The city of Tulsa, Oklahoma, uses Lake Eucha and Spavinaw Lake in the Eucha-Spavinaw Basin in northwestern Arkansas and northeastern Oklahoma for public water supply. The city has spent millions of dollars over the last decade to eliminate taste and odor problems in the drinking water from the Eucha-Spavinaw system, which may be attributable to blue-green algae. Increases in the algal biomass in the lakes may be attributable to increases in nutrient concentrations in the lakes and in the waters feeding the lakes. The U.S. Geological Survey, in cooperation with the City of Tulsa, investigated and summarized total nitrogen and total phosphorus concentrations in water samples and provided estimates of nitrogen and phosphorus loads, yields, and flow-weighted concentrations during base flow and runoff for two streams discharging to Lake Eucha for the period January 2002 through December 2009. This report updates a previous report that used data from water-quality samples collected from January 2002 through December 2006.

Based on the results from the Mann-Whitney statistical test, unfiltered total nitrogen concentrations were significantly greater in runoff water samples than in base-flow water samples collected from Spavinaw Creek near Maysville and near Cherokee City, Arkansas; Spavinaw Creek near Colcord, Oklahoma, and Beaty Creek near Jay, Oklahoma. Nitrogen concentrations in runoff water samples collected from all stations generally increased with increasing streamflow.

Nitrogen concentrations in base-flow and runoff water samples collected in Spavinaw Creek significantly increased from the station furthest upstream (near Maysville) to the Sycamore station and then significantly decreased from the Sycamore station to the station furthest downstream (near Colcord). Nitrogen concentrations in base-flow and runoff water samples collected from Beaty Creek were significantly less than base-flow and runoff water samples collected from Spavinaw Creek.

Based on the results from the Mann-Whitney statistical test, unfiltered total phosphorus concentrations were significantly greater in runoff water samples than in base-flow water samples for the entire period for most stations, except in water samples collected from Spavinaw Creek near Cherokee City, in which no significant difference was detected for the entire period nor for any season. Phosphorus concentrations in runoff water samples collected from all stations generally increased with increasing streamflow.

Based on results from a multi-stage Kruskal-Wallis statistical test, phosphorus concentrations in base-flow water samples collected from Spavinaw Creek significantly increased from the Maysville station to the Cherokee City station, probably because of discharge from a municipal wastewater-treatment plant between those stations. Phosphorus concentrations significantly decreased downstream from the Cherokee City station to the Colcord station. Phosphorus concentrations in base-flow water samples collected from Beaty Creek were significantly less than phosphorus in base-flow water samples collected from Spavinaw Creek downstream from the Maysville station.

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First posted August 5, 2010