Portable analyser provides a valuable supplement to laboratory data because large numbers of results can be obtained quickly over a wide area, whereas lab results can take days to arrive and cost more
In an attempt to create a clearer picture of what life was like in Roman Britain, Channel 4 television's Time Team chose ten sites with which to illustrate different aspects of Roman life.
Live programmes were broadcast during the first week of July 2005 from each of the sites, including Charterhouse, which lies in the Mendip Hills just south of Bristol, UK.
Charterhouse was a key site in Roman Britain.
It was one of the chief sources of lead in the province, brought quickly into large-scale production by AD49 at the latest.
A Roman fort existed here in the first century when the site was under imperial control and the military oversaw mining operations.
The Roman settlement covers some 60ha and is characterised by complex extant earthworks and voids created by the exploitation and extraction of lead from (possibly) prehistoric until Victorian times.
An earlier earthwork survey undertaken by Chert (Charterhouse Environs Research Team) revealed that the site is multi-period, with a street grid overlain by later earthworks.
However, the existence of buildings within the street grid was not established unequivocally.
To set the site in context, this is the largest Roman settlement visible as earthworks in the area, certainly in the west of Britain, where until the commencement of Chert's work little archaeological investigation had been undertaken.
It is clear therefore that a great deal had yet to be learned about the Charterhouse site and the programme makers were naturally excited about the prospects for the Big Roman Dig.
The archaeological investigation was comprised of a geophysical survey and excavation of two small trenches within the scheduled monument.
The geophysics revealed a network of enclosures and roadways on differing alignments, probably representing many different phases of settlement development, in addition to evidence for possible ore extraction 'rakes'.
The remains of at least a single masonry structure were revealed in one trench, while pottery recovered from the other trench, which lay over a pronounced rectangular earthwork enclosure, confirmed this feature to be of Roman date.
The level of contaminants and a small quantity of slag from both trenches confirmed that lead processing had almost certainly occurred in the immediate vicinity, and the limited ceramic evidence suggests that most activity ranged from the later 1st to 3rd century AD.
Previous investigations (carried out in the 1970s) had noted the death of livestock in the area as possibly due to contaminants disturbed by excavation.
Therefore an assessment of the Health and Safety implications was carried out prior to excavation.
Earlier contamination reports were obtained from Mendip District which revealed only 'standard background' contaminant levels in a service trench in the vicinity of Charterhouse Activity Centre.
Halcrow was commissioned to carry out a survey on the excavation area.
Laboratory analysis of ten auger samples taken from the site indicated the presence of lead (maximum level 54,000ppm) cadmium (maximum level 17ppm) and arsenic (maximum level 171ppm) and it was recommended that a number of precautions should be undertaken in the course of the dig.
These included a decontamination unit, the full use of personal protection equipment and the deployment of a portable X-ray fluorescence (XRF) analyser that was rented from Ashtead Technology Rentals.
The purpose of the XRF analyser was to retrieve real time readings of contaminant levels.
The analyser is a sophisticated, expensive analytical tool but the facility to rent such equipment renders high technology available to most budgets and is ideal for short-term projects such as this.
A portable analyser provides a valuable supplement to laboratory data because large numbers of results can be obtained quickly over a wide area, whereas laboratory results can take days to arrive and carry a significant cost.
As a result the number of samples obtained is kept to a minimum.
Richard Brown, senior project manager for Oxford Archaeology/archaeological co-ordinator for Time Team's Big Roman Dig, who supervised the excavations at Charterhouse, operated the XRF following brief but comprehensive training by Ashtead Technology Rentals.
Following completion of the project, Brown said: "The XRF analyser was very easy to use, the software was self explanatory and guided you very clearly through the analyser set up.
"It allowed for simple scrolling between readings and its filing for readings was very robust (ie no matter how times I accidentally prodded the wrong part of the PDA screen I never managed to wipe any information - extremely useful in the wind and rain with rubber gloves on)".
During excavation it became clear from the XRF analyser readings that contaminant levels below the topsoil were significantly higher than those recorded in the Halcrow survey and were also particularly high in archaeological features.
Levels recorded in archaeological features after the removal of topsoil peaked at 6176ppm for arsenic and 155ppm for cadmium.
Given the variation in contaminant levels between those recorded by Halcrow (on which health and safety mitigation measures were based) and those identified during the programme of work, it was considered inappropriate to continue excavation in the context of a live broadcast where the time constraints and inherent pressures ran contrary to a considered and formal process of re-assessment of the health and safety mitigation procedures.
Naturally, it was the source of great disappointment to everyone when the dig had to be terminated, but as Richard Brown reports: "Despite the severe limitations imposed on the excavations by contamination, the two small trenches combined with the extensive geophysical survey have gone some way in increasing our understanding of the chronology and character of the Roman settlement at Charterhouse".
Furthermore, the early closure of the site due to the pollutants in the soil may be indicative of why the site was abandoned.
A determination to extract the lead and silver in the Roman period may have outweighed the atrocity of the working conditions and the short life expectancy for those undertaking the extraction.
With the withdrawal of Roman protection in the 5th century, those who held power locally may have found it increasingly difficult to sustain the lead working in a drastically altered economy, where lead was available for scavenging in every Roman town and villa.
Oxford Archaeology will shortly be submitting draft publication text to English Heritage for review prior to publication.
