Investigation of the Effect of Epikarst on Recharge and Storage of
Groundwater in Karst Aquifers, 20-R8220
Principal Investigators
Ronald Green
F. Paul Bertetti
Ronald McGinnis
Hakan Basagaoglu
Inclusive Dates: 04/01/11 – 09/01/12
Background — Increased demand for groundwater has elevated the need to better understand the hydraulics and sustainable yield of aquifers, a major source of water. For karst aquifers, a critical need is to better understand how the epikarst layer affects recharge and storage of groundwater. The term epikarst describes the weathered horizon at the top of the vadose zone with high porosity and permeability. Karst aquifers exhibit complex hydraulic responses to recharge events due to development of dissolution flow pathways at the surface and at depth. The epikarst layer has a strong spatial and temporal influence on recharge of the aquifer. The objective of this project is to investigate the hydraulics of epikarst layers to understand the mechanisms of recharge and groundwater storage in regional-scale karst aquifers and to develop a model for the hydraulics of epikarst layers.
The hydraulic effect of epikarst on recharge is exhibited by the lag time between the occurrence of recharge events and times when the groundwater elevation and spring discharge respond to precipitation. Not accounting for the lag introduces large errors into groundwater flow models and renders water-resource management decisions inaccurate if not misleading. Recharge lag time can be determined using hydraulic or water chemistry responses observed in groundwater or spring discharge. Lag times can vary from hours to months depending on the size of the recharge zone, the climatic season of interest (rainy or dry), the physical nature of the epikarst, and related hydraulic features of the aquifer. In general, larger recharge zones or catchment areas with a more extensive or better developed epikarst layer will have longer lag times.
Approach — The objectives of this project were to investigate the hydraulics of epikarst, to understand the mechanisms of recharge and groundwater storage in regional-scale karst aquifers, and to develop a universal model for the hydraulics of epikarst. To achieve these objectives, the following tasks were undertaken:
- identify potential recharge zone for groundwater monitored at a representative index well and
- quantify the delayed aquifer response and storage time of the epikarst.
Accomplishments — The project achieved its objectives of characterizing the hydraulic properties of epikarst to identify potential recharge zone for groundwater monitored at a representative index well, and to quantify the delayed aquifer response and storage time of the epikarst. A multi-variate regression model was constructed to analyze the statistical correlation between precipitation at Edwards Aquifer Authority rain gauges and changes in groundwater elevation at the Uvalde index well. The multi-variate regression model is essentially an impulse-response model, which relates changes in groundwater elevation to 15-day moving-averaged and lagged precipitation events. The statistical relationships were analyzed using daily precipitation, groundwater elevation, and river stage data.
