STREAMWATER LITHIUM ISOTOPE VARIATIONS IN AUSTIN, TX WATERSHEDS: ASSESSING URBANIZATION VS. GEOLOGIC CONTROLS

The sustainability of water resources in urban environments is becoming increasingly important in the face of population growth and urbanization in the 21^st century. Urbanization can have multivariate impacts on water quality, including those resulting from impervious cover and industrial and domestic wastewater inputs. Watersheds in Austin, Texas span a range of urbanization. Streamwater ⁸⁷Sr/⁸⁶Sr values in Austin indicate that in more urbanized watersheds streamwater contains a large component of municipal water, reflecting significant contributions from irrigation and/or failing infrastructure. Whereas Sr isotopes are useful for tracking the overall input of municipal water into a natural stream, they are not as diagnostic for differentiating municipal supply from wastewater inputs to streams. The ⁷Li/⁶Li isotope system has been applied to tracing wastewater impacts in streams. A global dataset of stream waters shows an inverse trend, ranging from high δ⁷Li values and low Li concentrations to low δ⁷Li and high Li concentrations. This trend has been ascribed to either (1) impacts of urbanization, whereby low δ⁷Li and high Li concentrations are diagnostic of municipal wastewater, or (2) geologic control, whereby low δ⁷Li and high Li concentrations result from weathering of shales relative to carbonate rocks. Stream water and municipal supply water in most rural and urbanized Austin watersheds have high δ⁷Li and low Li concentrations, indicating that the wastewater component in these watersheds is not a significant source of Li. In one urbanized watershed, by contrast, stream water has low δ⁷Li and high Li concentrations. Our preliminary results indicate this watershed’s streamwater composition may be a function of either urbanization or geologic control (i.e., (1) or (2) above). Further analysis of streamwater and bedrock Li isotope compositions and concentrations may provide a means to distinguish between these controls on streamwater Li.