Assessing lake water balances across the USA using stable isotopes: Results from EPA’s National Lakes Assessments.
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Under the Clean Water Act, EPA is mandated with assessing the physical, chemical, and biological condition of lakes within the USA. The hydrologic water balance strongly influences the ecological condition of lakes through its control on lake volume, surface area, and water quality. Unfortunately, hydrological characteristics such as lake flow-through status and water residence time require measurements of water inflow and outflow that are not easily measured for most lakes. However, water isotopes (d2H and d18O) measured in a simple grab sample provide an integrated record of hydrologic processes and can be used to calculate evaporation:inflow ratios (E:I) and water residence time (RT) in lakes. They are particularly suited to large-scale surveys such as EPA’s National Lakes Assessment (NLA), where individual lakes are sampled on a single day visit. We assessed E:I and RT for a statistical sample of 1000 lakes distributed across the conterminous USA (CONUS) in each of the 2007, 2012, and 2017 NLA surveys, inferring our results to the population of ~60,000+ lakes ≥ 4 ha and ≥ 1 m deep across CONUS.
In all three surveys, most CONUS lakes were flow-through lakes (E:I<0.4). In 2007 and 2017, the median proportion of inflowing water that evaporated was 20% (0.2 E:I), while the remaining 80% flowed through the lake. In 2012, the median evaporated proportion of inflow was 26%. Less than 2% of lakes in 2007 and 2017 were closed basin lakes where evaporation exceeded inflow (E:I>1), whereas in 2012, that proportion was 4%. Natural lakes had a greater proportion of inflowing water evaporating compared to man-made lakes across all surveys. Residence times were also relatively stable across time, with median RT = 0.47, 0.55, and 0.39 yr in 2007, 2012, and 2017 respectively, and longer for natural vs. man-made lakes.
These hydrologic measures were related to lake morphology, where deeper lakes had longer residence times and lower proportions of inflowing water that evaporated. Lake hydrology was related to the chemical condition of the lake, another important assessment parameter in monitoring programs. For example, total nitrogen concentration in NLA lakes was positively correlated with E:I, more strongly than other predictors, and excess nitrogen presented the greatest relative risk to lake biota. Isotopically derived hydrologic measures allow for key insights into lake hydrologic function at a national scale and provide the context for individual lake studies relative to the national population of lakes.