archaeological recording and chemical stratigraphy applied to contaminated land studies
TRANSCRIPT
Science of the Total Environment 409 (2011) 5432–5443
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Archaeological recording and chemical stratigraphy applied to contaminatedland studies
Effie Photos-Jones a,⁎, Allan J. Hall b,1
a Analytical Services for Art and Archaeology (Scotland) Ltd. (SASAA), Glasgow, UKb Archaeology, University of Glasgow, Glasgow G12 8QQ, UK
Abbreviations: CLA, Contaminated Land Assessment;sultant; GUARD, Glasgow University Archaeological RIronWorks; NHSEL, Norwest Holst Soil Engineering Ltd;vices for Art and Archaeology; MUSF, Moffat Upper SteaISO, International Organisation for Standardisation.⁎ Corresponding author. Tel.: +44 141 337 2623.
E-mail addresses: [email protected] (E. Photos-Jones(A.J. Hall).
1 Tel.: +44 141 330 5690.
0048-9697/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.scitotenv.2011.08.035
a b s t r a c t
a r t i c l e i n f oArticle history:Received 29 March 2011Received in revised form 17 July 2011Accepted 15 August 2011Available online 2 October 2011
Keywords:Harris MatrixContaminated landMade groundPollutionChemical stratigraphyArchaeology
The method used by archaeologists for excavation and recording of the stratigraphic evidence, withintrenches with or without archaeological remains, can potentially be useful to contaminated land consultants(CLCs). The implementation of archaeological practice in contaminated land assessments (CLAs) is not meantto be an exercise in data overkill; neither should it increase costs. Rather, we suggest, that if the excavationand recording, by a trained archaeologist, of the stratigraphy is followed by in-situ chemical characterisationthen it is possible that much uncertainty associated with current field sampling practices, may be removed.This is because built into the chemical stratigraphy is the temporal and spatial relationship between differentparts of the site reflecting the logic behind the distribution of contamination.An archaeological recording with chemical stratigraphy approach to sampling may possibly provide ‘onemethod fits all’ for potentially contaminated land sites (CLSs), just as archaeological characterisation of thestratigraphic record provides ‘one method fits all’ for all archaeological sites irrespective of period (prehistor-ic to modern) or type (rural, urban or industrial). We also suggest that there may be practical and financialbenefits to be gained by pulling together expertise and resources stemming from different disciplines, notsimply at the assessment phase, but also subsequent phases, in contaminated land improvement.
CLC, Contaminated Land Con-esearch Division; GIW, GovanSASAA, Scottish Analytical Ser-m Forge; BS, British Standard;
rights reserved.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Archaeological excavations of the remnants of past industries areinevitably carried out within potentially contaminated land. This hastwo implications i) the potential requirement for site remediationprior to development which may involve the removal or preservationof the archaeological resource (EIA, 2005), and ii) the establishment,in advance of full scale excavation, of human health screening valuesacceptable to those working on site, be they archaeologists, staff orvisitors (Cooke and Konstantopoulos, 2005). It is often the case thatarchaeological and contaminated land assessments are carried out in-dependently of each other, despite the fact that both sets of specialistsare likely to undertake intrusive investigation on the same site.
Over and above the presence of archaeology within a brown fieldsite earmarked for development, is the soil sampling strategy, forwhich there are specific industry standards (BS 10175 (BS,
2001updated 2011) and BS 10381–2 (BS, 2002 updated 2009)). Acrucial question is: how many samples would need to be collectedand in what manner, for the sampling strategy to be deemed repre-sentative and in order to cover the requirement for a risk assessment.The reported results of the concentrations of various elements andcompounds on which risk assessments are based bring with them un-certainties which derive both from the process of analysis but also theact of sample taking. It is recognised that soil sampling is the moreimportant contributor to uncertainty (Eurochem/Citac Guide, 2007)and that small scale heterogeneity has been found to be the maincause of this uncertainty (Ramsey and Argyraki, 1997, 256). In com-paring uncertainties derived from sampling in the field and from an-alyses in the laboratory, Davidson and Williams (2009), havesuggested that “uncertaintywithin the laboratory varieswith themeth-od— generally inorganicmethodswill fall between±5 and 10% and or-ganicmethods between±5 and 30%. Uncertainty in the site sampling iscommonly ±50–200%”. Measurements are carried out both in-situwith portable analytical instrumentation such as the XRF or IR and ex-situ with conventional lab based analysis and issues arising from bothhave been presented in the literature (Ramsey and Boon, 2011; EA,2009). In attempting to deal with uncertainty associated with soil sam-pling, current approaches necessitate the devising of a sampling strate-gy that needs to be tailored ‘afresh’ for every new site.
Archaeological methodology for excavation involves recording thestratigraphy of a site, irrespective of whether the site is prehistoric or
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modern, rural, urban or industrial. The stratigraphic record providesthe relationship, both spatial and temporal, of the various deposits,even when there is no or minimal evidence for archaeological objectsor built structures within those deposits. Although deposits or ‘soils’play an important role, archaeologists, routinely do not samplethem, opting instead, to sieve and discard them, having previously re-covered, the artefacts and ecofacts within. When archaeological soilsare indeed sampled, (as bulk samples) and analysed in the laboratoryfor particle size, organic content, pH, phosphates or other chemical orphysical parameters, a wealth of information can be derived regard-ing a variety of anthropogenic activities particularly on prehistoricsites (Jones et al., 2010). This information is even more important inthe case of ‘soils’ from archaeological industrial sites since they pro-vide an insight into local practice, for example stages in a multistageindustrial process, not necessarily reflected in the extant artefactualevidence (Photos-Jones et al., 2008). These sites are often deemedbrown field sites and are invariably contaminated.
In this paper we present a strategy for soil sampling for contaminat-ed land assessments grounded on: an established archaeological meth-odology for excavation and recording; and the evaluation of the site'schemical stratigraphy. Sampling can therefore be standardised acrosssites and not invented anew, and uncertainty regarding the existenceof ‘hot spots’ that may have been missed can be lessened because ofthe development of understanding of stratigraphywithin the anthropo-genic layers. Remediation is therefore targeted accordingly and cost-effectively without necessitating the need to treat the whole site.
The aim here is to show contaminated land consultants (CLCs) thepotential benefits of ‘singing from the same hymn book’with archaeol-ogists. The problem and need for an interdisciplinary understanding ofgeological data by geotechnical engineers is similarly highlighted byMerritt et al. (2006). This paper presents both work already completedaswell as recommendations for future work. There is no attempt to rig-orously assess and compare the proposed approach of archaeologicalrecording with chemical stratigraphy with existing methodologies,but merely to present its merits and potential advantages on the basisof work to date. Contaminants whether of organic or inorganic originencountered in industrial sites differ substantially in terms of behav-iour, transport and fate. This article does not delve into aspects oftheir behaviour but is simply concerned with highlighting their pres-ence. The idea of chemical stratigraphy implemented within the studyof archaeological sites, has already been presented in the archaeologicalliterature as Holistic Context Analysis (HCA) (Photos-Jones et al., 2008).HCA deals with themeasurement of a number of parameters in archae-ological soils, both physical and chemical, which are aimed to facilitatethe archaeological enquiry. We believe that in reference to CLAs such aholistic approach may be data overkill particularly when under currentlegislation site assessment is primarily based on the values of specific el-ements/chemical compounds.
Soil sampling based on archaeological stratigraphy requires an un-derstanding of two basic archaeological concepts, namely ‘context’and the ‘Harris Matrix’. ‘Context’ is the base unit of archaeological re-cording. It is usually a deposit typically a layer but it can be an archae-ological feature such as a wall or a cut. A ‘context’ therefore,represents an event, or a particular time interval. Each ‘context’ isgiven a unique number. A site Harris Matrix is the visual representa-tion of all ‘contexts’ across all trenches on site.
Archaeological recording of site stratigraphy is based on the objec-tive evaluation of cuts and the variations in soil morphology, colour,texture and the material evidence of fills. Once a new context andits extent is identified and given a unique number then analysis in-situ or ex-situ can follow immediately with or without the removalof a bulk sample. This is not a judgmental sampling strategy but rath-er one related to stratigraphy or to a spatial temporal/sequence ofevents. It therefore has a far higher likelihood of decreasing systemat-ic errors thus decreasing uncertainty that hot spots have indeed beenmissed.
When soil sampling is based on a grid design laid on ground sur-faces, then it is a case of a geometric shape attempting to ‘reach out’to echoes of buried human activities. Even when these are ‘reached’,by sampling at depth, it is difficult to translate elemental composi-tions to events/activities, because the sampling design strategy isbased on an abstract shape, laid out on the surface; it is not targettedfor the pursuit of activities that may have caused contamination. Onlythe stratigraphic record can do that. In the proposed method of soilsampling, it is the evidence of disturbance of the natural that guidesthe process of sampling, rather than statistics, thus providing uswith the confidence that we have not missed a ‘hot’ spot.
Further, there is a difference in perception, between archaeolo-gists and CLCs, of what constitutes the ‘made ground’ and the ‘natu-ral’. What would the advantages be to both sets of specialists ifgeotechnical (boreholes) and archaeological trench data were dis-played in the same drawings? The point to be made here is, thatsoil sampling is integrally connected with how we record soils, thesheer act of recording them influencing our understanding of them.
To illustrate the differences of approach, we use archaeological re-cording and geotechnical trench logs obtained from the same trenchwithin a 19th century industrial site near Glasgow, Scotland. Follow-ing the identification of stratigraphy within the sections of a trenchand the generation of the Harris Matrix for the same trench we pro-ceed to outline the various stages in the derivation of chemical stra-tigraphy. These consist of a) measurements in-situ or ex-situ ofeach individual context, b) data treatment with a statistical packageand c) the superposition of the chemical data on the stratigraphic re-cord as it is represented by the Harris Matrix.
The final section of this paper presents some recommendations re-garding how archaeological recording and chemical stratigraphy canbe incorporated within the general framework of CLAs. Ultimatelythe merits of the approach lie in the value and extent of decision sup-port. Can the volume of the made ground to be removed or treated bereduced and by how much?
2. Archaeological context and the Harris Matrix: an illustration
The excavation of an archaeological site is usually a destructive pro-cess and as such, the onlyway that the archaeology can ‘survive’ is by re-cord. The process of archaeological excavation is therefore focused on therigorous and objective depiction of the site stratigraphy. This provides ameans of understanding both natural and man-made conditions thatled to the formation of archaeological deposits and their sequence intime. Stratigraphic correlation has long been used in geology and iswell established in the geotechnical world. Geologists have developedthe concepts of lithostratigraphy (correlation based on composition oflayers), chronostratigraphy (correlation based on time-markers) and se-quence stratigraphy (correlation based on matching sequences of sedi-ments). See definitions of the Society of Sedimentary Geology (2010).
In the UK and abroad, archaeological recording takes special accountof ‘contexts’ (MoLAS, 1994; Roskams et al., 2001). A ‘context’, usually adeposit forming a layer orfill, butmay be a cut or part of a buried featuresuch as a wall or a grave, is given a unique number and is described indetail on its own context sheet. Each deposit, or layer, that forms an in-dividual context is typically a composite material consisting of both amatrix and inclusions. Inclusions may be artefacts made of materialssuch as clay, stone, glass, metal or bone, or ‘eco-facts’ such as seeds,pips or charcoal. The soil matrix within a context is usually not whatsoil scientists define as soil, namely broken up rock, the result of erosionandweathering, and capable of supporting plant life; such soils are besttermed ‘agricultural soils’. Archaeological contexts that are called ‘soils’or ‘deposits’may therefore beman-made, such as those consisting of in-dustrial debris and found as fills in industrial sites. They are best knownas ‘archaeological soils’ rather than just ‘soils’. In this paper we use theword soils to mean ‘archaeological soils’
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In recording stratigraphy, archaeologists, usually use the HarrisMatrix, named after Edward Harris (Harris, 1979), for representingthe chronological sequence of contexts which represent past actionsor processes. Harris Matrices reveal not only the stratigraphy withinthe four walls of the trench, but also within its floor as well as the de-posits within. Contexts are often grouped into ‘phases’ which havegenerally formed over a relatively short time span. A phase is a‘time interval’ represented by one or more contexts. The constructionof the Harris Matrix for entire archaeological sites, particularly multi-period sites, is a complex and time consuming operation. However,the number of exploratory trenches is small. Harris Matrices can beparticularly useful since as it is shown below they carry both a recordand a narrative, regarding the sequence of events.
Fig. 1 provides an illustration of how contexts (boxed numbers) aredepicted in the framework of a Harris Matrix. Context numbers aregiven to every feature (e.g. deposit, wall, cut) as it is progressivelyrevealed during excavation. The numbers are provided by the site direc-tor and need not be in a simple sequence. The stratigraphy and contextsare first recorded, then the sequence of events established, but, for thefinal description, it is easier integrating observation and interpretationas has been done here. For Fig. 1 the narrative is as follows: once thetopsoil (400) had been removed the top of a wall (402) was exposed.The sequence of contexts deposited after the wall was constructedand presumably abandoned is different on either side of the wall. Tothe left of 402, contexts 403, 407, 408 and 410 represent a sequenceof four distinct deposits while to the right, only two deposits wereencountered, 404 and 409. The latter are horizontal and likely to befloor deposits inside a building, whereas the series of contexts, 410–403 is likely to represent deposits piling up against the exterior ofwall 402. Assuming 404 and 409 are floor deposits, a further wall, theother side of the building, would be anticipated to occur to the right ofthe trench. In the process of excavation, wall 402 was confirmed to beconstructed in the fill 413, of the cut 412. This cut 412, was noted tohave been made into 414, the natural. The trench was not excavatedfurther as both deposits 414 and 415 were considered to be natural.Each context is recorded in its own sheet and the relationship shownto its immediate neighbours. All relationships are finally assembled onthe Harris Matrix (inset in Fig. 1). This imaginary example is illustratedin a 2-dimensional section, but the observations in a 3-dimensionaltrench can be recorded and represented in the same way.
3. ‘Natural’ and ‘made ground’
Having explained archaeological ‘context’ and provided an illus-tration of the narrative underpinning the Harris Matrix, we proceed
Fig. 1. Demonstration of construction of a simple Harris Matrix (right) that represents the stburied wall, 402.
to consider ‘made ground’ and ‘natural’. The term ‘natural’ is used inarchaeology to signify natural undisturbed deposits that underlie ar-chaeological/anthropogenic features. ‘Natural’ can still be contami-nated via migration of pollutants. From the perspective of thearchaeological recording, a ‘natural’ deposit has been unaffected byvisually identifiable human disturbance. But from the perspective ofthe chemical stratigraphy, the apparently ‘natural’ deposit could beclearly identified as ‘contaminated’.
‘Made ground’ is a geotechnical term. It can contain archaeologicalmaterials that, if in-situ, can provide evidence of industrial practicebut otherwise can reflect post-industrial activity such as dismantlingor demolition. In archaeological recording, these are viewed and trea-ted very differently: The in-situ archaeology is carefully recorded andconsidered to be significant. The secondary or re-deposited layersoften consist of deposits resulting from demolition, levelling or sim-ply back-filling, and because the materials/finds within such depositsare un-stratified, i.e. not in-situ or undisturbed, they are usually con-sidered to be of lesser archaeological significance in elucidating thearchaeological resource.
Natural deposits are differentiated archaeologically from thosemade by or strongly influenced by humans, so-called ‘anthropogenicdeposits’ which include middens and refuse tips. Deposits in formerindustrial/mining areas, for example mine waste, process waste, andsediments in settling ponds, may look natural but are in fact ‘anthro-pogenic’. Clues that such sediments are not entirely natural in origincould come from the presence of man-made inclusions and/or a dis-tinctive chemical fingerprint. The recognition of ‘anthropogenic de-posits’ is very important in the course of site assessment.
The extent of the made ground and the ‘natural’, the former deriv-ing primarily from the archaeological approach and the latter fromthe geotechnical could be highlighted in a single section. As will beshown below, geotechnical trenches, borehole data and archaeologi-cally recorded trenches are often presented together in plan. It israre that they are presented in section. However, we suggest thatthere are benefits to be obtained from presenting the same data insection, as a means of providing a quick and effective visual insightinto the nature of the deposits.
To illustrate this point we present a case study where no such sec-tion drawings were implemented but, we argue, had they beenimplemented, a substantial insight would have been derived linkingthe archaeological and contaminated land assessments at minimalextra cost. These are, the demolished remains of the Govan IronWorks (GIW), an 19th century iron works in SE Glasgow, which layin the path of the M74 extension, a major motorway that feeds trafficfrom England in the south, into Glasgow, the major city and industrial
ratigraphy of an imaginary trench (section on left) that has encountered and removed a
Fig. 3. Plan of recording intrusive work at GIW, Glasgow. Position and size of GUARD'sarchaeological trenches (Tr_n) (green) projected on to line (red) denoting the courseof the planned motorway; Northwest Holst (NH) trial pits (T_n) (triangles — blue)and boreholes (B_n) (circles — blue). (For interpretation of the references to colourin this figure legend, the reader is referred to the web version of this article.)
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hub of Scotland. The motorway was opened in June 2011. GIWformed part of a greater 19th century industrial landscape on the SEside of Glasgow that included potteries and small engineering firms.
In the course of the archaeological assessment, sixteen trencheswere opened up by Glasgow University's commercial ArchaeologicalResearch Division (GUARD) and are shown in Fig. 2 superimposedon the first edition Ordnance Survey town plan (c.1857–8) (Dalglishand Driscoll, 2004, 63). Part of the site is detailed in Fig. 2 whichshows the location and distribution of some archaeological trenches(Tr_n) as well as boreholes (B_n) and investigative ‘soundings’(T_n) undertaken by Norwest Holst Soil Engineering Ltd (NHSEL)(Dalglish and Driscoll, 2004).
Both sets of intrusive approaches are shown in the single plan inFig. 3. Green colours denote the position and size of GUARD's archae-ological trenches; blue triangles and circles denote the positions ofNHSEL ‘soundings’ and boreholes respectively. The archaeologicaltrenches varied in size (ranging between 50 and 100 m2) and wereopened for the purpose of assessing archaeological features. Thelines extending from the archaeological trenches to the line of themotorway denote projections of the former on the plane of the latter.Such plans simply highlight trench distribution and do not purport toassist either the archaeological or contaminated land enquiry in anymeaningful way.
On the other hand, cross-sections as shown in Fig. 4, the cross-sectionof the ‘motorway’ as depicted by a red line in Fig. 3, can demonstratehow archaeological stratigraphic information can be related to boreholedata. Fig. 4 may be useful because from the archaeologist's perspectiveit is essential to assess the presence and depth of in-situ archaeologyandwhere it lies with respect to the ‘natural’. From the CLCs' perspective,the archaeological layers, albeit representing the shallow end of theirinterest, nevertheless can assist in pin-pointing source(s) of contamina-tion emanating from the archaeological resource
In Fig. 4, the section has been drawn to a depth of 10 m which iswell below the boundary of man-made deposits. The ground surfaceis assumed to be horizontal and is used as a reference level. The ver-tical scale used is approximate. The boreholes continue to depth torock-head. The boreholes and trenches indicate that made ground isdistinct to depths of 2–5 m (Fig. 4). The complete stratigraphic se-quence is seen in the boreholes and is best divided into three zones:
Fig. 2. Sixteen trenches (numbered), part of the archaeological assessment, opened by GUARDalglish and Driscoll, 2004, 63).
made ground (mainly building rubble and waste from the iron/cokeindustry); possible anthropogenic deposits (sandy deposits whichcontain evidence, albeit very limited, of human activity so may repre-sent material moved during construction); and distinct natural de-posits extending to N10 m depth (dominantly laminated silts andclays). A persistent layer that is rich in coke and dark oily materialnear the base of the made ground at GIW is a potential stratigraphicmarker deposit that may mark an original horizontal ground surfacefollowing first abandonment and demolition of the site.
D and illustrated here in blue are shown superimposed on the OS (1857–8) map (after
Fig. 4. GIW. Sketch cross-section (NW, left to SE, right) showing geotechnical borehole records (B_n) and trial pits (T_n) as well as archaeological trenches (Tr_n). The widths of thecolumns representing stratigraphy recorded in the ‘trial pits’ are arbitrary. The archaeological trenches are projected on to the plane of the section from the north and south. Theapproximate base of archaeological trenches is indicated for clarification. Significant materials (e.g. slag) and layer boundaries that were noted on the original records are given.Deposits are subdivided into ‘made ground (MG)’, ‘possible anthropogenic’ and ‘natural’. This method of combining archaeological data and deep borehole data can give a gener-alised and clearer picture of the site subsurface. The vertical scale is 1cm=1m.
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Looking at Fig. 4, trench T205N shows a downward sequence ofmade ground followed by a coke layer followed by sand/silt/gravelwhich may be anthropogenic and finally the definite ‘natural’, aclay/silt. The deposit identified as made-ground seemed to be mainlydistinctive building rubble. Sands and gravels below the made groundin one of the boreholes (B219N) contain fragments of slag, accordingto NHSEL data logs. If the fragment identification is correct then theremust have been human activity before or at the time of deposition ofthese sediments and the sediments may have been deposited throughhuman activities. Stratigraphic correlation implies that possible an-thropogenic deposits must be present throughout the section(Fig. 4). Such sediments are clearly problematic for archaeological in-terpretation, as it is not easy determining whether or not they repre-sent ‘the natural’. They have therefore been labelled here as possibleanthropogenic deposits.
The boreholes at GIW demonstrate that ‘the natural’ encounteredat depth below clear archaeology is a laminated silty sediment and
Fig. 5. Looking into Trench 5, MUSF.
not a solid rock. Being laminated and very fine-grained, this sedimentmust have been deposited in a relatively quiet large expanse of watersuch as the early Clyde estuary or a glacial lake in the early Clyde val-ley. Had industrial activities been taking place at the time this layerwas deposited, then it would need to be sampled for possible contam-ination. Regarding the rest of the layers, it is quite possible for naturalsands and gravels to overlie finer silts and sands. However, thesesands and gravels are possibly anthropogenic and could have beendeposited by the industrial development, for example when founda-tions were constructed, or pits excavated to recover material andthen backfilled with waste. Thus from a CLC's perspective, ‘natural’may not carry the same meaning as for an archaeologist.
Similar possible misunderstanding may arise from the use of theterm ‘made ground’. From an archaeologist's perspective the ‘madeground’ is in reality nuanced in that it can contain both in-situ archae-ology and demolition layers. Geotechnical recording is transparent tothose nuances, yet it is precisely those layers where contamination islikely to be found. Furthermore, the contaminated land survey need
Table 1Archaeological and geotechnical description of the same trench at MUSF (Trench 5).
Context Archaeologicaldescription
Geotechnicaldescription
Depth(m)
501 Turf and topsoil; between 0.12 m and0.26 m deep
Grass 0.30
502 Yellow-brown sandy clay with moderndebris; 0.5 m thick
Dark grey sand withrootlets and brickfragments
0.50
503 Dark grey sand with a notable marblingeffect; depth of up to 0.4 m
Dark brown gravelyclay; contains ironslag
0.70
504 Mid-brown clay; 0.07 m505 Loosely compacted Yellow-brown gritty
sand with brick inclusions included amoderate amount of both firebricks andregular bricks; 0.4 m deep
Dark grey sand 0.90
506 Pink-brown coarse sand507 Yellow sandstone fragments with clay
matrixBase of trench 1.30
Fig. 6. Babtie Trench Record Form, completed for trench 5, MUSF.
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not recognise possible anthropogenic deposits, i.e. deposits immedi-ately above cuts into the ‘natural’, possibly cut for the installation offoundations. Although contamination is unlikely within such de-posits, it is nevertheless possible.
The cross-section (Fig. 4) demonstrates how, in principle, it is possi-ble to bring all the data together to give amodel of the site's sub-surface
Fig. 7. Depiction of trench contents in south-facing section of trench 5 at MUSF; numbers regrey ashy cinder lens. Compare with Fig. 5.
in order for the CLCs to understand the full depth and variation of the ar-chaeological layers and for the archaeologist to gain a clearer under-standing of the differences between the various layers of what theyattribute to be natural, wherein the foundations of any structure or fea-ture is built. Fig. 4 therefore illustrates the importance of documentingunder one ‘umbrella’ archaeological and contaminated land invasive
present contexts. In context 503, ‘a’ is lighter grey marbling and in context 505, ‘b’ is a
Fig. 8. View from S, of E side of section, Trench 6, MUSF.
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survey for the purpose of clarifying the extent of human activities andthe nebulous interface between made ground and the ‘natural’. Withthe exception of the undisturbed true ‘natural’ all these layers are poten-tial sources of contamination.
4. Geotechnical vs archaeological recording: a case study
Having highlighted the advantages of illustrating jointly and insection geotechnical and archaeological data pertaining to the ‘natu-ral’ and made ground, we turn our attention to the nature and con-tents of real test trenches. The purpose is to compare methods oftrench recording for purposes of sample taking. We rely again on acase study where the two assessments CL and archaeological tookplace side by side. Moffat Upper Steam Forge, near Airdrie, in the Cen-tral belt of Scotland, between Glasgow and Edinburgh, was the site ofa puddling furnace operating in the mid 19th century. HistoricallyAirdrie, in the parish of Monklands was a major centre for the devel-opment of the iron and steel industry and was formerly surroundedby iron and steel works as well as the mines and quarries that provid-ed the necessary raw materials, in particular coal and Carboniferousiron ore (Thomson, 1982). Puddling was the way high carbon iron(pig-iron) from the blast furnace was treated to make more malleablelow carbon iron by the decarburization of the pig in a reverberatoryfurnace, the whole process being very labour intensive (Sexton,1902; Macfarlane, 1906). An archaeological assessment was carriedout at MUSF by SASAA funded by Historic Scotland (Coulter et al.,
Fig. 9. East and West sections of Trench 6, MUSF to illus
2005) for the purpose of clarifying Scottish puddling activities,given that little was known for such practices outwith England (Kill-ick and Gordon, 1987). MUSF consisted of two puddling furnaces,reheating furnaces and hammers for squeezing slag out of the ironmass (Photos-Jones et al., 2008). Following preliminary assessment,MUSF was reinstated to its original state, trenches were backfilled,with full excavation perhaps to take place sometime in the future.
As the work was initialised at MUSF, the aim, for the archaeologist,was to establish the level of preservation of the archaeological re-source while for the CLC it was to establish the level and type of pos-sible contamination, as it pertained to the Health and Safety ofworking staff. Two sets of investigations were carried out within thesame trench at MUSF, Trench 5, (Fig. 5) in parallel and independentlyby investigators at SASAA (www.sasaa.com) and at the then Jacobs–Babtie (www.jacobs.com) (Konstantopoulos and Cooke, 2004).Trench 5 (Figs. 5 and 7), measured 5.5 m E–W by 1.1 m N–S with afinal maximum depth of 1.45 m. It was excavated by machine up toc. 80 cm and then by hand (Table 1). Geotechnical recording of thistrench (Fig. 6) was based on the identification and description ofsoil texture, colour and consistency. Samples were taken at a particu-lar depth from the surface and duly recorded. Two types of layerswere identified, dark grey sand and dark brown soil with slag inclu-sions. Recording is only along a single axis, i.e. the vertical.
In archaeological recording, the contexts were identified, given aunique number, described in the manner shown in Table 1 and drawnon transparent paper (Fig. 7) (Coulter et al., 2005). The final depth ofTrench 5 was to the top of context 506. There was no attempt to reachthe ‘natural’, since Health and Safety considerations became an issue.In examining the two sets of data, presented in Table 1, it is difficult todraw a direct comparison. It is clear that both the level of descriptionof the archaeological contexts and their exact position with respect toeach other reflect a much more accurate representation of the section,than that in the geotechnical log (Fig. 6). This level of detail would beexpected from archaeological recording and is very useful, in the nextstep, i.e. that of soil sampling. There is no on the spot decision makingprocess as to which contexts will be analysed. ALL contexts are ana-lysed. From the perspective of CLC, there was decisionmaking involved.Soil sampling was carried out as a function of depth from surface and atspecified intervals. Effort was made to take samples from layers thatlooked ‘different’. At the time, the CLA was carried out on the basis ofvalues from geotechnically derived samples alone.
Further to the above we proceeded with the archaeological assess-ment of a new trench, trench 6. We use this trench to illustrate oursoil sampling strategy based on archaeological excavation and record-ing followed by the derivation of chemical stratigraphy. Trench 6,(Figs. 8–9) is a trench within MUSF with more complex stratigraphy
trate the complexity of the stratigraphic sequence.
601
630
602
657 PHASE IIIdpost IIIc levelling
631 PHASE IIIc 641post IIIb levelling
648613
649614 deposited from NE
644612
637
643
624 Same as 645
603 trample/spread
634 Same as bIII ESAHP406backfilling of
635 furnace
615623 deposited
from SW 616621
617
926026
619
606816
605
608
642 PHASE IIIadismantling of
607 Same as 647 furnace
226646Group 1
636Group 2 632
639Group 3 656
PHASE II056906 furnace
in use836156 Brick floor
640
foundation cut 652628
I ESAHP116Pre Furnace
KEY
610 Natural
Fig. 10. Harris Matrix for Trench 6, MUSF.
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Fig. 11. Discriminant Analysis plot (MUSF trench 6) showing good differentiation inthe dataset.
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and one that reflects more accurately the soils at MUSF. Both east andwest sections, were recorded in the section drawings of Fig. 9. Phasesof activity evident within this trench are listed here in chronologicalorder (starting from oldest at bottom):
Phase I The activity on the site prior to the construction of the build-ing housing the furnace, including the contexts above the‘natural’ (611) and the cut of the furnace foundations (628)
Phase II The period of the site associated with the construction andworking life of the furnace. This phase includes the work-shop's brick floor
Phase III The period after the working life of the furnace when it wasdismantled and backfilled:
IIId further site levelling following Phase IIIcIIIc levelling of the area following Phase IIIbIIIb backfilling of the furnaceIIIa dismantling of the furnace
Following the above, a Harris Matrix was constructed for trench 6(Fig. 10) in themanner described in Fig. 1. This is a visual representationof the stratigraphic sequence of the deposition of contexts and includesnot only deposits identified in the east and west sections of Trench 6but also contexts present within the trench and whose relationshipswere established during excavation. Furthermore it includes informa-tion regarding particular phases of activity as indicated by arrows.
5. The derivation of the MUSF's chemical stratigraphy
Having constructed the Harris Matrix for trench 6 we now proceedto the derivation of the chemical stratigraphy of the soils within trench6. This step is divided into four stages: i) analysis in-situ or ex-situ ofindividual deposits within the trench; ii) data treatment (ex-situ)with a statistical package for the identification of groups/clusters ofchemically similar/distinct deposits; iii) the ‘reinstatement’ of thesegroups within the Harris Matrix thus providing visual illustration ofthe ‘spread’ of contamination; and finally iv) the scrutiny of the abso-lute values of in-/ex-situ analysis regarding individual contaminants,their behaviour and distribution.
5.1. Analysis in-situ or ex-situ of individual deposits within the trench
In their original state, the contexts within trench 6 contained boththe soil (mostly industrial in nature, and consisting of sand and silt)as well as inclusions or aggregate (large fragments of slag, brick,cement and other non-friable/non-destructible material). Inclusionswere separated via sieving and it is only the soils that were subse-quently analysed by ICP-MS/AES to provide concentrations of majorand trace elements. It is the data from these analyses that are usedfor the generation of the annotated or colour coded Harris Matrixand not the data obtained from the combined analysis of soilsplus inclusions. In a study undertaken separately it was found thatmore often than not fragments of slag, brick, cement and other aggre-gates do not contribute to soil contamination (Photos-Jones et al.,2008).
5.2. Data treatment (ex-situ) with a statistical package for the identifica-tion of groups/clusters of chemically similar/distinct deposits
Considering the chemical compositions of the soils (trench 6),their classification (using SPSS v.11) by Principal Components Analy-sis (PCA) revealed three groups, and these three groups were thentested with Discriminant Analysis (using the Mahalanobis D-squared statistic). Fig. 11 shows that they are well discriminatedand contamination focuses primarily in contexts belonging to Group2. Each group has been colour-coded (Fig. 11).
• Group 1 contexts stand out on account of Fe and Mn; these contextsreflect puddling slag and by extension the puddling process itself
• Group 2 contexts stand out on account of the relatively high con-centrations of contaminants, namely Pb, Zn, Ni, Cd, Cu
• Group 3 contexts display low levels of metallic elements and reflectthe composition of the ‘natural’
5.3. The ‘reinstatement’ of these groups within the Harris Matrix thusproviding visual illustration of the ‘spread’ of contamination
Having, colour-coded the individual groups and by extension the in-dividual contexts within, in green, blue and red, respectively, we pro-ceeded to colour-code each context within the Harris Matrix of Trench6. Coding (in colour) the contexts (Fig. 12) according to their chemicalcomposition (group) gives added value to the stratigraphic informationdepicted in a conventional Harris Matrix (Fig. 10). Thus in Fig. 13 thethree groups appear in relatively well-defined clusters. In Phase I, thepre construction phase, some ‘soils’ originate from the ‘natural’ andsome have been imported to the site (Groups 3 and 2). In Phase IIwhich coincides with the period of the working of the site, which inci-dentally is known from company archives to have been rather short(c.15–20 years), the ‘soils’ derive from all three types of materialsnamely imported sands, natural clay and puddlying waste; but it is pri-marily ‘soils’ with Group 1 and 3 that are evident.
In Phases IIIb and IIIc (Fig. 12), there is considerable clustering ofthe groups. In Phase IIIb the majority of contexts contain slag-rich‘soils’ (Group 1); these contexts are mainly deposited from the SWof the site of the furnace structure. It follows that slag-rich withcoal-rich waste materials were ‘stored’ in close proximity to eachother and to the SW of the furnace, and therefore they were thefirst to be used once backfilling started. In Phase IIIc, the majority ofcontexts contain ‘soils’ belonging to Group 2; they were depositedfrom the NE of the furnace and their make-up and location suggeststhat they were used for the levelling of the area subsequent to thebackfilling of the furnace (Table 2).
Thus by monitoring the chemical make up of the contexts, it ispossible, to follow closely or re-construct the sequence of eventsfrom the construction to the operation and demolition of feature(s)
601
630
602
657 PHASE IIIdpost IIIc levell
631 PHASE IIIcpost IIIb levelling
613
614 deposited from NE
612
624 Same as
603 trample/spread
634 Same as bIIIESAHP406backfilling of
635 furnace
615623 deposited
from SW 616621
617
926026
619
606816
605
608
642
607 Same as 647
226646Group 1
636Group 2 632
639Group 3 656
056906
836156 Brick floor
640
foundation cut 652628
611
610 Natural
KEY
ling
641
648
649
644
637
643
645
PHASE IIIadismantling of
furnace
PHASE IIfurnace in use
PHASE IPre Furnace
Fig. 12. Annotated Harris Matrix for Trench 6, MUSF indicating chemical composition groupings (ICP-AES data). Compare with Fig. 10.
5441E. Photos-Jones, A.J. Hall / Science of the Total Environment 409 (2011) 5432–5443
Fig. 13. Principal Components Analysis plot: PC1 vs PC3 scores derived from the fullchemical dataset for samples from trenches 2,7 and 9 at GIW. The variance and ele-ments loading respectively PC1 and PC3 are shown. Attention is drawn to the samplesseparating from the main cluster along PC1 and PC3.
5442 E. Photos-Jones, A.J. Hall / Science of the Total Environment 409 (2011) 5432–5443
within the trench as well as the final backfilling of the area. All ofthese steps are an integral part of the archaeological enquiry. Butare they of interest to CLCs?
Contamination is perceived here, not as an accumulation of ‘hotspots’ but rather as the result of a very deliberate activity, i.e. theimporting of Group 2 yellow-reddish brown sands to the site, forthe purpose of laying foundations and/or backfilling. By providing ameaningful explanation for the source of contamination as well asfor its possible spread within the site, these light coloured sands,highly visible in distinct layers, could be selectively removed as partof a remediation strategy. Should, in the future, the site whereMUSF lies be targetted for further development then ONLY thecoloured sands forming distinct and compact layers across the siteneed be removed. The black sands which look similar in texture tothe coloured sands are NOT contaminated and could, because of thehigh charcoal content, be sold by the developers to garden centres. In-deed we were told by local residents that that black sand was mostbeneficial when added to their garden.
5.4. The scrutiny of the absolute values of in-/ex-situ analysis regardingindividual contaminants, their behaviour and distribution
Contaminated deposits are not always visually and chemically dis-tinct, as in the case of the light coloured sands of MUSF. Chemicalstratigraphy in the manner outlined above (stages i–iii) was appliedon three of the sixteen trenches in GIW (Trenches 2,7 and 9 inFig. 2); soil samples were removed from all contexts within these
Table 2Plot showing grouping of contexts in MUSF, trench 6, based on the elemental content ofthe total soil digests.
Group Contexts Outliers
1 605, 607, 609, 615, 617, 618, 619, 621, 623, 635, 646, 647, 656 –
2 601, 603, 604, 606, 611, 613, 624, 629, 634, 639, 642, 643, 644,648, 649, 652
612,654
3 602, 610, 616, 622, 636, 650, 651 608
trenches and analysed ex-situ, with ICP-AES, a total of 102 contexts(Photos-Jones et al., 2005); the data set was treated by principal com-ponent analysis (Fig. 13) revealed one tight cluster with some outliers
Although outliers 909, 203, 208, 237 and 223 are separated alongthe PC1 axis on the basis of rare earth metals, Sc and Sr, it is the con-tent of heavy metals as those shown in Table 3 that is of interest here.This is indicated to some extent by the PC3 axis where contexts 906,911, 931, 936 and 247 are separated on the basis of Pb, Bi and Zn.
The key outlying contexts shown in Table 3 were concentratedboth in in-situ archaeological deposits such as the floor levels ofTrench 2 or around the floor of the room in Trench 9 but also amidstthe demolition layers. This suggests to us that the high levels of Pb,Zn, Cr, Cd probably arose from activities at the floor level of theroom in trench 2. On demolition the material that infilled theserooms may have been subsequently re-deposited elsewhere, in away that may have been arbitrary and which, given the size, chrono-logical span and the complexity of the site, is impossible to trace.Thus, these contaminated contexts are not only texturally or visuallydistinct they are also dispersed across the site. Mitigating against suchdispersed contamination is difficult and uplifting of made ground toinclude both in-situ and re-deposited remains is possibly the onlyway forward.
To summarise the patterns of contamination distribution emerg-ing from the two sites are different. At MUSF, the results of chemicalanalysis showed that soil compositions fall into three distinct groups:those high in iron and manganese reflecting puddling activities(Group 1) and free of contamination; sandy soils (Group 2) with con-taminants, imported to the site and for the purposes of foundation/backfilling; and natural boulder clay-rich soils (Group 3), free of con-taminants. The Group 2 soils were visually distinct and selectiveuplifting would have resulted in cost cutting. At GIW, the results ofchemical analyses of the many contexts within the trenches showedthat the soils were similar throughout the site with only some con-texts rich in contaminants. The contamination started at the floorlevel of some in-situ archaeology. Following demolition and levelling,layers got mixed up resulting in ‘hot-spots’ of contamination dis-persed randomly throughout the site. In this case remediation cannotbe as easily targeted as in the case of MUSF. GIW has been fully exca-vated, and the final report pending publication.
6. Conclusions
The undertaking of measurements in CL is associated with theneed of a) understand the anthropogenically derived contamination,b) establish whether the measured values exceed or not regulatorythresholds and c) decide on the financial implications of site remedi-ation. This paper deals primarily with part a) and offers a methodol-ogy for the understanding of the distribution of contamination withdirect implications on sampling strategy. The archaeological methodof systematically recording stratigraphic relationships of individualcontexts for representation in the Harris Matrix combined withchemical analysis of each individual context (chemical stratigraphy)can provide a much better insight into site contamination than aprobabilistic model would, with obvious consequences on site reme-diation strategy. We often implement chemical stratigraphy with in-
Table 3Archaeological contexts at GIW with higher than average metal content evident on vi-sual inspection of the data. CrN1000 ppm; PbN1000 ppm; AsN100 ppm; NiN400 ppm;CdN10 ppm; and ZnN1000 ppm.
Context Elements Context Elements
906 Cr, Pb, Cd, Zn 936 Pb, Cd, Zn908 Pb, Zn 704 As, Zn, Pb911 Pb, Zn 234 Pb, Ni, Cd, Zn931 Cr, Pb, Cd, Zn 247 Pb, Ni, Zn, Cd
5443E. Photos-Jones, A.J. Hall / Science of the Total Environment 409 (2011) 5432–5443
situ analysis, in archaeological sites. Since for the majority of thesesites, land contamination is not an issue, there has been no need tocompare measured values against acceptable threshold ones, but itis understood that this is the next step in the development of themethodology.
We present below and in the form of a step by step process the im-plementation of the proposed methodology and argue that its under-taking does not necessitate the presence of archaeological remainswithin, simply embracing an archaeological approach.
• coring by geotechnical investigators to assess depth of made groundand depth and makeup of the natural; the generation of a sectiondrawing similar to Fig. 4.
• opening of small trenches by archaeologists along the proposedscheme to assess and record stratigraphy; the generation of a HarrisMatrix similar to Fig. 10.
• in-situ analysis of all recorded contexts• ex-situ treatment of data with statistical packages to assess group-ing of contexts based on their chemical make up; the generationof an annotated Harris Matrix similar to Fig. 12.
• establishing whether the measured values within the annotatedHarris Matrix exceed or not regulatory thresholds
• remediation proposal
The topics in this interdisciplinary paper have been tackled with theaim of bridging the practices of two professional groups in a way thatshould bring cost-effective benefits to both. Nevertheless a number ofissues/queries might arise and can be summarised as follows:
a. It might be argued that CLCs have little use of the level of detailgenerated by archaeological recording. We suggest that where ar-chaeology is present, archaeological recording takes place in anycase; furthermore, it is important to acquire an understanding ofthe nature of the made ground.
b. It might be argued that sampling within contaminated land shouldbe based on a strategy firmly grounded in statistics and the laws ofprobability of hitting a ‘hot’ spot. We suggest that while this is inprinciple correct, chemical stratigraphy can offer a lot more sinceit provides a guided ‘tour’ around the site, spatially and in time.
c. In-situ analysis with portable instrumentation i.e. XRF of soils fromindividual contexts is currently being carried out in many sites bythe authors. It of course deals with metallic elements only. Portableinstrumentation for organic components is also commercially avail-able and the authors are in the process of investigated that avenue aswell. BS10175 (BS, 2001, updated 2011) prescribes ‘representativesampling schemes’ which are often difficult to achieve in practicedue to limitations in time, personnel and budgets allocated for anal-ysis. Boon and Ramsey (2011) have aptly highlighted the advan-tages of using in-situ analysis, which to our view is indispensablegiven the large number of contexts needing to be measured.
d. It might be argued that understanding contamination in such de-tail as provided archaeologically, may not be deemed necessary.We argue that such an approach leads to remediation procedureswhich can often be an overkill; gradually a need will arise tostreamline what may currently be deemed conventional practice,both on account of the possibility and implementation of cost cut-ting practices but also for environmental reasons (recycling andfewer materials taken to landfill sites).
Acknowledgements
We are indebted to Dave Cooke and Chris Dalglish for reading andcommenting extensively on the original manuscript in a rigorous and
most constructive manner; to H James and R Jones for highlightingdetails that needed to be addressed; to Th. Konstantopoulos andS. Coulter, for their substantial contributions at the time of the inves-tigations of MUSF and GIW; to Historic Scotland for funding the pilotstudy at MUSF and Glasgow City Council, for the work at GIW. And tothe three anonymous referees for their perseverance in bringing themanuscript closer to contaminated land audiences.
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