Sr isotopes as tracers of anthropogenic influences on stream water in the Austin, Texas, area

The impact of urbanization on streams is multifold and affects both stream flow and stream-water quality. These impacts can lead to either 1) a decrease in stream base flow via increased runoff to streams and reduced diffuse recharge caused by the burial of soil by impervious cover, or 2) an increase in stream base-flow via addition of anthropogenic water sources. Identifying the sources of inputs to urbanized streams is essential to addressing this issue. In the Austin, Texas area, we find a strong contrast in Sr isotope composition between municipal water and stream water in relatively undeveloped watersheds. Dissolved Sr in stream water from eight watersheds in Austin has a wide range of Sr isotope compositions, from 87Sr/86Sr = 0.70778 to 0.70918. The mean 87Sr/86Sr value of stream water in each of the eight watersheds correlates highly to both 1) a variety of indices of the degree of urbanization within the watersheds, such as amount of impervious cover, and 2) mean stream-water concentrations of dissolved ions of anthropogenic origin, such as F, Cl, NO3, Na, and K. These 87Sr/86Sr values can be accounted for by the mixing of two endmembers: 1) the lowest values are similar to those for Cretaceous marine limestones that comprise nearly all of the bedrock in the Austin-area watersheds, and 2) the highest values are similar to Austin municipal water that is taken from the Colorado River of Texas. These relationships enable the novel application of Sr isotopes as a proxy for municipal water contributions to stream base-flow. In the more urbanized watersheds, Sr-isotope mass balance indicates that ~ 90% of base flow is composed of water from anthropogenic sources. Sr isotopes may also be applied to identify thresholds in the extent of urbanization, above which stream-water flow and quality is impacted.