Abstract
Estuaries are suitable places for both clay mineral accumulation and iron trapping. Flocculated and deposited Fe compounds in the estuaries can lead to neo-formed minerals which they have a basic role in reservoir quality estimation. The aqueous geochemistry of the Ravenglass estuary and its feeding rivers has been studied to assess if, where and when aqueous iron is lost from the river water and accumulates as part of the sediment in the estuary. Ravenglass estuary waters are conservative mixtures between river water and seawater in terms of chloride, sodium, potassium and magnesium. Alkalinity (bicarbonate), calcium and sulphate are locally non-conservative and are affected by biological and mineral processes. The River Irt contains twice as much dissolved iron as the River Esk but all iron concentrations are much lower in the estuary samples than in the feeding rivers. Aqueous iron undergoes large-scale accumulation in the Ravenglass estuary. Iron concentrations are lowest at high tide at all sampling sites on the Ravenglass estuary. Iron concentrations are highest at low tide for the Irt arm of the estuary but are highest on the falling tide between high and low tide. Iron concentrations in estuary samples decrease rapidly as salinity increases with low iron concentrations in all estuary samples once salinity exceeds 5,000 mg/lit. Iron concentrations also decrease as pH increases. The loss of iron is presumably due to flocculation of colloidal iron oxides, hydroxides and iron-organic complexes. Fluvial aqueous iron does not behave conservatively on mixing with seawater; most iron is lost from the water column at an early stage of river water mixing with estuary water. The site of primary iron-loss from the water occurs towards the heads of estuaries but this site will move as a function of time within the tide cycle. Given that the Esk has highest iron concentrations between high and low tide, it is likely that iron is swept from the iron-rich Irt arm of the estuary into the iron-poor Esk arm soon after high tide.
Highlights
IntroductionClay minerals and organic materials in association with iron colloids are transported to estuaries by rivers, Fe colloids become immobilised in presence of cation during the mixing of freshwater and seawater, estuaries are suitable places for both clay mineral accumulation and iron trapping [19,24,34]
The non-conservative behaviour of dissolved iron during estuarine mixing has been well documented [6,7,16,25,35].Clay minerals and organic materials in association with iron colloids are transported to estuaries by rivers, Fe colloids become immobilised in presence of cation during the mixing of freshwater and seawater, estuaries are suitable places for both clay mineral accumulation and iron trapping [19,24,34]
Low tide estuary samples, at least high up the estuary, appear to be river water-dominated while the high tide samples and the more sites closer to the sea tend to be seawater-dominated
Summary
Clay minerals and organic materials in association with iron colloids are transported to estuaries by rivers, Fe colloids become immobilised in presence of cation during the mixing of freshwater and seawater, estuaries are suitable places for both clay mineral accumulation and iron trapping [19,24,34]. Solid phase ferric oxides and hydroxides in the estuaries can be reduced to ferrous phases [11,12]. This ferrous phase depends on the density of organic materials and present or absent of sulphate behaves differently [1,12,33,34]. On mixing with seawater in an estuary, these colloids are believed to aggregate to create grains that are larger than the filter pore size [25]. The aggregation of the Fe colloids is due to an interaction with cations such as Mg2+ and Ca2+ which are introduced to the estuary on an incoming tide by seawater [7,14]
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