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 .


[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


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

  • Improving arsenopyrite oxidation rate laws: implications for arsenic mobilization during aquifer storage and recovery (ASR) 2018-04-25


    Aquifer storage and recovery (ASR) and aquifer recharge (AR) provide technical solutions to address water supply deficits and growing future water demands. Unfortunately, the mobilization of naturally present arsenic due to ASR/AR operations has undermined its application on a larger scale. Predicting arsenic mobility in the subsurface during ASR/AR is further complicated by site-specific factors, including the arsenic mobilization mechanisms, groundwater flow conditions, and multi-phase geochemical interactions. In order to ensure safe and sustainable ASR/AR operation, a better understanding of these factors is needed. The current study thus aims to better characterize and model arsenic remobilization at ASR/AR sites by compiling and analyzing available kinetic data on arsenic mobilization from arsenopyrite under different aqueous conditions. More robust and widely applicable rate laws are developed for geochemical conditions relevant to ASR/AR. Sensitivity analysis of these new rate laws gives further insight into the controlling geochemical factors for arsenic mobilization. When improved rate laws are incorporated as the inputs for reactive transport modeling, arsenic mobilization in ASR/AR operations can be predicted with an improved accuracy. The outcomes will be used to guide groundwater monitoring and specify ASR/AR operational parameters, including water pretreatment requirements prior to injection.

  • Heavy metal exposure has adverse effects on the growth and development of preschool children 2018-04-25


    The purpose of this study was to investigate the associations between levels of lead (Pb), cadmium (Cd), chromium (Cr), and manganese (Mn) in the PM2.5 and blood and physical growth, and development parameters including birth length and weight, height, weight, body mass index (BMI), head circumference, and chest circumference in preschool children from Guiyu (e-waste exposure area) and Haojiang (the reference area). A total of 470 preschool children from Guiyu and Haojiang located in southeast coast of China were recruited and required to undergo physical examination and blood tests during the study period. Birth length and weight were obtained by birth records and questionnaire. Pb and Cd in both PM2.5 and blood were significantly higher in Guiyu than Haojiang. Remarkably, the children of Guiyu had significantly lower birth weight and length, BMI, and chest circumference when compare to their peers from the reference area (all p value < 0.05). Spearman correlation analyses showed that blood Pb was negatively correlated with height (r = −0.130, p < 0.001), weight (r = −0.169, p < 0.001), BMI (r = −0.100, p < 0.05), head circumference (r = −0.095, p < 0.05), and chest circumference (r = −0.112, p < 0.05). After adjustment for the potential confounders in further linear regression analyses, blood Pb was negatively associated with height (β = −0.066, p < 0.05), weight (β = −0.119, p < 0.001), head circumference (β = −0.123, p < 0.01), and chest circumference (β = −0.104, p < 0.05), respectively. No significant association between blood Cd, Cr, or Mn was found with any of our developmental outcomes. Taken together, lead exposure limits or delays the growth and development of preschool children.

  • Contamination characteristics of trace metals in dust from different levels of roads of a heavily air-polluted city in north China 2018-04-24


    Concentrations of eight trace metals (TMs) in road dust (RD) (particles < 25 μm) from urban areas of Xinxiang, China, were determined by inductively coupled plasma mass spectrometry. The geometric mean concentrations of Zn, Mn, Pb, As, Cu, Cr, Ni and Cd were 489, 350, 114, 101, 60.0, 39.7, 31.6, and 5.1 mg kg−1, respectively. When compared with TM levels in background soil, the samples generally display elevated TM concentrations, except for Cr and Mn, and for Cd the enrichment value was 69.6. Spatial variations indicated TMs in RD from park path would have similar sources with main roads, collector streets and bypasses. Average daily exposure doses of the studied TMs were about three orders of magnitude higher for hand-to-mouth ingestion than dermal contact, and the exposure doses for children were 9.33 times higher than that for adults. The decreasing trend of calculated hazard indexes (HI) for the eight elements was As > Pb > Cr > Mn > Cd > Zn > Ni > Cu for both children and adults.