam modifications were underway during the study, downstream access was not always possible. On these occasions we integrated TOC and DOC profiles from !:he hypolimnion to estimate outlet concentrations. A comparison of average hypolimnetic concentrations and outlet concentrations on four occasions when both measurements were made showed that the two values were nearly identical. Our sampling was informally stratified to represent varying hydrologic conditions. Because the riverine water input was 97.5% of the total water budget, we assumed that TOC and DOC inputs from precipitation and the non-gauged portion of the watershed were negligible. Bartlett Reservoir was sampled more intensively to learn more about spatial and temporal variations in chemistry. Three sites along the midline of the reservoir (Fig. 1) were sampled throughout the year. At each site, samples were cc,llected at three to six depths using a Kemmerer sampler. Temperature, dissolved oxygen, pH, and Secchi disk were measured in the field (Parks, 1995).
1753
All TOC/DOC samples were collected in pre-ashed, amber glass bottles. DOC samples were filtered through Whatman GF/C filters that had been pre-ashed, cleaned by filtering 75 mL distilled, deionized water through them, and oven dried at 105°C. TOC and DOC samples were acidified with H2504 to pH < 2 for preservation. Samples collected by the USGS were stored on ice for up to 36 h prior to filtration and preservation in our lab. TOC and DOC were analyzed using one of two Rosemount DC-180 carbon analyzers. During the first half of the study, analyses were conducted at the City of Phoenix's Deer Valley Water Treatment Plant. Analyses in the second half of the study were conducted in our laboratory using a newly purchased Rosemount analyzer. An interlaboratory check for 18 samples revealed a bias of 0.6% for TOC and 5.1% for DOC. Coefficients of variation for TOC and DOC were 7 and 14%, respectively, in our lab, and 6% and 7% in the City lab. Sample preparation and analyses of nutrients (total phosphorus, ammonium, soluble reactive phosphorus), chlorophyll a, conductivity, and suspended solids generally followed APHA (1989); details are described in Parks (1995). TOC and DOC mass balances were computed in two ways. Both took advantage of the available continuous flow record. In the first method (a "simple" method; Dolan et al., 1981), loading within a time interval was computed from the total flow within the time interval multiplied by the concentration at the midpoint of the interval. Total loading was computed as the sum of loadings for all time intervals. We also computed loadings using the Beale's stratified ratio estimator (Beale, 1962 in Dolan et al., 1981; Young et al., 1988), in which the loading is calculated as follows: m,
( + ls~., \ 1
(l)
nm~m,.~ 1S~
where py = estimated load, /~ = mean daily flow for the year, m: = mean daily loading for the days on which concentrations were determined, rn~ = mean daily flow for the days on which concentrations were determined, n = number of days on which concentrations were determined, l
n
S~.,. - (n -
1) ~ x,y~ - nm~my "i=l
2_
1
S" - (n - 1)
z,'~x,:
'
- n m;
x, -- individual measured flow, y~ = daily loading for each day on which concentration was determined. The estimate is derived from a ratio, m, lm,., which is the ratio of the mean of measured loads to the mean of flows when loads were measured. This ratio is used with the overall mean flow,/~, to obtain the load. This method takes advantage of situations like ours in which loading is calculated with "sparse" concentration data but an abundance of flow data. The method also reduces the bias introduced when high flows are undersampled (nearly always). The bias in the estimate is removed by the term in brackets in equation (1). As n, the number of days concentration measurements are available, increases, the influence of the bias correction term decreases (Dolan et al., 1981). A third potential method for estimating loadings, regression analysis, was not used because TOC and DOC could not be predicted from flow (see Discussion below; also see Baker, 1996, for comparison of loading estimate methods).
S. J. Parks and L.A. Baker
1754 50 m 40 "T -J ¢rj
Salt River
30--
ee
E
(5
20 m lO-
• 0
~=lO
|0
' I'
I'
30
60
ee • •
I'l' 90
•
•
120
I'
I'
150
180
I'l 210
240
Flow, m 3 sec -'1
40
L
30
E
20
O~
•
Verde River
(5 10 . ~ t
@
•
00
o
I'
I'
0
30
•
I' 60
•
l' 90
I ' I 'I' 120
Flow,
150
180
I' 210
l 240
m 3 sec -1
Fig. 2. Flow versus TOC in the Salt River (top), and Verde River (bottom). TOC data are from USGS files.
RESULTS
T O C above the reservoirs
relationships that follow the pattern that we observed. Flow is clearly a factor in determining TOC concentration, but the relationship is not a simple stochastic relationship. In contrast, we previously reported a highly predictive log-log relationship between flow and arsenic at both of these sites (Baker et al., 1994). Close inspection of temporal trends in TOC and flow suggest that two hydrologic events explain much of the variation in TOC levels. First, in most years TOC levels are fairly high (5-10 mg L -~) in the springtime and decline in an exponential fashion during spring runoff (February-June; Fig. 3). Because this period is represented by only 3-5 TOC data points in a given year, only a few of the trends are statistically significant (P < 0.05 for 2yr on the Salt River and l y r on the Verde River). However the consistency of the trend in most years suggests that the phenomenon is "real". This phenomenon is probably caused by a buildup of DOC in the soil (vadose zone) during the fall and winter at higher elevations. As snowmelt starts, DOC is washed out of the vadose zone. The first flush, which occurs well before peak flow, is high in DOC; DOC concentrations then decline as the DOC pool becomes depleted. This phenomenon is common in forested watersheds (reviewed by Thurman, 1985) and has recently been modeled by Hornberger et al. (1994) for the Snake River in Colorado.
TOC in both the Salt and Verde rivers varied with flow in an unusual way (Fig. 2). For both rivers, TOC was fairly constant and nearly always less than 10 mg L -~ when flows were greater than 30 m 3 s-'. When flows were less than 30 m 3 s -1 TOC concentrations varied enormously, from approximately 1 mg L-' to greater than 30 mg L -t. Regression anaylses using common transformations did not yield meaningful equations that could be used to predict TOC from flow (Table 3). Although a few had slopes significantly different from 0, they all had very low r2 values and poorly distributed residuals. Several investigators report a lack of relationship between TOC and flow (e.g. Jones et al., in press; Hornberger et al., 1994), but we are not aware of other TOC-flow Table 3. Statistical analyses of flow versus TOC in the unregulated portion of the Verde and Salt rivers. The Salt River is represented by USGS Station 09498500 and the Verde River is represented by USGS Station 09508500 (also see Fig. 2)
Relationship
River
r2
Significance level for F-test
Flowrate-TOC Flowrate-log TOC Log flowrate-TOC Log flowrate-log TOC FIowrate-1/(TOC) FIowrate-TOC FIowrate-log TOC Log flowrate-TOC Log flowrate-log TOC Flowrate-1/(TOC)
Salt Salt Salt Salt Salt Verde Verde Verde Verde Verde
0.062 0.062 0.036 0.068 0.065 0.012 0.090 0.022 0.163 0.095
0.0l 3 0.013 0.059 0.009 0.011 0.477 0.046 0.334 0.006 0.040
10-•1' • • •
"7 -J
E
[] O A
1976 1978 1979 1980 1981 1982 1983 1984
• •
1982 1983
•
d
O
I--
"'l'"l'"l'"l'"l"'l Jan Feb Mar Apr May Jun
Jul
10-
L E " I--
1
,,
rill
Jan Feb Mar Apr May Jun
Jul
Fig. 3. TOC concentrations (log scale) during the snowmclt period for the Salt River (top) and Verde River (bottom). Downward
trends arc significant at the P = 0.05 level f o r
1976 and 1983 on the Salt River and for 1983 on the Verde
River. Other trends, though not statistically significant, are consistently negative.
Sources of organic carbon 4o ,50
3o
~o 14.
10 0
~o
E
1o
0
I--
0
Jun
Jul
Aug
Sep
Oct
Nov
Fig. 4. TOC concentrations and mean daily flows in the Salt
River during the summer and fall of 1977. Even with sparse data collection, late summer peaks were seen in sevenof nine years of the USGS record.
Flows and TOC concentrations declined to base levels by June. Early summer TOC concentrations generally remained at around 1-3 mg L -t until the monsoon rains started in late July or August. In most years, the first monsoon rain, indicated by a small but sudden increase in flow, was accompanied by an abrupt pulse in TOC, usually to concentrations well above 10 mg L -t. Figure 4 shows a typical trend for the Salt River. By early in the summer of 1977 TOC levels had declined to ~ 2 mg L -t. The first major monsoon storm event in late July (peak flow of 25 m 3 s -~) was accompanied by a TOC level of 33mg L -~. Eleven out of 12 of the TOC values > 10 mg L -~ in the USGS record for the Salt River occurred between July 25 and September 30. Late summer TOC ]pulses were observed for seven of the nine years of record. The phenomenon was less distinct in the Verde River, probably because upstream irrigation diversions alter natural patterns and the data record is shorter. These TOC pulses probably represent a "first flush" phenomenon. We postulate that organic matter builds up on the soil surface of desert watersheds because the annual vegetation which grows in the spring dies and starts to decay during the dry summer. After several months with no precipitation, the first monsoon rains flush this accumulated material from the watershed. Because the volume of this first flush is low, TOC concentrations are very high. Water balance for the Horseshoe-Bartlett reservoir system The water budget for the study period shows that the Verde River accounts for 97.5% of the inflow to the two-reservoir system (Table 4). The flow of the
1755
Verde River exiting Bartlett Reservoir is a bit larger than the Horseshoe Reservoir inflow because the reservoirs were drawn down during the course of the study. About 3.1% of the water entering the reservoirs leaves by evaporation. Seepage from Horseshoe Reservoir (16.6% of gauged outflow) enters Bartlett Reservoir, but net seepage for the two-reservoir system is only 1.7% of river inflow. Precipitation to the surface of the two reservoirs is very small, less than 1% of total inflow. The expectedly low seepage estimate suggests that there are no major errors in the hydrologic budget. Nearly all of the inflow to the two-reservoir system occurred during spring runoff. At the end of a peak storm flow (February 15-16, 1995) the outflow-based hydraulic retention time for the two-reservoir system was only 2.1 days. In contrast, there was no outflow from Bartlett Reservoir in the period March 23-30 (hydraulic retention time = oo). Rapid flushing during springtime is reflected by very low overall residence times for these reservoirs: 0.05yr for Horseshoe Reservoir and 0.15 yr for Bartlett Reservoir. The median daily hydraulic residence time for Bartlett Reservoir (0.67 yr) is considerably longer than the overall hydraulic residence time, reflecting the fact that water stored in this reservoir is released slowly after filling. Limnological characteristics of Bartlett Reservoir Bartlett Reservoir exhibits a monomictic stratification pattern, with stratification occuring in the summer. During spring runoff (represented by the March 16, 1995 sampling date), the reservoir was highly turbid along its entire length (Secchi disk < 20 cm) and had lower conductivity than at other times of the year. The reservoir was not stratified at this time. By June, distinct stratification was evident and hypolimnetic oxygen concentrations began to decline. By late summer the entire hypolimnion was anoxic (Fig. 5a). Chlorophyll a concentrations in the epilimnion averaged 4.5 keg L -~ and Secchi disk transparency averaged 2 m, placing the reservoir roughly in the "mesotrophic" category (Reckhow and Chapra, 1983). Nearly all of the TOC in the reservoir was DOC, a situation common for most lakes. This is true even for the highly turbid spring inflow period. At any given site and time TOC and DOC profiles were nearly uniform with depth (Fig. 5b). The lack of distinct spatial patterns is not unusual, because most
Table 4. Water budget from June 17, 1994to August02, 1995for Horseshoeand Bartlett reservoirs Two-reservoir Horseshoe Bartlett system Reservoir Reservoir AV/At (ra3day-I) -209,529 -79,150 -130,379 Verde River inflow(Qm, m3 day-') 2.279,098 2.279,098 2,008,122 Bartlett outflow(Qo=,m3day-t) 2,475,826 2,008,122 2,475,826 Evaporalion from reservoirsurface (m3day-') 71.812 26,490 45,322 Precipitationto reservoirsurface(m3 day-~) 18,247 9288 8959 Net seepage (m3 day-~) 40,764 -332,924 373,688 Seepage [% of Q=) 1.7 -14.5 16.5
S. J. Parks and L. A. Baker
1756
o
Table 5. Organic carbon mass balances for the Verde River two-reservoir system from June 17, 1994 to August 02, 1995. The Beale's stratified ratio estimator method was used to calculate mass loading rates
lO
20
A. Oxygen
3O
~
6/17194
+
7/21~
40
~
8/17/94
0
+
E ~.~
a
9~/~
O
12/18/94
Change in reservoir storage (kg day -~) Mass loading above Horseshoe Reservoir (kg day -~) Mass loading below Bartlett Reservoir (kg day ~) Net source (kg day -~) Net source (as % input)
TOC
DOC
POC
149
56
93
8077
6831
1246
9831
9570
261
1903 24%
2795 41%
-892 -72%
3/16/95
,
0
0
4
;
8
,
12
16
Dissolved Oxygen (mg L-1) B. T O C
_
- 4 " - 7/21/94 + --B0 -e-
10-
E .="
20-
8/17/94 12/18/94 3116/95 8•02•95
30a
4050 0
-'
I 4
'
' 8
I
12
TOC (mg L-1) Fig. 5. Chemical profiles for (a) oxygen and (b) T O C in
methods, our simple method calculations yielded similar results. Using this method, autochthonous DOC production was 62% of input DOC and POC retention was 65% of inflow POC. The similarity in results between the two methods of calculation provides support that the computed sources and sinks are "real". The fact that TOC concentrations in the outlet of Bartlett Lake were always higher than TOC concentrations in the inlet to Horseshoe Reservoir (Fig. 6) provides additional confidence in our conclusion that the reservoirs are a net source of TOC. Unmeasured inputs are unlikely to be of sufficient importance to alter our conclusions. Although precipitation TOC and DOC inputs were not measured, these inputs are unlikely to account for more than t % of the total input of either constituent. Runoff from the ungauged portion of the watershed (as noted, 4.9% of total watershed input) may possibly contribute significant amounts of POC, but are unlikely to be a major contributor of DOC (see 'Discussion'). If ungauged washes contributed significant POC, the effect would be a larger calculated POC Sink.
Bartlett Reservoir near the dam. DISCUSSION
of the DOC in lakes is relatively recalcitrant (Wetzel, 1983). We did not incur major algae blooms on any of our sampling trips, so we would have missed any sudden peaks of easily degradable DOC that might have accompanied algae blooms. Higher TOC and DOC concentrations were observed during the spring flooding period (Fig. 5b). This is the only time we observed differences in TOC and DOC levels along the main axis of the reservoir. Organic carbon mass balance
Mass balance calculations using the stratified ratio estimator showed that DOC comprised 85% of the TOC loading in the Verde River inflow (Table 5). The mass of DOC leaving the reservoir was 41% higher than the mass of DOC entering the reservoir. In contrast, 72% of the inflow particulate organic carbon (POC, computed as TOC-DOC) was retained within the two-reservoir system. Although the Beale's ratio method is considered to be a more accurate method of computing chemical loadings than simple
Watershed export of TOC from the Verde River (0.2 g C m -2 yr -~) is similar to the 5 yr average export from nearby Sycamore Creek (0.fg C m -2 yr-'; Jones et al., in press), but considerably lower than S-1
"7
6-
.-I O}
4-
--4k-- Above Horseshoe J i --Q-- BelowBartlett ~
E
o
ii,l,l,l,i,[,i,l,l,l,l,l,l,lCl
Fig. 6. T O C in the inlet to Horseshoe Reservoir and in the outlet from Bartlett Reservoir.
Sources of organic carbon TOC export from watersheds in more verdant regions. Brinson (1976, in Thurman, 1985) showed that annual TOC export from watersheds (g m-2 yr-~) is directly proportional to annual runoff (cm yr-~). The low watershed q?OC export rate observed in this study is consistent with Brinson's export-runoff relationship. The observed flow-TOC concentration relationship for both rivers was unusual when compared to typical flow-concentration relationships. Most of the high TOC values ( > 10 mg L -~) that occur during slightly elevated flows represent wash-off from the first monsoon rains. Most of the TOC entering Horseshoe Reservoir was in the form of DOC. POC concentrations were low, even during the flooding period when suspended solids concentrations exceeded 100mg L -~. In contrast, Cole et al. (1990) estimated that particulate organic matter was ~ 50% of total organic matter (TOM) for four perennial rivers and 93% of TOM for 14 intermittent streams in New Mexico. Fisher and Grimm (1985) found that particulate organic matter averaged 28% of TOM in four successive cloudburst storms in a small So~aoran catchment in Arizona. One reason for the relatively low contribution of POC to TOC loading in the Verde River is that the watershed is quite large (15,171 km2). Because of this, much of the suspended matter initially washed from the surface probably is trapped in the watershed. The ratio of sediment generated to sediment delivered, the sediment delivery ratio, can be less than 10% for watersheds larger l:han 1000km 2 (Walling, 1983). Sediment trapping would reduce the POC and POC:DOC ratio as watershed size increases. Net retention of POC in lakes and reservoirs has been observed in many other studies. Groeger and Kimmel (1984) showed that POC retention can be predicted from hydraulic residence times. The 72% retention of POC observed in our study is very close to the value predicl:ed by the Groeger and Kimmel regression (approximately 60%, based on a tworeservoir retention time of 0.26 yr). The fact that net POC retention is :~imilar to net suspended solids retention (78%; Baker et al., 1994) suggests that simple sedimentation is an important mechanism for POC removal. Net production of DOC is the difference between in-lake sources and sinks. Extracellular release of DOC from algae and macrophytes is well documented (reviewed in Wetzel, 1983; Thurman, 1985). The fraction of net primary productivity that is exuded as DOC is probably < 20% (Wetzel, 1983).
1757
Decomposition processes also release DOC, sometimes leading to a buildup of DOC in the hypolimnion of lakes. Several processes contribute to the loss of DOC within lakes and reservoirs. Photodegradation rates of DOC are significant. Valentine and Zepp (1993) estimated that production of carbon monoxide from degradation of DOC was 0.1q).5% day -~ for 15 high-DOC waters. In laboratory experiments using lakewater from the Superior National Forest in Minnesota, Engstrom (1987) determined DOC loss rates of 0.46yr -t under a "warm/light" condition (artificial light; 25°C) and 0.16yr -: under a "cold/dark" condition (no light; 4°C). Biological degradation of DOC is also important, particularly for fresh DOC released during decomposition or exudation of algae and macrophytes. Precipitation of DOC can be a significant sink in acidic waters with high concentrations of aluminium, iron, or calcium. Very few studies have quantified autochthonous TOC and DOC production simultaneously or compared autochthonous production with allochthonous production. Among the few lakes for which we do have this information, results are quite variable (Table 6). Broberg and Persson (1984) observed a net sink of DOC in acidic Lake Gardsjon, an oligotrophic, acidic (pH = 4.6) lake in Sweden. DOC inputs nearly equaled DOC outputs for Mirror Lake, an oligotrophic lake in New Hamshire. In contrast, DOC output exceeded input by 70% in Lawrence Lake, Michigan. Although both lakes are oligotrophic, littoral productivity was more important in Lawrence Lake, perhaps accounting for the difference. On an areal basis, production of DOC in the Horseshoe-Bartlett system (38 g m -2 yr t) is higher than reported for the lakes in Table 6, probably because algal productivity is higher. The relative importance of allochthonous versus autochthonous DOC production probably depends on at least three factors. The first is watershed production of DOC. As noted earlier watershed TOC export rates are greater in wetter regions than in drier regions. Since most TOC is DOC, the same relationship should occur. The second factor is water residence time. The relative contribution of autochthonous DOC production is probably low in very rapidly flushed lakes, simply because the water does not reside in the lake long enough for DOC to accumulate. At the other extreme, for lakes with very long residence times (e.g. many seepage lakes), most DOC probably results from autochthonous
Table 6. Autochthonous production of TOC and D O C in comparison with watershed inputs (allochthonous production) in several studies. Net production is indicated by positive ( + ) values; net losses are indicated by negative ( - ) values Lake and reference Lawrence Lake, MI (Wetzel et al., 1972, cited in Wetzel, 1983) Mirror Lake, NH (Jorden et al., 1982, cited in Wetzel, 1983) Gardsjon (Broberg and iPersson, 1984) Horseshoe and Bartlett reservoirs (this study)
POC (g m -2 yr -~) (% of input) -1.3 -0.37 --12.4
-32 -32 --72
DOC (g m--' y r - ' ) (% of input) 14.8 0.4 - 14.8 38
70 3.5 - 55 41
1758
S.J. Parks and L.A. Baker
production. The third factor is primary productivity. Although there is no demonstrated relationship between net areal DOC production and primary productivity for lakes, it is reasonable to postulate that such a relationship exists. These factors may explain the rather high proportion of DOC that is produced within the Horseshoe-Bartlett reservoir system. First, runoff in the unregulated portion of Verde River watershed is very low (3.2 cm yr-~), so watershed export of DOC is very low, around 0.2 g C m 2 yr-~. Thus, DOC production from the reservoirs is relatively important simply because export from the upstream watershed is so low. Second, although the nominal hydraulic residence time of the two-reservoir system is only 0.26 yr, these reservoirs are rapidly flushed during the flooding period and have relatively long residence times during the summer and fall (up to several hundred days). This gives refractory DOC time to accumulate in the water column. Finally, the net autochthonous DOC production rate of 38 g m -2 yr -~ is high, at least compared to the few lakes for which comparable data are available. Taken together, these environmental factors result in net autochthonous DOC production that is approximately half of aUochthonous input, even though the two reservoirs comprise only 0.14% of the watershed area. This study has shown that reservoir DOC production comprises a major fraction of total DOC export from the Verde River and suggests that reservoirs may generally be an important source of DOC in arid climates. Several important questions need to be addressed. First, how does the nature of the DOC change between the inlet and outlet of the reservoirs? Of particular interest in the context of drinking water supply reservoirs is the change in reactivity of DOC with respect to the formation of disinfection by-products. Second, would reductions in nutrient supply reduce DOC levels? If algae are responsible for autochthonous DOC production, then reductions in nutrient inputs, which are well known to reduce algae abundance, should also reduce DOC production in lakes and reservoirs. Finally, how does the relative importance of autochthonous and allochthonous DOC production change with respect to varying hydrologic conditions? We postulate that DOC export from both the watershed and the reservoirs changes dramatically from year-to-year, reflecting the extreme interannual variations in precipitation and runoff that characterize this region. CONCLUSIONS 1. Two distinct temporal trends in TOC concentrations in the unregulated portions of the Salt and Verde rivers were observed: (a) dilution during the smowmelt period, and (b) an abrupt pulse to very high concentrations during the first monsoon rains.
2. The two-reservoir system at the lower end of the Verde River retained 72% of input POC and produced 41% as much DOC as the entire upstream watershed. 3. On an areal basis, the reservoirs produced around 20 times more DOC than the watershed. 4. Although the Verde River reservoirs accounted for only 0.14% of the watershed, they produced nearly one-third (29%) of the total DOC exported from the system. Acknowledgements--This project was supported by the
Arizona Water Resources Research Center (P403983) and ASU's Faculty Grant-in-Aid Program (F-019-94). We would like to thank Leslie Farnsworth-Lee for her assistance with sampling and analysis; the City of Phoenix Water Services Department for running TOC and DOC analyses in the early part of the project; Gregg Elliott, Slavco Jovanovic, and Dallas Reigel from the Salt River Project for providing water budget data, maps, and insights; and Henry Sanger and Larry Young from the USGS for assistance in collecting samples from the upstream Verde River site.
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