On site determination of trace metals in estuarine sediments by field-portable-XRF.

A portable x-ray fluorescence (XRF) spectrometer and mobile test stand have been employed to examine the feasibility of on site measurements of trace metals in estuarine sediment. In the laboratory, the instrument was able to detect the trace metals: As, Cr, Cu, Pb, Sn and Zn; and the geochemical proxy metals: Ca, Fe, K and Rb; in both fresh and freeze-dried surficial samples from the Tamar and Tavy estuaries, southwest England, that had been emplaced in polyethylene bags over the detector window. The presence of interstitial water in fresh samples acted as both a diluent of sediment mass and an attenuator of incident and fluorescent x-rays, resulting in measured (fresh weight) metal concentrations that were significantly lower than corresponding (dry weight) concentrations derived from dry analyses. Gravimetric correction for fractional water content (fw ~ 0.2-0.6) gave rise to results that were within 20% of those derived from dry analyses with the exception of K, whose relatively low energy fluorescent x-rays were subject to significant attenuation from the aqueous medium; further x-ray attenuation was observed for both K and Ca through the sample bag, thereby limiting the usefulness of the approach for these metals. A relationship between the concentration of Rb and fw in fresh samples suggests that Rb may be used as a proxy for interstitial water content through its covariance with sediment grain size. Accordingly, on site measurements of trace metals of sufficient fluorescent x-ray energies may be corrected empirically with respect to Rb in order to simultaneously account for variations in granulometry and mass contribution of water. On this basis, results from an axial transect of the Tamar and an intertidal transect in the Tavy were able to detect variations in trace metal concentrations that were consistent with known sources and geochemical behaviours.