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Lake hydrologic indicators from a single sample: Using δ2H and δ18O in assessing evaporation and water residence time of lakes in EPA’s National Lakes Assessment.
Citation:
Brooks, J. Renee, Phil Kaufmann, E. Fergus, AND A. Pollard. Lake hydrologic indicators from a single sample: Using δ2H and δ18O in assessing evaporation and water residence time of lakes in EPA’s National Lakes Assessment. NWQMC National Monitoring Conference, virtual, OR - Oregon, April 19 - 23, 2021.
Impact/Purpose:
Lake hydrologic flows are a fundamental part of how lakes function, but a long-overlooked part of extensive monitoring programs. Here, we cover the basic theory on how δ2H and δ18O contained within water can provide an integrated record of hydrologic processes and can be used to assess the hydrologic condition of lakes in national surveys such as EPA's National Lakes Assessment.
Description:
Hydrologic water balance measurements are often excluded from spatially extensive lake monitoring programs because they require detailed gauging and monitoring of lake inflows and outflows. Yet lake flow-through status and water residence time are important parameters for understanding a lake’s chemical, physical and biological condition. For national surveys such as EPA’s National Lakes Assessment (NLA), where individual lakes are assessed on a single day, the δ2H and δ18O contained within water can provide an integrated record of hydrologic processes and can be used to calculate evaporation:inflow ratios (E:I) and water residence time. Here, we cover the basic theory and utility of these isotopically derived parameters for programs such as the NLA. Because the lighter isotopes of water (1H and 16O) preferentially evaporate relative to their heavier counterparts (2H and 18O), lake water becomes progressively enriched in the heavy isotopes with evaporation. This isotopic shift from evaporation is related to the proportion of water entering a lake that leaves through evaporation (E:I), as compared to water flowing out of the lake. Using climate data and estimates of input precipitation isotopes, lake water isotopes can be used to calculate E:I directly. E:I can then be used to calculate lake water residence time (I:V) if lake volume is known. The utility of these measures for assessing lake hydrology is illustrated by the 2007 NLA, representing ~50,000 lakes distributed across the USA. For half the US lake population, evaporation was 1). These hydrological measures were related to the chemical condition of the lake, another important assessment parameter in monitoring programs. For example, E:I was strongly correlated with Lake total nitrogen concentration, more than any other NLA metric. Lake hydrologic flows are a fundamental part of how lakes function, but a long-overlooked part of extensive monitoring programs. The cost of water isotope analysis is relatively low. While calculating E:I from water isotope values is not a trivial endeavor, the additional hydrologic data derived from isotopic analysis of water samples already being collected in extensive surveys could justify the additional expense, depending on survey objectives.