Name: Syeda Sadia Ali
Date: 04/10/2025
Time (EST/EDT): 11:00 am
Location: BFL 1101
Remote Access: email: mees@umd.edu

Committee Chair: Dr. Lee Cooper
Committee Members: Dr. David Nelson, Dr Keith Eshleman

Title: Assessing and understanding spatiotemporal variation in stable hydrogen and oxygen isotope values of Maryland’s streams and rivers

Abstract:  This study explores the spatiotemporal variability of stable hydrogen (δ²H) and oxygen (δ¹⁸O) isotope values in Maryland’s rivers and streams, emphasizing the influence of precipitation sources, physiographic features, and hydrological processes. Rivers in western Maryland exhibited more depleted δ²H and δ¹⁸O values, primarily due to long-distance moisture transport and altitude effects. In contrast, eastern rivers and streams displayed heavier isotopic compositions, influenced by local moisture recycling, higher temperatures, and greater warm-season precipitation inputs. A Local Meteoric Water Line (LMWL) was derived for Maryland as δ²H = 7.84·δ¹⁸O + 12.86 (R² = 0.99), deviating slightly from the Global Meteoric Water Line (GMWL) as a result of regional climatic influences such as atmospheric recycling. Elevation demonstrated a clear isotopic control, with heavy isotope depletion gradients of approximately –0.6‰ per 100 m for δ²H and –0.1‰ per 100 m for δ¹⁸O. Seasonal patterns were also evident, as winter precipitation was isotopically lighter due to cold-temperature isotopic fractionation and remote moisture sources, whereas summer precipitation was heavy isotope enriched as a result of convective storms and evaporation. Variability over two years (2022-2024) showed that in the drier year (2024) with a reduced moisture surplus, river systems relied more on stored winter precipitation, emphasizing the buffering role of groundwater. Deuterium excess (d-excess) values further showed regional differences in moisture sources. Higher d-excess in western Maryland pointed to potential lake-effect precipitation derived from the Great Lakes and long-distance transport, while lower values in the east reflected enhanced local evaporation. These findings establish a regional isotopic baseline that enhances our understanding of hydrological and climatic controls across Maryland. They also elaborate the potential impacts of ongoing climate change: a reduction in winter precipitation could diminish groundwater recharge and baseflow, affecting dry-season water availability. Meanwhile, more intense summer rainfall may increase surface runoff and nutrient loading, heightening flood risks and degrading water quality.