Amorphous silica analysis in terrestrial runoff samples

The correct analysis of amorphous silica concentration (CASi) in natural waters is crucial if one wants to correctly quantify terrestrial and/or riverine ASi fluxes. Soil ASi measurements are conducted with a constant solid to solution ratio (σ). As the suspended particulate matter concentration (CSPM), and therefore σ, cannot be exactly known a priori in river samples. It is important to understand how variations in σ effect analysed CASi. The objectives of this paper are (i) to investigate whether and how variations in σ values affect measured CASi in river runoff samples and (ii) to investigate whether or not it is possible to define a range of σ within which CASi in runoff and/or soil samples can be accurately measured. For the laboratory experiment 30 runoff samples with a wide range of CSPM, typical for the Belgian Loam Belt, were prepared and analysed using the alkaline digestion method (0.1 M Na2CO3). Our study confirmed that the alkaline digestion method proposed by DeMaster can be used for runoff samples provided that σ is within certain limits: at very low σ (< 0.1 kg m−3), subsample heterogeneity results in high variability of measured CASi while at higher σ values (> 0.8 kg m−3) incomplete dissolution of ASi as well as the reduction of mineral dissolution rates results in underestimated CASi. As both errors compensate one another, the range of applicable σ-values can be extended above the theoretically correct limit (1.6 kg m−3). The finding that reliable measurements can be made within a relatively wide range of σ values (0.1 ≤ σ ≤ 1.6 kg m−3) is important. It is now possible to propose a method for the measurement of ASi in runoff samples. We make recommendations for ASi analysis distinguishing samples with a low and high CSPM. For samples with a low CSPM (≤ 1.6 kg m−3) the standard procedure is proposed while for samples with a high CSPM (> 1.6 kg m−3) an adapted procedure is proposed, analogue to that for soil samples. However, one should be aware that the range and limits for σ proposed here may depend on the type of sediment to be analysed: it is therefore recommended to evaluate the performance of the method again before it is used in other environments.

► Alkaline digestion method can be applied to measure ASi in runoff samples.
► The method is applicable for a range of σ values.
► ASi is linear related to σ values.
► Error compensation extends the theoretically correct limit.