The Origin and MgCl2–NaCl Variations in an Athalassic Sag Pond: Insights from Chemical and Isotopic Data

The examination of past and new chemical–isotopic data (²H/¹H–¹⁸O/¹⁶O, ¹¹B/¹⁰B and ⁸⁷Sr/⁸⁶Sr ratios) shows the meteoric origin of the Sawa Lake (Muthanna Governorate, Iraq) and its connection with the local aquifers, which feed the lake via the groundwater emerging from its floor through fault systems. The chemical and isotopic evaporation models are traced by geochemical computer codes by using a different composition of some potential inflows to the lake (e.g., the Euphrates River and Dammam aquifer). The main product of the chemical evaporation models is gypsum, as confirmed by the mineralogical examination of the sediment and the surrounding outcrops. A strong ¹⁸O–²H enrichment is a consequence of the evaporation effect in arid regions; δ¹⁸O–Cl models and δ¹¹B?=?+?23.4‰ exclude the contribution of any seawater-derived fluids. This latter value along with ⁸⁷Sr/⁸⁶Sr?=?0.707989 suggests a mixed origin from the Eocene–Miocene aquifers. The isotope and chemical evaporation paths from the meteorically recharged sources match the lake composition. However, compositional switches from NaCl toward MgCl₂ occurred in the last decade and are related to post-drought periods, showing that the interaction of the recharging waters with the local soils (Na–Mg exchange and/or the leaching of the top layer salts) have a role in the chemical composition. This demonstrates that the lake is significantly influenced by climatic variations.