Simulations of Flow in Crystalline Rock and Recharge from Overlying Glacial Deposits in a Hypothetical New England Setting

Abstract

Cross‐sectional, finite‐difference, steady‐state ground‐water‐flow models of a generalized New England landscape were constructed to examine the effects of hydraulic conductivity and topography on bulk‐fluid flow processes in fractured crystalline rock. The generalized landscape consists of a flat hilltop, a hillside that has a slope of 0.17 ft/ft, and a flat river valley. It includes three hydrogeologic units, stratified drift in the river valley, glacial till, and underlying crystalline rock. Distribution of recharge to and discharge from bedrock is affected by the presence of small topographic features, such as terraces on the hillside. Most of these terraces consist partly or entirely of sand, and they create localized areas of recharge to and discharge from bedrock. Maximum recharge to bedrock commonly occurs at downward inflections of the water table, and discharge occurs at upward inflections of the water table. These inflections of the water table are associated with inflections in topography because the shallow depth of water on hilltops and hillsides resemble the configuration of the land surface. Regional ground‐water flow in crystalline rock and distribution of bedrock recharge and discharge are also affected by heterogeneities in hydraulic conductivity of the bedrock. Vertical heterogeneity creates short, shallow flow paths in high‐relief terrain. Horizontal heterogeneity causes large variations in the size of bedrock recharge areas; in some cases the difference from homogeneous conditions is more than 50 percent.

The effect of terrace geology on ground‐water movement and on the interaction of ground water and surface water on a mountainside near Mirror Lake, New Hampshire, USA

“…Terraces of similar size do not seem to be present in watershed NW, where only one known small terrace (FS3) is present directly adjacent to watershed W. To develop concepts of how the presence of terraces might affect ground-water movement in the watershed, Harte and Winter (1995) conducted numerical modelling experiments of two hypothetical settings. In one setting, a sand terrace is positioned on a continuous till unit, which, in turn, overlies bedrock (Figure 2A).…”

“…Small ground-water flow systems could be associated with these terraces, and the flow systems could have different characteristics depending on the size and geologic composition of the terraces (Harte and Winter, 1995); (2) Bedrock is near the surface in certain reaches of the stream channel, and the movement of ground water between bedrock, glacial deposits, and the stream is of interest to understanding the chemical characteristics of the streams, ground water, and the lake; (3) In the vicinity of the stream, the riparian zone has a variety of configurations, from being V-shaped, essentially having no flood plain in some reaches, to other reaches where the flood plain is as much as 20-m wide (Shattuck, 1991). These characteristics raise questions about how stream-flow is generated and how the stream interacts with ground water as the stream water moves down the channel.…”

Using Domestic Well Records to Determine Fractured Bedrock Watersheds and Recharge Rates

“…Importantly, this study reveals that bedrock drainage basins may not coincide very well with drainage Thomas et al (1967) 0.10-1.08 Harte and Winter (1995) 0.025-0.076 Tiedeman et al (1998) 0.18 Mullaney (2004) 0.10-0.19 Spera (2011) 0.09 Metcalf and Robbins (this study) 0.03-0.96 0.001-0.10 basins derived from surface topography owing to scale. It should be noted that the quality of such an analysis is only as good as the quality of the well completion reports and the distribution of the dataset.…”

Characterization of Crystalline Bedrock Contaminated by Dense Nonaqueous Liquid

“…Fractured till deposits have been observed in similar field settings (Harte and Winter 1995). The surficial geology comprises sandy fill over glacial till, which overlies bedrock.…”