SEGH Articles

How useful are on-site measurements in environmental geochemistry?

12 August 2012
How reliable are on-site measurements? Can sound decisions be made using them, or should we stick to measurements made in the remote lab?

 

There are an increasing number of portable instruments (and test kits) that environmental geochemists can take into the field to get rapid measurements of contaminant concentration. They can measure a range of different inorganic and organic contaminants in many different media, such as soils, wastes, waters and gases – and give results in a few minutes. This seems a very attractive way to enable rapid decisions on whether contaminants might pose a threat to human health, or just whether and where more measurements need to be taken. It seems to be a much better option than waiting weeks to get samples analysed in a remote lab. The key issue is how reliable are these on-site measurements? Can sound decisions be made using them, or should we stick to measurements made in the remote lab?

 

Recent research at University of Sussex has been investigating this issue, by focusing particularly
on the uncertainty in both the on-site and lab-based measurements. The key concept here is that all measurements are wrong to some extent, but the value of the uncertainty tells us the range of concentration within which the true value lies. We applied a range of different on-site techniques to a several different contaminated sites, and compared the results with those from remote labs. There are two main ways of applying on-site analytical techniques in the field. The ‘in situ’ approach leaves the sample material in its original position; for example a portable x-ray fluorescence spectrometer (PXRF – Fig.1) can be placed directly onto an area of bare soil, or a pH electrode placed in a river. The second more common ‘on-site’ approach, is to remove a sample from its position (e.g. in the top soil), prepare it in some way (e.g. mix a soil as far as possible within a polythene bag), and analyse this prepared sample near the original location, or in a local field-lab.

 

 

The results for the in situ approach, in an example for As determined by PXRF, showed that the areas of land identified as being contaminated by As above a threshold concentration value, were very similar to those found using the lab measurement. Somewhat unexpectedly, the uncertainty of both the in situ and lab measurements were also similar[1]. The was because the main source of uncertainty was shown to be the sampling, rather than the analytical process, and therefore caused by the small-scale heterogeneity of the As distribution in the soil. The optimal level of uncertainty was calculated, at a level that minimizes the overall potential financial loss. This cost arises from both the taking of the measurements, and from decision errors in the land classification caused by the uncertainty. Neither  the lab or the in situ measurements had this optimal level of uncertainty, which is required to achieve full fitness-for-purpose. Both type of measurement would therefore benefit from the taking of composite samples or measurements. This is achieved by taking several sub-samples across each small area, rather than just one sample, to reduce the effect of the small-scale heterogeneity to an acceptable level. The bias the in situ measurements was estimated as -43%, by comparison against the lab measurements across all locations. This was partially due to the moisture and pore spaces included in the soils measured in situ. Once quantified, this bias can be corrected for, to improve the agreement. Using this approach, it was argued that in situ measurements can be not only sufficiently reliable, but they can also be more fit than lab measurements for some purposes, such as hot spot delineation.

For another case study of on-site measurements, with local removal of the samples from two sites, the sampling process again dominated the uncertainty values[2]. For the organic contaminant total petroleum hydrocarbons (TPH), the uncertainty of the lab analysis (U = 54%) was too high to make an effective comparison of the on-site results from the test kit (U = 30%) against them. This study also indentified that the different definitions of terms, like TPH including aromatic but not aliphatic compounds, is another factor limiting the interpretation using such comparisons.

 

Overall the conclusion was that on-site (including in situ) measurements can have distinct advantages over lab-based measurements, especially in terms of their rapidity and lower overall cost. The consideration of the uncertainty of both types of measurements revealed that both can be very high (e.g. ~50 - 100%), but the reliable interpretation of both types of measurements requires the value of the uncertainty to be estimated for that application. Knowing this information, on-site measurements can be as useful to environmental geochemists as those made in the lab.



Professor Mike Ramsey, University of Sussex. 

 

 

 

 

[1] Ramsey M.H. and Boon K.A. (2012) Can in situ geochemical measurements be more fit-for-purpose than those made ex situ? Applied Geochemistry 27, 969-976 . http://dx.doi.org/10.1016/j.apgeochem.2011.05.022

 

[2] Boon K.A. and Ramsey M.H. (2012) Judging the fitness of on-site measurements by their uncertainty, including the contribution from sampling. Science of the Total Environment  419, 196–207 http://dx.doi.org/10.1016/j.scitotenv.2011.12.001

 

Keep up to date

Submit Content

Members can keep in touch with their colleagues through short news and events articles of interest to the SEGH community.

Science in the News

Latest on-line papers from the SEGH journal: Environmental Geochemistry and Health

  • Fertilizer usage and cadmium in soils, crops and food 2018-06-23

    Abstract

    Phosphate fertilizers were first implicated by Schroeder and Balassa (Science 140(3568):819–820, 1963) for increasing the Cd concentration in cultivated soils and crops. This suggestion has become a part of the accepted paradigm on soil toxicity. Consequently, stringent fertilizer control programs to monitor Cd have been launched. Attempts to link Cd toxicity and fertilizers to chronic diseases, sometimes with good evidence, but mostly on less certain data are frequent. A re-assessment of this “accepted” paradigm is timely, given the larger body of data available today. The data show that both the input and output of Cd per hectare from fertilizers are negligibly small compared to the total amount of Cd/hectare usually present in the soil itself. Calculations based on current agricultural practices are used to show that it will take centuries to double the ambient soil Cd level, even after neglecting leaching and other removal effects. The concern of long-term agriculture should be the depletion of available phosphate fertilizers, rather than the negligible contamination of the soil by trace metals from fertilizer inputs. This conclusion is confirmed by showing that the claimed correlations between fertilizer input and Cd accumulation in crops are not robust. Alternative scenarios that explain the data are presented. Thus, soil acidulation on fertilizer loading and the effect of Mg, Zn and F ions contained in fertilizers are considered using recent \(\hbox {Cd}^{2+}\) , \(\hbox {Mg}^{2+}\) and \(\hbox {F}^-\) ion-association theories. The protective role of ions like Zn, Se, Fe is emphasized, and the question of Cd toxicity in the presence of other ions is considered. These help to clarify difficulties in the standard point of view. This analysis does not modify the accepted views on Cd contamination by airborne delivery, smoking, and industrial activity, or algal blooms caused by phosphates.

  • Effects of conversion of mangroves into gei wai ponds on accumulation, speciation and risk of heavy metals in intertidal sediments 2018-06-23

    Abstract

    Mangroves are often converted into gei wai ponds for aquaculture, but how such conversion affects the accumulation and behavior of heavy metals in sediments is not clear. The present study aims to quantify the concentration and speciation of heavy metals in sediments in different habitats, including gei wai pond, mangrove marsh dominated by Avicennia marina and bare mudflat, in a mangrove nature reserve in South China. The results showed that gei wai pond acidified the sediment and reduced its electronic conductivity and total organic carbon (TOC) when compared to A. marina marsh and mudflat. The concentrations of Cd, Cu, Zn and Pb at all sediment depths in gei wai pond were lower than the other habitats, indicating gei wai pond reduced the fertility and the ability to retain heavy metals in sediment. Gei wai pond sediment also had a lower heavy metal pollution problem according to multiple evaluation methods, including potential ecological risk coefficient, potential ecological risk index, geo-accumulation index, mean PEL quotients, pollution load index, mean ERM quotients and total toxic unit. Heavy metal speciation analysis showed that gei wai pond increased the transfer of the immobilized fraction of Cd and Cr to the mobilized one. According to the acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) analysis, the conversion of mangroves into gei wai pond reduced values of ([SEM] − [AVS])/f oc , and the role of TOC in alleviating heavy metal toxicity in sediment. This study demonstrated the conversion of mangrove marsh into gei wai pond not only reduced the ecological purification capacity on heavy metal contamination, but also enhanced the transfer of heavy metals from gei wai pond sediment to nearby habitats.

  • Cytotoxicity induced by the mixture components of nickel and poly aromatic hydrocarbons 2018-06-22

    Abstract

    Although particulate matter (PM) is composed of various chemicals, investigations regarding the toxicity that results from mixing the substances in PM are insufficient. In this study, the effects of low levels of three PAHs (benz[a]anthracene, benzo[a]pyrene, and dibenz[a,h]anthracene) on Ni toxicity were investigated to assess the combined effect of Ni–PAHs on the environment. We compared the difference in cell mortality and total glutathione (tGSH) reduction between single Ni and Ni–PAHs co-exposure using A549 (human alveolar carcinoma). In addition, we measured the change in Ni solubility in chloroform that was triggered by PAHs to confirm the existence of cation–π interactions between Ni and PAHs. In the single Ni exposure, the dose–response curve of cell mortality and tGSH reduction were very similar, indicating that cell death was mediated by the oxidative stress. However, 10 μM PAHs induced a depleted tGSH reduction compared to single Ni without a change in cell mortality. The solubility of Ni in chloroform was greatly enhanced by the addition of benz[a]anthracene, which demonstrates the cation–π interactions between Ni and PAHs. Ni–PAH complexes can change the toxicity mechanisms of Ni from oxidative stress to others due to the reduction of Ni2+ bioavailability and the accumulation of Ni–PAH complexes on cell membranes. The abundant PAHs contained in PM have strong potential to interact with metals, which can affect the toxicity of the metal. Therefore, the mixture toxicity and interactions between diverse metals and PAHs in PM should be investigated in the future.