Department Water Resources and Drinking Water

Soils at the locality Erzmatt near the village of Buus in the Swiss Jura Mountains are part of the oxidation zone of a smallhydrothermal Fe–As–Tl ore occurrence in dolomite rock. The soils are characterized mostly by near-neutral pH values, buffered bythe dolomite and limestone bedrock. The concentrations of As and Tl in the soil profiles increase with depth and attain maximumvalues of ∼20 000 mg/kg for As and ∼10 000 mg/kg for Tl. Using a combination of mineralogical, geochemical, and spectroscopictechniques, we investigated soil samples from the Erzmatt with respect to the characterization of As- and Tl-bearing secondaryminerals using total digestions, powder X-ray diffraction, X-ray absorption spectroscopy (XAS), polarized light microscopy, electronmicroprobe analysis (EMPA), scanning and transmission electron microscopy (SEM, TEM). Arsenic K edge XAS data reveal littlevariation in bulk soil As speciation and suggest that most As is arsenate bound to iron oxides. Samples with As contents exceeding∼1 wt% As₂O₅ contain abundant crystals of bariopharmacosiderite [Ba_0.5Fe₄(OH)₄(AsO₄)₃·nH₂O] in the voids in the soil matrix. Lesscommonly, Ca–Fe arsenates (arseniosiderite) were identified. An earlier study also identified Tl(I)-containing jarosite and avicennite(Tl₂O₃) as secondary Tl-bearing minerals. The EMP analyses reveal the degree of jarosite–dorallcharite [(K,Tl)Fe₃(SO₄)₂(OH)₆] solidsolution formation, and indicate the presence of Tl₂O₃ with variable contents of As and P. Using electron diffraction, Tl₂O₃ with lowAs content was positively identified as avicennite. The platy morphology of the Tl₂O₃ aggregates is not compatible with the cubicmorphology of avicennite, indicating that they may be pseudomorphs after an unknown primary mineral, possibly carlinite, Tl₂S. Theidentification of bariopharmacosiderite, jarosite and avicennite in near-neutral soils points to their stability at near-neutral soil pH (incontrast to scorodite) and indicates their potential for the storage and retention of toxic As and Tl in mine wastes and contaminatedenvironments. The mineralization on the Erzmatt could be an example of a geochemical anomaly similar to the famous Lengenbachdeposit (Switzerland) before the metamorphic overprint at Lengenbach.

Chlorine dioxide (ClO₂) is commonly used as an alternative disinfectant tochlorine in drinking water treatment because it produces limited concentrations of halogenatedorganic disinfection byproducts. During drinking water treatment, the primary ClO₂ byproductsare the chlorite (50−70%) and the chlorate ions (0−30%). However, a significant portion of theClO₂ remains unaccounted for. This study demonstrates that when ClO₂ was reacting withphenol, one mole of free available chlorine (FAC) was produced per two moles of consumed ClO₂. The in situ formed FACcompleted the mass balance on Cl for inorganic ClO₂ byproducts (FAC + ClO₂¯+ ClO₃¯). When reacting with organic matterextracts at near neutral conditions (pH 6.5−8.1), ClO₂ also yielded a significant amount of FAC (up to 25%). Up to 27% of thisin situ formed FAC was incorporated in organic matter forming adsorbable organic chlorine, which accounted for up to 7% ofthe initial ClO₂ dose. Only low concentrations of regulated trihalomethanes were produced because of an efficient mitigation oftheir precursors by ClO₂ oxidation. Conversely, dichloroacetonitrile formation from ClO₂-induced generation of FAC washigher than from addition of FAC in absence of ClO₂. Overall, these findings provide important information on the formationof FAC and disinfection byproducts during drinking water treatment with ClO₂.

A source-to-tap evaluation of the origin and fate of chloramination N-nitrosodimethylamine (NDMA) precursorsat 21 full-scale drinking water plants was conducted. Upstream wastewater discharges accountedfor (on a median basis) ∼16 ng L⁻¹ NDMA formation potential (FP). A correlation between concentrations ofthe artificial sweetener sucralose (wastewater tracer) and NDMA FP was found within certain watersheds,with increased river flow decreasing sucralose and NDMA FP concentrations by diluting the wastewaterdischarges. The polymers polydiallyldimethylammonium chloride (polyDADMAC) and polyamine contributed(median) 6 and 14 ng L⁻¹ of NDMA FP to coagulated water (which was pre-chloraminated in twocases), respectively. Biofiltration increased NDMA FP by (median) 6 ng L⁻¹; biofiltration tended to increaseprecursor loading rather than reduce it. Although wastewater and polymers are known sources of precursors,biofilters as a source of precursors was an important finding. Ozonation of raw or settled water waseffective at destroying NDMA precursors (median 34%). As free chlorine exposure increased (from ≤3 minto >1 h), NDMA formation in the chloraminated distribution system decreased (from median removal of21% to 90% of the NDMA FP sampled prior to chlorination). For either oxidant, precursor abatement wastypically higher at pH ∼8–9 than at 7. Riverbank filtration, and powdered and granular activated carbon removed(median) 64, 47, and 64% of watershed-derived NDMA precursors, respectively. Each was able toremove NDMA FP better than that of the bulk total organic carbon. Granular activated carbon did not appearto be effective at removing polyDADMAC-derived precursors added during coagulation. The contributionof different watershed or in-plant sources to NDMA precursors varies by plant and over time and,depending upon precursor sources, different in-plant treatment strategies can effectively control NDMAformation.