Assessing And Understanding Spatiotemporal Variation In Stable Hydrogen And Oxygen Isotope Values Of Maryland’s Rivers And Streams

Abstract

This study explores the spatiotemporal variability of stable hydrogen (δ²H) and oxygen (δ¹⁸O) isotope values in Maryland’s rivers and streams during a two-year period (2022-2024), emphasizing the influence of precipitation sources, physiographic features, and hydrological processes. Rivers in western Maryland exhibited lower δ²H and δ¹⁸O values, likely due to long-distance moisture transport and altitude effects. In contrast, eastern rivers and streams displayed higher isotopic compositions, likely 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) because of regional climatic influences such as atmospheric vapor recycling, and sub cloud evaporation. Elevation demonstrated a clear isotopic control on river-water isotope values, with a 0.6‰ decrease per 100 m of increase in elevation for δ²H and 0.1‰ per 100 m for δ¹⁸O. Seasonal patterns were also evident, with lower isotopic values during winter due to cold-temperature isotopic fractionation and remote moisture sources and more positive values in summer as a result of convective storms and evaporation. During 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 Laurentian Great Lakes and long-distance transport, whereas lower values in the east reflected enhanced local evaporation. These findings establish a regional baseline that enhances our understanding of hydrological and climatic controls of river-water isotope values across Maryland. They also imply that 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.