Troy E. Gilmore

Groundwater transit time distribution and mean from streambed sampling in an agricultural coastal plain watershed, North Carolina, USA

We measured groundwater apparent age (τ) and seepage rate (v) in a sandy streambed using point-scale sampling and seepage blankets (a novel seepage meter). We found very similar MTT estimates from streambed point sampling in a 58 m reach (29 years) and a 2.5 km reach (31 years). The TTD for groundwater discharging to the stream was best fit by a gamma distribution model and was very similar for streambed point sampling in both reaches. Between adjacent point-scale and seepage blanket samples, water from the seepage blankets was generally younger, largely because blanket samples contained a fraction of “young” stream water. Correcting blanket data for the stream water fraction brought τ estimates for most blanket samples closer to those for adjacent point samples. The MTT estimates from corrected blanket data were in good agreement with those from sampling streambed points adjacent to the blankets. Collectively, agreement among age-dating tracers, general accord between tracer data and piston-flow model curves, and large groundwater age gradients in the streambed, suggested that the piston flow apparent ages were reasonable estimates of the groundwater transit times for most samples. Overall, our results from two field campaigns suggest that groundwater collected in the streambed can provide reasonable estimates of apparent age of groundwater discharge, and that MTT can be determined from different age-dating tracers and by sampling with different groundwater collection devices. Coupled streambed point measurements of groundwater age and groundwater seepage rate represent a novel, reproducible, and effective approach to estimating aquifer TTD and MTT.

Evaluating an unconfined aquifer by analysis of age-dating tracers in stream water

The mean transit time (MTT) is a fundamental property of a groundwater flow system that is strongly related to the ratio of recharge rate to storage volume. However, obtaining samples for estimating the MTT using environmental tracers is problematic as flow-weighted samples over the full spectrum of transit times are needed. Samples collected from the base flow of a gaining stream in the North Carolina Coastal Plain (West Bear Creek) that were corrected for exchange with the atmosphere yielded environmental tracer concentrations (SF6 and CFC-11) very similar to flow-weighted values from nine or ten streambed piezometers that directly sampled groundwater during low streamflow. At higher streamflow on the falling limb of the hydrograph, stream tracer concentrations (after correction for gas exchange) were significantly higher than the flow-weighted mean from piezometers, consistent with dominance of the stream tracer signal by transient influx of surface water and/or younger subsurface water. The apparent MTT derived from SF6 in low flow stream water samples was 26 years, suggesting a groundwater recharge rate of about 210 mm/yr, that is consistent with vertical profiles obtained by sampling nested piezometers in the aquifer. When sampled under low flow conditions when streamflow consists of a high component of groundwater discharge, West Bear Creek appears to act as a flow-weighted integrator of transit times and, streamflow samples can provide fundamental information regarding groundwater recharge rate and MTT. Our study suggests that watershed-scale evaluation of some groundwater flow systems is possible without utilizing monitoring wells.

Quantifying the fate of agricultural nitrogen in an unconfined aquifer: Stream-based observations at three measurement scales

We compared three stream-based sampling methods to study the fate of nitrate in groundwater in a coastal plain watershed: point measurements beneath the streambed, seepage blankets (novel seepage-meter design), and reach mass-balance. The methods gave similar mean groundwater seepage rates into the stream (0.3–0.6 m/d) during two 3–4 day field campaigns despite an order of magnitude difference in stream discharge between the campaigns. At low flow, estimates of flow-weighted mean nitrate concentrations in groundwater discharge ([ NO3−]FWM) and nitrate flux from groundwater to the stream decreased with increasing degree of channel influence and measurement scale, i.e., [ NO3−]FWM was 654, 561, and 451 µM for point, blanket, and reach mass-balance sampling, respectively. At high flow the trend was reversed, likely because reach mass-balance captured inputs from shallow transient high-nitrate flow paths while point and blanket measurements did not. Point sampling may be better suited to estimating aquifer discharge of nitrate, while reach mass-balance reflects full nitrate inputs into the channel (which at high flow may be more than aquifer discharge due to transient flow paths, and at low flow may be less than aquifer discharge due to channel-based nitrate removal). Modeling dissolved N2 from streambed samples suggested (1) about half of groundwater nitrate was denitrified prior to discharge from the aquifer, and (2) both extent of denitrification and initial nitrate concentration in groundwater (700–1300 µM) were related to land use, suggesting these forms of streambed sampling for groundwater can reveal watershed spatial relations relevant to nitrate contamination and fate in the aquifer.

Quantifying an aquifer nitrate budget and future nitrate discharge using field data from streambeds and well nests

Novel groundwater sampling (age, flux, and nitrate) carried out beneath a streambed and in wells was used to estimate (1) the current rate of change of nitrate storage, dSNO3/dt, in a contaminated unconfined aquifer, and (2) future [ NO3−]FWM (the flow-weighted mean nitrate concentration in groundwater discharge) and fNO3 (the nitrate flux from aquifer to stream). Estimates of dSNO3/dt suggested that at the time of sampling (2013) the nitrate storage in the aquifer was decreasing at an annual rate (mean = −9 mmol/m2yr) equal to about one-tenth the rate of nitrate input by recharge. This is consistent with data showing a slow decrease in the [ NO3−] of groundwater recharge in recent years. Regarding future [ NO3−]FWM and fNO3, predictions based on well data show an immediate decrease that becomes more rapid after ∼5 years before leveling out in the early 2040s. Predictions based on streambed data generally show an increase in future [ NO3−]FWM and fNO3 until the late 2020s, followed by a decrease before leveling out in the 2040s. Differences show the potential value of using information directly from the groundwater—surface water interface to quantify the future impact of groundwater nitrate on surface water quality. The choice of denitrification kinetics was similarly important; compared to zero-order kinetics, a first-order rate law levels out estimates of future [ NO3−]FWM and fNO3 (lower peak, higher minimum) as legacy nitrate is flushed from the aquifer. Major fundamental questions about nonpoint-source aquifer contamination can be answered without a complex numerical model or long-term monitoring program.