Geochemical variations in a core of hydrogeologic units near Freehold, New Jersey

Abstract

Solutes were determined for 26 pore-water samples extracted from Tertiary and Cretaceous core material from a 1,320-foot-deep test borehole at Freehold, New Jersey. The cored materials are sediments that form a multilayered aquifer system of seven aquifers and eight confining units in the New Jersey Coastal Plain. The sediments are of marine origin in the upper 650 feet of the core and primarily nonmarine below 650 feet. Total concentrations of silica and major anions (SO 4 -2 and Cl - ) and cations (Ca +2 , Mg +2 , total Fe, Mn +2 , Na + , and K + ) in the pore-water samples varied with depth. Three core intervals are defined by water- chemistry variations, the environment of deposition (marine or nonmarine), and the degree of alteration (weathered or unweathered) of the units. Calcium, magnesium, sulfate, and total-ion concentrations were greater in the midcore (unweathered marine) interval than in the upper (weathered marine) and lower (fluviodeltaicsilicate) intervals of the core. Generally, pore-water chemical types in confining units were distinct from those found in aquifers–particularly in the midcore interval. In this interval, observed variations in pore-water chemistry in part reflect carbonate dissolution and cation exchange reactions common in Coastal Plain sediments. However, high concentrations of sulfate in unweathered marine sediments indicate novel processes may be important factors influencing pore-water chemistry in confining units.