Fig. 1: Local-scale studies of submarine groundwater discharges and seawater intrusion.

These local-scale studies use a variety of approaches, including hydrological models, seepage meters, and solute concentration measurements (see review by Santos et al. ⁶⁵). We show locations where local-scale studies have demonstrated or simulated the occurrence of submarine groundwater discharge (blue points), or a landward gradient or seawater intrusion (red points). For primary results and discussion of important local considerations see refs. ^66,67,68. For reviews of studies of submarine groundwater discharge research and seawater intrusion prior to the year 2000 see refs. ^9,69. For primary references see Supplementary Table 5), which reviews 108 studies. We present a series of conceptual models based on previous studies as cross sections. a In Puget Sound^12,13,14 (Washington state), high relief, high recharge rates and modest groundwater use create seaward hydraulic gradients and concomitant submarine groundwater discharge. b In the Salinas Valley^15,16,17 (California), decades of extensive groundwater pumping and limited local recharge (due to a shallow aquitard) have increased landward hydraulic gradients and induced seawater intrusion. Seawater has moved several kilometers inland in both the shallow (180-foot) aquifer and the deeper (400-foot) aquifer. c In the West Coast Basin^18,19 (Los Angeles, California), high groundwater withdrawals have created landward hydraulic gradients and seawater intrusion, but the installation and operation of injection wells have increased hydraulic heads in the aquifer close to the coast. d In the Gulf Coast of Alabama²⁰, hydraulic heads lie just above sea level yet some intrusion has occurred. e Near Miami^39,40 (Florida), a shallow carbonate aquifer (Biscayne) has experienced seawater intrusion, likely exacerbated by the construction of leaky canals. f In Savannah²⁷ (Georgia), groundwater withdrawals have drawn down a piezometric surface that once lay above sea level to now lie below sea level, creating a landward hydraulic gradient. g In Cape May³⁵ (New Jersey), landward gradients are clearly reflected in piezometric data for confined aquifers (e.g., the Cohansey and Atlantic City 800-foot sand), highlighting the potential vulnerability of confined aquifers to seawater intrusion. Aquifer outlines are from water.usgs.gov/ogw/aquifer/map.html.