Transit Times In A Shallow Aquifer From Tracer Measurements In The Aquifer And A Gaining Stream

Abstract:

The mean transit time (MTT) is a fundamental property of a groundwater flow system 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) that is not usually determined directly. We studied the TTD and MTT in the baseflow of a gaining stream (West Bear Creek) and in the surrounding shallow aquifer in the North Carolina Coastal Plain. Groundwater seepage to the stream 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. Environmental tracers concentrations (CFCs, SF₆, ³H-³He) were measured in multi-level monitoring wells, in streambed piezometers, seepage meters, and stream samples. Apparent (piston flow) ages from monitoring wells are generally consistent with an exponential TTD. Discrete samples from streambed piezometers have concentrations from multiple age-dating tracers suggesting that mixing of a wide range of groundwater ages was not occurring as groundwater converged below the streambed and that piston flow apparent ages were reasonable estimates of the transit time of groundwater. Stream water samples that are corrected for exchange with the atmosphere yield SF₆ concentrations that are similar to flow weighted values from streambed piezometers. The MTT of 25-30 years derived from well samples is similar to that based on streambed piezometer samples and the exchange-corrected stream samples. The flow-weighted cumulative distribution of apparent ages from streambed samples 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 appears to act as a flow weighted integrator of transit times and samples from the stream and streambed can provide fundamental information regarding properties of the contributing aquifer.