EFFECT OF A POINT SOURCE INPUT ON STREAM NUTRIENT RETENTION.

Authors: Haggard, Brian; STORM, DANIEL - OKLAHOMA STATE UNIVERSITY; STANLEY, EMILY - UNIVERSITY OF WISCONSIN

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: The effect of point source-added nitrogen and phosphorus on water chemistry and the ability of the Spavinaw Creek, Arkansas to retain the added nitrogen and phosphorus was examined during summer low flows in 1998 and 1999. The distance required to remove approximately 63 percent of the point source-added nutrients was used to assess the impact of the point source inputs on the receiving stream. In order to assess background ability to remove phosphorus, we artificially added some phosphorus to the stream above the point source affected area. Spavinaw Creek required 0.75- km to remove 63 percent of the added phosphorus above the point source whereas the stream needed between 9 and 31-km to remove the same fraction of phosphorus added by the point source. Between 3 and 12-km of flowing waters were needed to remove 63 percent of the nitrogen added by the point source. We also looked at how the point source affected the amount of phosphorus loosely bound to the stream bottom sediments and how easily the sediments buffered phosphorus additions. The amount of loosely bound sediment phosphorus below the point source increased three times the amount in the sediments above the point source. The ability of the stream bottom sediments to buffer phosphorus additions decreased below the point source compared to above. Spavinaw Creek showed little ability to retain point source-added nitrogen and phosphorus because it took several kilometers (up to 30-km) to remove 63 percent of the added nitrogen and phosphorus.

Technical Abstract: The effect of a point source input on water chemistry and nutrient retention was examined in Spavinaw Creek, Arkansas, during summer base flow in 1998 and 1999. The nutrient uptake length concept was used to quantify the impact of nutrient inputs in the receiving stream. We used an artificial injection upstream of the point source inputs to estimate background nutrient uptake length and used the natural decline in nutrient concentrations below the point source to estimate the net nutrient uptake length. Uptake length for soluble reactive phosphorus (SRP) in the upstream reference section was 0.75 km, but net uptake length ranged from 9.0 31-km for SRP and 3.1 12-km for nitrate-N in the reach below the point source. Net uptake length for SRP was significantly correlated with discharge whereas net uptake length for nitrate-N was correlated with the amount of nitrate-N enrichment from the point source. In order to examine specific mechanisms of P retention, loosely exchangeable P and P Sorption 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 exchangeable P in the sediments also increased three folds below the point source; indicating sediments removed some water column P. The physical process of flow and sediment sorption apparently regulated P retention 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 point source-added nutrients because net nutrient uptake lengths were in the km range.