Concentrations and characteristics of organic carbon in surface water in Arizona: influence of urbanization

Journal of Hydrology 236 (2000) 202–222 www.elsevier.com/locate/jhydrol

Concentrations and characteristics of organic carbon in surface water in Arizona: influence of urbanization P. Westerhoff a,*, D. Anning b,1 a

Department of Civil and Environmental Engineering, Engineering Center Room ECG 252, Arizona State University, Tempe, AZ 85287-5306, USA b Central Arizona Basins NAWQA Program, US Geological Survey, Water Resources Division, 520 North Park Avenue, Suite 221, Tucson, AZ 85719, USA Received 10 September 1999; received in revised form 3 July 2000; accepted 6 July 2000

Abstract Dissolved (DOC) and total (TOC) organic carbon concentrations and compositions were studied for several river systems in Arizona, USA. DOC composition was characterized by ultraviolet and visible absorption and fluorescence emission (excitation wavelength of 370 nm) spectra characteristics. Ephemeral sites had the highest DOC concentrations, and unregulated perennial sites had lower concentrations than unregulated intermittent sites, regulated sites, and sites downstream from wastewatertreatment plants …p ⬍ 0:05†: Reservoir outflows and wastewater-treatment plant effluent were higher in DOC concentration …p ⬍ 0:05† and exhibited less variability in concentration than inflows to the reservoirs. Specific ultraviolet absorbance values at 254 nm were typically less than 2 m ⫺1(milligram DOC per liter) ⫺1 and lower than values found in most temperate-region rivers, but specific ultraviolet absorbance values increased during runoff events. Fluorescence measurements indicated that DOC in desert streams typically exhibit characteristics of autochthonous sources; however, DOC in unregulated upland rivers and desert streams experienced sudden shifts from autochthonous to allochthonous sources during runoff events. The urban water system (reservoir systems and wastewater-treatment plants) was found to affect temporal variability in DOC concentration and composition. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Organic carbon; Arid watershed; Hydrology; Urban; Ultraviolet; Fluorescence

1. Introduction Dissolved organic carbon (DOC) concentration and structural composition in many stream ecosystems depends on characteristics of the watershed (Aiken and Cotsaris, 1995). Gradients of DOC concentration * Corresponding author. Tel.: ⫹1-480-965-2885; fax: ⫹1-480965-0557. E-mail addresses: [email protected] (P. Westerhoff), [email protected] (D. Anning). 1 Tel.: ⫹1-520-670-6671, ext.: 263; fax: ⫹1-520-670-5592.

can be readily measured by current analytical techniques; however, it is considerably more difficult to determine changes in DOC composition. Terrestrial plant and soil sources (allochthonous sources) can contribute OC to a stream during runoff events as well as during low flow (i.e. base flow) periods. Bacteria and algae (autochthonous sources) can produce (e.g. exudation of extracellular by-products) and consume or alter (e.g. utilization as a respiratory energy source) the concentration and composition of OC. Many of these processes are well documented for temperate regions (McDowell and Likens, 1988; Leff

0022-1694/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-169 4(00)00292-4

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Fig. 1. Location of surface-water quality sites.

and Meyer, 1991); however, they are poorly understood for arid regions (Parks and Baker, 1997). The quantity (mg/l) and composition (i.e. chemical structure) of DOC and particulate organic carbon (POC) can affect ecosystems and municipal water supplies. DOC absorbs and reacts with sunlight energy, complexes metals, provides an energy source,

associates with hydrophobic organic chemicals, buffers low alkalinity waters, and reacts with microbial disinfectants during drinking-water treatment to form carcinogenic by-products. Arid regions throughout the world are experiencing rapid population growth and areal expansion (e.g. urban sprawl), leading to the development of large urban areas requiring

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extensive water infrastructure systems (reservoirs, conveyance systems, treatment plants, etc.). DOC concentrations can change and DOC compositions can be transformed as water moves through urban water infrastructure systems; however, these processes have not been well described in the literature. This study was a collaborative effort between Arizona State University (ASU) and the National Water-Quality Assessment Program (NAWQA) of the United States Geological Survey (USGS) (Gilliom et al., 1994; McKnight, 1994). The objective of this study was to determine the effects of seasonal watershed runoff events, variations in streamflows, water-management practices, and urban infrastructure on the concentration and composition of DOC in surface waters of Arizona, an arid state in the United States. A comparison of total organic carbon (TOC) and DOC was performed, but DOC is generally more mobile than POC in aquatic environments and was the primary focus of this study. Spatial and temporal OC concentrations, OC structural characteristics (e.g. aromatic OC content, TOC versus DOC fluxes), and probably sources (allochthonous vs. autochthonous) in Arizona surface waters are discussed in relationship to stream classification, reservoir management, and effects of continuous wastewater treatment plants (WWTPs) discharges to otherwise ephemeral streams.

2. Site description and analytical methods In Arizona, precipitation during the winter months is generally from broad regional storms and falls as rain at lower desert elevations or as snow at higher elevations, or during the summer from thunderstorms (i.e. monsoons). While runoff in uncontrolled streams generally occurs in both seasons, much of the annual runoff occurs in either one season or the other, which allows them to be categorized by their river regime. Winter-flow streams have much of their annual runoff during January through April, and generally drain the Mogollon Rim and the higher elevation mountain ranges throughout the state. Summer-flow streams have much of their annual runoff during July through September and generally drain the large alluvial basins south and west of the Mogollon Rim and the Colorado Plateau north of the Mogollon Rim (Fig. 1).

2.1. Data sources and site selection USGS data from various streamflow and waterquality monitoring networks were used to define spatial and temporal patterns in DOC concentrations and loads. Streams, reservoirs, and study sites are shown in Fig. 1, and the location, elevation, drainage area, and flow characteristics are listed in Table 1. Dates for samples collected by the USGS are illustrated by site in Fig. 2. Streams were classified as being uncontrolled, controlled (by impoundments/ dams), or effluent dependent (flow comprised mostly (⬎90%) of treated domestic wastewater except during runoff events). Uncontrolled sites were further classified as perennial, intermittent, or ephemeral. Samples collected under the USGS NAWQA were also analyzed by several spectroscopic methods (UV/Vis and fluorescence spectroscopy) to define short-term temporal and spatial variability of the characteristics and sources of DOC. Additional samples were collected from an uncontrolled intermittent stream (Sycamore Creek, I4) to characterize the variability of the concentration, chemical structure, and sources of OC during runoff events. This additional desert stream site was selected because other studies have characterized parafluvial flow patterns, nutrient cycling, microbial diversity, and DOC fluxes along several reaches (Jones et al., 1996; Valett et al., 1994; Holmes et al., 1994; Kaplan and Newbold, 2000). Samples collected during runoff events and during periods of low flow from Sycamore Creek (site I4) were archived (filtered, frozen, and stored) and later analyzed for DOC plus UVA and fluorescence properties. Synoptic sampling was also conducted at Sycamore Creek (site I4) before, during, and after a monsoon runoff event. Visual observation recorded a “wave” propagating downstream at the beginning of the monsoon event; no flow measurements were available. During on synoptic sampling effort in June 1998, glass containers (1 l) with 15-cm diameter glass funnels were placed outside just prior to a thunderstorm, during a dust storm, at three locations throughout the Phoenix metropolitan area. Samples were collected after only few minutes, to capture the initial rainfall, and then a second sample was collected 10 min after initiation of the rainfall event.

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Table 1 Surface-water quality monitoring site names, drainage area, and selected stream flow statistics Site identifier

Name

Uncontrolled ephemeral sites E1 Tanque Verde Creek at Tucson E2 Alamo Wash at Tucson E3 Indian Bend Wash at Curry Road, Tempe

Drainage area (km 2)

Mean daily discharge (m 3/s)

Median daily discharge (m 3/s)

567 24 212

0.700 NA 0.211

0.000 0.000 0.000

Uncontrolled intermittent sites I1 Gila River at Calva I2 San Pedro River at Winkleman I3 Wet Bottom Creek I4 Sycamore creek a,b

29,700 11,266 94.3 424

10.9 1.30 1.34 1.04

2.0 0.088 0.012 0.019

Uncontrolled perennial sites P1 San Pedro River at Charleston b P2 Salt River near Roosevelt b P3 West Clear Creek near Camp Verde b P4 Verde River above Horseshoe Reservoir b P5 Agua Fria River near Rock Springs

3196 11,150 624 15,172 2877

1.58 26.0 1.89 16.6 2.69

0.37 9.63 0.51 6.74 0.091

3779

462 c 391 d 393 351 21.5 c 13.6 d 30.0 24.4 c 16.7 d NA

222 348 376 331 5.66 8.15 23.1 8.07 8.44 NA

2600 3131 9072 – – 3807

NA 0.47 1.78 NA 5.14 1.50

NA 0.40 1.47 NA 5.13 1.13

Controlled sites C1

Colorado River at Lee’s Ferry

289,600

C2 C3 C4

Colorado River below Hoover Dam Colorado River below Parker Dam Gila River at Kelvin b

444,703 473,200 46,650

C5 C6

Salt River below Stewart Mountain Dam Verde River below Bartlett Dam

C7

Agua Fria River below Waddell Dam

Effluent-dependent sites ED1 Santa Cruz River near Rio Rico ED2 Santa Cruz River near Tubac b ED3 Santa Cruz River at Cortaro Road b ED4 91st Avenue WWTP outfall near Phoenix b ED5 Buckeye Canal near Avondale b ED6 Hassayampa River near Arlington b a b c d

16,140 15,960

Data from nearest USGS site. NAWQA samples analyzed for spectrometric properties. Pre-dam. Post-dam.

2.2. Description of analytical techniques Depth-integrated samples for analysis of TOC, DOC, or POC by the USGS were collected from the centroid of the stream. TOC samples were chilled and sent to the USGS National Water-Quality Lab (NWQL) for analysis by a wet oxidation method. DOC and POC samples were filtered (0.45-micron

silver filter) and then chilled. The filtrate and filter were sent to the USGS NWQL for wet oxidation analysis for DOC and POC, respectively. In some cases, TOC was calculated as the sum of DOC and POC. Detailed field and lab methods are described in Shelton (1994) and Wershaw et al. (1987). Spectroscopic analyses (absorbance and fluorescence) were conducted on field-filtered (Whatman

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Fig. 2. Sample-collection dates at USGS water-quality monitoring sites. Each (⫹) represents a sample that was collected and analyzed for DOC and TOC.

glass fiber filters, GF/F) and acidified (pH 2.0 with hydrochloric acid) samples that were stored at 4⬚C in pre-washed amber glass bottles. Ultraviolet and visible spectroscopic measurements were performed using a scanning spectrophotometer (Shimadzu UV1601) with a 1-cm quartz cell. Fluorescence intensity was measured with a scanning fluorimeter (Shimadzu 551 with PC control) with a 1-cm quartz cell. Ultraviolet and visible (UV/Vis) absorbance and fluorescence emission intensity were used to characterize the structural composition and likely sources of DOC. The absorbance of ultraviolet (UVA) and visible light energy by the material constituting DOC is a function of the molecular structure of the material. Carbon–carbon double bonds absorb energy in the range of 250–280 nm (Silverstein and Bassler, 1967; Korshin et al., 1997). Measurement of UVA normalized to DOC concentration was termed specific ultraviolet and visible absorbance (SUVA) with units of m ⫺1 (mg/l) ⫺1. SUVA measurements of DOC at

254 nm and 280 nm were positively correlated to sp 2-hybridized carbon content (e.g. carbon–carbon double bonds), aromatic carbon content, and molecular weight, and provided a molecular structure indicator for DOC (Westerhoff et al., 1999; Chin et al., 1994). Differences in SUVA for water samples collected at different locations or times reflect inherent structural differences in the material comprising the DOC. Fluorescence characteristics of DOC also depend on properties of the molecular structure and have been used to characterize DOC sources as either allochthonous or autochthonous (Donahue et al., 1998; Senesi et al., 1989). Fluorescence intensity depends on a complex set of DOC structural properties including the following: molecular weight, carbon bonding arrangements, and carboxyl functionality. The previously cited fluorescence research has used l ex values of 370 nm for characterizing DOC. Previous work has shown that two different spectra indicators could be used to infer the source of the

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Fig. 3. Concentration of TOC (a) and DOC (b). The number of samples is above the plot in parentheses. The top of the box is the 75th percentile, the middle bar is the median, and bottom of the box is the 25th percentile. The upper and lower hinges are data values less than or equal to 1.5 times the interquartile range outside the quartile. The asterisks are data values less than or equal to 3 times the interquartile range outside the quartile. Open circles are data values more than 3 times the interquartile range outside the quartile.

DOC (Donahue et al., 1998). First, the emission wavelength with the highest intensity, or peak wavelength (PW), indicated an autochthonous (PW less than 450 nm) or allochthonous (PW greater than 450 nm) DOC source. Second, the slope of fluorescence spectra between fluorescence intensities at emission wave-

lengths of 450 and 500 nm was termed the fluorescence ratio (FR). Values of FR greater than 1.8 were indicative of autochthonous origin while FR less than 1.5 were indicative of allochthonous origin. A reference OC source, a standard fulvic acid from the Suwannee River (SRF), Georgia, (International

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Table 2 Summary of Tukey honestly significantly difference test on ranked TOC and DOC concentrations. Stream classes: E — uncontrolled ephemeral; ED — effluent-dependent; I — uncontrolled intermittent; C — controlled; P — uncontrolled perennial. Stream classes connected by a line are not significantly different from each other Relative OC concentration

TOC concentration E

High l

ED

DOC concentration I

C

P

––

E

ED

I

C

P

–– –––––––––––––

––––––––––––––––––––– ––

Low

Humic Substances Society), was obtained. Lyophilized SRF material was dissolved in ultrapure water and pH adjusted with HCl. The SRF sample provided a reference characteristic against which UV/Vis and fluorescence data collected from sites in our study were compared against. 2.3. Statistical analysis Non-parametric statistical methods were used to detect differences in concentrations between two sites and also among the five different stream classes (uncontrolled perennial, intermittent, and ephemeral streams; controlled streams; and effluent-dependent streams). The Statistical Analysis System (SAS) (SAS Institute Inc, 1989) was used to perform the Wilcoxon rank sum test (Wilcoxon, 1945) to detect differences in concentrations between two individual sites. SAS also was used to perform the Tukey honestly significant difference (HSD) test (Helsel and Hirsh, 1992) on ranked data to identify differences in concentrations among the five different stream classes. 3. Results 3.1. Quantity of organic carbon in major surfacewater resources of Arizona TOC concentrations in major surface-water resources of Arizona varied between one and several hundred milligrams per liter (mg/l). TOC concentrations from several uncontrolled perennial, intermittent, and ephemeral streams, controlled streams, and effluent-dependent streams are presented in Fig. 3a. Results from the Tukey HSD test (Table 2) showed that ephemeral streams

––

––

had the highest TOC concentrations, and perennial streams had the lowest …a ˆ 0:05†: Intermittent and effluent dependent streams had greater TOC concentrations than controlled streams. Wilcoxon rank sum tests showed that river reaches downstream from large reservoirs on the Agua Fria, Salt, and Verde Rivers had higher TOC concentrations …p ˆ 0:0001; p ˆ 0:0023; and p ˆ 0:0001; respectively) compared to reaches upstream from the impoundments. Variability of TOC was generally low downstream from impoundments and reaches directly downstream from wastewater-treatment plants. TOC concentrations decreased downstream of treatment plants as illustrated by sites ED1 and ED2 …p ˆ 0:0001†; and ED4 and ED5 …p ˆ 0:0026†: Differences between upstream and downstream sites, and stream classes could have been due to unequal sampling frequencies and collection periods (discussed later). For the river systems illustrated in Fig. 3a, the percentage of TOC that consisted of DOC varied from less than 10% to nearly 100%, but was typically 90%. The ephemeral streams generally had the lowest fraction of DOC, and the water released from dams had the highest. DOC concentrations were generally between 1 and 10 mg/l and are presented in Fig. 3b. Ephemeral streams had the highest DOC concentrations and perennial streams had the lowest DOC concentrations. Relatively high DOC concentrations were observed in ephemeral and intermittent streams (Fig. 3b). High DOC concentrations also occurred in perennial streams; however, the sampling frequency for perennial streams was more consistent than for ephemeral or intermittent streams. Sampling frequency of the latter was biased toward periods shortly after rainfall

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Fig. 4. DOC concentration, 1982–1995, at P5 upstream from (W) and at C7 downstream from (X) Lake Pleasant, Arizona.

and runoff events, because that was typically the only opportunity to collect water samples. DOC concentrations from sites on the Agua Fria, Gila, Salt, and Verde Rivers were less upstream from impoundments than downstream (p ˆ 0:0001; p ˆ 0:0037; p ˆ 0:0001; and p ˆ 0:0001; respectively). As an example, DOC concentrations upstream and downstream of Waddell Dam on the Agua Fria River (P5 and C7) from 1982–1995 are presented in Fig. 4. The site downstream of the reservoir (C7) had higher DOC concentrations, for most of the period of record, than the uncontrolled perennial stream entering the reservoir (P5). Occasionally high DOC concentrations in the uncontrolled perennial stream occurred periodically. For large reservoir systems in Arizona, the DOC concentrations leaving arid-region reservoirs were statistically greater than concentrations in uncontrolled perennial streams flowing into the reservoir. One possible explanation, discussed in detail below,

could involve wet/dry atmospheric deposition of OC to the reservoir surface may account the observed increase in DOC concentrations in controlled, compared against uncontrolled perennial, streams. During a synoptic sampling of a dust storm that was proceeded by a monsoon rain event, DOC concentrations during the first 2 min of rainfall had DOC concentrations of 4–10 mg/l (average 6 mg/l; n ˆ 4†: Lower DOC concentrations (1–2 mg/l) were observed for samples collected later in the rainfall event. The results imply that OC in dust is soluble. Contact of dust with a lake surface, during wet or dry deposition, could be a source of OC. 3.2. Seasonal variation of DOC concentrations and loads Instantaneous load of DOC (g/s) was calculated as a

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Fig. 5. Seasonal distribution of streamflow, DOC concentration (W), and instantaneous load ( ⴱ) of DOC for water year 1997 on the San Pedro River — P1 (a); Verde River — P4 (b); Gila River — C4 (c); and Santa Cruz River — ED2 (d).

function of measured DOC concentration (mg/l) and gauged streamflow (m 3/s) for approximately a 1-year period (Fig. 5a and d). In uncontrolled streams (Fig. 5a and b), DOC concentrations and loads increased in response to runoff events; therefore, the seasonality of concentration and loads was coincident with the seasonality of streamflow.

In desert streams impacted by monsoons (summerflow), such as the San Pedro River, the highest concentrations and loads occurred during the summer (Fig. 5a). Similar patterns occurred for streams impacted primarily by snowmelt runoff and, to a lesser extent, winter rains (winter-flow), such as the Verde River (Fig. 5b). At sites on reaches of

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Fig. 6. Absorbance spectra and fluorescence emission spectra (370 nm excitation) for DOC samples from allochthonous (standard fulvic acid) and autochthonous (ED4) sources.

controlled streams, such as the Gila River at Kelvin (Fig. 5c), DOC concentrations had small seasonal variations and the DOC loading correlated mostly with streamflow. For the effluent-dependent stream, such as ED2 shown in Fig. 5d, DOC concentrations and loads demonstrated seasonal patterns. Monsoon activity in July and August increased streamflow and DOC loading.

3.3. Variations in DOC characteristics and sources DOC exhibited an exponential decrease in the UV/ Vis absorbance with increasing wavelength (Fig. 6). The UV/Vis absorption spectrum presented in Fig. 6a is a standard fulvic acid (10 mgDOC/l) from the Suwannee River (SRF) that has an allochthonous (terrestrial- or lignin-derived) source of DOC (Averett et al., 1989).

The second UVA spectrum, which absorbed less light energy than the SRF sample, is for a sample collected from a wastewater-treatment plant outfall (ED4, 10 mg/ l). In the range of 254 nm, the DOC-normalized SUVA value of 4.4 m ⫺1 (mg/l) ⫺1 for the SRF sample indicated that the DOC had a higher concentration of carbon– carbon double bonds than the sample of effluent from ED4, which had a SUVA value of 1.2 m ⫺1 (mg/l) ⫺1. Fluorescence emission spectra for two samples (SRF, ED4) are shown in Fig. 6b, and were obtained at an excitation wavelength (l ex) of 370 nm. DOC from the ED4 site (a wastewatertreatment plant) had lower PW (438 nm) and higher FR (2.2) values than the SRF sample (PW ˆ 455 nm; FR ˆ 1.4) (Fig. 6b), suggesting that DOC in the ED4 sample came from autochthonous (bacterial) sources. SUVA, PW, and FR data for samples collected at

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Table 3 Summary of DOC concentration and composition measurements Stream

Site identifier for location

Number of samples

DOC concentration a (mg/l)

Specific ultraviolet and visible absorbance a (m ⫺1 (mg/l) ⫺1)

Peak wavelength (PW)

Fluorescence ratio (FR)

Salt river

P2 b ED4 ED5 ED6 ED3 ED2 c P1 d P4 b P3 b C4

23 19 29 36 11 23 21 21 21 26

2.1 ^ 1.2 8.6 ^ 1.1 6.8 ^ 1.3 5.2 ^ 2.2 12 ^ 1.8 3.2 ^ 1.9 2.0 ^ 1.4 1.7 ^ 1.1 1.7 ^ 1.8 4.7 ^ 2.5

1.8 ^ 0.9 (0.6 – 4.9) 1.3 ^ 0.1 (1.2–1.6) 1.3 ^ 0.1 (1.1–1.5) 1.4 ^ 0.2 (0.93–1.7) 1.1 ^ 0.1 (1.0–1.2) 1.3 ^ 0.4 (0.8–1.6) 1.4 ^ 0.6 (0.6–2.8) 1.6 ^ 0.7 (0.54–2.6) 1.7 ^ 1.0 (0.72–3.2) 1.8 ^ 0.5 (1.3–2.6)

420–453 434–440 432–438 431–437 434–440 424–428 420–460 419–453 419–457 420–423

1.4–2.2 2.1–2.2 2.2–2.3 2.1–2.3 2.1–2.4 2.1–2.3 1.4–3.0 1.4–6.4 1.3– ⬎ 2 2.1–2.3

Buckeye canal Hassayampa river Santa cruz river San Pedro river Verde river West clear creek Gila river a b c d

(0.9–4.9) (6.8–11) (4.9–11) (2.9–15) (9.4–15) (1.0–7.4) (1.0–3.7) (0.5–4.4) (0.5–6.9) (2.2–12)

Mean and one standard deviation values are shown; range of measured values is given in parentheses. River regime is winter-flow dominant. SUVA, PW, and FR data not available during runoff event. River regime is summer-flow dominant.

Fig. 7. Temporal patterns in spectrometric data for several sites.

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Fig. 8. Spectrometric data for Sycamore Creek, Arizona (Site I4).

uncontrolled perennial, controlled, and effluentdependent sites were used to assess variability of the DOC characteristics and sources in streams with different water-management and climatic conditions (Table 3, Fig. 7). For all the sites studied, median SUVA values were lower for the Arizona waters

(1.5 m ⫺1 (mg/l) ⫺1) compared against SRF (4.4 m ⫺1 (mg/l) ⫺1). SUVA values in Arizona surface waters varied seasonally and ranged from 0.6 to 4.9 m ⫺1 (mg/l) ⫺1. However, the median SUVA values were similar at all sites. Perennial sites exhibited more seasonal variability than effluent-dependent and

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Fig. 9. DOC concentration, SUVA, and FR values for a high-flow event on Sycamore Creek-I4 (September 1997).

controlled sites. At the perennial sites, SUVA values were highest between January and February, corresponding to periods when snowmelt runoff contributed to streamflow. Fluorescence indicators (PW and FR) were less variable at effluent-dependent and controlled sites and were characteristic of autochthonous sources of DOC (Table 3; Fig. 7). Perennial sites exhibited more variability in PW and FR values. During baseflow conditions (November through January) PW values were near 420 nm, and then increased rapidly to approximately 455 nm during the period of snowmelt runoff. PW values remained elevated, indicating allochthonous carbon sources, through June. In contrast, SUVA values gradually declined during the same time period. These results suggest that a continual autochthonous source of DOC contributed to the stream (high PW), but that the material decreased in the fraction of aromatic carbon content. Similar patterns SUVA, PW, and FR values were observed in perennial sites dominated by either snowmelt runoff or monsoon rainfall runoff. More

aromatic and autochthonous carbon was detected during periods of high, non-baseflow, streamflow conditions. To further understand the variability of concentration, structure, and sources of DOC, archived samples were analyzed from an intermittent desert stream (Sycamore Creek, I4). In general, higher streamflows (termed flood periods) in Sycamore Creek were associated with winter rains, although summer thunderstorms often resulted in short-term high-flow conditions. DOC concentrations in samples during flood periods ranged from 8 to 30 mg/l and were higher than concentrations in samples collected during periods of low flow (i.e. baseflow), which ranged between 2 and 4 mg/l (Jones et al., 1996). In general, SUVA values during runoff events were higher than during periods of low flow (Fig. 8a). The increased SUVA values during runoff events indicated an input of more aromatic and larger molecular weight organic material. The PW and FR data also exhibited a strong dependency on flow conditions, with values indicative of allochthonous sources

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215

Fig. 10. Predicted increase for DOC concentration within Bartlett Reservoir on the Verde River due to dry atmospheric deposition (75 mg OC/ m 2/d) between 1/1/96 and 4/30/98 based upon actual inflow/outflow and physical reservoir data.

occurring during runoff events and of autochthonous sources during periods of low flow (Fig. 8b and c). The variability and changes in concentration, structure, and source of DOC that occurred during a single runoff event were defined by the analysis of synoptic samples collected prior to and during runoff on Sycamore Creek in September 1997 (Fig. 9). Prior to the runoff event, the DOC was approximately 3 mg/l, and SUVA, FR, and PW values were 1.5 m ⫺1 (mg/l) ⫺1, 2.2 and 420 nm, respectively. The DOC increased to above 30 mg/l as the first part of the flood crest passed the sampling station, and decreased by nearly one half to approximately 15 mg/l over the next 2 h while the floodwaters receded. The SUVA increased by approximately 30% during the flood period, potentially a result of DOC mobilization from upstream channel sediment that was originally input as terrestrial debris, such as leaves, during the antecedent period. The FR (Fig. 8b) values were relatively constant during the entire event. Similar to FR values, PW values were relatively constant …455 ^ 1 nm; n ˆ 71† throughout the runoff event. Both FR and PW values indicated a steady DOC input probably of allochthonous origin.

4. Discussion 4.1. Significance of stream classification on DOC level Watershed management policies and regional watershed climate patterns affected the seasonality of flow, and OC concentrations and loads in streams. The relationships between stream classification and OC concentrations/loadings are discussed below. 4.1.1. Uncontrolled perennial versus controlled sites Controlled reservoir release sites had statistically higher DOC concentrations in the outflow compared against the inflow. Three potential explanations are discussed below. First, TOC and DOC concentrations transported into reservoirs during runoff events may have been under represented by the samples. The sites upstream from the reservoirs were biased towards samples collected at lower streamflows (i.e. baseflow), and these tended to have a lower concentration of OC. Baseflow may dominate streamflow for up to nine nearly continuous months, while higher flow may only occur for less than two or three months. For watersheds in Arizona, stream flow rates can be highly

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sporadic, as inferred from the difference between mean and median flowrates presented in Table 1. Patterns in high flow events and DOC loading for uncontrolled perennial sites were illustrated in Fig. 5a and b. Longer-term patterns for site P4 are discussed in Parks and Baker (1997). Reservoir storage, and retention within the reservoir, may have attenuated high OC loading events from the upstream watershed. Residence times for Horseshoe and Bartlett Reservoirs on the Verde River had median daily residence times (daily outflow/reservoir volume) of 25 and 244 days, respectively (Parks and Baker, 1997), while reservoirs on the Colorado River had residence times of more than one year. The reservoir on the Gila (Fig. 5c), like many reservoirs in arid regions, released the most water and also the largest portion of the annual load of OC during the summer months. In winter-flow streams, such as the Gila River, a seasonal shift of the transport of OC in the reach downstream from the reservoir occurred, with inputs during the spring and releases during summer and fall periods. Therefore, samples collected below reservoirs at controlled sites exhibited less monthly variability in OC concentrations, and therefore probably were not undersampled. Secondly, in-reservoir algae growth may have been responsible for the increased autochthonous DOC concentrations in reservoirs. Parks and Baker (1997) estimated that autochthonous production of DOC accounted for 41% of the total DOC input to the Horseshoe/Bartlett reservoir system, and documented that a higher annual loading of DOC exited the reservoir system than entered through streamflow. However, in a subsequent work on the same reservoir system we have observed that on an annual basis reservoirs can also be a net sink for DOC, and was based largely upon the timing of reservoir filling and releases (Nguyen et al., 1999). Loss of DOC was attributed to bacterial consumption, especially in the hypolimnion. In the current study, additional data (e.g. reservoir DOC concentrations and changes in reservoir storage) would have been required to compute annual loading into and out of the reservoirs. Arid-region reservoirs were typically constructed by damming deep canyons. The resulting reservoirs have surface area to volume ratios lower than for shallow lakes commonly found in more temperate zones. Thus, as arid region reservoirs stratify, the

hypolimnion represented a large portion of the reservoir volume. The zone capable of supporting photosynthetic activity, hence autochthonous DOC production, comprised a small portion of the total reservoir volume. Further work is needed to verify that alga growth was a significant process for DOC production in large, deep arid region reservoirs. Finally, atmospheric deposition (e.g. dust storms) may have transported a sufficient quantity of soluble OC to cause an increase in DOC concentrations in reservoirs. Large dust storms are common in Arizona, and can precede rainfall events. Dust deposition rates have been reported in the range from 195 to 550 kg dust/ha/year for sand-dune and central European ecosystems, respectively (Littman, 1997). However, only a portion of the dust would comprise soluble OC. Shachak and Lovett (1998) reported carbondust input rates ranging from 5 to 100 mg OC/m 2/d for fine and coarse particles, with coarse particles being responsible for higher carbon-input rates. No data for Arizona was available on OC content in dust. However, based upon the synoptic rainfall study presented above, atmospheric dust contained OC that was soluble in water. Therefore, dry or wet atmospheric deposition has the potential to transport soluble OC to lake surfaces. Given the potential for soluble OC input via dry deposition, the amount of dry deposition required to effect DOC levels in reservoirs was estimated. For example, Bartlett Reservoir on the Verde River, downstream from site P4, has a nominal surface area of 1488 ha, volume of 2.2 × 10 8 m 3, and nominal retention time of 245 days (Parks and Baker, 1997); relationships between depth, surface area, and volume were also available. Historic streamflow data above and below the reservoir over a 2.3 year period (1/1/96 through 4/30/98) was used to account for changes in residence time, volume, and surface area. Changes in DOC levels due solely to atmospheric deposition within the reservoir was predicted by assuming a null concentration of DOC entering the reservoir (Fig. 10). A prediction using OC deposition rate of 75 mg soluble OC/m 2/d (274 kg soluble OC/ha/year) increased DOC concentrations by 0.3–0.8 mg/l. Reservoir storage time, volume, and surface area changed during the simulations based upon inflow from the perennial Verde River and outflow to the

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Fig. 11. Changes in DOC characteristics through an urban system.

controlled stream below the reservoir. The prediction assumed a completely mixed reservoir, but it stratifies at a depth of 10 m (maximum depth ˆ 54 m). Therefore, atmospheric deposition may cause a larger DOC increase in epilimnion relative to the hypolimnion. Estimated quantities of dry soluble OC deposition suggested that a small, probably seasonal, change in DOC could have occurred from dry deposition on surfaces of arid-region reservoirs. 4.1.2. Effluent dependent sites In effluent-dependent streams, DOC concentrations were a function of proximity to the wastewater-treatment plant and the season of the year. DOC concentrations at sites directly downstream from treatment plants (ED1, ED3, and ED4 in Fig. 3a and b) exhibited small seasonal variability. For sites further downstream from WWTPs, such as the Santa Cruz River at Tubac which is located approximately 15 km downstream from the lagoon-treatment WWTP in Nogales Arizona, streamflows were lower during warmer

Table 4 POC and DOC mass balance through the Phoenix, AZ urban infrastructure system for 1 January 1996 through 31 March 1998 based upon USGS daily flows and monthly NAQWA DOC and POC concentrations using Beale’s stratified ratio estimator function (Equation 1) Cumulative volume of water (m 3)

Cumulative DOC loading (kg)

Inputs Salt river (P2) Verde river (P4) CAP canal a

12.6 × 10 8 10.4 × 10 8 10.5 × 10 8

4.7 × 10 6 5.2 × 10 6 3.2 × 10 6

8.9 × 10 6 5.8 × 10 6 1.6 × 10 6

Outputs Salt river (ED4) Percentage output

2.91 × 10 8 9%

2.4 × 10 6 18%

0.30 × 10 6 2%

a

Cumulative POC loading (kg)

Assumed constant DOC (3.0 mg/l) and POC (0.15 mg/l) concentrations (GeoSystems Analysis, 1998); volumes of water obtained from Central Arizona Water Conservation District (Phoenix, AZ).

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months (May through July, Fig. 5d) because of riparian evapotranspiration. During these summer months, DOC concentrations were less than during winter months, probably because of increased microbial activity in the stream channel and sediments associated with warmer temperatures. The effluent-dependent streams (e.g. ED2, Fig. 5d) responded to summer monsoon rains during August and September. During these runoff periods, the Santa Cruz River (ED2) contained a high percentage of natural runoff, non-wastewater origins. Upstream of the Nogales WWTP, the Santa Cruz River was ephemeral and followed similar OC loading trends as illustrated in Fig. 3 for ephemeral streams. Therefore the elevated DOC concentrations, above the wastewater effluent DOC level, during August and September (Fig. 5d) was attributed to DOC in the runoff and resulted in elevated DOC loadings. 4.2. DOC characteristics: allochthonous versus autochthonous sources Changes in DOC characteristics and sources were impacted by upstream hydrology and management decisions, and feedback impacted downstream ecosystems and water uses. For example, samples from winter-flow streams — the Salt River (P2), West Clear Creek (P3), and Verde River (P4) — exhibited higher SUVA values and increasing PW values from January through April 1997 as compared to values for the remainder of the year. These data corresponded with runoff events that occurred in response to snowmelt and suggested a shift from autochthonous to allochthonous sources of DOC. Runoff and infiltration processes may have been responsible for the transport of allochthonous DOC into the river. In one study, melting snow resulted in rising groundwater table, saturating soil horizons, and mobilizing organic soil matter that subsequently recharged a surface stream (Boyer et al., 1995a,b; Grieve, 1991). The SUVA values decreased during the summer of 1997 at these sites as streamflow decreased, suggesting a shift towards DOC with lower carbon–carbon double bond density. For the same samples, the PW (Fig. 7b) and FR values remained relatively constant and were indicative of an allochthonous source. One hypothesis for this observation was that mounded

groundwater continued to leach organic soil material from the saturated zone near the rivers. The spring, summer, and fall of 1996 were much dryer than the corresponding 1997 seasons, and most rivers had lower base flows during the fall dry season. It was possible that during 1997, riparian groundwater levels were higher than during 1996, resulting in saturation of organic soils that leached soil organic matter as DOC. This would explain the higher PW values and more allochthonous characteristics of DOC observed during the fall of 1997 compared to 1996. Several samples were collected from a summerflow stream, the San Pedro River (P1), during a runoff event in August 1997. Median values, and ranges were included in Table 3. During the runoff event, the SUVA and PW values from these samples were higher, and FR values lower, than values in samples collected during periods of low flow, implying that allochthonous sources of DOC were dominant during summer runoff events. In comparison to the controlled and effluent-dependent streams, both the summer-flow (i.e. monsoon impacted) and winter-flow (i.e. snowmelt and winter-rain impacted) perennial, intermittent, and ephemeral streams showed wide variations in the quantity and/or composition of DOC. Although not directly measured, these shifting DOC sources transport both biologically labile DOC and more refractory humic acid-type DOC into the rivers that could provide an important energy source for microbial communities (McKnight et al., 1993; Jones et al., 1994). In contrast, the controlled site (C4) exhibited low variability (Table 3) in SUVA, PW, and FR values. These data suggested an autochthonous source, potentially the result of algal and bacterial activity in the upstream reservoir. The effluent-dependent sites also exhibited little variation throughout the sampling period (ED4, Fig. 7). The effluent-dependent reaches consistently had an autochthonous source characterized by lower SUVA values (i.e. less complex carbon content), and less seasonal variation in DOC composition. The lack of variation in DOC concentration and structural characteristics for controlled and effluent dependent streams may limit the diversity of organisms that respond to dramatic changes in environmental conditions, and provided demonstrated impacts of water management decisions on DOC characteristics.

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4.3. Effect of urban areas on the concentration and composition of OC In Arizona and other arid regions, water managers impound nearly the entire flow of rivers, except during large runoff events, so that urban populations can be supplied with water throughout the year. Groundwater is used during droughts and in areas where surface-water supplies are insufficient or undeveloped. Wastewater is typically returned to dry river channels, creating effluent-dependent streams. Unlike receiving streams in more temperate climates, the treated wastewater is not diluted by natural streamflow and comprises most or all of the streamflow. The urban water system dramatically affects the patterns of the DOC concentration and composition in streams (Table 3; Fig. 11). The Phoenix metropolitan area uses surface water from reservoirs on the Salt and Verde Rivers for drinking water, plus water imported by the Central Arizona Project (CAP) canal system from the Colorado River at Lake Havasu. As discussed above, DOC concentrations from sites upstream located above reservoirs were lower compared against downstream controlled sites, and suggested that arid-region reservoirs generate DOC (this work; Parks and Baker, 1997). Since reservoirs were considered part of the urban infrastructure system, this was the first, most upstream, noticeable effect of urbanization. Impounded water was conveyed from sites, via concrete-lined canals, for municipal and agricultural users. Consequently, the Salt River through the Phoenix metropolitan area was a dry riverbed most of the year. Agriculture operations removed water from the canals for irrigation. Drinking-water treatment plants removed water from the canal and pass the water through a series of physical–chemical processes designed for particulate and DOC removal, preferentially removing higher SUVA material (American Water Works Association, 1990). Approximately one third of the distributed drinking water was later collected as sewage, which typically has DOC concentrations ranging from 100 to 300 mg/l (Viessman and Hammer, 1993). Biological processes in WWTPs convert soluble organics into microbial biomass during intensive aeration, and the biomass is removed and taken to landfills. DOC effluent concentrations range from 8 to 20 mg/l for common

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activated sludge WWTPs, and is lower (6–12 mg DOC/l) in effluent from advanced nitrification–denitrification (NDN) WWTPs (Pinney, 1998). Much of the sewage from the Phoenix metropolitan area was treated at the 91st Avenue NDN-WWTP and subsequently released to the Salt River (ED4). This release created an effluent-dependent reach on the Salt River, actually the river contained 100% WWTP effluent (i.e. no native streamflow), with DOC concentrations ranging from 6.8 to 11 mg/l, which were several times higher than the DOC concentrations in rivers upstream of the reservoirs. Thus, urban development in arid-regions had dramatic impacts on the concentration and composition of DOC moving through a watershed. A mass balance for the Phoenix urban system over a 2.4 year period was conducted using monthly DOC data and daily streamflow data for sites above (P2 and P4) and directly below (ED4) the Phoenix urban infrastructure (Table 4). Mass balance calculations were performed using the Beale’s stratified ratio estimator (Dolan et al., 1981; Parks and Baker, 1997). Additional flow and OC data for the CAP canal was also used (Table 4). Only 9% of the surface water entering the urban system was released at from the 91st Avenue WWTP (site ED4). The remaining water was used for agriculture irrigation, park/golf course irrigation, nuclear/thermoelectric cooling, or other internal urban uses. Only 18% of the DOC, and 2% of the POC, load to the urban region as surface water was discharged at site ED4. Reservoirs within the urban infrastructure system probably achieved high POC removal through sedimentation processes. DOC discharged at site ED4 probably comprised a mix of the following: (1) refractory DOC in the drinking water; (2) refractory DOC added by the urban population (e.g. pharmaceutical compounds); and (3) DOC added as soluble microbial products from biological processes (e.g. bacteria within WWTP) (Drewes and Fox, 1999; Drewes and Fox, 2000). In a different urban system served primarily by groundwater, as opposed to surface water, different OC patterns were observed. Groundwater is used as the primary source of water for the residents of Nogales and Tubac because surface-water supplies from the Santa Cruz River were insufficient. Near the United States/Mexico border, the City of Nogales, Arizona, used groundwater with low DOC concentrations and

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discharged aerated-lagoon treated wastewater into the ephemeral Santa Cruz River channel with DOC concentrations ranging from 12 to 18 mg/l, and values for SUVA of 1.5 m ⫺1 (mg/l) ⫺1, PW of 440 nm, and FR of 1.4 (Pinney, 1998). The sampling site at Tubac (ED2) is approximately 25 km downstream of Nogales, where the was still effluent-dependent (Cordy et al., 1998; Arizona Department of Environmental Quality, 1995). Although the DOC concentrations (average DOC at ED2 was 3.2 mg/l) were lower than levels immediately downstream from the WWTP in Nogales, the structural characteristics of the DOC did not vary significantly over the reach. The watershed does not have impoundments, so runoff events (e.g. August 1997; Fig. 5d) resulted in high flows that exceeded average wastewater discharges (0.6 m 3/s). The highest DOC concentrations at Tubac (ED2) corresponded with summer monsoon thunderstorms and increased streamflow. Spectrometric data were not available during the high-flow event. However, on the basis of data from Sycamore Creek, DOC in runoff events (including events on the Santa Cruz River) would have allochthonous sources rather than the autochthonous source that attributed to the WWTP effluents that would dominate during lowflow periods. The high DOC concentrations in the Santa Cruz River during periods of low flow were in contrast to the DOC concentrations in the perennial (non-effluent-dependent) San Pedro River (approximately 2.0 mg/l) under similar non-runoff conditions. The San Pedro and Santa Cruz watersheds are in close proximity to each other (Fig. 1) and had similar geology and climate. However, above the San Pedro site, the watershed did not have as much urban development as in the Santa Cruz basin. Runoff events from summer thunderstorms, however, have similar impacts on the two systems, resulting in elevated DOC concentrations and a shift from DOC with autochthonous sources to more allochthonous sources.

5. Conclusions Surface waters in arid regions, such as Arizona, can exhibit large spatial and temporal variations in POC and DOC concentrations. Ephemeral sites had the highest DOC concentrations, and unregulated peren-

nial sites had lower concentrations than unregulated intermittent sites, regulated sites, and sites downstream from wastewater-treatment plants …p ⬍ 0:05†: Urban infrastructure, such as reservoir dams attenuate seasonal fluxes of allochthonous DOC and may support processes (atmospheric deposition, alga growth) that increase DOC levels. Uncontrolled perennial streams exhibited rapid runoff response to winter/spring snowmelt for watersheds draining higher elevations or to late summer monsoons for watersheds in lower desert elevations. Controlled and effluent-dependent streams had the lowest fraction of TOC as POC. POC loads were on a similar order of magnitude as DOC loads in two perennial streams. POC would be efficiently removed via sedimentation processes within reservoirs, which served as the first upstream consequence of urban development. Sonoran desert streams exhibited low SUVA values, typically less than 2.0 m ⫺1 (mg/l) ⫺1, compared to DOC found in more temperate stream systems. Higher SUVA values occurred during snowmelt in higher elevation basins and in runoff from summer thunderstorms in the lower-elevation desert basins. During periods of runoff response (overland flow or rising groundwater tables that recharge streams) uncontrolled perennial streams contained allochthonous DOC. However, during most of the year perennial streams under baseflow conditions exhibited autochthonous DOC characteristics. DOC in Arizona surface waters were largely predominantly autochthonous, based upon fluorescence PW and FR, and characterized by low average SUVA values. Therefore, DOC in Arizona surface waters may be considered an “end-member” relative to DOC that has been characterized by most researchers (Aiken et al. 1992; USGS). As an end-member DOC source, further structural characterization should be undertaken and a more comprehensive understanding of its role in biogeochemical processes developed. Urban development without upstream impoundments (e.g. Santa Cruz River) permitted a more “natural” seasonal variation in DOC concentration and composition compared to urban developments with upstream reservoirs (Salt and Verde Rivers). An unanticipated observation from this study was that urban infrastructure (e.g. reservoirs or WWTPs) resulted in less variable patterns with higher DOC

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