OR/18/052 Results from GC-MS and LC-MS datasets
|Lapworth, D J, Crane, E J, Stuart, M E, Talbot, J C, Besien, T, and Civil, W. 2018. Micro-organic contaminants in groundwater in England: summary results from the Environment Agency LC-MS and GC-MS screening data. British Geological Survey Internal Report, OR/18/052.|
Top 50 GC-MS substances by frequency of detection
The top 50 substances from the GC-MS dataset selected by frequency of detection are shown in Figure 3.1. A statistical summary of the data is shown in Table 3.1. Figure 3.2 shows the distribution of the results for the 50 most frequently detected substances by GC-MS using box plots. Due to the low frequency of detections, the box plots highlight the outlier values for each substance rather than the interquartile range, which for all of these substances is below the detection limit and was not computed.
|Ranking||CAS Number||Analyte||Short name||Use code *||LOD||95th percentile concentration||Maximum concentration||Number of analyses||Number of positive detections||% analyses with positive detections|
|G04||106650||Butanedioic acid, dimethyl ester||Dimethyl succinate||PPCL||0.01||0.02||74||11368||769||6.76|
|G14||117817||Bis(2-ethylhexyl) phthalate (DEHP)||DEHP||Plast||1||1||62||11368||452||3.98|
|G15||627930||Dimethyl adipate||Dimethyl adipate||Indu||0.01||0.01||150||11368||360||3.17|
|G30||115968||Tri-(2-chloroethyl) phosphate||Tri-(2-chloroethyl) phosphate||Indu||0.01||0.01||65||11368||202||1.78|
|G35||1241947||2-Ethylhexyl diphenyl phosphate||2-ethylhexyl diphenyl phosphate||Indu||0.01||0.01||1||11368||178||1.57|
* Key to use codes given in Table 2.2.
Discussion of results Pesticides (Pest)
Eight out of the top 50 most frequently detected compounds using the GC-MS method are pesticides. The triazine herbicides remain prominent. Atrazine is the most frequently detected compound with 1069 detections corresponding to 9.4% of samples, although the concentrations are relatively low, with the 95th percentile (p95) at the LOD and a maximum concentration of 0.75 μg/L. Its TP desethyl-atrazine is also prominent, the 5th most frequently detected compound, with a slightly higher maximum concentration. Simazine is also in the top 50, the 11th most frequently detected compound, but with lower maximum concentrations. Following their withdrawal from use in the UK, in 1993 for amenity use and in 2005 for agricultural uses, concentrations of atrazine and simazine have declined considerably, but concentrations not meeting the drinking water standard still persist in a few places.
The next most frequently detected compound, 2,6-dichlorobenzamide (commonly referred to as BAM) is a TP of the herbicide dichlobenil. The p95 at the LOD, but a number of high concentrations have been measured with a maximum concentration of 70 μg/L.
Metaldehyde is a molluscicide and has been a good example of an emerging contaminant with unanticipated widespread elevated environmental concentrations found over the past decade following development of a suitable analytical method. It has been detected in 2.5% of samples with a maximum concentration of 6.4 μg/L. A ban on the outdoor use of metaldehyde is to be introduced across Great Britain from Spring 2020.
Other pesticides are oxadixyl, a fungicide used in fruit growing, diphenylamine, widely used as an industrial antioxidant and reagent and also employed in agriculture as a fungicide and antihelminthic, and the herbicides metazachlor and bromacil. These are detected in fewer than 2% of samples and have maximum concentrations in the range 3–6 μg/L except metazachlor which has a maximum concentration of 44 μg/L.
These are all compounds with a well-established track record of persistence in groundwater.
Halogenated solvents (Hsol)
Seven out of the top 50 most frequently detected compounds using the GC-MS method are halogenated solvents. Trichloroethene is the most frequently detected compound in this category, and the second overall, with 948 detections corresponding to 8.34% of samples, although the concentrations are relatively low, with a p95 of 0.13 μg/L and a maximum concentration of 184 μg/L. Tetrachloroethene and the THM bromodichloromethane are found with similar maximum concentrations but in fewer samples. Other brominated THMs detected were chlorodibromomethane and bromoform in about 2% of samples. 1,1,2-trichloroethane and chlorobenzene were also found but at lower concentrations. All compounds except trichloroethane and tetrachloroethane had censored distributions with p95 concentrations at the LOD.
Five compounds classified as plasticisers were ranked in the top 50. Of these bisphenol A was the most frequently detected, and was the third most frequent in the dataset, with 939 detections corresponding to 8.26% of samples, with a p95 of 0.09 μg/L and a maximum concentration of 100 μg/L. DEHP was detected in 4% of samples with a maximum concentration of 62 μg/L. N- butylbenzene sulphonamide was much less frequently detected (2.66%) but had a very high maximum concentration (4000 μg/L). Dimethyl phthalate and ATBC were the least frequently detected, but ATBC had a maximum detected concentration of 45 μg/L.
This is a diverse group of 13 compounds comprising non-chlorinated solvents, industrial intermediates and flame retardants, many of which are also used as plasticisers. All compounds in this group had censored distributions with the p95 concentration at the LOD except cyclohexanone.
Two compounds classed as non-halogenated solvents were detected in the top 50; 1,4-dioxane was detected in 4.6% of samples with a maximum concentration of 63 μg/L and dimethyl adipate in 3.17% of samples but at with a higher maximum concentration of 150 μg/L. As well as a solvent 1,4-dioxane is used as a stabilizer for the transport of halogenated hydrocarbons in aluminum containers. Dimethyl adipate is a nylon precursor and it is also used as a plasticiser as well as a solvent.
Cyclohexanone is the most frequently detected compound in the Industrial group, found in 5.61% of samples with a maximum concentration of 300 μg/L. This has a wide range of applications and has been used in the manufacture of nylon. Other compounds were found in fewer than 2.5% of samples, TMDD, 1(3H)-isobenzofuranone ((phthalide) can also be used as a food additive), TTT, 2,4-DTBP, dibenzofuran and nerolin. They had maximum concentrations of 6 μg/L or less except TTT, which was found at 62 μg/L.
The aryl phosphate esters are mainly used as flame-retardant plasticisers in PVC and other polymers. Triphenyl phosphate is the most widely detected of these, found in 2.91% of samples with a maximum concentration of 2 μg/L. Tri-(2-chloroethyl) phosphate and 2-ethylhexyl diphenyl phosphate were found in 1.78% and 1.57% of samples with maximum concentrations of 65 μg/L and 1 μg/L respectively.
Polyaromatic hydrocarbons (PAHs)
There are 8 compounds in this category. PAHs are generally not very water soluble and are found at low concentrations. Fluoranthene and pyrene are the most soluble of these compounds and are detected in 5.47% and 5.33% of samples respectively with concentrations of 3.4 μg/L and 2 μg/L. Others are detected in fewer than 2% of samples with maximum concentrations of 2 μg/L or less except for acenaphthene which had a maximum concentration of 5 μg/L.
Pharmaceutical, personal care products, lifestyle (PPCL)
Nine of the top 50 compounds were in this category. The p95 of concentration for all of these compounds was below the LOD.
Compounds in this group tend to be applied to the skin. The insect repellent DEET is the most frequently detected compound in the PPCL category and the 7th overall. It was detected in 5.85% of samples with a maximum concentration of 17 μg/L. Benzophenone, benzophenone-3 and drometrizole are UV absorbers and can be used as cosmetics, such as sunscreen, and were found in 2.53% of samples at a maximum concentration of 0.8 μg/L, 1.32% of samples at a maximum concentration of 45 μg/L and 1.05% of samples at a maximum concentration of 18 μg/L respectively. The anti-microbial propyl paraben was detected in 1.28% of samples with a maximum concentration of 36 μg/L.
Relatively few pharmaceuticals are detected by the GC-MS method. Carbamazepine was detected in 1.5% of samples with a maximum concentration of 0.26 μg/L.
Butanedioic acid, dimethyl ester was the most frequently detected in this group, and the 4th most frequently overall, being detected in 6.76% of compounds with a maximum concentration of 74 μg/L. It is used as a flavouring agent but also has a wide range of industrial applications. The food antioxidant BHT was detected in 1.43% of samples with a maximum concentration of 8.7 μg/L.
Caffeine was detected in 4.26% of samples with a maximum concentration of 1.8 μg/L.
Top 50 GC-MS substances by maximum concentration
The top 50 GC-MS substances ordered by maximum concentration are shown in Figure 3.3. It is worth noting that some of these maximum concentrations (i.e. outliers) are considerably higher than the next highest concentration detected (see Figure 3.2), and they may represent a point source of contamination.
The top 10 highest maximum concentrations in these frequently detected compounds were for benzenesulfonamide, benzotriazole, ethylhexanoic acid, cyclohexanone, cholesterol, mercaplobenzothiazole, benzothiozole, TCE, chlorodibromomethane, and tetrachloroethene.
Top 50 LC-MS substances by frequency of detection
The top 50 substances from the LC-MS dataset selected by frequency are shown in Figure 3.4. A statistical summary of the data is shown in Table 3.2. Box plots indicating data distribution are shown in Figure 3.5.
The top 18 compounds have sufficient detection data for the statistical distribution to be calculated. Many of these compounds have a median concentration considerably below the LOD. For the majority of these distributions, the p95 is at or close to the LOD. The LOD for the LC-MS method used here are much lower than the GC-MS method and this may well be the main reason why the percentage of detections is much higher.
Figure 3.5 shows the distribution of the results for the 50 most frequently detected substances by LC-MS using a Tukey box plot. The box plots show the interquartile range (IQR), box whiskers (values +/- 1.5 IQR) and outliers (values >1.5 IQR). In many cases the proportion of detections is too low to calculate the IQR and in these instances, the box-plots simply illustrate the outlier concentrations.
Cumulative frequency plots for the 50 most frequently detected substances by LC-MS screens are presented in Figure 3.6.
|Ranking||CAS Number||Analyte||Short name||Use code *||LOD||Median concentration||Mean concentration||Standard deviation||95th percentile concentration||Maximum concentration||Number of analyses||Number of positive detections||% analyses with positive detections|
|L28||115286||1,4,5,6,7,7-Hexachloro-5- norbornene-2,3-dicarboxylic acid||Chlorendic acid||Indu||0.005||NA||NA||NA||0.09||2.1||267||34||12.73|
|L36||142459583||Flufenacet (Fluthiamide) (BAY FOE 5043)||Flufenacet||Pest||0.001||NA||NA||NA||0.001||0.057||267||25||9.36|
|L46||87674688||Dimethenamid (SAN 582H)||Dimethenamid||Pest||0.001||NA||NA||NA||0.001||0.01||267||18||6.74|
* Key to use codes given in Table 2.2.
Discussion of results
Thirty six of the top 50 compounds most frequently detected are pesticides. The top 4 are all triazine herbicidal compounds: atrazine, the 2 main atrazine TPs and simazine. All 4 are detected in over 50% of samples. Median concentrations ranged from 0.001 to 0.002 μg/L with maximum concentrations in the ranging from 0.03 μg/L for simazine to 0.31 μg/L for atrazine.
Three ‘uron’ herbicides are in the top 15 most frequently detected pesticides: diuron in 49.8% of samples, monuron in 25.1% and isoproturon in 22.5%. Other ‘urons’ detected were chlorotoluron, cycluron and chlorsulfuron.
Other top 50 compounds were the neonicotinoid insecticide clothianidin, the herbicide bentazone and the herbicide TP chloridazon-desphenyl-methyl, all detected in more than 20% of samples. Many of the most frequently detected pesticides are no longer approved for use. The most frequently detected pesticide that is currently approved is bentazone and that was detected in 28% of samples.
Of the other pesticide compounds detected, 2 were insecticides (fipronil and imidacloprid), 9 were fungicides (epoxiconazole, propiconazole, boscalid, carbendazim, azoxystrobin, tebuconazole, fluopicolide, thiamethoxam and the TP prothioconazole-desthio) and the remaining 12 compounds were herbicides (including the parent chloridazon). All were detected in 6% of samples or more. It is interesting that the parent prothioconazole does not appear in the top 50 compounds, although it is detectable by this method. This may relate to the rapid breakdown of the parent molecule in the environment. It was introduced in 2002 as a foliar treatment for fungal diseases in cereals and is still used in the UK.
Compounds with maximum concentrations above the 0.1 μg/L drinking water limit, in descending order of concentration were chloridazon-desphenyl-methyl (1.1 μg/L), bentazone, metazachlor, 2- methyl-4-chlorophenoxyacteic acid (MCPA), atrazine, atrazine-desethyl, carbendazim, boscalid, trietazine, chlorotoluron, propiconazole and flutriafol.
Industrial compounds (Indu)
There are 6 compounds in this group, 5 of which are perfluorinated acids and esters. As might be anticipated, PFOA is the most frequently detected, in 39.7% of samples with a median concentration of 0.0006 μg/L and a maximum concentration of 0.05 μg/L, and PFOS is next, in 30% of samples with a median concentration of 0.0003 μg/L and a maximum concentration of 0.44 μg/L. The other three are the shorter chain acids, PFPeA, PFHxA and PFHpA, present in between 18.7 and 22.4% of samples with maximum concentrations of 0.08, 0.13 and 0.03 μg/L respectively.
Chlorendic acid is an industrial intermediate used in the synthesis of flame retardants and polymers. It is also a common breakdown product of several organochlorine insecticides. It was detected in 12.7% with a maximum concentration of 2.1 μg/L.
Pharmaceutical, personal care products, lifestyle (PPCL)
There are eight compounds in this group. The most widely detected compound, and 6th overall is carbamazepine, one of a number of compounds used to treat epilepsy/convulsions/bipolar disorder. This was detected in 43.8% of samples at a median concentration of 0.0006 μg/L and a maximum of 0.61 μg/L. Lamotrigine was also detected in 28.5% with a median concentration of 0.003 μg/L and a maximum of 0.036 μg/L.
Antibiotic and antibacterial compounds were detected: sulfamethoxazole in 29.6% of samples with a median concentration of 0.0004 μg/L and a maximum of 0.03 μg/L, and sulphanilamide in 13.1% with a maximum concentration of 0.037 μg/L.
The analgesics/anaesthetics tramadol and lidocaine were detected in 10.9% and 7.4% of samples respectively at maximum concentrations of 0.048 and 0.008 μg/L respectively.
The veterinary antiprotozoal substance clopidol was detected in 39.7% of samples with a median concentration of 0.0003 μg/L and a maximum concentration of 3.5 μg/L.
The artificial sweetener sucralose was detected in 19.5% of samples with a maximum concentration of 2.3 μg/L. Sucralose has been suggested as an indicator of wastewater ingress to groundwater.
Top 50 LC-MS substances by maximum concentration
The top 50 LC-MS substances ordered by maximum concentration are shown in Figure 3.3. It is worth noting that some of these maximum concentrations are considerably higher than the next highest concentration detected (see Figure 3.5), and although they may represent a highly contaminated sample.
The top 10 highest maximum concentrations were for clopidol, followed by sucralose, chlorendic acid, triallate, pentobarbital, chloridazon-desphenyl-methyl, bentazone, tridimefon, metazachlor and mecoprop, with 3 of the top 5 being PPCL compounds.
Overlap of substances in GC-MS-LC-MS screens
Comparing the compounds listed in the Top 50 by frequency of detection and Top 50 by concentration for the GC-MS and LC-MS screens identifies 5 compounds which are detected by both methods. These compounds are listed in Table 3.3.
Analysis of the complete dataset indicates that the GC-MS method detected 663 substances in samples and the LC-MS detected 178 substances. Of these, 81 substances were detected by both methods: these are listed in Table 3.4.
|67564914||Fenpropimorph (Ro 14-3169)|
|87674688||Dimethenamid (SAN 582H)|
|142459583||Flufenacet (Fluthiamide) (BAY FOE 5043)|
Spatial plots of occurrence of selected substances
This section presents a series of spatial plots of key substances. These were selected on the basis of frequency of detection and/or being classed as PPCL, a large group of substances of potential emerging concern which is receiving growing attention in Europe. Concentrations (maximum per site) are shown as proportional symbols. Results are plotted on a background of a simplified 1:625 000 scale geological map as requested by the EA: the legend is provided in Figure 3.8.
Atrazine remains the most frequently detected compound by GC-MS. Since the database contains data collected throughout the period that this method has been used, it may give a misleading impression of current conditions. A spatial plot of these data (Figure 3.9) shows that atrazine is predominantly detected at concentrations of >0.1 µg/L in the principal aquifers of the Chalk and the Permo-Triassic sandstone. This may reflect historical usage and particularly for the Chalk the travel time from the surface through the unsaturated zone to the water table and the slow rate of flushing of these aquifers.
Historically atrazine was used for weed control, both in agriculture and for amenity use until 1992 when it was withdrawn from non-agricultural uses in the UK. There was concern that amenity use potentially allowed pesticide to enter the subsurface via soakaways, bypassing the soil. Limited agricultural uses were permitted until an EU-wide ban for all uses followed in 2003. The elevated concentrations seen in this dataset demonstrate that a precautionary approach is needed to protect groundwater from pesticides as degradation rates in the subsurface are typically very slow.
BPA is the most frequently detected plasticiser in the dataset. The spatial plot (Figure 3.10) shows it to be widely detected across England. There are areas of concentrations >1 µg/L in the urban areas of Greater London and the Thames Estuary, Birmingham, Liverpool and Hull areas and in the Jurassic limestones of Lincolnshire and perhaps surprisingly in the far southwest.
There are likely to be multiple sources of BPA in the environment, including wastewater, septic tanks and landfills. It is also a common component of many plastic items and care is needed to exclude these from sampling equipment. Plasticisers can be leached into groundwater from plastic well casing and pipework associated with groundwater sampling and this could explain BPA detections at some sites rather than BPA occurrence in the aquifer.
DEET is the second most frequently detected PPCL in the dataset. Figure 3.11 shows higher concentrations to be distributed with areas of central southern England, the north Yorkshire coast and the Manchester-Liverpool area with groups of values over 0.1 µg/L. The Lincolnshire Limestone and the south east of England also show scattered higher concentrations.
When used as an insect repellent, DEET is topically applied and may therefore be found in the wastewater stream (Aronson et al., 2012). DEET can be readily absorbed into the body of plastic objects. The spatial coherence of DEET observations in some regions suggests that some detections may be due to contamination from samplers, but this has not been verified.
Butanedioic acid, dimethyl ester (dimethyl succinate) is detected widely across England, with surprisingly few detections in the Chalk of south-east England, the Carboniferous and Devon and Cornwall (Figure 3.12).
Caffeine is also widely detected in groundwater across England with pronounced clusters of detections in the London area, Cornwall and North Yorkshire (Figure 3.13).
There are considerably fewer LC-MS data and these are confined to selected regions of the EA. Carbamazepine is the most frequently detected PPCL in the LC-MS database. The spatial plot (Figure 3.14) shows sampling to be mainly restricted to the central and southern areas of England, with some sampling in the northeast. Carbamazepine is detected in the London area, in the Chalk of the southeast and in the Permo-Triassic sandstone. There is insufficient data to comment on controls on spatial distribution.
Detections of clothianidin were found in Gloucestershire and Oxfordshire, in London and in Sussex/Hampshire (Figure 3.15). Presumably, this is associated with the distribution of the target insect pests, but there are too few datapoints to allow comment on spatial distribution.
PFOS and PFOA were mainly detected in the London area, some of which are possibly associated with the Buncefield fire in 2005 and the extensive use of foams to supress the fire (Figure 3.16 and Figure 3.17). Other localised detections include in Hampshire, two sites near the coast in north east England, and a site in Birmingham. PFOS and PFOA are also degradation products of other precursor substances not reported in the LC-MS target screening method.
Clopidol was detected in µg/L concentrations at only a small number of sites, possibly associated with livestock farming and the veterinary use of this substance to treat Coccidia parasites. A high proportion of detections at low concentrations give rise to its prominence in Figure 3.18, but initial results do not indicate a wide distribution in groundwater at µg/L concentrations, however the spatial sampling is limited.
- ARONSON, D, WEEKS, J, MEYLAN, B, and HOWARD, P. 2012. Environmental release, environmental concentrations, and ecological risk of N,N-Diethyl-m-toluamide (DEET). Integrated Environmental Assessment and Management, 8, 1, 135–166.