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Lead in ancient Rome's city waters

Lead in ancient Rome's city waters
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  Lead in ancient Rome ’ s city waters Hugo Delile a,b,1 , Janne Blichert-Toft b,c , Jean-Philippe Goiran d , Simon Keay e , and Francis Albarède b,c a Université Lumière Lyon 2, Centre National de la Recherche Scientifique-Unité Mixte de Recherche (CNRS UMR) 5600, 69676 Bron, France;  b Ecole NormaleSupérieure de Lyon, Université Claude Bernard Lyon 1, CNRS UMR 5276, 69007 Lyon, France;  c Department of Earth Science, Rice University, Houston,TX 77005;  d Maison de l ’ Orient et de la Méditerranée, CNRS UMR 5133, 69365 Lyon Cedex 7, France; and  e Archeology, Faculty of Humanities, Universityof Southampton, Southampton SO17 1BF, Great BritainEdited by Thure E. Cerling, University of Utah, Salt Lake City, UT, and approved March 19, 2014 (received for review January 3, 2014) It is now universally accepted that utilization of lead for domesticpurposes and water distribution presents a major health hazard.The ancient Roman world was unaware of these risks. How far thegigantic network of lead pipes used in ancient Rome compromisedpublic health in the city is unknown. Lead isotopes in sedimentsfrom the harbor of Imperial Rome register the presence of a stronganthropogenic component during the beginning of the CommonEra and the Early Middle Ages. They demonstrate that the leadpipes of the water distribution system increased Pb contents indrinking water of the capital city by up to two orders of magni-tude over the natural background. The Pb isotope record showsthat the discontinuities in the pollution of the Tiber by lead areintimately entwined with the major issues affecting Late AntiqueRome and its water distribution system. harbor geoarcheology  |  paleopollution  |  Late Holocene  |  ore provenance  | sedimentology S tatistics on demography, money supply and metal circulation,life and health standards, and many other social parametersrequired to understand modern history are largely missing fromthe written record of the ancient past. For example, the ap-parently simple question of how the population of ancientRome evolved is still unresolved (1, 2), prompting the design of indirect estimates (3). Another well-publicized problem illus-trating the lack of primary sources of accurate information isthe decade-old debate on Pb poisoning of the high society of Rome, either by lead water pipes or grape juice concoctionsprepared in lead cups (4 – 9). Here we focus on the condition of Pb in the public waters of ancient Rome. Lead is regarded asa powerful and ubiquitous indicator of the manufacturingstatus of a society. For example, a surge in Pb concentrations inthe Greenland ice-core record was correlated with the heightof the Roman Empire (10). Three out of the four existing Pbisotopes are rapidly modified by the radioactive decay of nat-ural uranium or thorium over geological time. The mining of ores from geologically diverse areas produces metallic Pb with variable isotopic abundances that depend on the tectonic ageand the Th/U and U/Pb ratios of the mining district. Arche-ologists interested in the provenance of artifacts routinely tapthis wealth of information (11). To explore how the supply of metals from all over the Roman world and their utilization may have affected the nearby environment of ancient Rome, thepresent work sets out to investigate the isotope compositionsof Pb in sediment cores from the Trajanic harbor basin atPortus, the maritime port of Imperial Rome, and the channelconnecting Portus with the Tiber (Canale Romano) (Fig. S1).Harbors are excellent sedimentary traps. The record of humanPb pollution from the time that the harbor basin was excavated(  ca.  112 AD) and well into the Middle Ages offers a new his-torical,  ca.  1,000 y-long perspective on the evolution of Pbreleased by Rome, its water distribution system, and the majordisruptive events that affected the life of the capital city andits harbor.In 42 AD Claudius started the construction of an open coastalport to compensate for the long-standing shortcomings of theexisting system for supplying Rome from the Mediterranean,notably the small scale of the harbor and anchorage facilities atOstia and the long route of communication with the principalmaritime port at Puteoli (Pozzuoli) on the Bay of Naples (12).The inland  ∼ 0.4 km 2 Trajanic basin, which was excavated in theearly years of the second century AD in response to the growingdemands of an expanding population in Rome, offered bothsafe mooring to sea-going merchant ships and immense ware-houses and other buildings (13 – 15). Communication between theClaudian and Trajanic basins was facilitated by an entrancechannel, into which the  ca.  9-m-long core TR14 was drilled. Upuntil the Middle Ages, the Trajanic basin was also accessed froma man-made branch of the Tiber (Fossa Traiana; what is now theFiumicino Canal) by means of the Canale Traverso. The trans-port of sand and silt sediments from this channel to the TrajanicHarbor has been attested to by sedimentological, geochemical,and ostracod analyses (16 – 18). The now filled-in Canale Romano, which ran past the southwestern side of the Trajanic basin to- ward the Tiber, was used to carry cargoes transshipped on to river-going craft bound for Rome (15). A 13-m-long core labeled CN1 was drilled into the sediments of the Canale Romano. The de-tailed sedimentology and geochemistry of core TR14 are givenelsewhere together with  14 C ages (18). Some  14 C dates likewise were obtained for core CN1 (Table S1). For reference and mod-eling purposes, the bedload of the modern Tiber between Romeand the Tiber delta was also sampled (Table S2), as were fivedifferent Roman Pb water pipes (  fistulæ ) collected in Rome anddating to between the first and the second centuries AD (Fig.S2). In all, 42 samples from TR14, 37 samples from CN1,6 samples from the Tiber bedload, and 10 samples from the fiveRoman  fistulæ  were measured for their Pb isotope compositionsat the Ecole Normale Supérieure de Lyon. Significance Thirty years ago, Jerome Nriagu argued in a milestone paperthat Roman civilization collapsed as a result of lead poisoning.Clair Patterson, the scientist who convinced governments toban lead from gasoline, enthusiastically endorsed this idea,which nevertheless triggered a volley of publications aimed atrefuting it. Although today lead is no longer seen as the primeculprit of Rome ’ s demise, its status in the system of waterdistribution by lead pipes ( fistulæ ) still stands as a major publichealth issue. By measuring Pb isotope compositions of sedi-ments from the Tiber River and the Trajanic Harbor, the presentwork shows that  “ tap water ”  from ancient Rome had 100 timesmore lead than local spring waters. Author contributions: H.D., J.B.-T., J.-P.G., S.K., and F.A. designed research; H.D., J.B.-T., andJ.-P.G. performed research; J.B.-T. contributed new reagents/analytic tools; H.D. and F.A.analyzed data; and H.D., J.B.-T., and F.A. wrote the paper.The authors declare no conflict of interest.This article is a PNAS Direct Submission. 1 To whom correspondence should be addressed. E-mail: hdelile@gmail.com.This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1400097111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1400097111 PNAS Early Edition  |  1 of 6      E     N     V     I     R     O     N     M     E     N     T     A     L     S     C     I     E     N     C     E     S     A     N     T     H     R     O     P     O     L     O     G     Y  Results and Discussion The TR14 core can be broken down into successive sedimentary units corresponding to different time slices: ( i ) preharbor up to  ca.  100 AD, ( ii ) Early Empire up to  ca.  250 AD, ( iii ) LateEmpire up to  ca.  500 AD, ( iv ) Early Middle Ages up to  ca.  800 AD, and (  v ) Late Middle Ages (see the  “ Historical period age-depth model ”  columns in Fig. 1). Age boundaries between unitsmay be uncertain by up to 100 y. Silts and sands dominate sed-iment mineralogy. The preharbor sequence attests to depositionin an environment of deltaic progradation (19). The constructionof the harbor brings about a sharp sedimentological changeand marks the beginning of the harbor mud deposits. A well-stratified ∼ 50-cm-thick layer within the Early Empire deposits (753 с m) displaying well-preserved shells does not appear in othercores and may signal local dredging (18, 20). The layer corre-sponding to the Early Middle Ages contains more carbonatesand ostracods of brackish affinity than the rest of the core. At thetop, the sediments from the Late Middle Ages horizon arecharacteristic of flood plain deposits (17, 18).In Fig. 1, two different representations of lead isotope varia-tions in TR14 and CN1 have been used: First (Fig. 1  A ), theconventional raw isotopic ratios in which  206 Pb is kept as thedenominator; and, second (Fig. 1  B ), a derived set of geologically informed parameters which will now be explained. In compliance Fig. 1.  Chronostratigraphic evolution of (  A ) the raw isotopic ratios of the cores TR14 and CN1 and ( B ) the geological parameters  T  mod ,  238 U/  204 Pb ( μ ), and 232 Th/  238 U ( κ ) derived from the raw isotope ratios as described in the main text. The different time slice boundaries (indicated with black arrowheads andhighlighted by alternating light and dark gray shading for better visibility) derived from the age-depth model of core TR14 (18) coincide with major Pbisotope compositional shifts prominent in both cores. Hercynian Pb shows up during the Early Roman Empire and Early Middle Ages. In contrast, low-Th/Unatural Pb dominates both the preharbor sequence and the flood plain deposits at the top of TR14. 2 of 6  |  www.pnas.org/cgi/doi/10.1073/pnas.1400097111 Delile et al.   with literature (e.g., ref. 21), we searched Pb isotope databasesfor potential sources of ores matching the Pb isotope composi-tions of archeological artifacts and sediment samples (Fig. S3).In addition, the geological province to which a particular Pbsample belongs can often be inferred from a conversion of itsisotope compositions into a set of geologically informed parame-ters, the Pb model age  T  mod , and the apparent  238 U/  204 Pb ( μ ) and 232 Th/  238 U ( κ ) ratios (e.g., refs. 22 – 24).  T  mod  reflects the tec-tonic age of the crustal segments in which ore deposits occur. InEurope, crustal segments of Alpine ages (30 – 120 Ma) contrast withHercynian (240 Ma and older) and early Paleozoic ( > 450 Ma)segments.  μ  and  κ  are parameters that tend to increase withcrustal depth. Typically,  κ  is higher in crustal segments that losttheir shallow levels by erosion or tectonic denudation, such asin Iberia, southern France, and the eastern Alps. Fig. S4 showsthat these parameters can be used to divide Europe into co-herent regions, which justifies using  T  mod ,  μ , and  κ  for provenancepurposes (24 – 26).  T  mod ,  μ , and  κ  in turn provide a rapid char-acterization of the geological environment in which the oresformed. Ores formed by remobilization of metal from the un-derlying basement and hosted in sediments, such as MississippiValley type deposits, may to some extent challenge a simpleinterpretation of model ages. Fig. S4 shows, however, that,overall, the connection between Pb model ages and the tectonicage of the local crystalline basement remains very strong. Thebroad relationship between  T  mod ,  μ , and  κ  tectonic provinces iscompelling and holds particularly true for southern Europe (27). Fig. 2.  (  A ) Lead isotope ratios ( 207 Pb/  206 Pb vs. 208 Pb/  206 Pb) and ( B ) geological parameters ( κ  vs. T  mod ) for the leached samples from cores TR14 (red)and CN1 (blue), the modern Tiber bedload (yellow),and Rome  fistulæ  (green). The gray fields corre-spond to the light and dark gray shaded time slicebands of Fig. 1 and overlap the samples from coreTR14 in accordance with the respective historicalperiods. The two mixing lines (gray dashes) connect,respectively,  α  and  β  on the one hand, and  α ′  and  α ″ on the other. The  α  end-member corresponds tounpolluted Tiber water and is composed of theMediterranean outflow water ( α ″ , blue ellipse) (30)and volcanic rocks from the Alban Hills ( α ′ , orangeellipse) (28, 29).  β  is the anthropogenic end-member. Delile et al. PNAS Early Edition  |  3 of 6      E     N     V     I     R     O     N     M     E     N     T     A     L     S     C     I     E     N     C     E     S     A     N     T     H     R     O     P     O     L     O     G     Y  Principal component analysis of the 3D Pb isotopic data showsthat  > 99% of the variance is accounted for by two principalcomponents and, therefore, that the data plot in a plane span-ning any 3D space of Pb isotopic ratios. Cores TR14 and CN1define indistinguishable planes, which allows the Pb isotopedata to be merged into a single dataset. As illustrated by the 208 Pb/  206 Pb vs.  207 Pb/  206 Pb plot of Fig. 2  A , the isotope com-position of Pb in leachates form two coplanar alignments, whichare most straightforwardly accounted for by the mixing of com-ponents of different srcins.In Fig. 2  A  and  B , the component labeled  α  located at theintersection (the kink) of the two trends is ubiquitous in bothcores including the preharbor and Late Middle Ages deposits. Itis also an end-member in plots of Pb isotope ratios from leachingresidues (Fig. S5  A  and  B ). This component therefore reflectsPb naturally present in Tiber water. It can itself be broken downinto a mixture of two local low- 207 Pb/  206 Pb sources, the compo-nent  α ′  srcinating from the recent volcanic rocks of the AlbanHills (28, 29), and the component  α ″ , which is very similar to Pbdissolved in modern Mediterranean seawater (30) and releasedby erosion of recent limestones from the Apennines.The anthropogenic nature of the third component  β  becomesapparent when plotting Al/Pb (data from ref. 18) as a functionof   207 Pb/  206 Pb (Fig. 3). The  α − β  alignment intersects the  x  axis atthe value of   207 Pb/  206 Pb of the  fistulæ  (Al/Pb  ≈ 0), which showsthat the contaminant is essentially pure lead from Al-free andtherefore suspension-free water. As with raw isotopic ratios, aplot of   κ  vs.  T  mod  (Fig. 2  B ) shows a bundle of alignments con-sistent with the observations from isotopic ratios. The alignmenttrending toward high  207 Pb/  206 Pb and old model ages reveals thatPb component  β  is of Hercynian (or Variscan;  T  mod  ≈ 250 Ma)affinity with rather high  κ  values. Hercynian Pb is absent frompeninsular Italy, and the Apennines formed less than 20 Ma ago(31) from recent sediments and volcanic rocks. The Pb component β  therefore, clearly being foreign to peninsular Italy, shouldrather be traced to southwestern Spain, the Massif Central of France, the eastern Alps, Eifel in Germany, the Pennines in Eng-land, and Macedonia (Fig. S3). Among these potential sources,only some of them are consistent with the known maritimefreight routes, which are punctuated by frequent shipwrecksloaded with Pb ingots (32 – 34), and with the known period andoutput of mine exploitation during the Late Republican Periodand the Early Roman Empire (e.g., refs. 21 and 35). An un-expected observation is the lack of signal from the productiveand geologically young mining areas of the Spanish Betics(Carthagene). It is most likely that the Pb used for water man-agement in Rome had been mined in the Spanish Sierra Morena,the English Pennines, the German Eifel, or the FrenchMassif Central.The isotope composition of component  β  is remarkably con-sistent with the data on four of the five lead  fistulæ  analyzed inthis work. Component  β  is still conspicuous in the leachates fromthe modern Tiber bottom sediments, which suggests that to thisday old Pb pollution still permeates the bedload sediment. Theanthropogenic srcin of the Hercynian Pb component  β  isalso attested to by the comparison of leachates and residues: 207 Pb/  206 Pb is, with the sole exception of the deepest sample,higher in leachates than in residues (Fig. 4). Leaching there-fore releases older labile Pb from a solid residue of much youngergeological age.Lead pollution of the Tiber River can be evaluated in a simple way by using φ  fist = ð 207 Pb = 206 Pb Þ  riv − ð 207 Pb = 206 Pb Þ  nat ð 207 Pb = 206 Pb Þ  fist − ð 207 Pb = 206 Pb Þ  nat ;  where  φ  fist  is the fraction of Pb in river water derived from Pb  fistulæ . It has been estimated that the proportion  f   fist  of Tiber water running through the aqueducts was about 3% at the peak of the Roman Empire (36). It can therefore be deduced that  fistulæ  increased Pb in the water distributed in Rome over thenatural level by a factor of about 40, 14, and 105 for the Early Empire, Late Empire, and High Middle Ages, respectively ( SI  Materials and Methods ). Although the value of   f   fist  is only aneducated guess pertinent to a given period, using different num-bers does not significantly affect the relative levels of Pb pollu-tion deduced. These levels are maximum values because they characterize the final output of the water system to the Tiber while most Roman citizens would have used drinking water that was tapped, whether legally or illegally, all along the water distribu-tion system (36). The inferred increases of Pb in the water of theRoman distribution system unquestionably attest to general leadpollution of Roman drinking water but the Pb concentrations atissue are unlikely to have represented a major health risk (9).Evidence bearing on the timeline of anthropogenic pollutionin the Rome area can be derived from the sequence of Pb iso-tope characteristics (Fig. 2). Lead in preharbor sediments is of  Fig. 3.  Al/Pb vs.  207 Pb/  206 Pb showing the TR14 (red)and CN1 (blue) core leachates, the modern Tiberbedload (yellow), and the Rome  fistulæ  (green). Thebeige field corresponds to the anthropogenic com-ponent characterized by the  fistulæ . Symbols andparameters are as in Fig. 2. This plot shows that theAl-free, and therefore suspension-free, water com-ponent has the same Pb isotope composition as the fistulæ  and therefore corresponds to clear waterfrom the city water distribution system. 4 of 6  |  www.pnas.org/cgi/doi/10.1073/pnas.1400097111 Delile et al.  natural srcin (trend  α ′ − α ″ ). Excavation of basin deposits dating tothe period of the Early Roman Empire coincides with both a surgeof Hercynian Pb in leachates and a dramatic drop in the isotopiccontrast between the residue and the leachate (trend  α – β ) (Fig. 4). At this time, the Roman Empire reached the height of its conquests,especially in western European territories such as Britain (Fig.S3). The isotopic contrast between the fractions rapidly dimin-ishes, although quite smoothly, from the Early to the Late Ro-man Imperial periods. This change is largely accounted for by the dramatically smaller contribution of anthropogenic Pb toleachates and therefore by a lesser pollution of Tiber water. Oneinterpretation of this may be a redirection of spring water away from the lead pipes of Rome, in some way related to thecontroversial decline of the population (3, 37) or to a poorly documented deterioration of the water distribution system. At the end of the Late Roman Empire and throughout theEarly Middle Ages section of TR14, the isotopic difference be-tween the leachate and the residue bounces back and the pres-ence of a rather homogenous anthropogenic component rich inHercynian Pb again becomes prevalent in leachates (Figs. 2 and4). The discontinuity appears in both the TR14 and CN1 cores(Fig. 1). The end of the Early Middle Ages section ( ∼ 800 AD) isalso brutal and signals the return of uncontaminated Pb in theTiber (trend  α ′ − α ″ ). The persistence of Hercynian Pb in thebedload of the modern Tiber nevertheless indicates that centu-ries of contamination, possibly in the form of Pb carbonates, lefta lasting imprint on the river sediments.The consistency of the Pb isotope results from the CN1 core, which is expected to carry a straightforward Tiber signal, with thosefrom the TR14 core is rather good despite the latter beingsusceptible to both harbor activity and input of water from thePortus aqueduct, which has its source in the vicinity of modernPonte Galeria. Both cores reflect the presence of a Hercynian end-member and coincide on the timing of major isotopic shifts.  κ  valuesin the CN1 core may, however, be marginally higher than those inTR14, especially during the Early Empire. It is unfortunate that thePb isotope database on pipes used for water distribution is still toolimited to identify such small differences with confidence.The extensive nature of the harbor installations calls for ad-ditional work beyond the 95 samples of sediment core, bedload,and Pb pipes from Portus and the Tiber analyzed in this study todemonstrate unambiguously that the observed discontinuities inthe Pb isotope and overall geochemical record correspond tocatastrophic disruptions of Portus activity. Although the coastalposition of the port leaves the Trajanic basin vulnerable to river vagaries and maritime hazards, the lack of coarse gravels andsediment sorting, combined with the good preservation of thedelicate ostracod shells, are strong evidence against exceptionalfloods, storms, and tsunamis. The age-depth model (18) is cer-tainly evocative of some critical dates of Roman history. Asspeculated by Delile et al. (18) based on the  14 C record of TR14and adjacent cores, transitions between units may be correlated with the initial excavation of the Trajanic basin (by   ca.  112 AD),the continued use of the port during the third century (  ca.  250 AD), the gradual fortification and contraction of the port in thelater fifth and earlier sixth centuries (  ca.  500 AD), and thetransition to the post-Byzantine period. The later fifth andsixth century transition is coeval with Belisarius ’  fixing of thedecommissioned aqueducts of Rome (38) at the end of theGothic Wars (535 – 554 AD). Byzantine repairs of the waterdistribution system may have remobilized massive amounts of corrosion products from abandoned lead pipes in which watermay have stagnated for protracted lengths of time. Although acausal relationship cannot be formally demonstrated, the dis-continuities in the cores at Portus seem contemporaneous withhistorically documented events such as the struggle for the controlof the port between Gothic and Byzantine forces (536 – 552 AD)and the damages inflicted to the water distribution system duringthe Arab sack of Rome in the mid-ninth century. Further work is needed to learn whether the causes of Portus ’  demise werenatural, with the harbor finally falling into disuse on account of flood plain deposits, possibly after the major floods of 856 AD(39 – 41), or a consequence of military events (39). Conclusions This work has shown that the labile fraction of sediments fromPortus and the Tiber bedload attests to pervasive Pb contaminationof river water by the Pb plumbing controlling water distributionin Rome. Lead pollution of   “ tap water ”  in Roman times is clearly measurable, but unlikely to have been truly harmful. The dis-continuities punctuating the Pb isotope record provide a strongbackground against which ideas about the changing character of the port can be tested. Materials and Methods After removal of the coarse gravel fraction, 500 mg of sample were crushedand treated with chloroform to remove most of the abundant organicfraction. The residue was rinsed and leached in dilute HBr. Because lead pipecorrosion products, such as Pb carbonates (42, 43), were suspected to bepresent in the sediments and carry a signal from aqueducts, no attempt wasmade at using the more specific protocols developed to selectively extracthydroxide coatings (e.g., ref. 44). Lead from the leachates was purified on anion exchange resin using HBr as eluent of the sample matrix and HCl to elute Pb.The amounts of Pb extracted were large ( > 1  μ g) and orders of magnitudeabove the blank of the procedure ( ∼ 20 pg). Lead isotope compositions wereanalyzed by multicollector inductively-coupled plasma mass spectrometry onboth the residues and the leachates of the samples from TR14 and the resultswere so systematic that no further attempt was made to also measure theresidues from CN1 (Table S2). Fig. 4.  TR14 downcore behavior of the isotopic contrast ( Δ Pb isotope ratios)between the residue and the leachate. Increasing distance from the zerodashed line (left side of the figure) indicates increasingly predominant an-thropogenic impact, which is strongest during the Early Roman Empire andthe Early Middle Ages. Time slice model as in Fig. 1. Delile et al. PNAS Early Edition  |  5 of 6      E     N     V     I     R     O     N     M     E     N     T     A     L     S     C     I     E     N     C     E     S     A     N     T     H     R     O     P     O     L     O     G     Y
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