Impact assessment of a recent road opening as recorded in nearby lacustrine sediments : local versus regional heavy metal pollution.

ARIES Sébastien^1.3, POLVE Mireille¹, JOUANNEAU Jean Marie²

¹ LMTG, UMR CNRS 5563, 38, rue des trente six ponts, 31400 Toulouse, France

² DGO, UMR CNRS UMR 5805, Avenue des facultés, 33405 Talence, France

³ e-mail : aries@lucid.ups-tlse.fr

Abstract

The aim of this study is to determine if an event such as a road opening in a non polluted area (protected mountain area) can have a significant influence in terms of metal pollution with respect to the global metal pollution. A road has been built in the middle of the 20^th century and borders two mountain lakes. Several cores have been collected from these two lakes. Here are the preliminary results obtained by analysing two cores collected in the central part of the two lakes.

Introduction

Lake Orédon is located in the Néouvielle Massif, Pyrénées mountains (France) at 1850 m height. This lake has a surface of 43 ha, a maximum depth of 59 m and a mean depth of 40m. Lake Aumar is located near Lake Orédon, at 2192 m height. The lake has a surface of 27 ha, a maximum depth of 24 m and a mean depth of 16 m. These tow lakes are natural lakes digged by the Néouvielle glacier during the Würm glaciation period. They have been converted to water supply reservoirs at the beginning of the 20^Th century. Then roads had been built for tourism and hydroelectric system maintenance purposes.

Methods

Several sediment cores were collected on summer 1999, in the basins of the two lakes. Cores have been sliced immediately after coring and stored in polycarbonate pétri boxes. Sediment samples were dried at ambient temperature in a hood under a laminar flux of filtered air and were grinded to 100 µm. Sample sediment mineralizations have been carried out in teflon beakers by adding suprapure (H₂O₂) and bi-distilled reagents (HF, HNO₃). The closed beakers were submitted to a microwave treatment (MDS 81 microwave oven, CEM). The were then diluted and analysed for total element concentrations by ICP-MS (Elan 6000, Perkin Elmer). The method has been validated for all the elements of interest (within 5% of the certified values 2s) by analysing several certified reference materials of marine (MAG-1, BCSS-1, MESS-2) and lacustrine (LKSD-1) sediment samples.

Lead isotope ratios have also been measured by ICP-MS. This measurement technique is now widely used and provides precise (0.2% RSD) and accurate results when dealing with ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb isotopes. Recent accumulation rates have been estimated by determining ²¹⁰Pb and ¹³⁷Cs activities by gamma spectrometry.

Results and discussion

Eh values indicate that waters are oxic in the two lakes. A sharp gradient of Eh has been observed at the sediment/water interface. Eh remains constant along the two cores indicating anoxic conditions. Furthermore, according to Mn and Fe concentrations profiles, no redox front can be observed in the sediment column : Mn/Ti and Fe/Ti ratios are constant in the Aumar sediment column and Mn and Fe concentrations are constant in Orédon sediment column).

Accumulation rates determined by ²¹⁰Pb profiles yield values of 1.7 mm per year for lake Aumar and 1.3 mm per year for lake Orédon. First results of ¹³⁷Cs activity, determined in the top samples of the Aumar core could indicate the 1963 and the 1986 (1 cm depth) events but only one peak is detected in the Orédon core. Further measurements will allow to determine absolute dating.

Figure 1 Ti, Fe and Pb relative concentrations versus depth in Lake Aumar

Figure 2 Pb Enrichment Factor normalised to Ti versus depth in Lake Aumar

Figure 3 ^206Pb/^207Pb versus depth in Lake Aumar

Figure 4 Fe and Pb relative concentrations versus depth in Lake Orédon

Figure 5 ^206Pb/^207Pb versus depth in Lake Orédon

On Figure 1 total concentrations of Ti, Fe, and Pb normalised to their bottom concentrations are showed. The observed enrichment in Ti, representing the lithogenic fraction, indicates change in sedimentation conditions which can be explained, owing to dating estimation, by the building of the dam at the beginning of the 20^th century.

When possible heavy metal profiles were expressed as enrichment factors to avoid possible dilution effects by organic matter (Particulate Organic Carbon can achieve concentrations of about 10% in several samples) and to estimate the non natural contribution of the trace metals. We did not observe any enrichment in Co, Ni, Cu, Zn, Cd as it could be expected (Boutron et al. 1991, Von Gunten et al. 1997) for industrial pollution. Conversely, slight Pb enrichment were observed and are presented in figures 2 for lake Aumar. An enrichment can be observed at 30 cm depth (dating estimation middle of the 19^th century) could indicate the beginning of the industrial revolution. This assertion could be confirmed by the enrichment of lead concentration observed at 25 cm depth (same dating estimation) as shown in figure 4.

Profiles of lead isotope ratios are presented in figure 3 for lake Aumar and figure 5 for lake Orédon. Difference between pre anthropogenic and present lead isotope ratio also indicate that the area is poorly contaminated by lead. Aumar lead isotope profile looks like some other European studies (Farmer et al. 1996, Monna et al. 1999, Weiss et al. 1999) with a decrease in ²⁰⁶Pb/²⁰⁷Pb at the top of the core during suggesting use of leaded gasoline from 1950.

For lake Orédon a relatively good anti correlation is observed between ²⁰⁶Pb/²⁰⁷Pb and lead profiles (figure 4 and 5), suggesting a major (unique ?) source of lead contamination.

These preliminary results indicate that (i) this area is poorly polluted and (ii) the only visible pollution is related to leaded gasoline. At this time we can not conclude on the role played by the road in the lead supply even if lake Aumar profiles plead for a global lead pollution. However, a more precise dating and further analyses on other cores situated nearer the road will permit us to assert if the road has a recordable impact in lacustrine sediments.

References

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Monna F., Dominik J., Loizeau J-L., Pardos M., Arpagaus P. (1999), Environmental Science and Technology 33 : 2850-2857.

Farmer J.G., Eades L.J., MacKenzie A.B., Kirika A., Bailey-Watts T.E. (1996), Environmental Science and Technology 30 :3080-2083.

Von Gunten H.R., Sturm M., Moser R.N. (1997), Environmental Science and Technology 31: 2193-3197.

Weiss D., Shotyk W., Appleby P.G., Kramers J.D., Cheburkin A.K. (1999), Environmental Science and Technology 33:1340-1352.