Edwards Aquifer Urban Hydrology Network

Changes in Nitrate Concentrations and Sources in Edwards Aquifer Groundwater

Summary of findings from Musgrove and others, 2016

The Edwards aquifer region is undergoing rapid growth, particularly in urban areas such as San Antonio and Austin. Such growth can increase anthropogenic sources of nitrate to groundwater. Nitrate is an important nutrient but excess concentrations can affect aquifer ecosystems. Additionally, consumption of drinking water with excess nitrate can have health effects (nitrate has a U.S. Environmental Protection Agency primary drinking water standard of 10 milligrams per liter [mg/L]). There are many sources of nitrate and other nutrients in water—both natural and anthropogenic. Natural sources include soils, plant decomposition, animal waste, and rain. Anthropogenic sources include fertilizers, sewage, wastewater, and vehicle exhaust. This study investigated sources, variability, and transformation of nitrate in Edwards aquifer groundwater to assess current and future concerns associated with urbanization.

Some key findings are:

• While nitrate concentrations in Edwards aquifer groundwater are generally low (relative to the 10 mg/L drinking water standard) concentrations are increasing regionally.
• Nitrogen from anthropogenic sources, such as septic systems and land-applied treated wastewater effluent, are a probable source of nitrate in the Edwards aquifer.
• Groundwater nitrate concentrations are elevated relative to surface water recharge.
• Modern recently recharged groundwater is particularly vulnerable to nitrate contamination.
• Nitrification—the conversion of other forms of nitrogen to nitrate—within the aquifer is a potential source of nitrate in Edwards aquifer groundwater.
• A nitrogen mass balance that considers other nitrogen forms, such as organic nitrogen, can account for the majority of nitrate in groundwater.

The Edwards aquifer (San Antonio segment and Barton Springs segment)

Timeseries (A) showing increasing nitrate concentrations in groundwater discharging from Comal Springs (in the San Antonio segment of the Edwards aquifer) and Barton Springs (in the Barton Springs segment of the Edwards aquifer). Discharge values are daily mean values for USGS stations 08155500 (Barton Springs at Austin, Texas) and 08168710 (Comal Springs at New Braunfels, Texas). Concentrations relative to spring discharge values (B) indicate that increased contributions cannot be attributed to hydrologic conditions.