Freshwater Contamination (Proceedings of Rabat Symposium S4, April-May 1997). , cn

IAHSPubl.no. 243, 1997 153

Spatial and temporal variability in nutrient concentrations in surface waters of the Chattahoochee River basin near Atlanta, Georgia, USA

NORMAN E. PETERS, GARY R. BUELL & ELIZABETH A. FRICK US Geological Survey, 3039 Amwiler Rd., Atlanta, Georgia 30360-2824, USA

Abstract Nutrient concentrations from the early 1970s through 1995 were evaluated at several sites along the Chattahoochee River and its tributaries near Atlanta, to determine general patterns and processes controlling nutrient concentrations in the river. A spatial analysis was conducted on data collected in 1994 and 1995 from an intensive nutrient study of the Chattahoochee River and its tributaries by the Georgia Department of Natural Resources, Environmental Protection Division. The 1994-1995 data show step increases in ammonium (NH4-N), nitrite plus nitrate (N02 + N03-N), and total-phosphorus (Tot-P) concentrations m the river. The step increases occur downstream of two wastewater treatment facilities (WWTFs) and Peachtree Creek, a small tributary inflow with degraded water quality draining a predominantly urban and industrial area. Median N02 + NO3-N and Tot-P concentrations in the mainstem increase downstream of these inputs from 0.5 to 1 mg l"1 and from 0.04 to 0.13 mg T1, respectively. NH4-N concentrations were typically low with 95% of the 2575 observations less than 0.2 mg l"1 throughout the river system, except some high values (>1 mg l"1) in some tributaries, particularly near the central part of Atlanta. High NH4-N concentrations are attributed to sewage discharge as they also are associated with high biological oxygen demand and faecal coliform bacteria concentrations. Nutrient concentrations vary temporally. An assessment of four sites, two mainstem and two tributaries, from 1970 to 1995 indicates a progressive increase and variability in N02 + N03-N concentrations during the period. The progressive increase in N02 + N03-N concentrations and their variability is similar to that reported for surface waters throughout the world and for which increased fertilizer usage has been attributed. Tot-P concentrations increase at mainstem sites through the middle to late 1980s and decrease markedly thereafter, due to improvements to WWTFs and a 1990 phosphate detergent ban. NH4-N concentrations, although less pronounced than Tot-P, display a similar decrease from the late 1980s to 1995 at the four sites. Tot-P concentration variability has increased at the tributary sites since 1993, although recent concentrations, on average, are the lowest since 1970 at each of the four sites.

INTRODUCTION

Human activities have had a profound impact on the environment. Alteration of the land surface for a variety of uses including light and heavy industry, urbanization, and suburban developments has changed water pathways and induced changes to natural processes. Human activities are accompanied by sources of nutrients that are contributed to the landscape and receiving waters through various pathways, including atmospheric deposition, and solid and liquid waste disposal (Puckett, 1995). In addition, mechanisms for waste disposal and quality of waste are not static.

154 Norman E. Peters et al.

Improved understandings of the effects of nutrient forms or species on the environment have affected practices for land and water management.

The threat of degradation with respect to land-use change and waste disposal from previous and ongoing activities is quite high. Synoptic water-quality sampling provides an overview of water-quality conditions in an area, and when taken periodically and augmented with routine monitoring at selected sites, provide the basis for assessing patterns in a variety of environments, i.e. precipitation, soils, groundwater and surface waters, which in turn, contributes information to effectively manage the environment (Heathwaite et al, 1996). The need remains, therefore, to

30 KILOMETERS

EXPLANATION

Metropolitan Atlanta

Basin Boundary

Stream

Water-quality Monitoring Site

Wastewater-Treatment Facility

Chattahoochee River at Franklin

Fig. 1 Location map.

Variability in nutrient concentrations in surface waters — Chattahoochee basin, Atlanta 155

continually assess the status of stream ecosystems to provide feedback on resource management decisions. To this end, the spatial and temporal patterns in nutrient concentrations for surface waters of the Chattahoochee River basin in the Atlanta region from the early 1970s to 1995 are reported herein.

BACKGROUND

Chattahoochee River basin

The Chattahoochee River and some of its tributaries (Fig. 1) provide public water supplies for the Atlanta region. The study area is in the Atlanta region and extends from the Chattahoochee River at Buford Dam, the base level for Lake Sidney Lanier, to the Chattahoochee River at US Highway 27 at Franklin, upstream of West Point Lake. The climate of the Atlanta region is warm, temperate, and subtropical, and annual air temperature averages 16°C. Average annual precipitation is 1250 mm, primarily as rainfall (Hodler & Schretter, 1986). Annual runoff averages approximately 400 mm (Carter & Stiles, 1983). Flow in this section of the river is regulated by dam releases.

Urbanization, forest and agriculture are the dominant land cover and land use in the study area. Urbanization in the Metropolitan Atlanta area increased from 1140 km2 for 1972-1978 to 1660 km2 for 1990, as a result of rapid population growth from about 1.5 million people in 1970 to about 2.6 million in 1990, a 65% increase (Atlanta Regional Commission, written comm., 1996). Water releases from the Buford Dam are regulated primarily for power generation, but also for flood control, water supply for navigation in the lower Apalachicola-Chattahoochee-Flint (ACF) River basin, recreation, fish and wildlife, and pubic water supply (Fanning et al., 1991; Marella et al, 1993). Approximately 1.2 million m3 day"1 of freshwater were withdrawn for public supply from the surface waters of the Chattahoochee River basin near Atlanta in 1990; a similar amount was withdrawn for thermoelectric power generation (Marella et al, 1993). The Atlanta region accounts for 80% of all public water supply withdrawals from surface water in the entire ACF basin (drainage area of 51 300 km2). In contrast, approximately 0.92 million m3 day"1 (77% of water withdrawals) of treated municipal effluent was discharged to surface waters in the study area in 1990 (Marella et al., 1993; Frick et al., 1996).

Nutrient water-quality data

Nutrient water-quality data have been collected within the Chattahoochee River basin by Federal, State, and local governments, colleges and universities, and other organizations. These water-quality data were obtained to meet diverse objectives over varying temporal and spatial scales. Data for this report were obtained from readily available computerized data bases. Data limitations and handling procedures are detailed in Frick et al. (1996). The data analyses herein focus on concentrations of ammonium as N (NH4-N), nitrite plus nitrate as N (N02 + N03-N), and total phosphorus (Tot-P), due to data availability.

156 Norman E. Peters et al.

Table 1 Site ID, river kilometre (RK) upstream from the mouth, station name, drainage area upstream of the station and number of 1994 and 1995 nutrient samples (n) for a spatial assessment of water quality in the Chattahoochee River basin near Atlanta. Long-term monitoring sites used for a temporal assessment are in bold.

Site ID RK Station Drainage area (km2) n CR0015 TR0010 TR0020 CR0060 TR0030 CR0100 TR0040 TR0050 TR0060 CR0130 TR0070 TR0071 TR0080 CR0160 TR0090 TR0091 TR0100 CR0210 TR0110 TR0112 TR0120 CR0320 TR0130 CR0350 TR0140 TR0141 CR0370 TR0150 TR0160 CR0400 TR0170

TR0171 TR0174 CR0490 TR0180 TR0190 TR0200 CR0530 TR0210 TR0220 TR0221 CR0600 TR0230 TR0240 CR0640

TR0250 TR0260 TR0270 TR0280 TR0290 TR0291 TR0300

560.45 559.93 557.52 556.39 555.91 552.84 550.68 549.23 544.04 532.21 529.84 529.84 525.32 523.63 523.17 523.17 515.99 515.85 510.65 510.65 507.01 502.97 500.06 499.48 496.42 496.42 492.98 490.23 489.78 487.48 483.57

483.57 483.57 480.72 478.68 475.78 475.64 474.20 469.09 464.29 464.29 460.29 456.20 455.38 453.40

453.01 452.87 446.18 444.00 441.93 441.93 440.05

Chattahoochee River at Buford Dam Tailwater Haw Creek at Parker Road Richland Creek at Suwanee Dam Road

Chattahoochee River at Highway 20 James Creek at James Burgess Road

Chattahoochee River at McGinnis Ferry Road Level Creek at Settles Bridge Road Dick Creek at Old Atlanta Road Suwanee Creek at US Highway 23

Chattahoochee River at Medlock Bridge Road John's Creek at State Bridge Road John's Creek at Buice Road Unnamed Creek at Rivermont Parkway

Chattahoochee River at Holcomb Bridge Road Crooked Creek at Spalding Drive Crooked Creek at Peachtree Corners Circl Ball Mill Creek at Spalding Drive

Chattahoochee River at Eves Road Big Creek at Roswell Water Intake Big Creek at Holcomb Bridge Road Willeo Creek at Highway 120

Chattahoochee River at Morgan Falls Dam March Creek at Brandon Mill Road

Chattahoochee River at Johnson Ferry Road Sope Creek at Columns Drive Sope Creek at Lower Roswell Road

Chattahoochee River at Powers Ferry Road Long Island Creek at Northside Drive Rottenwood Creek at Akers Mill Road

Chattahoochee River at Paces Ferry Road Peachtree Creek at Ridgewood Road (long-term

site 6 km upstream) Nancy Creek at West Wesley Road Peachtree Creek at Moore's Mill Road

Chattahoochee River at Highway 280 Proctor Creek at Highway 280 Nickajack Creek at Highway 278 Sandy Creek at County Road 1288

Chattahoochee River at Highway 139 Utoy Creek at Highway 70 Sweetwater Creek at East Point Intake Sweetwater Creek at Blairs Bridge Road

Chattahoochee River at Highway 166 Camp Creek at Cochran Road Deep Creek at Cochran Road

Chattahoochee River at Highway 92 near Fairburn

Anneewakee Creek at Highway 166 Tuggle Creek at Highway 70 Pea Creek at Highway 70 Bear Creek (Douglas County) at Highway 166 Bear Creek at Woodruff Road Bear Creek (Fulton County) at Highway 70 Dog River at Dog River Dam

2693.6 4.5

22.8 2745.4

39.3 2843.8

21.3 18.3

119.5 3038.1

30 24.9

6.3 3133.9

13.1 21.9

8.3 3159.8 252.8 266.5 41.7

3548.3 12.6

3626 83.8 91.7

3677.8 16.1 50.5

3755.5 95.8

95.9 339.2

4118.1 38.3 82 11

4283.9 87.5

633 681.2

5128.2 75.5 75.5

5335.4

72.8 8.2

35 44 68 66.6

169.6

74 5

31 75 30 59 30

1 87 59 4

31 4

56 45 4 2

72 82 4

30 56 30 56 26 4

56 29 72 72 54

27 3

56 48 33 32 56 70 82 5

55 30 28 70

50 5

16 28 43

4 23

Variability in nutrient concentrations in surface waters — Chattahoochee basin, Atlanta 157

Table 1 continued.

Site ID RK Station Drainage area (km2) TR0301 CR0710 TR0310 TR0320 TR0330 TR0340 TR0350 TR0351 TR0352 CR0770

TR0360 TR0361 TR0370 TR0380 TR0390 TR0400 TR0410 TR0430 TR0450 TR0460 TR0470 CR0940

440.05 436.28 434.38 430.15 428.93 421.11 420.35 420.35 420.35 418.10

412.79 412.79 411.23 410.97 407.51 403.28 400.08 392.11 387.95 386.48 380.56 378.86

Dog River at Highway 5 Chattahoochee River at Capps Ferry Road

Hurricane Creek at Highway 5 Wolf Creek at Wilson Road White Oak Creek at Highway 70 Snake Creek at Black Dirt Road Cedar Creek at Sewell Mill Road Panther Creek at Sewell Mill Road Cedar Creek at Brimer Road

Chattahoochee River at US Highway 27A near Whitesburg

Wahoo Creek at Wagers Mill Road Wahoo Creek at Welcome-to-Sargent Road Thomas Creek at Payton Road Moore Creek at Sitton Road Acorn Creek at Highway 5 Whooping Creek at Highway 5 Yellowdirt Creek at Old Lowell Mill Road Hilly Mill Creek at Enon Grove Road Nutt Creek at Nutt Road Harris Creek at Highway 34 Centralhatchee Creek at US Highway 27

Chattahoochee River at US Highway 27 near Franklin

203.1 5879.3

14.5 43.3 41.7

123.6 102

11.4 111.9

6293.7

67 86.1 20.6

8.9 28 70.2 66.6 28.4 13.2 15.9

147.2 6941.2

4 55 4

27 27 28 24 24 4

71

63 4 4 1

28 67 4

27 4 1

28 72

The spatial patterns of nutrient concentrations were evaluated from data collected from May through October in 1994 and 1995 for a Georgia Environmental Protection Division (EPD) nutrient study of the Chattahoochee River (17 sites) and its tributaries (57 sites) from Buford Dam to Franklin, as listed in Table 1 (Roy Burke, EPD, written comm., 1996). Temporal patterns of nutrient concentrations from the early 1970s to 1995 were evaluated at four streamwater sites. Two sites were on the mainstem of the Chattahoochee River; one is at Atlanta and the other is downstream of Atlanta at State Highway 92 near Fairburn. The other two sites were on tributaries to the Chattachoochee River; Peachtree Creek drains a developed highly urbanized basin and Big Creek drains a rapidly developing area in the northern part of the study area.

VARIABILITY IN NUTRIENT CONCENTRATIONS

Tot-P concentrations of more than 40% of the 1994-1995 EPD surface-water samples exceed U.S. Environmental Protection Agency (USEPA) recommendation of 0.1 mg T1, a concentration threshold established to control eutrophication in flowing waters, and 75% of samples exceed USEPA's recommendation of 0.05 mg l"1, a concentration threshold recommended to control eutrophication where streams enter a lake or reservoir. However, Tot-P concentrations, although generally high, are much lower than in the 1970s and 1980s. In contrast, N02 + N03-N concentrations in surface-water samples from the Chattahoochee River basin in the Atlanta region

158 Norman E. Peters et al.

generally are much less than the USEPA concentration limit of 10 mg l"1 in drinking water (USEPA 1986, 1990, 1995) with a median for all 2 575 EPD samples of 0.44 mg T1; N02 + N03-N concentrations of the mainstem are slightly higher (0.54 mg l"1) than the tributaries (0.32 mg l"1). A very small percentage (< 1%) of the stream samples exceed the NH4-N concentration threshold of 2.1 mg l"1 (USEPA, 1986), a limit associated with ecosystem degradation due to chronic exposure of aquatic organisms to unionized ammonia.

Urban development has affected the nutrient concentrations in the Chattachoochee River basin. Major sources of nutrients in the Chattahoochee River basin include municipal wastewater effluent, animal manure, fertilizer, and atmospheric deposition. Point sources discharging to surface waters in the area include municipal- and industrial-storm drains, wastewater effluent, sanitary and combined sewer overflows (SSOs and CSOs, respectively), and untreated wastes or runoff from illegal outfall pipes. Effluent from municipal wastewater generally contributes a small percentage to loads of N and P to an entire watershed, but is a very important source because it is discharged directly to surface waters. SSOs and CSOs can discharge a mixture of raw sewage and storm runoff directly to streams typically during storms when the storm runoff exceeds the capacity of the sewers.

Nonpoint-source inputs have broad source areas, ranging in areal extent from less than 1 km2 to thousands of km2. A small, but often unknown, percentage of nonpoint-source inputs of nutrients enters the hydrologie system by leaching, runoff, or atmospheric deposition on water surfaces. Anthropogenic nonpoint-source inputs of nutrients for the study area are derived primarily from animal manure, fertilizer, and atmospheric deposition. Leaching of fertilizer into groundwater can provide a sustained nutrient-enriched, i.e. as N03, source as groundwater is discharged to streams. The fertilizer in the Atlanta region is primarily applied to grasses and shrubs in residential and commercial areas including golf courses and parks.

Spatial patterns

Nutrient concentrations for the 74 sites sampled by EPD in 1994 and 1995 vary markedly in the basin (Fig. 2). Nutrient concentrations in the mainstem of the Chattahoochee River for the EPD study increase markedly downstream of river kilometre (RK) 485 (Fig. 2 and Table 1). A tributary, Peachtree Creek, draining a highly urbanized and industrial area, and two major WWTF outfalls, R. L. Sutton and R. M. Clayton, are relatively major contributors of nutrients to the mainstem. Median Tot-P concentration of all mainstem sites from RK 485 to 378 was 0.13 mg l"1 exceeding the USEPA limit of 0.1 mg l"1; whereas, median concentrations of the tributaries downstream of RK 485 were 0.09 mg l"1. Upstream of RK 485, the tributaries have much higher median Tot-P concentrations (0.10 mg l"1) than the mainstem (0.04 mg l"1).

For the N parameters, median N02 + N03-N concentration of tributaries and mainstem were similar upstream of RK 485 (0.43 and 0.45 mg l"1, respectively); whereas downstream of RK 485, median concentration in the tributaries was much lower than that of the mainstem (0.2 and 1.9 mg l"1, respectively). Median N02 + N03-N concentration, like Tot-P concentration, increases markedly

Variability in nutrient concentrations in surface waters — Chattahoochee basin, Atlanta 159

downstream of RK 485 from 0.45 to 1.9 mg l"1, possibly due to the WWTF discharges (no N02 + N03-N concentrations were available for the WWTFs). Median NH4-N concentrations show a similar pattern of increases in the mainstem downstream of RK 485, and are higher in tributaries upstream than downstream (0.03 mg 1"! upstream and <0.03 mg l"1 downstream).

Nutrient concentration increases in the mainstem of the Chattahoochee River downstream of Atlanta were expected because of urban and suburban point sources, including sewage discharge, and nonpoint nutrient sources. As occurs at some

100

USEPA EUTROPHICATION THRESHOLD , FOR FLOWING WATER AND , FOR LAKES AND RESERVOIRS °

OOOOOOOOO OO OOOO OOO OOOOOO CO CO OOOO OOOOOO CO CO OO OOOO OO OOOOOOOOO OO OOOOOO OOOOOOO fT rrvv rvry (YfV r v v tvrv fVfr O^n Vrr r-'rvry p™ ^ ^ ^^ rwvrvrvtvwrvn'CY'rY rvrv ryry nvr fYtv-tYcrfv'rrtYCY'fYn-n'rfr.'rfrvn'of tYtY rm* rV(Y rftrrvtrrvrr rviv ty Ol * I I -I -l-l I I I h-O-f-h - h f - O H H - O - H - ( J - h O H - O - O - H H O H - h-OH-l-OH- HQi-Oh-HO

co u^a^cKNuxnc^i^r*^ coco cor-oi^-^ojocxn coo T-<OO^ CO CXD r ^ O C O h ^ T ^ C N Î C O C O C ^ CO C ^ ( ^ C ^ T ^ r^COCO COOT CX3 O r - T - T - T - ^ ( \ J C \ I C N K \ i C N C T C O c O ^ ^ ^ ^ ^

t t Fig. 2 Spatial variation in streamwater nutrient concentrations among sampling sites in the Chattahoochee River basin near Atlanta during May through October of 1994 and 1995. Mainstem sites are denoted by vertical grey lines and grey background in boxes, and long-term monitoring sites are denoted by arrows.

160 Norman E. Peters et al.

distance downstream of Atlanta, it is common for NH4-N concentrations to decrease and N02 + N03-N concentrations to increase because of nitrification in surface water downstream of the WWTF outfalls. The samples with high NH4-N concentrations also have high biological oxygen demand and faecal coliform bacteria concentrations, which is consistent with higher sewage content in them than in other samples.

Although N02 + NO3-N concentrations increased as expected below RK 485 for 1994-1995, NH4-N concentrations did not decrease because additional NH4-N discharges into this reach. Median NH4-N concentrations in the mainstem immediately upstream and downstream of RK 485 for 1994-1995 are 0.03 and 0.04 mg l"1, respectively. Conversion of NH4 to N02 and then to N03 occurs in well-oxygenated surface water (nitrification) and is a likely explanation for the much smaller increase in NH4-N compared to N02 + N03-N concentrations upstream of RK 485. However, N02 + N03-N concentrations remained relatively constant in the Chattahoochee River at sites downstream of Atlanta; whereas median NH4-N concentrations continue to increase, reaching a site maximum of 0.12 mg l"1 at the Chattachoochee River at State Highway 92 near Fairburn (RK 453) and progressively decreased to 0.04 mg l"1 at the downstream end of the study area, Chattahoochee River at Franklin (RK 378). It is not clear what the dominant controls are on nutrient concentration variations in the Chattahoochee River downstream of Atlanta, but it is likely that solar radiation, temperature and streamflow are important controls on species conversion as are the additional nutrient inputs.

Temporal patterns

As a result of improvements to several WWTF in the middle to late 1980s and a Statewide phosphate detergent ban in 1990, nutrient concentrations, particularly Tot-P, decreased at several sites from the late 1980s to 1995 (Figs 3 and 4). Amendments to the Georgia Water Quality Control Act in 1990 legislated restrictions limiting the amount of phosphorus in various household and commercial detergents, and directed major WWTF that discharge effluent into the Chattahoochee River below Buford Dam to reduce the average Tot-P concentration in effluent to less than or equal to 0.75 mg l"1 beginning 1 January 1992. By 1993, nine municipal and one industrial WWTF were in compliance. Three remaining WWTFs in this reach of the Chattahoochee River, which could not meet the compliance date, are owned by the city of Atlanta, which negotiated an extension until 4 July 1996, in exchange for agreeing to meet a more restrictive limit of 0.64 mg l"1 average phosphorus concentration in the effluent. Concentration variability increases also were observed, particularly after 1993, in two of the four long-term sites examined, a tributary draining an urban area and one undergoing rapid urbanization (Figs 5 and 6). These results were not reflected in 1980-90 trend tests (Wangsness et al, 1994; DeVivo et al, 1995; Frick et al, 1996). Combined annual P and NH4-N loads from the six largest Metropolitan-Atlanta-area municipal WWTFs decreased from 1988 to 1992, although the effluent discharge increased (Wangsness et al., 1994; Frick et al., 1996). The improvements to WWTFs, which improved treated wastewater, also may have improved NH4-N concentrations because NH4-N concentrations in the

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Variability in nutrient concentrations in surface waters — Chattahoochee basin, Atlanta 163

Chattahoochee River are lower and less variable in the 1990s than during the late 1980s.

N02 + NO3-N concentrations at the two Chattahoochee River sites and the two tributaries increased and became more variable from the early 1970s to 1995. Improvements to WWTFs targeted decreases in NH4-N and P-species by more effective biological removal. NH4 is converted to N03 through nitrification in the WWTF and is reflected in a N03 increase downstream of the WWTF outfalls, as noted in the spatial analysis. In addition, fertilizer use has progressively increased during the last few decades in the U.S. (Puckett, 1995) and in other areas of the world (Heathwaite et al., 1996). The increased fertilizer use has resulted in marked increases in surface and groundwater N02 + N03-N concentrations (Heathwaite et al, 1996) and is a likely contributor to the temporal increases observed herein.

The 1.5 year smoothing window for the LOcally WEighted Scatterplot Smoothing (LOWESS) curves in Figs 3-6 accentuates some seasonal variations, but the variations and timing of extreme concentrations are not consistent among sites. In particular, the LOWESS curve for N02 + N03-N concentrations for 1990-1995 of the Chattahoochee River at State Highway 92 near Fairburn has a maximum in late summer and early fall, and Peachtree Creek has a maximum in winter (Fig. 7). Nutrient concentrations in streams can vary seasonally because of the growth and decay of vegetation, and temperature variations, which in turn effects phytoplankton growth (Cherry et al, 1980) and nitrification of NH4 to N03 (Ehlke, 1978). Another factor affecting seasonal variations is fertilizer application, which is almost exclusively applied during the growing season.

Flow is another important variable affecting nutrient concentrations. For relatively constant nutrient discharges as occurs from WWTFs, increasing streamflow dilutes the high concentrations in the discharges. Dam releases affect low to medium flows and rainstorms affect high flows. During rainstorms, streamflow

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164 Norman E. Peters et al.

increases markedly in those tributaries receiving the rainfall, and depending on the source of the nutrients, nutrient mobility also can occur in these tributaries. The relations (linear regression) between nutrient concentrations and daily discharge for the long-term sites from 1990-1995 generally were not statistically significant nor consistent (e.g. positive or negative) among sites or parameters. However, N02 + NO3-N concentrations of the two mainstem sites show a statistically significant (a < 0.01) negative relation with flow. Tot-P concentration shows a negative relation with flow at the downstream mainstem station, but was positively related to flow at the Peachtree Creek site. To determine the factors affecting the nutrient concentration variations in the river will require additional data analysis including the examination of variations during storms and incorporation of travel times to elucidate sources and processes affecting species transformation.

CONCLUSIONS

An analysis of ammonium as N (NH4-N), nitrite plus nitrate as N (N02 + N03-N) and total phosphorus (Tot-P) concentrations was conducted for the mainstem of the Chattahoochee River and its tributaries from river kilometre (RK) 560.4 at Buford Dam upstream of Metropolitan Atlanta on Lake Sidney Lanier, a water-supply reservoir, to RK 378.9 at Franklin downstream of Atlanta. A spatial analysis was conducted on data collected from May through October in 1994 and 1995 for a nutrient study conducted by the Georgia Department of Natural Resources, Environmental Protection Division (EPD). Forty percent of the 2575 Tot-P concentrations from the EPD study exceeded the U.S. Environmental Protection Agency recommended limit of 0.1 mg l'1, for controlling eutrophication, and Tot-P concentrations typically are much lower during 1994-1995 than during the 1970s and 1980s. Step increases occur in ammonium (NH4-N), nitrite plus nitrate (N02 + NO3-N), and total-phosphorus (Tot-P) concentrations in the mainstem of the Chattahoochee River, downstream of wastewater treatment facility (WWTF) outfalls and tributary inflows with degraded water quality. In particular, N02 + N03-N and Tot-P concentrations increase from 0.45 to 1.9 and from 0.04 to 0.13 mg l"1, respectively, downstream of the combined inputs of two major WWTFs, which contribute 10% of the streamflow, and Peachtree Creek, a tributary draining a highly urbanized and industrialized basin.

Data from 1970 to 1995 for four long-term monitoring sites were evaluated for temporal patterns. Two sites are on the mainstem including one just upstream of the two major WWTFs and Peachtree Creek, and one 34 km downstream. Two sites are on tributaries, one upstream of Atlanta on Big Creek in a developing area and one on Peachtree Creek. Tot-P and NH4-N concentrations increased through the middle to late 1980s at the mainstem stations and then decreased to a minimum for the most recent years. As a result of improvements to several WWTF in the mid to late 1980s and a Statewide phosphate detergent ban in 1990, Tot-P and to a lesser extent NH4-N concentrations decreased at mainstem sites from the late 1980s to 1995. Meanwhile, N02 + NO3-N concentrations generally increased and became more variable at all four sites, with the most pronounced concentration increases occurring at mainstem sites. Also, nutrient concentrations vary seasonally, but the variations and in

Variability in nutrient concentrations in surface waters — Chattahoochee basin, Atlanta 165

particular the extremes, are not consistent among constituents or sites. Despite the general observation that streamflow affects concentrations, relations between nutrient concentrations and daily streamflow for a sub-period (1990-1995) at the four long-term monitoring sites generally were not statistically significant.

Acknowledgements The authors wish to thank various Federal and State agencies and in particular, R. Burke, Georgia Department of Natural Resources, Environmental Protection Division, for their cooperation in providing information and data that were used in preparing this report.

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