In groundwaters

Metal speciation studies in groundwaters are still rather scarce when compared to those in surface waters, mainly due to the lack of macroorganisms, and of non-equilibrium conditions, making it difficult to assess which metals are truly bioavailable. Thus, health criteria developed for metals in groundwaters to be used as drinking water are still based on total concentrations. However, there is an increasing number of studies concerned with the rehabilitation of metal-contaminated sites (soils, groundwaters), where speciation provides the information needed to find proper technical measures, to reduce metal mobility and bioavailability. This type of research also considers site characteristics and their influence on long-term metal mobility, e.g. to promote the natural attenuation capacity in contaminated groundwater aquifers.

Information on the speciation of metals in groundwaters is still rather scarce when compared to surface waters. This may be partly because of the fact that bioavailability aspects seem to be of minor importance due to the lack of macroorganisms. However, we know today that groundwater systems (aquifers) are a vital habitat for many microorganisms, which can interact in a still rather unknown way with abiotic groundwater parameters and the speciation of metals. We also know that solubility and partitioning reactions of trace metals in groundwaters are more characterized by long-term, non-equilibrium conditions, which make it difficult to describe or predict the particular chemical form, in which metals may occur, when site-characteristics change. For this reason, current health criteria set for metals in groundwater are still based on total concentrations. While one of the main focus of previous groundwater studies was more directed in assessing the influence of acidification on release and fate of trace metals in rather uncontaminated systems, e. g. from natural soils, aquifers or tubing materials, current efforts seem more dedicated to establish the impact of metals released from contaminated sites or landfills into groundwaters (see recent studies published in Environmental Science & Technology). There is indeed an overwhelming number of research in the scientific literature now describing the capacity of site-specific properties and conditions in groundwater aquifers, to naturally attenuate the effect of toxic metals, and technologies to strengthen these natural processes by particular on-site engineering measures. As we know that metals occur in different forms, which respond differently to distinct physico-chemical treatments and situations, there is an increasing need to take the true species composition of metals in groundwaters into account, when assessing the natural attenuation potential of soils, or when designing proper technical rehabilitation measures to limit or counteract groundwater contamination (see also section 5.5.7).

As an example, Santos et al. (2001, 2002) recently studied the distribution and speciation of trace metals in groundwater wells along the Guadiamar river (Spain), two years after the "Aznalcollar" mine tailing spill, to follow up the environmental impact of spill-induced metal releases. The use of chemical extraction and anodic stripping voltametry (see section

5.4.4) proved that Cd and Zn in the contaminated soil occurred in available forms (i. e. labile and H+-exchangeable), in contrast to Pb and Cu, which existed as less available, strongly inert species. Metals in groundwater wells more far away from the spill were mainly associated with suspended matter and dissolved organic material, while in wells close to the accident an increase of exchangeable metals especially for Zn was observed. The results were consistent with what was expected, i. e. that newly formed metal species are more labile bound than species already present in groundwater systems.

Jensen et al. (1999) investigated the behaviour of trace metals in metal-spiked anaerobic leachate polluted groundwater samples at concentrations within ranges typical for landfill leachates. They found a highly varying part of the added "dissolved" metals (Cd, Cu, Pb) bound to colloidal and organic fractions, except for Zn, which occurred mainly associated with inorganic (carbonate) fractions, and as free metal ion. Also Ni was present, like Zn, as a carbonate complex. As most of these metals have been sampled as dissolved fraction (<0.45 ^m), the results showed that their behaviour in the environment may actually differ from what is tradionally believed as truly dissolved metal phase.

To investigate the influence of aquifer material characteristics on the partitioning (and hence mobility) of Cu and Zn, Christensen et al. (2000) established ^-values for these metals by means of batch experiments for sandy aquifer samples at realistic environmental concentrations. The correlation between experimental ^-values and aquifer material characteristics showed a good agreement with pH at values between 5.3 and 8.9. But the observed mobility of Cu and Zn in the sandy aquifer seemed restricted to pH values < 6, according to the measured ^-values.

The study of Christensen and Christensen (2000) is an example to demonstrate the limitations we are faced when using geochemical speciation codes. The authors tried to determine the occurrence and formation of metal-DOC complexes at different DOC and pH levels in a leachate-polluted groundwater by means of a resin equilibrium method, and to compare results to simulations done by two geochemical speciation models ('WHAM' and 'MINTEQA2'). While the WHAM model gave a good prediction of the Cd-and Zn-DOC complexation at pH values between 5 and 8, and likewise for Ni at pH 5 and 7, the model overestimated Ni-DOC complexation at pH 8. In contrast, the MINTEQA2 predicts no pH-dependence of the metal-DOC complexation and was for this reason in poor agreement with the experimental results. Instead, the authors estimated the relationship existing between the conditional complex formation constant (log Kcond) and pH for each particular metal. In using this relationship as an alternative to estimate metal complexation by DOC, it was stated that a deviation in the calculated free metal ion activity up to a factor of 2 may occur.

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