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15 Despite the prevalence of density-driven flow systems in brine-rich aquifers of arid climates 16 and coastal aquifers, the impact of realistic geologic conditions remains poorly constrained re17 garding interface geometry in arid regions and time-sensitive density-dependent dynamics in 18 brine-bearing aquifers in general. Salar de Atacama provides an analog for exploring interface 19 dynamics in arid regions. A site-specific 2D hydrostratigraphic interpretation is used to ex20 amine the dynamics of the brine-to-freshwater interface. With the same simulation framework 21 and core data, a separate parametric series of hydraulic conductivity distributions with vary22 ing horizontal continuity provides a mechanistic explanation for observed dynamics. Compar23 ing modeled interfaces and their sensitivity to perturbations in recharge in each realization yields 24 insight into interface dynamics coupled with horizontal continuity in subsurface heterogene25 ity. Recharge fluctuation is introduced to each distribution following the interface reaching a 26 dynamic steady state. Metrics for results evaluation include migration length, interface slope 27 geometry, and response rate. Analyses suggest that the slope of the modeled interface shal28 lows or decreases by 0.01 to 0.05 m · m-1 for every increase in continuity of highly perme29 able pathways by a factor. Increasing continuity also increases both the overall response times 30 and the variability in response. Results indicate that accurate representations of transient dy31 namics in modeling density-driven brine-to-freshwater interface dynamics requires the con32 sideration of heterogeneity, as saline intrusion in the highest continuity group extends over twice 33 as far on average and the modeled interface takes over 43 percent more time on average to reach 34 a new dynamic steady state when compared to their homogeneous counterparts. 35 Plain Language Summary 36 Differences in the density of groundwater from dissolved salt causes impacts ground37 water flow which behaves differently under different subsurface conditions. This paper focuses 38 on how spatial differences in porosity affects flow behavior and changes the risk of saline ground39 water intrusion. Core and groundwater data from the southeastern edge of the salt flat in Salar 40 de Atacama provides information on subsurface physical characteristics and groundwater chem41 istry. This data is used to create an interpretation of how flow properties vary in time and space. 42 This provides a means for assessing the role of spatially variable geologic units on the geom43 etry and the sensitivity of the brine-to-freshwater interface to changes in recharge to an aquifer. 44 We use distributions of the area’s geology to test different amounts of geologic variability in 45 the horizontal versus the vertical direction. Results indicate that more horizontal continuity 46 shallows the geometry and increases the the time required for a brine-to-freshwater interface 47 to reach a new position following a change in recharge. These model results suggest that it is 48 important to consider how differences in subsurface porosity impact the response time and the 49 extent of saline intrusion for deserts as well as any other areas that might have salty ground50 water. 51