A large-scale tracer test in a SAT system verifying residence time of effluents in the subsurface

The largest managed aquifer recharge operation in Israel is the Shafdan system where ~ 130*10⁶ m³ of secondary effluents percolate from surface and recovered through ~ 150 wells annually, to be used for unlimited irrigation (i.e. as freshwater).  The residence time of effluents in the subsurface, during their passage in a Soil Aquifer Treatment (SAT) system, from the infiltration pond to the pumping wells' screens is a concern of health regulators. Therefore, a large-scale tracer test in a segment of the Shafdan SAT system was initiated in July 2024 and still ongoing. Various estimates of the residence time were done during the >40 years of operation of the Shafdan SAT, nevertheless, this is the first time a tracer test is performed, to directly derive travel-time distributions from soil surface to the first ring of recovery wells (200-500 m laterally, 50-80 m vertically).

A subgroup of 4 infiltration ponds with a total area of 5.6 hectare was chosen for spread of tracers. The first ring of working production wells surrounding these infiltration basins includes 6 wells. Four observation wells at smaller distances than 200m from the infiltration basins were also used for monitoring. Two tracers were applied: the anion Bromide (Br^-) and the fluorescent organic salt known as Uranine (Na-fluorescein). Additionally, the cheaply monitored water characteristic, electrical conductivity (EC), is used as a precursor for the tracers. A total of 47 tons of the 3 salts NaBr, NaCl and CaCl₂ (introducing Br^-, elevating EC and keeping SAR low) and 75 kg of Uranine were spread evenly over the 5.6 ha of the infiltration pond's surface using a mechanized broadcast fertilizer.

Uranine is still observed in pumped water of this sandy aquifer 18 months after application. Nevertheless, it was retarded in comparison to Br- by ~10 days in the closest observation well and by 30-115 days in the furthest production wells. First arrival (Br-) in a production well was observed 164 days after tracer application at surface, easing health regulators concern (90 days). EC was found correlated good with Br- in most production wells, but not in observation wells. Only ~ 1% of tracers' mass was recovered in wells after 280 days, and ~ 10% after 400 days. The main conclusion so far is residence time in sub surface is long enough, mixing and dilution of effluents in the recovery wells is very high enhancing the effectiveness of the physical and bio-chemical processes of the SAT system. Stay tuned for more results in May at Vienna.