sions initial geologic framework is developed, hydrogeologic data, geophysics, groundwater chemistry, and monitoring trends can refine the seawater intrusion conceptual model. This conceptual model supports practical decisions and can serve as the basis for numerical models. Where Seawater Intrusion Is Occurring in California
A
Seawater intrusion has affected many of California’s coastal groundwater basins (Figure 2), but the causes and management responses vary by region. A useful contrast is between agricultural basins in Northern and Central California and municipal-dominated basins in Southern California.
B Figure 1: Coastal Geologic Cross Sections in the Long Beach Area: Comparison of an oversimplified aquifer designation (A) to a robust stratigraphic interpretation (B). From Ehman and Edwards (2014)
A robust geologic model is therefore essential to developing the seawater intrusion conceptual model. Groundwater levels and chemistry data are difficult to interpret without knowing which hydrostratigraphic units the wells monitor, how those units are connected, and where they are hydraulically separated. For complex coastal basins, a simplified “layercake” aquifer interpretation is rarely sufficient because it can obscure the discontinuous sands, facies transitions, paleochannels, confining-unit breaches, and structural controls that govern saline water migration (Figure 1). The geology provides the subsurface “plumbing” that explains why one well shows increasing salinity while another well, sometimes nearby, does not. Developing this framework requires basin-scale geologic characterization, including depositional history, stratigraphic relationships, aquifer continuity, confining units, faults, and facie changes. After the
Hydro Visions
Figure 2: Groundwater basins impacted by seawater intrusion
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