Modeling groundwater transit time distributions and means across a Nebraska watershed: Effects of heterogeneity in the aquifer, riverbed, and recharge parameters

Abstract

Groundwater transit time distributions (TTDs), the travel times through the aquifer from recharge at the water table to discharge at the surface water body, provide critical information on the timescales of hydrologic response of subsurface flow systems. We investigated the effects of spatial patterns of recharge, aquifer heterogeneity, and systematic variation in riverbed hydraulic conductivity on the mean transit times (MTTs) and TTDs of groundwater discharge to the Upper Middle Loup River (UMLR) and headwaters using a 3D-steady-state MODFLOW USG-MODPATHD3DU model. This 5436 km2 watershed overlies the High Plains Aquifer in the Sand Hills of Nebraska, USA. Modeled MTTs differed by up to three orders of magnitude from upstream to downstream in a 158 km section of the UMLR under varying recharge, aquifer, and riverbed heterogeneity scenarios. The simulated MTTs ranged from 1 to 397 years for upstream sites and 820 to 7968 years for the downstream sites. The TTDs at upstream sites were dominated by young groundwater from shallow flow paths and were sensitive to changes in riverbed hydraulic conductivity. Recharge parameterization had greater influence on the shape of the TTDs and magnitude of MTTs at the downstream sites, where much older groundwater discharged to the UMLR. Overall, spatial trends in transit times under varying model scenarios provided important information for refined conceptualization and calibration of future numerical models.