USING FLOW AND REACTIVE TRANSPORT MODELING TO ESTIMATE DIFFUSION AND ADVECTION OF PHOSPHORUS FROM LAKEBED SEDIMENT INTO UPPER KLAMATH LAKE, OR

Management of water and biological resources in the Klamath Basin of Oregon and California is an ongoing challenge. Hypereutrophic Upper Klamath Lake (UKL) suffers from annual cyanobacterial blooms that contribute to low dissolved oxygen, high pH and high ammonia concentrations that affect endangered fish. Primary production may be phosphorus limited. Sources of phosphorus input to lake water include delivery with surface water inflow and precipitation as well as internal loading by phosphorus release from lakebed sediment. Prior studies indicate that groundwater advection, diffusion, bacterial recycling and benthic organisms influence lakebed phosphorus input. Limnologic water-quality studies generally include diffusion of nutrients from sediment pore water to overlying lake water, but neglect transport of phosphorus to or from the lake water column via groundwater advection through lakebed sediment. Time-varying upward flow of anoxic groundwater opposing downward diffusion of oxygen from the overlying oxic lake water can lead to time-varying redox conditions in lakebed sediment that influence the remobilization of sorbed phosphorus by iron reduction. Lakebed nutrient and trace element concentration profiles were monitored weekly from April 15 to Oct. 16, 2014 in the upper 10 cm of the UKL lakebed. Flow and reactive transport modeling is being used in conjunction with these data to characterize and estimate temporally variable advection and diffusion of phosphorus from the lakebed to the overlying water column. Preliminary model results indicate that simulated silica, ammonia and dissolved organic carbon concentrations reproduce observed concentrations when only diffusion and time-varying groundwater advection are simulated suggesting that these elements are relatively inert during transport through the sediment. However, the simulated phosphorus, manganese and iron concentrations (bioreactive solutes for a cyanobacterial dominated aquatic system) do not match observed concentrations suggesting that reactions and/or biological processes are occurring in the upper 10 cm of sediment that influence the transport of these elements. Ongoing modeling is exploring the nature of these reactions/processes and the role of redox conditions in the lakebed on phosphorus release to the lake.