Accuracy of travel time distribution (TTD) models as affected by TTD complexity, observation errors, and model and tracer selection

Water Resources Research
By: , and 

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Abstract

Analytical models of the travel time distribution (TTD) from a source area to a sample location are often used to estimate groundwater ages and solute concentration trends. The accuracies of these models are not well known for geologically complex aquifers. In this study, synthetic datasets were used to quantify the accuracy of four analytical TTD models as affected by TTD complexity, observation errors, model selection, and tracer selection. Synthetic TTDs and tracer data were generated from existing numerical models with complex hydrofacies distributions for one public-supply well and 14 monitoring wells in the Central Valley, California. Analytical TTD models were calibrated to synthetic tracer data, and prediction errors were determined for estimates of TTDs and conservative tracer (NO3−) concentrations. Analytical models included a new, scale-dependent dispersivity model (SDM) for two-dimensional transport from the watertable to a well, and three other established analytical models. The relative influence of the error sources (TTD complexity, observation error, model selection, and tracer selection) depended on the type of prediction. Geological complexity gave rise to complex TTDs in monitoring wells that strongly affected errors of the estimated TTDs. However, prediction errors for NO3− and median age depended more on tracer concentration errors. The SDM tended to give the most accurate estimates of the vertical velocity and other predictions, although TTD model selection had minor effects overall. Adding tracers improved predictions if the new tracers had different input histories. Studies using TTD models should focus on the factors that most strongly affect the desired predictions.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Accuracy of travel time distribution (TTD) models as affected by TTD complexity, observation errors, and model and tracer selection
Series title Water Resources Research
DOI 10.1002/2014WR015625
Volume 50
Issue 7
Year Published 2014
Language English
Publisher American Geophysical Union
Contributing office(s) National Research Program - Western Branch, California Water Science Center
Description 23 p.
First page 6191
Last page 6213
Country United States
State California
Other Geospatial Central Valley