WHICH CONNECTIVITY METRICS CAN BE USED TO PREDICT SALINITY PATTERNS IN HETEROGENEOUS COASTAL AQUIFERS

Geologic heterogeneity is an important feature affecting variable-density groundwater flow and solute transport in a coastal aquifer. “Connectivity” is a primary characteristic of heterogeneity and refers to the spatial connectivity of geologic features and flow. Connectivity results in preferential flow, varying travel times, and complex subsurface salinity distributions. Several connectivity metrics have been proposed to quantify this characteristic. In this study we examine the ability of several of these metrics to predict the simulated steady-state salinity patterns from a series of simulated heterogeneous coastal aquifers. We consider three types of connectivity that differ in the variables used to calculate them: statistical connectivity, transport connectivity, and flow connectivity. Statistical connectivity is based primarily on the geometry of lithofacies arrangement and geostatistical metrics of connectivity. Transport connectivity indicators are based on advective particle tracking results. Flow connectivity is quantified with the hydraulic conductivity. We show that these global statistics fail to predict patterns of salinity in a series of 2D cross-sections simulated with SEAWAT. We suggest a new proxy based on flow tubes developed from single-density flow simulation with MODFLOW. The primary advantage of this proxy is its ability to quickly predict salinity distributions so that uncertainty related to subsurface heterogeneity can be calculated from multiple models in a short time period relative to that required using variable-density flow and transport simulators.