Department Water Resources and Drinking Water

Mitigation of undesired byproducts from ozonation of dissolved organic matter (DOM) such as aldehydes and ketones is currently hampered by limited knowledge of their precursors and formation pathways. Here, the stable oxygen isotope composition of H₂O₂ formed simultaneously with these byproducts was studied to determine if it can reveal this missing information. A newly developed procedure, which quantitatively transforms H₂O₂ to O₂ for subsequent ¹⁸O/¹⁶O ratio analysis, was used to determine the δ¹⁸O of H₂O₂ generated from ozonated model compounds (olefins and phenol, pH 3-8). A constant enrichment of ¹⁸O in H₂O₂ with a δ¹⁸O value of ∼59‰ implies that ¹⁶O-¹⁶O bonds are cleaved preferentially in the intermediate Criegee ozonide, which is commonly formed from olefins. H₂O₂ from the ozonation of acrylic acid and phenol at pH 7 resulted in lower ¹⁸O enrichment (δ¹⁸O = 47-49‰). For acrylic acid, enhancement of one of the two pathways followed by a carbonyl-H₂O₂ equilibrium was responsible for the smaller δ¹⁸O of H₂O₂. During phenol ozonation at pH 7, various competing reactions leading to H₂O₂ via an intermediate ozone adduct are hypothesized to cause lower δ¹⁸O in H₂O₂. These insights provide a first step toward supporting pH-dependent H₂O₂ precursor elucidation in DOM.

Recent studies suggested that long-lived photooxidants (LLPO), which are reactive intermediates formed during irradiation of dissolved organic matter (DOM), may consist of phenoxyl radicals derived from phenolic moieties of the DOM. Besides the well-studied excited triplet states of chromophoric DOM (³CDOM*), LLPO presumably are important photooxidants for the transformation of electron-rich contaminants in surface waters. The main objective of this study was to further test the potential role of phenoxyl radical as LLPO. Suwannee River fulvic acid (SRFA) as a model DOM was pre-oxidised using the phenol-reactive oxidants chlorine and ozone, followed by its characterization by the specific UV absorption at 254 nm (SUVA₂₅₄), the ratio of absorbance at λ = 254 nm and λ = 365 nm (E2:E3), and the electron donating capacity (EDC). Subsequently, the photoreactivity of pre-oxidized SRFA was tested using 3,4-dimethoxyphenol (DMOP) as a LLPO probe compound at two initial concentrations ([DMOP]₀ = 0.1 and 5.0 μM). Linear inter-correlations were observed for the relative changes in SUVA₂₅₄, E2:E3, and EDC for increasing oxidant doses. Pseudo-first-order transformation rate constants normalized to the changing SRFA absorption rate (i.e., and for 0.1 and 5.0 µM, respectively) exhibited the following distinct trends: The LLPO-dominated ratio decreased with increasing oxidant dose and with decreasing SUVA₂₅₄ and EDC, while the ³CDOM*-dominated ratio positively correlated with E2:E3. Finally, it was concluded that precursors of ³CDOM* and LLPO are chemically modified differently by pre-oxidation of DOM, and LLPO precursors likely consist of phenolic moieties of DOM, suggesting phenoxyl radicals as LLPO.

Background: Changes in climate and anthropogenic activities have made water salinization a significant threat worldwide, affecting biodiversity, crop productivity and contributing to water insecurity. The Horn of Africa, which includes eastern Ethiopia, northeast Kenya, Eritrea, Djibouti, and Somalia, has natural characteristics that favor high groundwater salinity. Excess salinity has been linked to infrastructure and health problems, including increased infant mortality. This region has suffered successive droughts that have limited the availability of safe drinking water resources, leading to a humanitarian crisis for which little spatially explicit information about groundwater salinity is available.
Methods: Machine learning (random forest) is used to make spatial predictions of salinity levels at three electrical conductivity (EC) thresholds using data from 8646 boreholes and wells along with environmental predictor variables. Attention is paid to understanding the input data, balancing classes, performing many iterations, specifying cut-off values, employing spatial cross-validation, and identifying spatial uncertainties.
Results: Estimates are made for this transboundary region of the population potentially exposed to hazardous salinity levels. The findings indicate that about 11.6 million people (∼7% of the total population), including 400,000 infants and half a million pregnant women, rely on groundwater for drinking and live in areas of high groundwater salinity (EC > 1500 µS/cm). Somalia is the most affected and has the largest number of people potentially exposed. Around 50% of the Somali population (5 million people) may be exposed to unsafe salinity levels in their drinking water. In only five of Somalia's 18 regions are less than 50% of infants potentially exposed to unsafe salinity levels. The main drivers of high salinity include precipitation, groundwater recharge, evaporation, ocean proximity, and fractured rocks. The combined overall accuracy and area under the curve of multiple runs is ∼ 82%.
Conclusions: The modelled groundwater salinity maps for three different salinity thresholds in the Horn of Africa highlight the uneven spatial distribution of salinity in the studied countries and the large area affected, which is mainly arid flat lowlands. The results of this study provide the first detailed mapping of groundwater salinity in the region, providing essential information for water and health scientists along with decision-makers to identify and prioritize areas and populations in need of assistance.