Edwards Aquifer Urban Hydrology Network

Timescales and Drivers of Water-Quality Change in the Edwards Aquifer

Summary of findings from Musgrove and others, 2019

Understanding the timescales on which groundwater quality changes, and what drives these changes, informs groundwater management, use, and protection. This study combined high-frequency monitoring and discrete sample collection for a range of geochemical constituents (including selected isotopes and age tracers) in a small well network over several years to understand the timescales on which groundwater quality changes.

Some key findings are:

• Water quality in the Edwards aquifer is dynamic and can change over both short and long timescales.
• Because of the hydrogeology of the aquifer—its structure and flow patterns—different parts of the aquifer change at different timescales and, thus, differ in their vulnerability to changes in the water quality of recharge.
• In shallower wells in the recharge zone, water quality varies rapidly in response to changes in recharge and hydrologic conditions. Water quality is potentially vulnerable to urban influences on recharge quality in the short term.
• In relatively deep, confined, and downgradient parts of the aquifer, water quality is less responsive to recharge events and changes in hydrologic conditions. While water quality changes slowly (years to decades) and is decoupled from recent hydrologic responses, it is vulnerable to changes in the water quality of recharge over longer (decadal) timescales.
• The aquifer is characterized by updip-to-downdip (that is from higher to lower elevation) trends in geochemistry, mineral-solution reactions, and groundwater residence time (age).
• Groundwater ages provide insight into timescales of aquifer vulnerability. Mean groundwater ages, interpreted from measured age tracers and lumped parameter models were mostly modern, but ranged from years to centuries (where mixed with premodern downdip water).

Timeseries of continuously measured specific conductance and water level for wells and specific conductance and spring discharge for Comal Springs. From top to bottom the sites shift from shallow unconfined recharge zone wells (Shavano and Parkwood) with highly variable water levels and physicochemistry, to deeper confined wells (Z-DED, PSW1, PSW2) and a downgradient discharge site (Comal Springs) with more steady and less variable physicochemistry.

Well locations shown on the interactive map

Timeseries of age-tracer lumped parameter model results, showing mean tracer-based groundwater age during periods of both dry and wet conditions.