CHARACTERIZING UNCONFINED AQUIFERS USING SEEP- AND STREAM-BASED MEASUREMENTS OF GROUNDWATER AGE (Invited Presentation)

The mean transit time (MTT) in unconfined aquifers is a fundamental property that is related to recharge rate and storage volume. However, estimating the MTT using environmental tracers is problematic as flow-weighted samples over the full spectrum of transit times are needed and computed MTTs depend on the transit time distribution (TTD). We studied the TTD and MTT in the baseflow of two gaining streams and their surrounding aquifers; West Bear Creek in the North Carolina Coastal Plain (NC) and the West Fork of the Duchesne River, in the Upper Colorado River Basin, Utah (UT). In NC, groundwater seepage was quantified using the dilution of an injected Br tracer, velocity-area flow measurements, seepage meters, and Darcy calculations from discrete point measurements of vertical K and gradient. In UT, seep and spring discharge were quantified by bucket gaging, flumes, and/or velocity-area flow measurements. CFCs, SF₆, and ³H-³He were measured at a variety of discharge sites. Comparison of multiple age-dating tracer concentrations in samples suggested that mixing of a wide range of groundwater ages was generally not occurring as groundwater converged below the streambed or into springs. Piston flow apparent ages were reasonable estimates of the MTT for all of the NC streambed samples, and 70 percent of the UT spring samples. In NC, the MTT of 25-30 years derived from well samples was similar to that based on streambed piezometer samples and the exchange-corrected stream samples. In UT, flow-weighted ³H concentrations in springs were similar to baseflow ³H in the stream; lumped parameter modeling of multiple tracers (CFCs, SF₆, ³H-³He) from 21 springs yields a MTT of 40 years. The flow-weighted cumulative distribution of apparent ages from both sites can be modeled with a gamma distribution having a shape factor (α) that is greater than 1. Numerical modeling of some ideal cases indicates that spatial variations in recharge might be discerned from the cumulative TTD with α >1 for recharge concentrated away from a stream and α < 1 for recharge concentrated near the stream. West Bear Creek and springs feeding the West Fork of the Duchesne appears to act as a flow weighted integrator of transit times and samples from the streams, streambed, and springs can provide fundamental information regarding properties of the contributing aquifer.