This study uses tracer experiments to characterize the fate of a cold plunging river into a Mediterranean reservoir under different stratification conditions. Three tracer experiments in Lake Béznar, Spain, demonstrate that a significant fraction of the inflow may be quickly entrained into the surface mixed layer (SML), rather than flowing to lower depths. We observe that a fraction (and possibly all) of river inflow entrains into the SML when the density current forms intrusions at the top of the metalimnetic layer. Intrusions near the base of the SML form due to lighter interfacial fluid at the top of the density current, with intermediate properties between riverreservior water when there is sufficient dilution of the cold inflow, and abrupt density changes at the base of the SML. Consequently, a layer of fluid splits from the denser part of the current and intrudes just below the SML, while any remaining fluid flows to lower depths. We parameterize this splitting behavior in terms of the transition Richardson number, Ri 12 5 C 3 (r 2 2 r 1 )/(r 0 2 r 1 ), where r 0 , r 1 , and r 2 are densities of the inflow, the SML, and the metalimnetic layer underneath, respectively; and C is the total dilution through the plunge zone and subsequent entrainment. Splitting occurs when Ri 12 $ 1, consistent with previous laboratory experiments. We use this theory to predict the seasonal fate of river water during the stratification of Lake Béznar and identify the conditions that favor the formation of intrusions near the base of the SML.

Lake restoration by in-lake precipitation of PO4(3-) with Al(OH)3 is commonly used but there is currently no good guidelines for calculating doses (amounts and application) that can ensure long lasting effects. We studied the effect of aging of Al(OH)3 on PO4(3-) adsorption and desorption properties with and without PO4(3-) in solution and found that Al(OH)3 aged without PO4(3-) lost 75% of the maximum adsorption capacity in 90 days after which no further changes occurred. Al(OH)3 aged in presence of PO4(3-) maintained the adsorption capacity through 6 months and even increased it for PO4(3-) concentrations < 150 microM. On this basis, we suggest that repeated dosing of smaller Al-aliquots may be more efficient than adding a single big dose. Also, Al should be added at the time when PO4(3-) availability in the lake is highest. At laboratory conditions we obtained molar P:Al binding ratios of 0.12-0.19 at PO4(3-) concentrations similar to those in eutrophic lake sediments, but when examining Al(OH)3 aged in situ in two lake sediments lower ratios (approximately 0.1) were found. We suggest that total Al-dosage should be calculated relative to the pool of potential mobile P in the lake with a molar ratio not less than 10 Al: 1 P.

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