ABSTRACT

The Barton Creek watershed west of the Balcones Fault Zone in Austin, Texas is primarily underlain by the Glen Rose Formation. Groundwater discharges from springs in 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 springs with urban development in contributing recharge areas.

Comparisons between springs in rural settings and springs in urban settings indicate significant differences in nitrate, ammonia, total Kjeldahl nitrogen, calcium, magnesium, sodium, potassium, chloride, sulfate, alkalinity, specific conductance, and total dissolved solids. Mean concentrations of nitrates in urban springs are over 500% higher and ion concentrations range from 25% to over 700% 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 appear to be affected by effluent irrigation in recharge areas. Chemistry of some springs varies considerably depending on antecedent moisture conditions. Wet weather typically dilutes nutrients and ions associated with effluent irrigation.

The relationship between urbanization and groundwater chemistry does not appear to be caused by changes in host rocks as most springs discharge from the Glen Rose Formation. 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 municipal drinking water and high ion concentrations in irrigated wastewater effluent.