Abstract: Urban Impacts on Chemistry of Shallow Groundwater, Barton Creek Watershed, Austin, Texas

JOHNS, DAVID, A., and SYLVIA R. POPE
City of Austin, Austin, TX

The Barton Creek watershed west of the Balcones Fault Zone in Austin is primarily underlain by the Glen Rose Formation. Groundwater commonly discharges from springs in Member 3, a thin-to-medium bedded dolomitic unit, of the Glen Rose limestone and from small contact springs at the base of terrace deposits along the creek mainstream. Analysis of spring samples indicates that the chemistry of shallow groundwater is altered in areas with urban development in contributing recharge areas.

Comparisons between springs in rural settings and springs in urban settings indicate pronounced differences in nitrates, calcium, potassium, sodium, chloride, sulfate, alkalinity, and total dissolved solids. Average concentrations in urban springs of nitrates are over 400% higher and ion concentrations range from 27% to 2000% greater than those in rural springs. Results of heavy metal analyses are statistically inconclusive although heavy metals are more frequently detected in urban springs. In some cases, spring chemistry and discharge rate appears affected by effluent irrigation in recharge areas. Chemistry of these springs varies considerably depending on antecedent moisture conditions. Wet weather typically dilutes nutrients and ions associated with effluent irrigation.

Potential sources for increasing nitrogen concentrations in urban areas include landscape fertilizers and wastewater effluent. Increases in ions in urban areas may result from enhanced carbonate dissolution by irrigation with undersaturated municipal drinking water and high ion concentrations in wastewater effluent.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas