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

  • Geophagy among East African Chimpanzees: consumed soils provide protection from plant secondary compounds and bioavailable iron 2019-12-01

    Abstract

    Geophagy, the intentional consumption of earth materials, has been recorded in humans and other animals. It has been hypothesized that geophagy is an adaptive behavior, and that clay minerals commonly found in eaten soil can provide protection from toxins and/or supplement micronutrients. To test these hypotheses, we monitored chimpanzee geophagy using camera traps in four permanent sites at the Budongo Forest Reserve, Uganda, from October 2015–October 2016. We also collected plants, and soil chimpanzees were observed eating. We analyzed 10 plant and 45 soil samples to characterize geophagic behavior and geophagic soil and determine (1) whether micronutrients are available from the soil under physiological conditions and if iron is bioavailable, (2) the concentration of phenolic compounds in plants, and (3) if consumed soils are able to adsorb these phenolics. Chimpanzees ate soil and drank clay-infused water containing 1:1 and 2:1 clay minerals and > 30% sand. Under physiological conditions, the soils released calcium, iron, and magnesium. In vitro Caco-2 experiments found that five times more iron was bioavailable from three of four soil samples found at the base of trees. Plant samples contained approximately 60 μg/mg gallic acid equivalent. Soil from one site contained 10 times more 2:1 clay minerals, which were better at removing phenolics present in their diet. We suggest that geophagy may provide bioavailable iron and protection from phenolics, which have increased in plants over the last 20 years. In summary, geophagy within the Sonso community is multifunctional and may be an important self-medicative behavior.

  • Accumulation of uranium and heavy metals in the soil–plant system in Xiazhuang uranium ore field, Guangdong Province, China 2019-12-01

    Abstract

    Plants that have grown for many years in the special environmental conditions prevailing in mining areas are naturally screened and show strong capacity to adapt to their environment. The present study investigated the enrichment characteristics of U and other heavy metals (As, Cu, Pb, Mn, Mo, Zn, Cd, Co, and Ni) in the soil–plant system in Xiazhuang uranium mine. Four dominant plants (Castanopsis carlesii, Rhus chinensis, Liriodendron chinense, and Sapium discolor) and soil samples were collected from the mined areas, unmined areas, and background areas away from the ore field. U, As, Cu, Pb, Mn, Mo, Zn, Cd, Co, and Ni concentrations were analyzed by ICP-MS. The results demonstrate that (1) The highest concentrations of U (4.1–206.9 mg/kg) and Pb (43.3–126.0 mg/kg) with the geoaccumulation index (Igeo) greater than 1 show that they are the main soil pollutants in the research area. (2) The biological accumulation coefficient (LBAC) values for Cd, Mn, and Cu are greater than zero in S. discolor, L. chinense, and C. carlesii and these three plants indicate that they can be used for remediation of the soil in the ore field. (3) R. chinensis inhibits the accumulation of heavy metals and shows sensitive pigment responses to the accumulation of U in the leaves. L. chinense has the strongest enrichment effect on heavy metals but exhibits weak biochemical responses under U stress. C. carlesii demonstrates strong adaptation to U and can maintain healthy pigment characteristics in case of high U enrichment. (4) S. discolor, L. chinense, C. carlesii and R. chinensis have strong tolerance to U toxicity and different biochemical responses.

  • Distribution, sources and health risk assessment of contaminations in water of urban park: A case study in Northeast China 2019-12-01

    Abstract

    This case study was performed to determine whether the pollutants in water of urban park could bring health risk to human engaging in water-related activities such as swimming and provide evidence demonstrating the critical need for strengthened recreational water resources management of urban park. TN, NH4+-N, TP, Cu, Mn, Zn, Se, Pb, As, Cd and Cr(VI) contents were determined to describe the spatial distribution of contaminations; sources apportionment with the method of correlation analysis, factor analysis and cluster analysis were followed by health risk assessment for swimmers of different age groups. The results reveal that element contents in all sites do not exceed Chinese standard for swimming area and European Commission standard for surface water; all detected elements except Cr(VI) have a tendency to accumulate in the location of lake crossing bridge; Mn and Zn are considered to have the same pollution source including geogenic and anthropogenic sources by multivariable analysis. Carcinogenic risks of different age groups descend in the same order with non-carcinogenic risks. Among all elements, Zn and Mn contribute the lowest non-carcinogenic risk (5.1940E-06) and the highest non-carcinogenic risk (7.9921E-04) through skin contact pathway, respectively. The total average personal risk for swimmers in swimming area is 1.9693E-03, and this site is not suitable for swimming. Overall, it is possible that swimmers are exposed to risk via the dermal route when carrying out water-related activities, it is recommended that necessary precautions and management should be taken in other similar locations around the world.