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Dalkmann, P. (2014): Fate and natural attenuation of pharmaceuticals during long-term wastewater irrigation in Central Mexico.  137 S.,  19 Abb., 20 Tab.



Wastewater reuse for irrigation and artificial groundwater recharge is increasingly practiced worldwide to improve the production of food and to alleviate water shortages. However, the concomitant release of human pharmaceuticals, pathogens, and resistance determinants with wastewater poses a potential risk for environment and human health. Little is known regarding the processes and rates that control the distribution and accumulation of wastewater-derived pharmaceuticals in the environment, particularly such information is missing for prolonged irrigation with untreated wastewater as it is common, for instance, in the peri-urban agriculture of most tropical and subtropical megacities. The aim of my work was to elucidate (i) whether, how fast, and to which degree pharmaceuticals accumulate in wastewater irrigated soils. To unravel the underlying processes, I studied (ii) shifts in sorption and desorption behavior of the compounds on soils under different duration of wastewater irrigation, (iii) evaluated potential changes in their dissipation rates, and assessed (iv) the potential release and subsequent dissipation of pharmaceuticals in the flooded soils at declining redox potentials. To achieve these aims, I sampled a soil irrigation chronosequence (Leptosols, Phaeozems, Vertisols) irrigated for zero to 100 years in the Mezquital Valley, Mexico. The pharmaceuticals were sequentially extracted from soil with (i) CaCl2 solution to account for the bioaccessible fraction, and (ii) accelerated solvent extractions (ASE) to access a strongly sorbed, sequestered compound fraction. Furthermore, I conducted a batch sorption as well as laboratory incubation experiments to reveal the impact of long-term irrigation on sorption and desorption processes, dissipation rates, and potential release of the compounds under aerobic and anaerobic conditions, respectively. The results showed that ciprofloxacin (CIP), sulfamethoxazole (SMX), and carbamazepine (CAR) accumulated in soil with increasing duration of irrigation until after 19 - 28 years a plateau was reached at approximately 1.4 µg kg-1 (CIP), 4.3 µg kg-1 (SMX), and 5.4 µg kg-1 (CAR). Acidic pharmaceuticals (diclofenac (DIC), naproxen (NAP), bezafibrate (BEZ)) were not retained and thus did not accumulate in the soils. Besides the accumulation of pharmaceuticals in soils, long-term wastewater irrigation affected sorption and desorption processes of SMX, while sorption of CIP was always strong and largely irreversible irrespective of the duration of wastewater irrigation and of the soil organic carbon content (Freundlich KF: 346 - 979 mg1-1/n L1/n kg-1, 1/n: 0.62 - 0.76). Sorption of SMX was stronger in a non-irrigated soil (KF: 4.14 mg1-1/n L1/n kg-1 ± 0.02, 1/n: 0.69 ± 0.02) than in irrigated ones (KF:0.65 - 1.38 mg1-1/n L1/n kg-1; 1/n: 0.68 - 0.75). Additionally, its sorption hysteresis was more pronounced in the non-irrigated soil than in irrigated soils. The effect of the soil organic carbon content on sorption of the anionic antibiotic SMX was small compared to the effect of changing soil organic matter quality (i.e., increasing fraction of carboxylic moieties with increasing time of irrigation) and the effect of irrigation (e.g., due to competition with accumulated SMX or other wastewater-derived compounds for sorption sites). The observed reduction of sorption in combination with an increased biomass and activity of a potentially adapted microbial community in irrigated soils was hypothesized to accelerate dissipation of pharmaceuticals with increasing time of irrigation. However, dissipation half-lives (DT50) of SMX (2 - 33 days), DIC (< 0.1 - 1.4 days), BEZ (< 0.1 - 4.8 days), NAP (6 - 19 days), as well as of CAR (355 - 1624 days) and CIP were not affected by wastewater irrigation. Dissipation of trimethoprim (TRI) was even slower in soils irrigated for 100 years (DT50: 45 - 72 days) than in non-irrigated soils (DT50: 12 - 16 days), being negatively correlated with soil organic carbon content and soil-water distribution coefficients. The application of a kinetic fate model suggested that the cationic or uncharged TRI and uncharged CAR were sequestered more efficiently in soils with a long history of irrigation, which reduced their bioaccessibility, while no such enhanced sequestration was observed for fast dissipating zwitterions or negatively charged pharmaceuticals. Low redox potentials likely promoted the dissipation of SMX, but did not lead to a release of formerly sorbed pharmaceuticals from the wastewater irrigated soil. Long-term wastewater irrigation can lead to the accumulation of pharmaceuticals in soils. This accumulation is not counteracted by an adaptation of the soil system during the course of long-term irrigation leading to an accelerated dissipation of pharmaceuticals. Changes in soil properties such as quality and quantity of soil organic matter can even delay dissipation of cationic pharmaceuticals and reduce sorption of anionic compounds thus challenging the stability of the soil aquifer treatment system in the long run. Future research should, therefore, assess the possibility of enhancing the sustainability of wastewater irrigation for example by different treatments of the wastewater prior to its use in agriculture.