INVESTIGATING SOURCES AND PROCESSES CONTROLLING URBAN STREAM WATER COMPOSITIONS USING GEOCHEMICAL MODELING AND MASS BALANCE APPROACHES

Identifying and quantifying the sources and processes that influence stream water quality in urban environments is critical to watershed management, especially where stream water is a major source of aquifer recharge. The association between higher concentrations of anthropogenic constituents (i.e., Na, Cl, SO₄) in streams draining watersheds with higher densities of urban land use is well documented. There have been few studies, however, that have quantified the specific sources and processes responsible for water quality impairment in urban environments. This study investigates factors that affect stream water compositions in five streams that contribute the majority of recharge to the Barton Springs segment of the karstic Edwards aquifer in central Texas. Stream water samples collected every 3 to 4 weeks from November 2008 to March 2010 were compared to 30 years of historical data. Results indicate that recent (2008-2010) stream water constituent concentrations were significantly greater than concentrations measured prior to 2000. Geochemical modeling of end-member mixing and mineral-solution reactions was used to account for recent stream water compositions. Modeling results indicate that stream water compositions are not readily accounted for by natural sources (i.e., overland flow and shallow spring seepage); compositions instead require mixing with substantial portions of anthropogenic sources (i.e., municipal water, wastewater, or septic leachate). Estimated volumes of anthropogenic sources required to account for stream water compositions at high discharges are unrealistically high, which indicates that anthropogenic sources cannot be direct sources of stream water. Mass balance estimates indicate that anthropogenic sources are likely indirectly contributing solutes to the stream. Furthermore, stream water constituent concentrations are constant relative to large changes in discharge. The lack of dilution of stream water indicates that solute fluxes to streams increase with discharge. This likely occurs due to the rapid transport (flushing), during natural rain events, of anthropogenic solutes that accumulate in the shallow soil/epikarst zone. These results have implications for watershed management, especially in areas where surface water readily recharges groundwater.