Richard Jones and Brendan Derham from the University of Glasgow along with Roger Doonan from the University of Sheffield have conducted a small-scale geo-chemical survey over specific tracts of the plateau in an effort to detect signs of different activities (residential, industrial, agricultural) in ancient times. The selection of the tracts for geo-chemical survey was based on the results of the surface and geo-physical surveys. NP31 and NP104 are at the heart of the residential blocks of the city with the latter showing signs of a possible sanctuary. Geophysical survey over both tracts has revealed the outline of the insulae and the streets, and even inner divisions within specific insulae. SP21 and SP22 are at the heart of the area of ceramic production as betrayed by the number of wasters and ceramic kiln debris observed on the surface. In addition, geophysical survey over part of SP22 has produced spots with high magnetic readings, tentatively interpreted as the actual kiln sites. UP77 showed higher ceramic concentrations with relation to its surroundings and may represent the site of an installation, probably of agricultural or pastoral nature outside the city proper (but still within the city walls). Geochemical survey of sections of these tracts has generally followed the grid of the surface survey with squares of 20 by 20 m, where sub-surface soil cores, approximately 20 cm deep, were taken every 5 m (except in UP77 where they were taken every 10 m). Analyses were undertaken on the cores using a portable XRF (X-ray fluorescence) manufactured by Thermo Scientific NITON XL3t analyzers. A helium purge was included to enable the detection of lighter elements, such as phosphorous, that are normally obscured by atmospheric interference.
Geochemical survey of NP104 produced the most meaningful results: the main feature of this area was the elevated phosphorous and potassium concentrations in the spaces associated with the interior of the insulae the streets themselves having lower levels similar to background. This indicates the on-site disposal of human and animal waste in the vicinity of the individual buildings. Elevated levels of metals such as zinc and, to some extent, copper were also associated with the buildings, as was the presence of sulphur. It was re-assuring to notice that elevated levels of phosphorous and heavy metals were not recorded along the line of the ancient ‘street’. On the contrary in the other tracts the levels of all significant elements fluctuated around normal ‘background’ levels. This is rather surprising in the industrial district (SP21 and SP22) where the likely presence of ceramic kilns would have been expected to yield higher concentrations (with relation to the background) of phosphorous and other heavy metals.
Overall the significant contrast in phosphorous and potassium levels between the transient farmstead and the settled urban area clearly demonstrates the ability of the technique to locate significant and enduring residential areas and the associated waste and distinguish them from more transient activities.