Science Center Objects

Hydrogeology and Groundwater Flow, Fractured Mesozoic Structural-Basin Rocks, Stony Brook, Beden Brook, and Jacobs Creek Drainage Basins, West Central New Jersey

 

by Jean C. Lewis-Brown & Eric Jacobsen

 

MODEL VERSION/TYPE: MODFLOW-88, steady state

AREA STUDIED: Mercer, Somerset, and Hunterdon Counties

AQUIFERS SIMULATED: Passaic, Lockatong, and Stockton Formations

MOST RECENT WITHDRAWALS SIMULATED: none

MODEL SIZE: 2 layers, 122 rows, 150 columns

MINIMUM GRID SPACING:500 feet x 500 feet

 

This study was undertaken to characterize groundwater flow in the Stony Brook, Beden Brook, and Jacobs Creek drainage basins in west-central New Jersey. The study area, an 89-square-mile area, is underlain by dipping beds of fractured siltstone, shale, and sandstone and by massive diabase sills. The density of fractures in all the rocks decreases with depth. Rocks on both sides of the major fault that extends through the study area are extensively fractured.

The average annual rates of precipitation and groundwater recharge in the study area are 45.07 inches and 8.58 inches, respectively. The rate of recharge to the diabase rocks is about one-half the rate of recharge to other rocks. Part of the surface runoff from the diabase rocks flows downslope and recharges the groundwater system where more permeable rocks crop out.

The decrease in the density of fractures with depth is reflected in specific-capacity data. The specific capacity per foot of open hole of wells that are less than 76 feet deep is two to six times greater than that of wells 76 to 100 feet deep. Because water-bearing units dip, they are more extensive in the strike direction than in the dip direction, and groundwater flow is skewed toward the strike direction. Groundwater divides generally coincide with surface-water divides, and most groundwater flow in the study area follows short flow paths from the point of recharge to a nearby stream. Most groundwater flow in the study area occurs between the water table and 75 feet below land surface. When the system is unstressed, only about 6 percent of the recharge at land surface reaches depths greater than 75 feet below land surface.

A three-dimensional digital model of steady-state, prepumping groundwater flow was developed to test hypotheses concerning the geologic features that control groundwater flow in the study area. The decrease in the density of interconnected fractures with depth was simulated by dividing the model into two layers of different hydraulic conductivity. Over most of the model area, the upper layer represents the part of the system between the water table and 75 feet below land surface, and the lower layer represents the part of the system deeper than 75 feet below land surface. The pinching out of water-bearing units in the dip direction at land surface and at depth was represented by setting the hydraulic conductivity in the dip direction 2 times lower than in the strike direction for the upper layer and 10 times lower than in the strike direction for the lower layer. The vertical conductivity was slightly higher than the dip-direction horizontal conductivity. This model is appropriate for the analysis of groundwater flow in areas greater than about 0.5 square mile in size and if analysis of flow in discrete water-bearing units is not needed.