Can River Bank Sediment Be a Carrier and Then Source of Available P In Lake Pepin?.

Sediments are a major carrier of P to rivers and lakes. Although only a small fraction of sediment bound P (particulate P) is available at any time, sediments in lakes and river bottoms can act as a continuous source of available P over a long period of time. One such case is Lake Pepin, a large floodplain lake on the upper Mississippi River about 80km south of St. Paul, where severe eutrophication has been known to occur during dry years. Even though it is well accepted that a majority of sediments in Lake Pepin are coming from river banks, there is a perception that P is coming from agricultural lands. This research is geared to answer a question: Do the bank sediments have ability to adsorb soluble P from river waters during transport and then release it after being deposited at the bottom of Lake Pepin? For this study, we collected a series of river bank materials representing various parent materials and analyzed them for equilibrium phosphorus concentration (EPC₀) as well as their potential to adsorb and desorb soluble phosphorus.  In addition, we ran adsorption experiments of these materials with water samples collected both from treatment plants and various river reaches.  Results show that these bank materials have a low EPC₀ (0.155 mg/L) value, a high P adsorption potential but minimal desorption capacity. We conclude that these bank materials can adsorb soluble P from the river waters and then hold onto it until settled.  In this presentation, we also show that phosphorus concentrations in Lake Pepin sediment, prior to 1830, can be explained as enrichment of bank sediments. Similarly, the subsequent increase in sediment P concentrations, especially early to mid-1900s, are likely from P adsorption from high soluble P concentration in river water as a result of high P wastewater treatment plant discharges and/or dumping of raw sewage in rivers during high flows conditions.