DISSOLUTION INDUCED GROUND-WATER FLOW WITHIN FRACTURED BEDROCK AQUIFERS OF THE NEWARK BASIN

Ground water exhibits complex flow behavior in the fractured bedrock aquifers of the Mesozoic Newark Basin. Sedimentalogical and structural variations in the aquifer framework affect both water table and confined-flow conditions. Ground water is reportedly stored and transmitted along fractures. However, the Brunswick aquifer and the Lockatong Formation include stratified intervals with abundant calcium sulfate and calcium carbonate mineralization that is prone to dissolution and the development of conduits of significant flow. The stratified orientation of these dissolution zones helps account for the alignment of maximum hydraulic conductivity commonly along bedding strike. Regional analysis of ambient ground-water quality from bedrock wells shows that calcium-bicarbonate and calcium-sulfate waters dominate. Variations in the carbonate and sulfate water chemistry appear to reflect regional sedimentalogical trends of authigenic mineralization.

Secondary fractures mapped in outcrop generally appear as open and potentially conductive structures. However, they are mostly healed with calcite and gypsum when observed in deep bedrock excavations and rock cores. Geometric and petrographic analysis of the non-bedding fractures shows that they commonly occur as arrays of en echelon, calcite veins that were healed as they formed. Core samples of the Lockatong Formation and Brunswick aquifer show minimal dissolution of these veins below near-surface depths of less than 6 to 15 meters. Stockton Formation sandstone generally displays a deeper weathering profile with dissolution of vein-fill material to depths over 60 meters. This contrast probably occurs because sandstone has higher matrix porosity and matrix compositions that are less effective in buffering recharged, acidic ground water than the carbonate- and sulfate-rich lacustrine rocks of the Lockatong Formation and Brunswick aquifer. The near-surface dissolution of vein-fill material provides additional pathways for groundwater flow under water-table conditions, and helps explain why hydraulic gradients mapped in shallow bedrock commonly mimic topographic slope. Characterization of water-table flow conditions should include ground water monitoring within the weathered bedrock interval underlying regolith and other unconsolidated sediment.