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

SEGH Events

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

  • Assessment of the toxicity of silicon nanooxide in relation to various components of the agroecosystem under the conditions of the model experiment 2018-08-18

    Abstract

    Investigation of SiO2 nanoparticles (NPs) effect on Eisenia fetida showed no toxic effect of the metal at a concentration of 250, 500 and 1000 mg per kg of soil, but conversely, a biomass increase from 23.5 to 29.5% (at the protein level decrease from 60 to 80%). The reaction of the earthworm organism fermentative system was expressed in the decrease in the level of superoxide dismutase (SOD) on the 14th day and in the increase in its activity to 27% on the 28th day. The catalase level (CAT) showed low activity at average element concentrations and increase by 39.4% at a dose of 1000 mg/kg. Depression of malonic dialdehyde (MDA) was established at average concentrations of 11.2% and level increase up to 9.1% at a dose of 1000 mg/kg with the prolongation of the effect up to 87.5% after 28-day exposure. The change in the microbiocenosis of the earthworm intestine was manifested by a decrease in the number of ammonifiers (by 42.01–78.9%), as well as in the number of amylolytic microorganisms (by 31.7–65.8%). When the dose of SiO2 NPs increased from 100 to 1000 mg/kg, the number of Azotobacter increased (by 8.2–22.2%), while the number of cellulose-destroying microorganisms decreased to 71.4% at a maximum dose of 1000 mg/kg. The effect of SiO2 NPs on Triticum aestivum L. was noted in the form of a slight suppression of seed germination (no more than 25%), an increase in the length of roots and aerial organs which generally resulted in an increase in plant biomass. Assessing the soil microorganisms’ complex during introduction of metal into the germination medium of Triticum aestivum L., there was noted a decrease in the ammonifiers number (by 4.7–67.6%) with a maximum value at a dose of 1000 mg/kg. The number of microorganisms using mineral nitrogen decreased by 29.5–69.5% with a simultaneous increase in the number at a dose of 50 mg/kg (+ 20%). Depending on NP dose, there was an inhibition of the microscopic fungi development by 18.1–72.7% and an increase in the number of cellulose-destroying microorganisms. For all variants of the experiment, the activity of soil enzymes of the hydrolase and oxidoreductase classes was decreased.

  • Seasonal characteristics of chemical compositions and sources identification of PM 2.5 in Zhuhai, China 2018-08-16

    Abstract

    Fine particulate matter is associated with adverse health effects, but exactly which characteristics of PM2.5 are responsible for this is still widely debated. We evaluated seasonal dynamics of the composition and chemical characteristics of PM2.5 in Zhuhai, China. PM2.5 characteristics at five selected sites within Zhuhai city were analyzed. Sampling began on January 10, 2015, and was conducted for 1 year. The ambient mass concentration, carbon content (organic and elemental carbon, OC and EC), level of inorganic ions, and major chemical composition of PM2.5 were also determined. Average concentrations of PM2.5 were lower than the National Ambient Air Quality Standard (NAAQS) 24-h average of 65 μg/m3. The daily PM2.5 concentration in Zhuhai city exhibited clear seasonal dynamics, with higher daily PM2.5 concentrations in autumn and winter than in spring and summer. Carbon species (OC and EC) and water-soluble ions were the primary components of the PM2.5 fraction of particles. Apart from OC and EC, chemical species in PM2.5 were mainly composed of NH4+ and SO42−. There was a marked difference between the summer and winter periods: the concentrations of OC and EC in winter were roughly 3.4 and 4.0 times than those in summer, while NH4+, SO42−, NO3, and Na+ were 3.2, 4.5, 28.0, and 5.7 times higher in winter than those in summer, respectively. The results of chemical analysis were consistent with three sources dominating PM2.5: coal combustion, biomass burning, and vehicle exhaust; road dust and construction; and from reaction of HCl and HNO3 with NH3 to form NH4Cl and NH4NO3. However, additional work is needed to improve the mass balance and to obtain the source profiles necessary to use these data for source apportionment.

  • Estimates of potential childhood lead exposure from contaminated soil using the USEPA IEUBK model in Melbourne, Australia 2018-08-14

    Abstract

    Soils in inner city areas internationally and in Australia have been contaminated with lead (Pb) primarily from past emissions of Pb in petrol, deteriorating exterior Pb-based paints and from industry. Children can be exposed to Pb in soil dust through ingestion and inhalation leading to elevated blood lead levels (BLLs). Currently, the contribution of soil Pb to the spatial distribution of children’s BLLs is unknown in the Melbourne metropolitan area. In this study, children’s potential BLLs were estimated from surface soil (0–2 cm) samples collected at 250 locations across the Melbourne metropolitan area using the United States Environmental Protection Agency (USEPA) Integrated Exposure Uptake Biokinetic (IEUBK) model. A dataset of 250 surface soil Pb concentrations indicate that soil Pb concentrations are highly variable but are generally elevated in the central and western portions of the Melbourne metropolitan area. The mean, median and geometric soil Pb concentrations were 193, 110 and 108 mg/kg, respectively. Approximately 20 and 4% of the soil samples exceeded the Australian HIL-A residential and HIL-C recreational soil Pb guidelines of 300 and 600 mg/kg, respectively. The IEUBK model predicted a geometric mean BLL of 2.5 ± 2.1 µg/dL (range: 1.3–22.5 µg/dL) in a hypothetical 24-month-old child with BLLs exceeding 5 and 10 µg/dL at 11.6 and 0.8% of the sampling locations, respectively. This study suggests children’s exposure to Pb contaminated surface soil could potentially be associated with low-level BLLs in some locations in the Melbourne metropolitan area.