Confining Unit Effects on Water Quality in the New Jersey Coastal Plain

Abstract

Sediment and pore‐water samples from Tertiary and Cretaceous strata in the New Jersey Coastal Plain were collected from cores from Freehold Township, Monmouth County; Howell Township, Monmouth County; and from Clayton Township, Gloucester County. These strata form a multilayered aquifer system in which upper units are marine origin and lower units are predominantly nonmarine. Concentrations of major constituents in pore‐water samples from confining units and aquifers were highly variable: calcium (1.7–660 mg/I), magnesium (0.3–140 mg/l), sulfate (1.52200 mg/l), and dissolved inorganic carbon (9–290 mg/l); with large differences between marine confining units and aquifers. Where the aquifers are unconfined and oxic, well waters in the Potomac‐Raritan‐Magothy aquifer system in the northern Coastal Plain have higher concentrations of nitrate, chloride, and dissolved oxygen. In areas where the aquifer system is confined and anoxic, there are higher concentrations of sulfate and bicarbonate. Differences in bicarbonate concentrations between two wells (12 and 67 mg/l) along a regional flow path can be caused by mixing of water in a nonmarine aquifer with leached sulfate‐enriched, marine confining‐unit water, and subsequent reduction of sulfate. Bacterially mediated organic carbon oxidation, iron reduction, and sulfate reduction, and ion‐exchange reactions may explain constituent variations. Bacteriological assays of core sections from the Howell and Clayton Township coreholes showed viable sulfate‐reducing bacteria as deep as 558 feet below land surface. Sediments from a depth of 210 feet showed populations of sulfate‐reducing bacteria of 103–l06 organisms per gram of soil.

Sr-isotopic evidence for leakage of pore water from clay-silt confining units to the Atlantic City 800-foot sand, Atlantic City, New Jersey

“…Results of a recent study of the quality of pore water in finegrained sediments in the New Jersey Coastal Plain show elevated concentrations of SO 4 , Ca, and Na (Pucci et al, 1992); these results agree with the findings of Chapelle and McMahon (1991) for claysilt confining units in other parts of the Atlantic Coastal Plain. The chemistry of water in the confined aquifers in the New Jersey Coastal Plain, such as the Atlantic City 800-ft sand, likely is affected if leakage of mineralized pore water from the confining units occurs.…”

“…Understanding of the chemical processes that control the relation between dissolved Ca and SO 4 would be facilitated if the source of the dissolved SO 4 were known. Various possible sources of SO 4 in pore water from clay-silt confining units in the Atlantic Coastal Plain have been proposed, but the data collected to date are inadequate to definitively determine the source (Chapelle and McMahon, 1991;Pucci et al, 1992;Pucci et al, Chapter 24, this volume).…”

“…Another possible source of Sr is a SO 4 -bearing evaporite mineral such as gypsum; however, such a mineral is unlikely to be present in Kirkwood Formation sediments, which are marine in origin. No primary deposits of gypsum have been found in sediment from other boreholes drilled in the New Jersey Coastal Plain (Pucci et al, 1992).…”

Reply to comment by Javier Rodríguez on “Sulfate transport in a Coastal Plain confining unit, New Jersey, USA” (Pucci 1999)

“…The author and co-investigators designed QC and QA protocols for the collection of these data in anticipation of many of the concerns that are expressed in the commentary. The field-investigation team that collected these data was concerned that elevated concentrations of sulfate in confining-unit sediments should not be an ªartifact.º The authors anticipated the occurrence of elevated sulfate concentrations because of previously published investigations of Atlantic Coastal Plain hydrogeochemistry, as discussed in the introduction of the paper (Pucci and Owens 1989;Chapelle and McMahon 1991;Chapelle and Lovley 1992;Pucci et al 1992;Wicks et al 1995).…”

Trends in groundwater quality

“…If so, then kriging or other techniques can be used to develop adequate maps. However, in many instances, the spatial variability of groundwater quality and aquifer characteristics is large (Hoyle, 1989;Rogers, 1989;Pucci et al, 1992;Smith and Ritzi, 1992) and the number of points required to map groundwater quality in a useful way would also be large, perhaps much larger than the available number of monitoring wells. In any case, it is best to be cautious in making statements about regional groundwater quality conditions and in interpreting the results of regional studies.…”