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Highlights • The δ 15N of submerged biomass is correlated to changes in the quality of wastewater treated effluent. • A two sources simple mixing model for NH 4 + and NO 3 − was explored for estimating DIN assimilation. • N isotopic fractionation (α assimilation) in macrophytes was estimated between 1.005 and 1.012. • The δ15N of submerged biomass can be used complementarily to water quality monitoring. Abstract We measured the changes in concentration and isotopic composition of the two major dissolved inorganic nitrogen species (ammonium and nitrate) and the isotopic composition of submerged biomass, in order describe the changes in the nitrogen cycling at a river receiving treated effluent before and after upgrades at a wastewater treatment plant in southwestern Ontario (Canada). We observed that changes in the δ 15N of macrophytes and periphyton are correlated to changes in the quality of treated effluent. We explored two scenarios, differential assimilation of dissolved nitrogen species without isotopic fractionation using a two end-member mixing model, and isotopic fractionation due to assimilation (α assimilation) in macrophytes. The two end-member mixing model suggested different contribution per N specie, and α assimilation was estimated to be around 1.005 and 1.012 before and after upgrades, respectively. Thus, we provided evidence that the δ15N of submerged biomass can be used as a proxy for in-stream nitrogen cycling and complementary to water quality monitoring for evaluating changes in the nitrogen cycling in urban catchments. [ABSTRACT FROM AUTHOR]

Sinkholes are common karst forms were we can access the aquifer. This study describes an 85 m deep sinkhole in north Quintana Roo (Mexico), located at 45 km from the coast, allowing us to explore an open section of the aquifer. The profile of the water column was realized to evaluate the physical chemistry, hydrochemistry and water isotopes (δ2H, δ 18O and 3H) in order to distinguish the stratification. It was identified water-rock interaction (limestone) in the upper section (0–40 m), typical from karst groundwater; a mixing zone approximately 10 m thick, and saline intrusion in the deep section (50–80 m). Salinization of the freshwater was observed, likely due to diffusion of sodium and chloride from the deep water. The hydrochemistry suggests seawater intrusion contributing as much a 60% of the water in the deep section. With the stable isotopes, meteoric water, groundwater and seawater mixing was attested. The tritium measurements suggest that the aquifer has short residence time and fast recharge. Few deep sinkholes in Mexico have been studied, the information obtained in Yumkin contributes to the knowledge and understanding of the Yucatan aquifer system. [Display omitted] • Meteoric water, groundwater and seawater mixing occurs in an 85 m deep sinkhole. • Seawater intrusion is about 60% at 80 m. • Tritium suggest fast groundwater recharge and short residence time. • Data contributes to a better understanding and long-term aquifer management. [ABSTRACT FROM AUTHOR]