ESTIMATING GROUNDWATER TRANSIT TIMES THROUGH A COASTAL AQUIFER USING MODPATH

Groundwater discharge to streams and estuaries is a large contributor to coastal nutrient budgets. Long groundwater transit times introduce a lag between when solutes enter the aquifer and when they discharge to coastal waters, so characterizing transit times of groundwater discharging to streams and bays is necessary for management of coastal watersheds. As part of an investigation to estimate nutrient fluxes to Indian River Bay, DE, advective particle tracking with MODPATH was used to estimate transit times of groundwater discharge to coastal streams and bays. Although this approach has been widely applied, implications of alternate particle tracking-approaches on the resulting transit time distributions are not well understood. Mean transit times calculated with forward-tracked particles (92-yr average) were insensitive to the number of particles released in each cell. Transit times calculated by reverse-tracking one particle from each stream and bay boundary cell with groundwater discharge (86-yr average) were slightly shorter than forward-tracked transit times. However, calculated mean transit times decreased toward an asymptote of 57 years as additional particles (up to 125 particles per cell) were reverse-tracked. Forward-tracked particles did not travel to all stream and bay cells with groundwater discharge, so transit times could not be assigned to each cell, whereas the reverse-tracking approach allowed transit time calculation for every groundwater discharge cell. We calculated mean transit time for each stream and bay cell with groundwater discharge as the mean transit time of all particles tracked from that cell weighted by particle velocities to account for greater flux associated with high-velocity flowpaths. Watershed-wide transit time distributions were calculated by flux-weighting these transit times by groundwater discharge to streams and bays from each cell. With reverse-tracking, straight, velocity-weighted, and flux-and-velocity-weighted model-wide mean transit times were 272, 68, and 57 years, respectively. Transit times were insensitive to weighting in forward-tracked scenarios. In summary, transit times may be effectively calculated with MODPATH, but care must be taken to select an adequate tracking scheme because transit times are sensitive to the chosen method.