76-9 Lithological Controls on the Storage and Transport of Bomb-pulse 129I in a Small Catchment in Central Indiana.

Session: Contaminants Near Groundwater-Surface Water Interfaces

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Abstract:

Naturally occurring radioisotopes such as tritium (³H, t_1/2 = 12.32 yrs) have been widely used to detect bomb-pulse recharge from the nuclear weapons testing era of 1950s and 1960s in aquifers and in groundwater discharge to streams and springs. However, it is becoming increasingly difficult to use ³H for this purpose since atmospheric ³H activities are approaching pre-bomb or background activities. Chlorine-36 (³⁶Cl, t_1/2 = 3.01×10⁵ yrs) offers an advantage in preserving the isotopic fingerprint of bomb-pulse recharge due to its much longer half-life; however, ³⁶Cl/Cl ratios can be diluted in groundwater and surface water by the presence of geologically old chloride such as geogenic chloride (magmatic and evaporites) and anthropogenic (road salt, fertilizers, water softeners). Here, we investigate using iodine-129 (¹²⁹I, t_1/2 = 15.7x10⁶ yrs) as an alternative technique to identify components of bomb-pulse recharge in groundwater and surface water. We measured ¹²⁹I, ³⁶Cl, and ³H concentrations in samples of spring and stream water from the Devil’s Punch Bowl catchment (0.44 km²) in Shades State Park, Indiana. The geology in the catchment, from top to bottom, includes a thin layer of glacial sediment, a thick permeable sandstone unit, a thin and spatially discontinuous fossiliferous limestone, and a thick shale unit. Our results show that ¹²⁹I is effective at capturing the bomb-pulse signal which had been previously observed with ³⁶Cl/Cl measurements. The ¹²⁹I/I ratios of groundwater ranged from 4000 to 23,000 ×10^-15 and from 5400 to 23,000 x 10^-15 in stream samples. These ratios are considerably higher than the expected pre-anthropogenic ratio of 1500 ± 250 × 10^-15. ^129I primarily enters an aquifer through recharge, rock leaching, and spontaneous fission of uranium-238, but these mechanisms cannot account for the very high ¹²⁹I/I ratios in our groundwater samples. However, ¹²⁹I dilution was observed in seeps from the upper shale. Dilution was not observed in the sandstone unit. We infer that groundwater is primarily stored within the sandstone unit and is subsequently discharged either at the base of the sandstone unit or from the bedding-parallel fractures in the upper shale. These data demonstrate that ¹²⁹I is very effective for tracing bomb-pulse recharge in groundwater and its discharge to streams; however, dilution from lithological sources of iodine must be considered.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6727

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