RADIOGENIC 4HE AS A TRACER FOR YOUNG (102-104 YEARS) GROUNDWATER IN BURIED-VALLEY AQUIFERS

Dissolved helium can be an excellent tracer for hydrogeologic processes because it is inert and its sources in groundwater are relatively well understood.  Radiogenic ⁴He (produced by the radioactive decay of U and Th) has traditionally been used as an age indicator and conservative tracer in modeling of old (>10⁴ years old) groundwater.  The lower age limit of this technique is commonly assumed to be controlled by the concentration of U and Th in the aquifer materials.  However, recent evidence indicates that release of radiogenic ⁴He from some aquifer materials can exceed the U/Th production rate by several orders of magnitude.  One setting where ⁴He release rates may be high is in recently eroded sediments of >100 MA rocks, characteristics commonly found in glaciated terranes.

This presentation reports the results of dissolved helium studies conducted in the Mahomet Aquifer, a regionally extensive buried-valley aquifer in central Illinois, USA.  The sediments filling and overlying the paleovalley are Pleistocene age glacial deposits.  Previous studies of the aquifer include ¹⁴C ages which indicate that groundwater ranges from modern to 13,000 radiocarbon years before present. Groundwater samples collected from the aquifer and overlying units were analyzed for dissolved helium and other noble gases.  Laboratory helium release rate experiments were also performed on aquifer sediments.  Radiogenic ⁴He concentrations ranged from 0 to 3044 mcm³(STP)kg_water^-1, and generally increase away from the main recharge area along inferred flow paths.  Although the flux of ⁴He from the underlying bedrock is not yet known, a bedrock flux is suspected in several areas where there is anomalously high ⁴He concentrations.  Release rates ranged from 0.17 to 0.53 mcm³(STP)kg_sediment^-1yr^-1 at 12^oC.  Based on these data, radiogenic ⁴He is likely to be useful as a conservative tracer in transport modeling of the aquifer, providing a calibration parameter independent of hydraulic head.  Additional data needed before a model can be constructed include estimates of ⁴He fluxes from the bedrock and additional measurements of ⁴He release rate from sediments.