1. PROBLEMÁTICA DE LA INVESTIGACIÓN
1.1 Delimitación del problema
1.2.2 El papel que juega la imagen en la lectura comprensiva
The temporal variation and spatial distribution of tritium in surface sediments was studied using three methods:
• A single large-scale survey of tritium activities at specific sites around the estuary (Figure 3.1), undertaken in May 2003.
• Monthly sediment samples from seven mud flat sites on the northern shore of the estuary, within 25 km of the Amersham plc discharge point (Figure 3.1), collected between February 2000 and May 2004.
• Sediment traps deployed at five sites for 6 months, to study tritium activities in sediment that had settled out of suspension.
From February 2000 to October 2001, the monthly sediment surveys were conducted by K. Doucette, Dr A.B. Cundy and Dr P.E. Warwick. From October 2001 samples were collected by J.E. Morris with the assistance of Dr F.M. Dyer, Dr P. Teasdale, L. Janes or J. Burnett.
Sampling sites Sampling sites were selected using the following criteria: • Distance from the Amersham plc. discharge point.
• Presence of fine sediment
• Ease of access to the intertidal zone via roads and public rights of way.
Seven mud flat sites were selected on the northern shore of the Severn estuary, within 25 km of the discharge point (Figure 3.1; Table 3.1), for the monthly sampling programme. Fifteen other sites located further from the discharge point were also sampled to assess the spatial distribution of tritium on a larger scale.
3 .2 . S a m p lin g 7 5 CARDIFF NEWPORT BRISTOL SWANSEA GLOUCESTER WESTON-SUPER-MARE Amersham plc. Berkeley Oldbury Hinkley Point X X N 20 km 0 Bristol Channel Se vern Estu ary Town/city
Nuclear Power station Amersham plc.
Amersham plc. discharge point Monthly sampling site
Estuary survey sampling site Bedrock Gravel Sand Mud Estuarine alluvium 7 E D C A B 1 23 4 5 6 G H J K L M N F P Q Depositional environments
Figure 3.1: The location of sampling sites, sources of tritium and the distribution of estuarine depositional environments and alluvium (after Allen, 1988; 1990) in the Severn estuary. Inset: Location of the Severn estuary in SW Britain.
3 .2 . S a m p lin g 7 6 Site num- ber
Site name Grid refer- ence
Number of times sampled
Site description Sediment description Special features
1 Barry Island ST 10834
66525
36 Mud flat in small harbour.
Patches of Spartina salt marsh (Fig. 3.2)
Surface to 2 cm: light brown oxic silty mud, 2 to 10 cm: dark grey anoxic sediment, sandy sub- strate
Near sewage outfall
2 Orchard
Ledges SW
ST 20018
73418
35 At western end of 2.5 km of ex- posed beach (Fig. 3.2)
Gravelly sands containing particulate coal and an- thropogenic debris, mud drapes in the lee of rip- ples
Within ABP Cardiff docks, regularly scoured of fine sed- iments by tidal currents or waves
3 Orchard
Ledges NE
ST 20730
74177
34 At eastern end of 2.5 km of ex- posed beach (Fig. 3.2)
Surface to variable depth: semi-consolidated patches of muddy deposits. Sandy gravel sub- strate with boulders and large blocks of building debris
Within ABP Cardiff Docks, regularly scoured of fine sed- iments by tidal currents or waves
4 Maerdy Farm ST 23170
77635
36 Mud flat overlying relict salt marsh in inlet (Fig. 3.2)
Light brown silty fluid mud layer of variable thick- ness overlaying light grey, well consolidated sub- strate
Scoured of fine sediments 5 times during sampling, probably by storms
5 Peterstone ST 27016
79755
34 Mud flat on seaward edge of eroding Spartina salt marsh (Fig. 3.2)
Unconsolidated light brown silty mud of variable thickness overlying light grey, well consolidated substrate
Scoured of fine sediments 5 times during sampling, probably by storms
6 St Brides ST 30083
81537
38 Mud flat in front of seawall Silty mud layer of variable thickness, overlying an irregular substrate of gravelly sand, pebbles and boulders
Scoured of fine sediment 4 times during sampling, probably by storms
7 Goldcliff ST 37504
82123
28 Mud flat in front of seawall Silty mud layer of variable thickness overlying an irregular substrate of pebbles with some boulders
A Pen-clawdd SS 53545
95961
2 Bank of small creek (Fig. 3.2) Mud
B Llanrhidian
Marsh
SS 47110
94546
1 Small creek that drains salt marsh
Surface: light brown oxic mud. Below 5 mm: dark grey anoxic mud
C Swansea West SS 62210
90700
1 Eastern side of a stream on the beach (Fig. 3.2)
Thin veneer of fresh mud overlying a sandy sub- strate
D Swansea East SS 65965
92215
1 100 m from marina steps
(Fig. 3.2)
Thin veneer of fresh mud overlying a sandy sub- strate
3 .2 . S a m p lin g 7 7 Site num- ber
Site name Grid refer- ence
Number of times sampled
Site description Sediment description Special features
E Neath SS 72056
93104
1 Bank of muddy creek, sur-
rounded by sand dunes
(Fig. 3.2)
Mud overlying a sandy substrate Opposite oil refinery
F East
Aberthaw
ST 30398
16595
2 Edge of eroding salt marsh, sur- rounded by sand dunes on the west side of an inlet. (Fig. 3.3)
Thin veneer of fresh mud overlying a sandy sub- strate
G Collister Pill ST 45450
85670
2 Mud flat overlying relict salt marsh
Surface: unconsolidated light brown sediment. Ir- regular light grey consolidated substrate
H Sudbrook ST 50156
87245
2 Beach (Fig. 3.3) Unconsolidated fresh brown mud overlying a
rocky substrate
J Beachley ST 54965
90250
2 Mud flat on beach Unconsolidated sediment overlying a shingle sub- strate
Under old Severn Bridge
K Lydney SO 64750
01215
1 Mud flat on rocks (Fig. 3.3) Surface: unconsolidated light brown sediment. Substrate of consolidated dark brown mud and rocks
L Longney SO 75491
12448
1 Muddy sand flats between reed beds and small stream (Fig. 3.3)
Thin layer of mud overlying a sandy substrate
M Slimbridge SO 71450
05160
1 Phragmites and Spartina salt marsh, adjacent to mud flats at edge of estuary (Fig. 3.3)
Top 1 cm of core fromSpartina marsh Salt marsh sediment
N Severn House
Farm
ST 64342
98290
1 Mud flat in front of sea wall Mud overlying a rocky substrate
P Burnham on
Sea
ST 30135
48567
1 Beach Muddy sand
Q Steart ST 26460
45740
1 Beach in front of salt marsh (Fig. 3.3)
3 .2 . S a m p lin g 7 8
Site 2 - Orchard Ledges SW
Site 3 - Orchard Ledges NE
Site 5 - Peterstone
Site A - Pen-clawdd
Site D - Swansea East
Site E - Neath
3 .2 . S a m p lin g 7 9 Site L - Longney Site K - Lydney Site H - Sudbrook
Site F - East Aberthaw Site M - Slimbridge
Site Q - Steart
Sampling methods Surface sediments were usually collected from an area of approxi- mately 25 x 25 cm to a depth of between 0.5 and 2 cm (typically 1 cm), using a spade or a plastic dustpan. The resulting fresh sediment sample weighed ∼500 g. At some sites, samples obtained by this method tended to contain a mixture of sediment types, e.g. mud and sand. When a very thin veneer of mud overlay a sandy substrate, a similar volume of sample was sometimes collected over a larger surface area, to maximise the amount of mud collected. Occasionally samples could not be collected because high tides prevented access to sampling sites or scouring had removed the surface fine sediment layer.
Samples were homogenised, sub-sampled, and frozen at − 40◦
C within 48 hours of collection. Cross-contamination was avoided by wrapping collected samples in a second sealed plastic bag and then freezing sub-samples in sealed plastic bags. When required for analysis, samples were thawed, homogenised and an aliquot of fresh sediment removed for analysis. About half of each sample (∼200 g) was freeze-dried forγ-spectrometry and X-ray fluorescence analysis.
Sediment traps were constructed from sections of plastic drainpipe (0.4 to 0.6 m in length) and inserted vertically into the sediment, leaving the top about 20 cm from the sediment surface, although this height varied over time as a result of mud flat mobility. A plastic bag was fixed inside the top of the drainpipe to collect sediment deposited vertically out of suspension, as the trap limited the effect of the lateral scouring of fine sediment. These traps were deployed at Sites 2, 4, 5, 6 and 7 for six months.
Analytical methods All samples were analysed for total tritium (3Htotal) and selected samples for exchangeable tritium (3H
ext), using the methods described in Chapter 2. The 3H
total and 3Hext data discussed in this chapter were collated from measurements pre- dominantly made by J.E. Morris, but also by K. Doucette, Dr F.M. Dyer, Dr P. Teasdale and Dr J. Oh. All 3H activities are reported in Bq/g dry weight. For compositional analysis, samples were selected from those collected from a range of sampling sites and dates, in addition to their sediment composition (from visual inspection). These samples were freeze-dried and analysed for:
• Elemental composition using X-ray fluorescence (XRF) analysis. • Mineralogical composition using X-ray (XRD) diffractometry. • Gamma-emitting radionuclides usingγ-spectrometry.
• Total organic carbon (TOC) using coulometry.
• Elemental carbon content using an aqua regia and HF acid digestion.
These methods are described in Chapter 2. Analytical support was provided by Dr I.W. Croudace, R. Williams, Dr F.M. Dyer, D. Green and Dr P.E. Warwick.