Presentation on theme: "Paper Review Interstitial Ion Concentrations as an Indicator of Phosphorus Release and Mineral Formation in Lake Sediments G C Holdren and D E Armstrong."— Presentation transcript:
Paper Review Interstitial Ion Concentrations as an Indicator of Phosphorus Release and Mineral Formation in Lake Sediments G C Holdren and D E Armstrong
Objective To understand the processes that control canal water chemistry by comparison with other observations. To decide on the best methods of data analysis
Purpose of paper Explain difference between P release from sediment in two lakes. Two sites Lake Mendota, eutrophic, calcareous, anoxic in summer Lake Minocqua, mesotrophic, non- calcareous, some oxygen depletion
Basic Site Chemistry Lake Mendota TOC7-7.5% Carbonate2.5-3% Total Fe0.7 – 1.5% Lake Minocqua TOC10-25% Carbonate0 Total Fe0.8-1%
Method Seasonal deep cores collected before and after overturn Data reported from 2 and 3 cm depth intervals in sediment Pore water extracted by centrifugation, depends on low temperature to avoid oxidation Concentrations measured by different techniques
Observations Difference in concentrations due to T and depth. pH decreases with increasing T Concentrations of most ions increase with depth
Observations P may peak at 7 cm depth (not noted in paper) P, Fe and S important in terms of P release Low Fe/P ratios result in P release in the shallow part of Lake Mendota High Fe/P ratios and oxidation result in P retention in Lake Minocqua and deep Lake Mendota
Explanation – pH and T At higher T, bacterial processes degrade organic matter more quickly liberating CO 2. CO 2 dissolves producing a lower pH.
Explanation – P concentrations P is immobilised by iron oxides and the iron phosphate vivianite. High Fe/P ratios result from low sulphide activities in porewaters Sulphide (HS - ) activities pH dependent
Lake Mendota Higher pH at Lake Mendota promoted high dissolved sulphide Iron concentrations limited by Fe sulphide (e.g. pyrite) are low hence P released
Lake Minocqua Lower pH Lower HS - activity Therefore higher Fe 2+ activity and lower dissolved P, limited by vivianite solubility.
Value P release depends on reactions involving Fe. Vivianite is an important P fixing mineral in organic-rich sediments Saturation indices and Fe/P ratios provide a means of assessment.
Problems Comparison with data in paper limited by scale of observation Canal sediments are not so deep and contain more Fe and P. Authors have not considered fluxes or rates but have adopted a steady state approach.