JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION VOL. 37, NO. 5 AMERICAN WATER RESOURCES ASSOCIATION OCTOBER 2001 EFFECT OF A POINT SOURCE INPUT ON STREAM NUTRIENT RETENTION' Brian E. Haggard, Daniel E. Storm, and Emily H. Stanley2 ABSTRACT: We examined the effect of a point source (PS) input on water chemistry and nutrient retention in Spavinaw Creek, Arkansas, during summer baseflows in 1998 and 1999. The nutri- ent uptake length (5w) concept was used to quantify the impact of nutrient inputs in the receiving stream. We used an artificial injec- tion upstream of the PS inputs to estimate background S and used the natural decline in nutrient concentrations below the PS to estimate the net nutrient uptake length (Snet). S for soluble reac- tive phosphorus (SRP) in the upstream reference section was 0.75 km, but 5net ranged from 9.0 to 31 km for SRP and 3.1 to 12 km for NO3-N in the reach below the PS. SnetSRP was significantly corre- lated with discharge whereas SfletNO3N was correlated with the amount of N03-N enrichment from the PS. In order to examine spe- cific mechanisms of P retention, loosely exchangeable P and P Sorp- tion Index (PSI) of stream sediments were measured. Sediments exhibited little natural P buffering capacity (low PSI) above the PS, but P loading from the PS further reduced PSI. Loosely exchange- able P in the sediments also increased three fold below the PS, indi- cating sediments removed some water column P. The physical process of flow and sediment sorption apparently regulated P reten- tion in Spavinaw Creek, whereas the level of N enrichment and possibly biotic uptake and denitrification influenced N retention. Regardless of the mechanism, Spavinaw Creek demonstrated little ability to retain PS-added nutrients because net nutrient uptake lengths were in the km range. (KEY TERMS: aquatic ecosystems; phosphorus; nitrogen; nutrient retention; sediments; point source pollution.) INTRODUCTION The Clean Water Act in 1972 initiated control mea- sures on point sources of pollution. As further regula- tion of point source (PS) pollution became less viable economically and significant water quality impair- ment remained, more attention was given to diffuse sources of pollution, especially agricultural nonpoint source (NPS) pollution (Smith et al., 1987; Carpenter et al., 1998). However, recent investigations have shown whole-stream nutrient retention is greatly reduced in PS impacted systems compared to less impacted streams (MartI et al., 2001). Several investi- gators have shown substantial decreases in phospho- rus buffering capacity and increases in P content of sediments below wastewater treatment plants (WWTP) (Dorioz et al., 1998; House and Denison, 1998). Furthermore, PS releases caused sediment deoxygenation up to 20 km below a PS release (Rutherford et al., 1991). In short, despite extensive regulation, PS inputs continue to have pronounced impacts on stream ecosystems. Impacts of PS inputs may result from effluent exceeding permitted levels for parameters such as ammonia or BOD, or from the input of chemicals that are not subject to regulation. Few WWTPs have strict regulations regarding nutrient loading, especially for P. This is true of facilities in rural areas of the Ozark Plateaus of Oklahoma and Arkansas; however, streams potentially impaired by elevated nutrient concentrations will be required to develop Total Maxi- mum Daily Loads (TMDLs) that would potentially limit PS contributions. Recent eutrophication of reser- voirs in this region has underscored the need to iden- tify sources of added nutrients, and to devise management strategies for reducing N and P loading to streams and rivers. As a case in point Spavinaw Creek (Arkansas and Oklahoma) is a primary tribu- tary of Lakes Eucha and Spavinaw (Oklahoma), the source of half the drinking water supply to the city of Tulsa, Oklahoma. Recently, the cost of chemicals used in drinking water treatment has doubled and taste 1Paper No. 00066 of the Journal of the American Water Resources Association. Discussions are open until June 1, 2002. 2Respectively, USDA-ARS, 203 Engineering Hall, University of Arkansas, Fayetteville, Arkansas 72701; Associate Professor, Biosystems and Agricultural Engineering Department, Oklahoma State University, 121 Agricultural Hall, Stillwater, Oklahoma 74078; and Assistant Professor, University of Wisconsin, Center for Limnology, 680 North Park Street, Madison, Wisconsin 53706 (E-Mail/Haggard: haggard@uark.edu). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1291 JAWRA