Abstract
The application of the cosmogenic radioisotope sulfur-35 (35S) as a chronometer near spreading basins is evaluated at two well-established Managed Aquifer Recharge (MAR) sites: the Atlantis facility (South Africa) and Orange County Water District’s (OCWD’s) Kraemer Basin (Northern Orange County, CA, USA). Source water for both of these sites includes recycled wastewater. Despite lying nearer to the outlet end of their respective watersheds than to the headwaters, 35S was detected in most of the water sampled, including from wells found close to the spreading ponds and in the source water. Dilution with 35S-dead continental SO4 was minimal, a surprising finding given its short ~3 month half-life. The initial work at the Atlantis MAR site demonstrated that remote laboratories could be set up and that small volume samples—saline solutions collected after the resin elution step from the recently developed batch method described below—can be stored and transported to the counting laboratory. This study also showed that the batch method needed to be altered to remove unknown compounds eluted from the resin along with SO4. Using the improved batch method, times series measurements of both source and well water from OCWD’s MAR site showed significant temporal variations. This result indicates that during future studies, monthly to semi-monthly sampling should be conducted. Nevertheless, both of these initial studies suggest the 35S chronometer may become a valuable tool for managing MAR sites where regulations require minimum retention times.
Highlights
An increasingly important water supply option is Managed Aquifer Recharge (MAR) combined with wastewater reuse
Field campaigns were conducted at the Atlantis and Orange County Water District (OCWD) MAR operations, respectively, during August 2010 and 2012–2013
Assuming a mean initial activity based on the ponds of 15.0 mBq/L and the simple piston flow model, the apparent ages of the groundwater ranged between 0 and 17 wks (Table 2)
Summary
An increasingly important water supply option is Managed Aquifer Recharge (MAR) combined with wastewater reuse. While many MAR facilities are currently in development, a number of them have been in operation for more than a half a century These sites include the well-established dune replenishment systems of the Netherlands [1,2,3], numerous Riverbank Filtration sites along European rivers such as the Rhine, (e.g., [4]) and the Elbe, (e.g., [5,6]), the Montebello Forebay Spreading Grounds of Los Angeles County, CA, USA [7], and the Orange County Forebay, located in Northern Orange. Many potential contaminants, such as most infective microorganisms (pathogens) and some trace organic compounds, may persist in recycled water even after tertiary treatment at above ground facilities; many contaminants are naturally removed or become inactive with time in the subsurface (e.g., [12,13,14,15,16])
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