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Using δ2H and δ18O in assessing water quality condition of the nation’s water
Citation:
BROOKS, D. V. PECK, J. VAN SICKLE, S. G. PAULSEN, K. RODECAP, AND M. H. WEBER. Using δ2H and δ18O in assessing water quality condition of the nation’s water. Presented at The roles of stable isotopes in water cycle research, Keystone, CO, March 29 - 31, 2011.
Impact/Purpose:
The Clean Water Act mandates reporting on the condition of the nation’s waters.
Description:
The Clean Water Act mandates reporting on the condition of the nation’s waters. The Environmental Protection Agency implemented National Aquatic Resource Surveys (NARS) to address this mandate, including the National Lakes Assessment conducted in 2007. This study focuses on whether stable isotopes of water (δ2H and δ18O) can be useful in assessing the condition of the nation’s lakes and potential causes of lake impairment. Stable isotopes of water provide a means for estimating evaporation and water residence time within a lake. These direct measures are not feasible to make during the half day assessment that crews conduct at each sampled lake. In the summer 2007, 1157 lakes were sampled for biological, chemical and physical condition. After chemical analysis, water samples were measured for δ2H and δ18O using a laser isotope system. Water isotopes for precipitation inputs were estimated using the spatially explicit models found at Waterisotopes.org. Lake water isotopes ranged from 5 to -20 for δ18O and 20 to -135 for δ2H with d-excess values ranging from 13 to -43. Most lakes were more enriched than the local precipitation with dual-isotope slopes less than 8, indicating evaporation as the cause for this difference. Using d-excess and lake–precipitation differences as indices of evaporation, we rated lakes for high evaporation by comparing them to reference lakes within the same ecoregion. These data were then compared to the biological condition of each lake. Natural lakes that were highly evaporated were four times more likely to be in poor biological condition compared to less evaporated lakes. Evaporation was not correlated with biological condition in man-made lakes. High evaporation or low turnover times could be significant stress factors for lakes in the mid-west and the western United States, whereas the degree of evaporation was not related to biological condition in eastern lakes.