New estimation to the turnover time of Lake Ohrid: an environmental isotope and noble gas study

<p>We present noble gas composition and environmental isotope signature (&#948;¹⁸O, &#948;²H, &#948;¹⁵N_NO3-, ³H) of waters sampled from Lake Ohrid. The lake is one of Europe's deepest (max. depth ~288 m) and oldest lake, situated in southeastern part of the continent at the border between Albania and North Macedonia.</p> <p>The sampling campaign was in the summer of 2017, when 100 water samples were taken from 19 depth profiles (at depths of 5 m, 25 m, 50 m, 100 m, 150 m, 200 m and 250 m) for water stable isotopes, noble gases, and tritium. Samples for &#948;¹⁵N_NO3- measurement of nitrate were collected at 7 m and 27 m depth.</p> <p>Lake Ohrid, situated between Gali&#269;ica Mt. to the east and Jablanica and Mokra Mts to the west, is a large (surface area ~350 km²), oligotrophic, and one of the most voluminous lakes (~55 km³) and together with Lake Prespa represents the biggest water system in the Balkan region. Numerous studies have been carried out on the hydraulic connection between the two lakes using stable isotopes and hydrological modelling. The water balance of Lake Ohrid is dominated by inflow from karst aquifers, direct precipitation and with slightly smaller shares from river runoff. Lake Ohrid is strongly influenced by karstic springs, adjacent to large part of the coastline, sub-aquatic as well as surface springs which are particularly cool, clean and oxygen-rich inflowing water. The springs are fed by aquifers that are recharged from precipitation and, along the eastern shoreline, also by Lake Prespa.</p> <p>The lake sediment covers a record of the last 1.5 million years. To better understand the link between the atmosphere and the sediment, our goal is to estimate the water turnover time of Lake Ohrid and to give an isotopic overview about the lake system. Our measured stable isotope data provide background information about hydrogen and oxygen isotope variability of the lake. The stable isotope results together with tritium data present a prospect for estimating evaporation and mixing proportions. The noble gas results detail the layers of the estimated mixing processes. Nitrogen stable isotope data provides additional information about the locality and the type of potential pollution sources.</p>