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NH4-N Concentration 03 km Downstream from WWTP Input (mgL)
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Stream Nutrient Retention EfficiencyStream Nutrient Retention Efficiencyat an Enriched System in theat an Enriched System in the
Eucha ndash Spavinaw BasinEucha ndash Spavinaw Basin
BE HaggardBE Haggard11 EH Stanley EH Stanley22 DE Storm DE Storm33
11USDA ndash ARS Poultry Production and Product Safety Research Unit Fayetteville AR USAUSDA ndash ARS Poultry Production and Product Safety Research Unit Fayetteville AR USA22University of Wisconsin Center of Limnology Madison WI USAUniversity of Wisconsin Center of Limnology Madison WI USA
3Oklahoma State University Biosystems Engineering Department Stillwater OK USA3Oklahoma State University Biosystems Engineering Department Stillwater OK USA
IntroductionIntroduction
In the last decade the effect of application of animal manure to pastures on nutrient concentrations and fluxes in surface runoff have been the focus of scientific investigation in the Ozark Plateaus USA Nutrient concentrations and fluxes in Ozark streams often increase with the proportion of pasture or agricultural land use in the catchment (Haggard et al 2003a Petersen et al 1999) and these observations have overshadowed the potential impacts of municipal wastewater treatment plants (WWTPs) inputs on aquatic systems Despite nationwide efforts to reduce WWTP nutrient inputs WWTPs continue to pose a threat to regional water quality Wastewater treatment plants and other point sources contribute almost 50 of the total nutrient load to some aquatic systems in this region (Haggard et al 2003b)The effects of WWTPs on stream nutrient concentrations and cycling are substantial the distance required to temporarily retain over 63 of WWTP nutrient inputs has been reported to be as long as 30 km (Haggard et al 2001a Martiacute et al 2004) Effluents can cause sediment deoxygenation several kilometers downstream (Rutherford et al 1991) and decreases in the phosphorus (P) buffering capacity of benthic sediments (Dorioz et al 1998 House and Denison 1998 Haggard et al 2001a) In short WWTPs can have significant effects not only on nutrient loads but on nutrient transformations and general limnological conditions in the stream that may persist for several km Unfortunately we have limited measures of nutrient uptake using spiraling methods (Stream Solute Workshop 1990) in highly enriched systems especially in streams receiving nutrient enriched WWTP effluent
Study Site DescriptionStudy Site Description
The focus of our study was Columbia Hollow a 3rd order tributary to Spavinaw Creek in the Ozark Plateau of northwest Arkansas USA (Fig 1) Spavinaw Creek drains into Lakes Eucha and Spavinaw which are drinking water supply reservoirs in northeast Oklahoma This reservoir system is the primary municipal water supply to the City of Jay Oklahoma and surrounding communities it also supplies over half of the municipal drinking water to Tulsa Oklahoma Columbia Hollow receives effluent from a municipal WWTP with secondary treatment in the City of Decatur Arkansas This facility receives wastewater from a poultry processing plant and a residential population of approximately 1000 inhabitants Mean discharge from the Decatur WWTP is ca 5000 m3d (60 Ls) and the effluent has limits of 15 and 10 mgL on ammonium (NH4-N) and nitrate (NO3-N) respectively Currently no regulations exist for P The Decatur WWTP contributes almost 25 of the annual P load to Lake Eucha (Storm et al 2002) and is approximately 9 km from Spavinaw Creek and over 30 km from Lake Eucha following the stream channels Within the study reach Columbia Hollow is a typical Ozark Mountain stream with a chert gravel bed and karst topography in the upland areas The catchment area above the most downstream site was 18 km2 with 73 of the land used as agriculture and pasture
Field MethodsField Methods
We sampled 7 sites at Columbia Hollow almost monthly from June 1999 through February 2000 We estimated discharge at each site from depth and velocity measurements at approximately 03 m intervals across a transect perpendicular to streamflow At each site we measured electrical conductivity temperature and pH at a single point and surface water samples were collected at 3 points perpendicular to streamflow Water samples were filtered immediately on site using 07 m pore diameter Whatman GFF glass fiber filters and acidified with H2SO4 to pH lt2 Water samples were put on ice stored in the dark and analyzed within 48 h of collection Upon return to the laboratory dissolved inorganic nutrient (SRP NH4-N and NO3-N) and chloride (Cl-) concentrations were determined
Study ObjectivesStudy Objectives
In a stream receiving municipal WWTP effluent discharge we evaluated net nutrient retention efficiency using metrics from the nutrient spiraling concepts and calculations of nutrient transformation Specifically we evaluated
1) The ability of the study reach to retain soluble reactive phosphorus (SRP)2) The nitrification and uptake of ammonium (NH4) within the study reach3) The ability of the study reach to retain nitrate (NO3)
Fig 1 Map of the Columbia Hollow catchment (GPS Lat 36 206 Long 94 286) Eucha-Spavinaw Basin and water-quality sampling sites
Nutrient Spiraling CalculationsNutrient Spiraling Calculations
We used nutrient spiraling methods (Stream Solute Workshop 1990) to estimate transport and retention of nutrients from the WWTP Short-term solute additions are often used to estimate the nutrient uptake length (Sw) the average distance a nutrient molecule travels downstream in the dissolved form before uptake by benthic abiotic and biotic processes Nutrient uptake length is an indicator of stream nutrient retention efficiency the shorter the length the greater the efficiency and vice versa We used this same principle of determining downstream declines in nutrient concentrations to estimate a whole-reach measure of nutrient retention However because the nutrient addition from the WWTP was substantial (see Results) and continuous the observed longitudinal pattern in nutrient concentrations downstream from continuous WWTP inputs is the net result of nutrient uptake and release processes Therefore we used downstream declines in nutrient concentrations to estimate a different parameter the net nutrient uptake length (Snet Haggard et al 2001a) Changes in the dilution-corrected nutrient concentration downstream from the WWTP were used to estimate Snet on each sampling date using the following equations
Cx = Coe-kx
ln(CxCo) = -kxSnet = -1k
where Cx is the dilution-corrected nutrient concentration at distance x (km) from the WWTP (mgL) Co is the nutrient concentration at the most upstream site below the WWTP (mgL) and k is the nutrient change coefficient (1m) Simple linear regression was used to determine if the relation between dilution-corrected nutrient concentrations and distance downstream (ie Snet) was significant at = 005 The mass transfer coefficient (vf) is the vertical velocity at which a nutrient molecule travels from the water column to the stream substrate (Stream Solute Workshop 1990) The parameter is related to Snet through average depth and velocity
vf -net = huSnet = Q(Snetw)
where h is the average water depth (m) u is the average water velocity (ms) Q is the average discharge (m3s) and w is the average stream width (m) Net nutrient uptake rate (Unet mg m-2 s-1) of nutrients added by the WWTP can also be calculated from Snet as follows
Unet = (CoQ)(Snetw) = vf-netCo
Because in this study we calculate Snet we have re-defined these terms as net mass transfer coefficient (vf-net) and net nutrient uptake rate (Unet) for this study We estimated the level of nutrient addition (ie SRP DIN NO3-N and NH4-N) as the difference between concentrations upstream and downstream from the WWTPLinear nitrification rate (KNIT 1m) was modeled using equations and methods described by Bernhardt et al (2002) in which KNIT was determined by fitting the model to longitudinal changes in NO3-N concentration Nitrate uptake per m (KNO3-N) and KNIT were simultaneously solved for using MS Excel Solver and minimizing
the sum of squares between predicted and observed NO3-N concentration MS Excel Solver was constrained so that KNIT KNO3-N and NH4-N uptake per m (KNH4-N) were greater than or equal to zero The fraction of NH4-N
nitrified and modeled nitrification rate (g m-2 d-1) were estimated using the following equationsNH4-N Nitrified = kNITkNH4-N
Modeled Nitrification Rate = (kNITkNH4-N)Unet-NH4-N
Aug 1999
Distance from WWTP Effluent Discharge (km)
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DINNO3-N
NH4-N
SRP
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SRP Concentration 03 km Downstream from WWTP Input (mgL)
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NO3-N Concentration 03 km Downstream from WWTP Input (mgL)
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ResultsResults
Soluble Reactive Phosphorus Concentration and RetentionSoluble Reactive Phosphorus Concentration and Retentionbull The Decatur WWTP effluent discharge increased downstream SRP concentration
(plt0001) resulting in up to 50 fold increasebull The greatest SRP concentrations was 99 mgL in February 2000bull SRP concentration decreased with increasing distance from the WWTP input but
concentrations 3 km downstream were still 30 fold greater than at the upstream sitebull Snet-SRP lengths are substantially greater than that observed in other streams in the basin
that drain forested or agricultural catchments (02ndash09Haggard et al 2001b) bull Unet-SRP was high at Columbia Hollow while the demand (vf-net) for this nutrient relative to
its supply was low compared to unenriched streamsbull A P exchange mechanism existed in the study reach that apparently helped maintain high
ambient SRP concentrations when WWTP inputs were low (SRP lt3 mgL) and may be related to benthic sediment equilibrium P concentrations
Ammonium Transformation and RetentionAmmonium Transformation and Retentionbull NH4-N concentrations below the WWTP were 10ndash100 fold greater than upstream
concentrations (plt001)bull NH4-N concentration showed a consistent longitudinal decrease along the study reachbull At the most downstream site NH4-N concentration declined to lt1 mgL and was lt01
mgL on most sampling datesbull The longitudinal decrease in NH4 was significant and Snet ranged from 04ndash14 km bull Modeled nitrification rates were 7ndash31 g NO3-N m-2 d-1 and several orders of magnitude
greater than rates in prairie and agricultural stream sediments (Kemp and Dodds 2002) or a desert stream with extensive hyporheic sediments (Jones et al 1995)
bull Modeled nitrification often exceeded NH4 uptake suggesting additional reduced N sources were nitrified in addition to NH4 inputs from the WWTP
bull Apparent retention of NH4 was counter balanced by increases in NO3 indicating overall retention of DIN was minimal on most sampling dates
Nitrate Concentrations and RetentionNitrate Concentrations and Retentionbull DIN was dominated by NO3 upstream from the WWTP ranging from 30ndash55 mgLbull In contrast to NH4 NO3-N concentrations gradually increased downstream of the WWTP
on all sampling datesbull Downstream changes in NO3 resulted in negative Snet-NO3-N vf-net and Unetbull NH4-N concentration showed a consistent longitudinal decrease along the study reachbull The transformation of NH4 to NO3 made Columbia Hollow a significant downstream
source of NO3 as was the case for SRP
SUMMARYSUMMARY
Point source nutrient loading to Columbia Hollow has created an unusual mixture of biogeochemical characteristics that includes high areal uptake rates that contrast with extremely low vf-net for nutrients a P exchange mechanism that apparently helped maintain high ambient P concentrations and distorted N cycling in which absolute and relative rates of transformation are substantially different from those in unenriched streams Thus rather than acting as a nutrient sink Columbia Hollow appears to act more as a short-term storage zone for added P and a transformer of added N Limited retention emphasizes that point source inputs may have long-term and large-scale effects on water quality (see also Mart et al 2004) and that the key to managing point sources will be to understand the biogeochemical distortions created by focused N and P loading and the circumstances that foster a return to more normal nutrient cycling conditions
References cited in text are available upon request and a more comprehensive manuscript has been submitted to the Journal of the North American Benthological Society Corresponding author Brian Haggard Research Hydrologist USDA ndash ARS PPPSRU 203 Engineering Hall Fayetteville AR 72701 USA tele 4795752879 fax 4795752846 email haggarduarkedu
