Geochemical processes and solute transport at the seawater/freshwater interface of a sandy aquifer

Geochemical processes occurring at a seawater/freshwater interface were studied in a shallow coastal siliclastic aquifer containing minor amounts of calcite. Data were collected from 106 piezometers in a 120-m transect from the coastline and landward. In the first 40 m from the coastline, a wedge of saltwater is intruding below the freshwater aquifer. The aquifer is strongly reduced with mineralization of organic matter by methanogenesis in the freshwater aquifer, and sulfate reduction dominating in the most seaward part of the saline aquifer. The spatial separation of cations in the aquifer indicated a slow freshening process where Ca2+ from freshwater displaced the marine cations Na+ and Mg2+ from the exchanger complex. The resulting loss of Ca2+ from solution decreases the saturation state for calcite and possibly causes calcite dissolution. A storm-flooding event was recorded where pulses of dense seawater sank through the fresh aquifer. As a result, the terminal electron accepting process switched from methanogenesis to sulfate reduction. The pulses of sinking seawater also triggered cation exchange reactions where Ca2+ was expelled from the exchanger by seawater Na+ and Mg2+. The released Ca2+ is being flushed from the aquifer by groundwater flow, and this export of Ca2+ will, in the long term, cause decalcification of the sediment. The water composition in the aquifer is in a transient state as the result of various processes that operate on different timescales. Oxidation of organic matter occurs continuously but at a rate decreasing on a geological time scale. The freshening of the aquifer operates on the timescale of a few years. The episodic flooding and sinking of seawater through the aquifer proceeds in the course of days to weeks, but occurs irregularly with years in between.