Groundwater flowing into estuaries can carry nutrients and pollutants. By studying an estuarine site, researchers found that the geology of the area influences where and how much fresh groundwater discharges. They used seismic data and resistivity profiling to map underground features and measured groundwater flow directly. They discovered that fresh groundwater flows offshore beneath low-permeability sediment, mixing with saltwater before reaching the estuary. In areas without this sediment, fresh groundwater discharges at the shoreline. The type of sediment also affects the exchange of saltwater and the amount of fresh groundwater discharged. This study shows how the geology of an area can impact the quality and quantity of groundwater flowing into estuaries, which can affect the health of coastal waters.