Chemical and Carbon Isotopic Characterization of a Karst-Dominated Urbanized Watershed: Case of the Upper San Antonio River.

Urbanization and agriculture are two key factors that place demands on water resources and serve as sources of anthropogenic pollution into inland waterways. The San Antonio River, which is sourced from a karst aquifer, plays an important recreational and scenic role, yet effective management is often hampered by the lack of understanding of the chemical characterization of the water system. The karst-dominated Edwards Aquifer watershed in south-central Texas is an ideal watershed to understand water-rock interaction (carbonate dissolution) and anthropogenic impact on our water resources. In order to understand groundwater-surface water interactions, we made chemical and isotopic measurements over a 17-km stretch of the San Antonio River beginning at the headwater sanctuary and moving downstream. The chemistry of the headwaters and at along the longitudinal profile of the river showed that the Edwards Aquifer is dominated by Ca, Mg and HCO ions resulting from carbonate dissolution. The carbon isotopic signature of dissolved inorganic carbon (δC) showed that the Edwards Aquifer is in chemical and isotopic equilibrium with soil CO. The relationships between δC and solutes (Cl, Na, F, NO) showed that anthropogenic sources of these solutes are associated with low δC values, indicating that carbon isotopic composition of dissolved inorganic carbon can be a useful tracer for contaminants in the environment. The anthropogenic inputs into the San Antonio River were sourced mainly from effluents of the San Antonio Zoo, waste discharge from the River Walk in downtown San Antonio and from fertilizers and animal waste in the less urbanized section of the sampled area (Mission Concepcion to Mission Espada). To protect and sustain the water quality of urban waterways and karst aquifers, urban sewage and effluents must be treated and controlled.