Regional assessment of status and trends in PCB concentrations in sediment

Overview

This document describes a regional assessment of the status and trends of PCB concentrations in sediment in Scottish marine waters. It builds on the 2020 assessment of data from the UK’s Clean Seas Environmental Monitoring Programme conducted by the Clean Safe Seas Evidence Group. In this, assessments of status and trends were made for a large number of PCB time series, each of a single PCB at a single monitoring station. The regional assessment synthesises the results of the individual time series to assess status and trends at the biogeographic regional level.

The regional assessment considers the following PCBs: CB28, CB52, CB101, CB118, CB138, CB153, CB180.

The regional assessment synthesises data from monitoring stations that are in estuarine, coastal and offshore waters. The results might therefore differ from regional assessments for Marine Strategy purposes, which exclude estuarine stations.

The following sections:

• summarise the individual time series results
• describe the methods used to combine the individual time series results to estimate regional status and trends
• present the estimates of regional trends
• present the estimates of regional status

Summary of individual time series results

A time series of PCB concentrations is assessed for status if:

• there is at least one year with data in the period 2014 to 2019
• there are at least three years of data over the whole time series
• a parametric model can be fitted to the data and used to estimate the mean concentration in the final monitoring year (or, occasionally, if a non-parametric test of status is applied)

The conditions are more stringent for trends. Specifically, a time series is assessed for trends if:

• there is at least one year with data in the period 2014 to 2019
• there are at least five years of data over the whole time series
• a parametric model can be fitted to the data and used to estimate the trend in mean concentration

Note that all trend assessments for individual time series and most status assessments are based on the fit of a parametric model. This is important because only the parametric results are passed into the regional assessments in the following sections. 33 of the 196 PCB time series were assessed for status using a non-parametric test.

The first tab below shows all the monitoring stations where there are individual time series assessments of trend or status for PCBs. The purple circles are stations where there is a trend assessment for at least one PCB, and the light blue cirles are stations where there are only status assessments.

The next set of tabs show the trend and status assessments for each PCB in turn. The colours have the following meaning:

• blue: the mean concentration is significantly (p < 0.05) below the Background Assessment Concentration (BAC)
• green: the mean concentration is significantly (p < 0.05) below the Environmental Assessment Criterion (EAC)
• red: the mean concentration is not signficantly below the EAC

And the shapes have the following meaning:

• downward triangle: the mean concentration is significantly decreasing (p < 0.05)
• circle: there is no significant change in mean concentration (p > 0.05) or there are too few years to test for trends
• upward triangle: the mean concentration is significantly increasing (p < 0.05)

The final set of tabs give:

• the number of time series with upwards, downwards or no trend in each biogeographic region
• the same information presented as proportions
• the number of time series with blue, green or red status in each biogeographic region
• the same information presented as proportions

Stations

CB28

CB52

CB101

CB118

CB138

CB153

CB180

Trend summary (numbers)

Number of time series with upwards, downwards or no trend by biogeographic region

region	status	CB28	CB52	CB101	CB118	CB138	CB153	CB180	total
Northern North Sea	upward trend	1	1	1	1	0	2	1	7
	no trend	4	8	9	9	8	9	4	51
	downward trend	1	1	1	1	3	1	3	11
Minches & W Scotland	upward trend	0	0	0	0	0	0	0	0
	no trend	3	3	3	4	4	5	3	25
	downward trend	0	0	0	0	1	0	0	1
Irish Sea	upward trend	0	1	1	1	0	1	0	4
	no trend	7	4	4	4	6	6	7	38
	downward trend	0	2	2	2	1	0	0	7
total	upward trend	1	2	2	2	0	3	1	11
	no trend	14	15	16	17	18	20	14	114
	downward trend	1	3	3	3	5	1	3	19

Trend summary (proportions)

Proportion of time series with upwards, downwards or no trend by biogeographic region

region	status	CB28	CB52	CB101	CB118	CB138	CB153	CB180	total
Northern North Sea	upward trend	16	10	9	9	0	17	12	10
	no trend	67	80	82	82	73	75	50	74
	downward trend	17	10	9	9	27	8	38	16
Minches & W Scotland	upward trend	0	0	0	0	0	0	0	0
	no trend	100	100	100	100	80	100	100	96
	downward trend	0	0	0	0	20	0	0	4
Irish Sea	upward trend	0	14	14	14	0	14	0	8
	no trend	100	57	57	57	86	86	100	78
	downward trend	0	29	29	29	14	0	0	14
total	upward trend	6	10	10	9	0	13	5	8
	no trend	88	75	76	77	78	83	78	79
	downward trend	6	15	14	14	22	4	17	13

Status summary (numbers)

Number of time series with each status by biogeographic region

region	status	CB28	CB52	CB101	CB118	CB138	CB153	CB180	total
Northern North Sea	blue	4	0	1	2	1	3	3	14
	green	11	11	14	9	15	13	12	85
	red	1	5	1	5	0	0	1	13
Minches & W Scotland	blue	2	0	0	0	0	0	0	2
	green	3	4	5	4	5	5	5	31
	red	0	1	0	1	0	0	0	2
Irish Sea	blue	0	0	0	0	0	0	0	0
	green	5	5	6	2	7	7	7	39
	red	2	2	1	5	0	0	0	10
total	blue	6	0	1	2	1	3	3	16
	green	19	20	25	15	27	25	24	155
	red	3	8	2	11	0	0	1	25

Status summary (proportions)

Proportion of time series with each status by biogeographic region

region	status	CB28	CB52	CB101	CB118	CB138	CB153	CB180	total
Northern North Sea	blue	25	0	6	13	6	19	19	12
	green	69	69	88	56	94	81	75	76
	red	6	31	6	31	0	0	6	12
Minches & W Scotland	blue	40	0	0	0	0	0	0	6
	green	60	80	100	80	100	100	100	88
	red	0	20	0	20	0	0	0	6
Irish Sea	blue	0	0	0	0	0	0	0	0
	green	71	71	86	29	100	100	100	80
	red	29	29	14	71	0	0	0	20
total	blue	21	0	4	7	4	11	11	8
	green	68	71	89	54	96	89	86	79
	red	11	29	7	39	0	0	3	13

Regional assessment - methods

Tabulating the number of time series with each status category by region provides a quick summary of the individual time series results. However, it does not provide an objective regional assessment of status. Similarly, tabulating the number of time series with an upward or downward trend does not provide an objective regional assessment of trend. This section describes how the individual time series results can by synthesised in a meta-analysis to assess both status and trend at the regional level. For PCBs, the meta-analysis is conducted in two parts: an analysis of the non-planar compounds CB28, CB52, CB101, CB138, CB153, CB180 and a separate analysis of the mono-ortho CB118. The results from the two parts are then combined.

Trends

For a regional trend assessment, the trend in each time series is summarised by the estimated change in log concentration over the last twenty years (or shorter if the time series doesn’t extend that far back). Regional trends for non-planar PCBs are then estimated by fitting the following linear mixed model by restricted maximum likelihood:

• response: trend (yearly change in log concentration)
• fixed model: region
• random model: PCB + region.PCB + station + trend estimation variation + residual variation

The fixed model means that a trend is estimated for each region. Trends in the different non-planar PCBs are assumed to be broadly similar, because inputs and control measures are likely to affect all non-planar PCBs in a similar way. Each regional trend can thus be interpreted as the common trend in non-planar PCB concentrations in that region (i.e. the trend averaged across all non-planar PCBs).

The random model has five terms:

• PCB allows for variation in trend between PCBs common to all regions
• region.PCB allows for additional variation in trend between PCBs within regions
• station allows for variation in trend between stations common to all PCBs
• trend estimation variation is the variance of the trend estimates from the individual time series analysis, assumed known and fixed
• residual variation is the variation that cannot be explained by any of the fixed effects or the other random effects

The model for CB118 is simpler because there is only one compound to consider:

• response: trend (yearly change in log concentration)
• fixed model: region
• random model: trend estimation variation + residual variation

The meta-analysis is restricted to time series from monitoring stations that are classified as ‘representative’ or ‘baseline’ (near pristine conditions or only subject to very diffuse inputs). ‘Impacted’ stations (those close to a point source) are omitted because trends there will likely reflect changes due to the point source and will be ‘unrepresentative’ at the regional level.

The meta-analysis is further restricted to:

• regions with at least three trend stations with good geographic spread. Three stations is considered the minimum required to provide some sort of evidence base at the regional level
• PCBs with trend information for at least three stations (over all regions): non-planar PCBs are treated as random effects in the meta-anlaysis, so information about trends in one PCB can be ‘borrowed’ from the trend information in the other PCBs

Status

For a regional status assessment, the status of each time series is summarised by the difference between the estimated mean log concentration in the final monitoring year and the log assessment concentration. This ensures that status is always measured on the same scale, even though the assessment concentration might vary between PCBs and regions. Essentially the same linear mixed models as for trends are then fitted. For non-planar PCBs:

• response: status (mean log concentration - log assessment concentration)
• fixed model: region
• random model: PCB + region.PCB + station + status estimation variation + residual variation

and for CB118:

• response: status (mean log concentration - log assessment concentration)
• fixed model: region
• random model: status estimation variation + residual variation

where status estimation variation is the variation in the status estimates from the individual time series analysis, assumed known and fixed.

There are no restrictions on the time series used in the status meta-analysis based on the classification of the monitoring station; time series from baseline, representative and impacted stations are all included. However, the few time series with a non-parametric assessment of status must be excluded, because there is no summary measure of status to use in the mixed model.

Again, the meta-analysis is restricted to regions with at least three status stations with good geographic spread and to PCBs with status information for at least three stations (over all regions).

Presentation

The results of both the trend and status regional assessments are back-transformed for presentation. The estimated regional trend is then interpreted as the percentage yearly change in concentration and the estimated regional status as the ratio of the mean regional concentration to the assessment concentration. To illustrate the latter, a value of 1 indicates that the mean regional concentration is equal to the assessment concentration; a value of 0.5 indicates that it is half the assessment concentration, and a value of 2 indicates that it is twice the assessment concentration.

Regional assessment - availability of time series

This section provides more detail on the number and geographic spread of the time series available for the meta-analyses. The map shows the stations with parametric trend and status assessments having excluded trend assessments at baseline and impacted stations. The purple circles are stations where there is a trend assessment for at least one PCB, and the light blue cirles are stations where there are only status assessments.

Regional trends

The first 4 tabs show:

• Regional trends: estimates of the regional trend for non-planar PCBs and CB118 with pointwise 95% confidence intervals
• Regional trends by PCB: regional trend estimates for each PCB with pointwise 95% confidence intervals plotted by PCB
• Regional trends by region: regional trend estimates for each PCB with pointwise 95% confidence intervals plotted by region
• Individual time series: the trend estimates from the individual time series on which the regional assessment is based. The estimates for regions with insufficient geographic spread (and excluded from the meta-analysis) are also shown.

The symbols in all these plots have the following interpretation:

• downward triangle: the mean concentration is significantly decreasing (p < 0.05)
• circle: there is no significant change in mean concentration (p > 0.05)
• upward triangle: the mean concentration is significantly increasing (p < 0.05)

The final 2 tabs show the estimates of the regional trend for non-planar PCBs and CB118 and the regional trend by PCB.

Regional trends

Regional trends by PCB

Regional trends by region

Individual time series

Estimates by region

This table shows the estimates of the regional trend:

• trend: the estimated mean yearly change in log concentration across the region (multiplied by 100 for presentation)
• se: the corresponding standard error
• lower, upper: the corresponding pointwise 95% confidence limits
• % yearly change: the estimated mean % yearly change in concentration across the region
• %yc lower, %yc upper: the corresponding pointwise 95% confidence limits

region	PCB	trend	se	lower	upper	% yearly change	%yc lower	%yc upper
Northern North Sea	non-planar PCBs	-1.78	1.47	-4.67	1.10	-1.77	-4.57	1.11
	CB118	-1.36	1.54	-4.37	1.66	-1.35	-4.28	1.68
Minches & W Scotland	non-planar PCBs	-0.51	2.33	-5.07	4.04	-0.51	-4.95	4.13
	CB118	-1.26	3.00	-7.14	4.62	-1.25	-6.89	4.73
Irish Sea	non-planar PCBs	-1.14	2.12	-5.29	3.00	-1.14	-5.15	3.05
	CB118	-0.96	1.77	-4.43	2.51	-0.96	-4.34	2.54

Estimates by region and PCB

This table shows the estimates of the regional trend by determinand:

• trend: the estimated mean yearly change in log concentration across the region (multiplied by 100 for presentation)
• se: the corresponding standard error
• lower, upper: the corresponding pointwise 95% confidence limits
• % yearly change: the estimated mean % yearly change in concentration across the region
• %yc lower, %yc upper: the corresponding pointwise 95% confidence limits

region	PCB	trend	se	lower	upper	% yearly change	%yc lower	%yc upper
Northern North Sea	CB28	-1.56	1.56	-4.61	1.49	-1.55	-4.51	1.50
	CB52	-1.42	1.56	-4.48	1.64	-1.41	-4.38	1.65
	CB101	-2.31	1.55	-5.35	0.73	-2.28	-5.21	0.74
	CB118	-1.36	1.54	-4.37	1.66	-1.35	-4.28	1.68
	CB138	-2.21	1.52	-5.19	0.76	-2.19	-5.06	0.77
	CB153	-1.34	1.50	-4.28	1.61	-1.33	-4.19	1.62
	CB180	-1.87	1.54	-4.88	1.13	-1.86	-4.77	1.14
Minches & W Scotland	CB28	-0.29	2.38	-4.96	4.38	-0.29	-4.84	4.48
	CB52	-0.15	2.39	-4.83	4.53	-0.15	-4.71	4.64
	CB101	-1.04	2.38	-5.71	3.63	-1.03	-5.55	3.70
	CB118	-1.26	3.00	-7.14	4.62	-1.25	-6.89	4.73
	CB138	-0.94	2.34	-5.53	3.64	-0.94	-5.38	3.71
	CB153	-0.07	2.34	-4.65	4.51	-0.07	-4.54	4.62
	CB180	-0.60	2.37	-5.25	4.04	-0.60	-5.11	4.12
Irish Sea	CB28	-0.92	2.16	-5.16	3.32	-0.91	-5.03	3.38
	CB52	-0.78	2.17	-5.04	3.48	-0.77	-4.91	3.54
	CB101	-1.67	2.16	-5.91	2.58	-1.65	-5.74	2.61
	CB118	-0.96	1.77	-4.43	2.51	-0.96	-4.34	2.54
	CB138	-1.57	2.15	-5.79	2.64	-1.56	-5.62	2.68
	CB153	-0.70	2.15	-4.91	3.51	-0.70	-4.79	3.58
	CB180	-1.23	2.16	-5.47	3.00	-1.23	-5.32	3.04

Regional status compared to the Environmental Assessment Criterion (EAC)

The first 4 tabs show:

• Regional status: estimates of the regional status for non-planar PCBs and CB118 with pointwise upper one-sided 95% confidence intervals
• Regional status by PCB: regional status estimates for each PCB with pointwise upper one-sided 95% confidence intervals plotted by PCB
• Regional status by region: regional status estimates for each PCB with pointwise upper one-sided 95% confidence intervals plotted by region
• Individual time series: the status estimates from the individual time series on which the regional assessment is based. The estimates for regions with insufficient geographic spread (and excluded from the meta-analysis) are also shown.

The symbols in all the plots have the following interpretation:

• blue: the mean concentration is significantly (p < 0.05) below the Background Assessment Concentration (BAC)
• green: the mean concentration is significantly (p < 0.05) below the EAC
• red: the mean concentration is not significantly below the EAC

Note that to see why some points might be coloured blue, it is necessary to look ahead to the next section, where status is compared to the BAC. Unfortunately, it isn’t easy to present the assessment of status relative to the EAC with the assessment of status relative to the BAC because the ratio of the BAC to the EAC can vary between PCBs and regions.

The final 2 tabs shows the estimates of the regional status for non-planar PCBs and CB118 and the regional status by PCB.

Regional status

Regional status by PCB

Regional status by region

Individual time series

Estimates by region

This table shows the estimates of the regional status:

• status: the estimated mean log concentration minus the log EAC
• se: the corresponding standard error
• upper: the corresponding pointwise upper one-sided 95% confidence limit
• concentration: the estimated mean concentration relative to the EAC
• conc upper: the corresponding pointwise upper one-sided 95% confidence limit

region	PCB	status	se	upper	concentration	conc upper
Northern North Sea	non-planar PCBs	-3.61	0.49	-2.81	0.03	0.06
	CB118	-1.26	0.23	-0.89	0.28	0.41
Minches & W Scotland	non-planar PCBs	-3.84	0.57	-2.91	0.02	0.05
	CB118	-1.28	0.42	-0.59	0.28	0.55
Irish Sea	non-planar PCBs	-2.04	0.53	-1.17	0.13	0.31
	CB118	0.08	0.29	0.56	1.08	1.75

Estimates by region and PCB

This table shows the estimates of the regional trend by determinand:

• status: the estimated mean log concentration minus the log EAC
• se: the corresponding standard error
• upper: the corresponding pointwise upper one-sided 95% confidence limit
• concentration: the estimated mean concentration relative to the EAC
• conc upper: the corresponding pointwise upper one-sided 95% confidence limit

region	PCB	status	se	upper	concentration	conc upper
Northern North Sea	CB28	-2.55	0.23	-2.17	0.08	0.11
	CB52	-2.53	0.25	-2.13	0.08	0.12
	CB101	-2.95	0.24	-2.56	0.05	0.08
	CB118	-1.26	0.23	-0.89	0.28	0.41
	CB138	-3.77	0.23	-3.40	0.02	0.03
	CB153	-5.08	0.22	-4.71	0.01	0.01
	CB180	-4.76	0.23	-4.38	0.01	0.01
Minches & W Scotland	CB28	-2.93	0.39	-2.28	0.05	0.10
	CB52	-2.81	0.40	-2.15	0.06	0.12
	CB101	-3.20	0.40	-2.54	0.04	0.08
	CB118	-1.28	0.42	-0.59	0.28	0.55
	CB138	-3.92	0.38	-3.30	0.02	0.04
	CB153	-5.25	0.37	-4.63	0.01	0.01
	CB180	-4.96	0.38	-4.33	0.01	0.01
Irish Sea	CB28	-1.06	0.31	-0.54	0.35	0.58
	CB52	-1.12	0.32	-0.59	0.33	0.55
	CB101	-1.45	0.32	-0.93	0.23	0.39
	CB118	0.08	0.29	0.56	1.08	1.75
	CB138	-2.12	0.31	-1.61	0.12	0.20
	CB153	-3.50	0.31	-2.98	0.03	0.05
	CB180	-2.99	0.31	-2.48	0.05	0.08

Regional status compared to the Background Assessment Concentration (BAC)

The first 4 tabs show:

• Regional status: estimates of the regional status for non-planar PCBs and CB118 with pointwise upper one-sided 95% confidence intervals
• Regional status by PCB: regional status estimates for each PCB with pointwise upper one-sided 95% confidence intervals plotted by PCB
• Regional status by region: regional status estimates for each PCB with pointwise upper one-sided 95% confidence intervals plotted by region
• Individual time series: the status estimates from the individual time series on which the regional assessment is based. The estimates for regions with insufficient geographic spread (and excluded from the meta-analysis) are also shown.

The symbols in all the plots have the following interpretation:

• blue: the mean concentration is significantly (p < 0.05) below the BAC
• green: the mean concentration is significantly (p < 0.05) below the Environmental Assessment Criterion (EAC)
• red: the mean concentration is not significantly below the EAC

The final 2 tabs shows the estimates of the regional status for non-planar PCBs and CB118 and the regional status by PCB.

Regional status

Regional status by PCB

Regional status by region

Individual time series

Estimates by region

This table shows the estimates of the regional status:

• status: the estimated mean log concentration minus the log BAC
• se: the corresponding standard error
• upper: the corresponding pointwise upper one-sided 95% confidence limit
• concentration: the estimated mean concentration relative to the BAC
• conc upper: the corresponding pointwise upper one-sided 95% confidence limit

region	PCB	status	se	upper	concentration	conc upper
Northern North Sea	non-planar PCBs	0.11	0.26	0.54	1.12	1.71
	CB118	0.01	0.23	0.38	1.01	1.46
Minches & W Scotland	non-planar PCBs	-0.11	0.39	0.53	0.89	1.71
	CB118	-0.02	0.42	0.67	0.98	1.95
Irish Sea	non-planar PCBs	1.68	0.33	2.23	5.37	9.29
	CB118	1.34	0.29	1.82	3.82	6.18

Estimates by region and PCB

This table shows the estimates of the regional trend by determinand:

• status: the estimated mean log concentration minus the log BAC
• se: the corresponding standard error
• upper: the corresponding pointwise upper one-sided 95% confidence limit
• concentration: the estimated mean concentration relative to the BAC
• conc upper: the corresponding pointwise upper one-sided 95% confidence limit

region	PCB	status	se	upper	concentration	conc upper
Northern North Sea	CB28	-0.47	0.23	-0.10	0.62	0.91
	CB52	0.54	0.24	0.94	1.72	2.57
	CB101	0.11	0.24	0.51	1.12	1.66
	CB118	0.01	0.23	0.38	1.01	1.46
	CB138	0.19	0.23	0.57	1.21	1.76
	CB153	0.26	0.22	0.63	1.30	1.87
	CB180	0.03	0.23	0.41	1.03	1.50
Minches & W Scotland	CB28	-0.79	0.39	-0.15	0.45	0.86
	CB52	0.27	0.39	0.92	1.31	2.51
	CB101	-0.12	0.39	0.53	0.89	1.69
	CB118	-0.02	0.42	0.67	0.98	1.95
	CB138	0.04	0.37	0.65	1.04	1.92
	CB153	0.08	0.37	0.69	1.08	2.00
	CB180	-0.16	0.38	0.46	0.85	1.59
Irish Sea	CB28	1.04	0.31	1.55	2.82	4.71
	CB52	1.99	0.32	2.51	7.28	12.29
	CB101	1.62	0.32	2.14	5.07	8.52
	CB118	1.34	0.29	1.82	3.82	6.18
	CB138	1.83	0.31	2.34	6.25	10.41
	CB153	1.84	0.31	2.35	6.30	10.49
	CB180	1.77	0.31	2.28	5.87	9.80

PCB profiles

The profile of concentrations across PCBs can sometimes be useful for examining sources, and for comparing concentrations across regions where assessment criteria are not available. Profiles were examined by fitting the following linear mixed model to the non-planar PCBs by restricted maximum likelihood:

• response: mean log concentration
• fixed model: region
• random model: PCB + region.PCB + station + estimation variation + residual variation

where, for each time series, the mean log concentration is the estimated mean log concentration in the final monitoring year and the estimation variance is the variance associated with this estimate. The usual simplified model was also fitted to CB118 and the results combined.

The estimated PCB concentration profiles by region are shown below with pointwise 95% confidence limits. The units are ug/kg dw normalised to 2.5% organic carbon.