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Groundwater transit time distribution and mean from streambed sampling in an agricultural coastal plain watershed, North Carolina, USA
Author(s) -
Gilmore Troy E.,
Genereux David P.,
Solomon D. Kip,
Solder John E.
Publication year - 2016
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1002/2015wr017600
Subject(s) - groundwater , hydrology (agriculture) , aquifer , sampling (signal processing) , environmental science , groundwater flow , watershed , groundwater model , groundwater discharge , geology , modflow , geotechnical engineering , filter (signal processing) , machine learning , computer science , computer vision
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 .