Quantifying the Distribution and Dynamics of Managed Aquifer Recharge Using Mass- Balance and Time-Series Thermal Methods
Abstract
Managed aquifer recharge (MAR) is becoming increasingly popular for supplementing fresh water resources, helping to limit ground water overdraft and improve both the quantity and quality of available water. Most MAR systems are operated above a vadose zone, and usually recharge rapidly during an initial phase of diversion, but recharge typically slows considerably within subsequent weeks to months as sedimentation, biofouling, soil compaction, drainage at the base of the wetting front, and other processes reduce the hydraulic conductance below the percolation pond. We instrumented a MAR recharge pond above a shallow aquifer in central coastal California, to quantify variations in rates and locations of recharge, and to measure changes in soil properties with time during a recharge season. Careful measurements and calculations of diversion rates, precipitation, evaporation, pond water levels, and pond volume and area were used to construct a history of whole-pond seepage. Shallow piezometers were installed in the base of the pond and instrumented with autonomous temperature sensors and loggers prior to the start of the recharge season, allowing us to use heat as a tracer of fluid flow based on time-series analysis. Autonomous pressure loggers installed in the same locations allow quantification of head gradients with time. When this information is combined with seepage rates, we can determine absolute values of the hydraulic conductance of the saturated soil at the base of the pond, including changes in these values with time. The recharge pond is 3 km2 in area and typically recharges at the start of the MAR operating season at a mean rate of 2 m/day, based on a whole-pond mass balance and bottom area, but this rate tapers off abruptly after 4-8 weeks of operation. Point-specific seepage rates vary enormously throughout the recharge cycle across the pond base, with some areas allowing recharge at rates in excess of 10 m/day, and other areas being virtually stagnant. Collection of soil samples before and after the seepage season allows assessment of accumulating fine-grained sediment during MAR, and geochemical data from the aquifer below the pond help to resolve the fraction of the pond that contributes most to recharge. Studies such as these generate improved understanding of recharge processes in general, with MAR systems providing controlled windows into subsurface conditions and processes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H13A0888R
- Keywords:
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- 1829 Groundwater hydrology;
- 1830 Groundwater/surface water interaction;
- 1838 Infiltration;
- 1875 Vadose zone;
- 1876 Water budgets