EXPLORING POTENTIAL PHOSPHOROUS POLLUTION OF A RECREATIONAL LAKE NEAR COUNCIL BLUFFS, IOWA

The objective of this work was to determine the internal phosphorus load within Lake Manawa, near Council Bluffs, Iowa using phosphorous (P) loading models such as the Canfield-Bachmann 1981 Total P Estimation and the Vollenweider Shallow Lake and Reservoir models. Lake Manawa is one of the most popular recreational lakes in the Council Bluffs, Iowa and Omaha, Nebraska area. Lake Manawa is shallow with an average depth of 1.86 meters. The lake has poor water clarity and is hyper-eutrophic due to the continual mixing of the sediments within the lake by recreational watercraft in combination with the lakes high external pollutant loads. This project utilized Geographic Information Systems (GIS) software to delineate the land use features of the Lake Manawa watershed to determine potential sediment and nutrient loads entering the lake. Loads were determined using the Spreadsheet Tool for Estimating Pollutant Loads (STEPL). STEPL is a black box model that calculates pollutant loads based upon USLE soil parameters, and human land cover/use acreages. The majority of the water entering Lake Manawa is diverted from Mosquito Creek. Mosquito Creek flows north to south in close proximity to the lake. In order to acquire an accurate estimation of the total external P load entering the reservoir, the Mosquito Creek P load was taken into consideration, in addition to the estimate of pollutants entering the reservoir from the lake Manawa watershed. Mosquito Creek delivers an estimated 1.8 billion gallons of water per year to the lake with an average base-flow P concentration of 0.25mg/L. This diversion transports nearly 3000 lb. of P into the lake every year. Understanding the effect of land use in the surrounding watershed as well as the Mosquito Creek diversions on the pollutant loads entering the reservoir will assist in determining an accurate estimation of the external and internal P load of the lake.