Origins of seawater intrusion in a coastal aquifer — A case study of the Pajaro Valley, California

Seawater may enter and contaminate stratified coastal aquifers through a number of different pathways. These pathways and their relative contribution are examined in the Pajaro Valley, California, a coastal area with extensive groundwater development. This study considers three pathways of possible intrusion of the primary confined aquifer: (1) onshore leakage from brackish sources, the estuary and sloughs, through the confining layer; (2) near-shore leakage from the ocean through the confining layer; and (3) offshore flow from the ocean through the submarine canyon outcrop of the aquifer. Groundwater flow and seawater intrusion are simulated using an areal, two-dimensional solute-transport computer model. This analysis indicates that leakage through confining layers is the principal mechanism of recharge to the aquifer. Although lateral flow through the offshore outcrop contaminates the aquifer, as a whole, at a higher rate, vertical leakage through the sea floor initially is the main pathway of seawater intrusion to the onshore portion of the aquifer. It is likely that leakage generally is the dominant mechanism of recharge and initial cause of seawater intrusion for poorly-confined, stratified coastal aquifers. This analysis suggests that a significant time interval follows the initial observation of seawater intrusion, during which remedial action can be taken to control lateral flow through the offshore outcrop, which ultimately will be the largest component of future intrusion in these aquifers.