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

Abstract

Abstract:The west watershed of Mirror Lake in the White Mountains of New Hampshire contains several terraces that are at different altitudes and have different geologic compositions. The lowest terrace (FSE) has 5 m of sand overlying 9 m of till. The two next successively higher terraces (FS2 and FS1) consist entirely of sand and have maximum thicknesses of about 7 m. A fourth, and highest, terrace (FS3) lies in the north-west watershed directly adjacent to the west watershed. This highest terrace has 2 m of sand overlying 8 m of till. All terraces overlie fractured crystalline bedrock. Numerical models of hypothetical settings simulating ground-water flow in a mountainside indicated that the presence of a terrace can cause local ground-water flow cells to develop, and that the flow patterns differ based on the geologic composition of the terrace. For example, more ground water moves from the bedrock to the glacial deposits beneath terraces consisting completely of sand than beneath terraces that have sand underlain by till. Field data from Mirror Lake watersheds corroborate the numerical experiments. The geology of the terraces also affects how the stream draining the west watershed interacts with ground water. The stream turns part way down the mountainside and passes between the two sand terraces, essentially transecting the movement of ground water down the valley side. Transects of water-table wells were installed across the stream's riparian zone above, between, and below the sand terraces. Head data from these wells indicated that the stream gains ground water on both sides above and below the sand terraces. However, where it flows between the sand terraces the stream gains ground water on its uphill side and loses water on its downhill side. Biogeochemical processes in the riparian zone of the flow-through reach have resulted in anoxic ground water beneath the lower sand terrace. Results of this study indicate that it is useful to understand patterns of ground-water flow in order to fully understand the flow and chemical characteristics of both ground water and surface water in mountainous terrain.

Characteristics and controls of variability in soil moisture and groundwater in a headwater catchment

“…These locations were not related to the gully/ridge features in the catchment, in conflict with our prior hypothesis, but instead may indicate preferential groundwater flow paths which channel water from the upper slopes. Such preferential paths were previously reported at Maimai catchment where there is a clearly defined soil-bedrock interface Woods and Rowe, 1996); our results suggest a similar outcome in the Langs Gully catchment despite the gradual transition from soil to broken bedrock. The cross-slope gradients needed to generate the preferential paths could be caused by deeper bedrock structures, or by local areas with high permeability such as the gravel-rich soil layers observed during installation of the soil moisture sensors.…”

“…Winter et al (2008) and Tiedeman et al (1998) monitored 31 bedrock wells and found water table gradients caused by different geological units within a catchment. Even in headwater catchments, variability in groundwater dynamics has been found due to multiple underlying aquifers (Kosugi et al, 2008(Kosugi et al, , 2011.…”

“…The cross-slope gradients needed to generate the preferential paths could be caused by deeper bedrock structures, or by local areas with high permeability such as the gravel-rich soil layers observed during installation of the soil moisture sensors. At Maimai, Woods and Rowe (1996) suggested that preferential flow paths were caused by temporary hydraulic gradients in the soil, and variations in vertical drainage due to patterns of soil moisture deficit. Figure 9a (third panel) shows distinct differences in the timing of the groundwater response between locations.…”

“…Model descriptions of relationships between mean soil moisture and drainage must therefore be altered to take account of soil moisture variability (e.g. Moore, 2007;Wood et al, 1992) and organisation (Lehmann et al, 2007), and may need to change seasonally as soil moisture variability changes (McMillan, 2012). Similarly, averaging of soil texture or water-holding properties should take spatial organisation into account.…”

Impactos del nitrógeno agrícola en los ecosistemas acuáticos

“…Las zonas de interacción entre las aguas subterráneas y superficiales presentan características físico-químicas específicas que afectan a la hidrología, la ecología y a los procesos biogeoquí-micos del nitrógeno. Winter et al (2008) vieron que el agua subterránea presentaba condiciones anóxicas y una composición química diferente al atravesar el cauce de un arroyo e interaccionar con los materiales del lecho. Desde un punto de vista ecológico, la presencia de microalgas bentónicas puede estimular la nitrificación durante el día y la desnitrificación durante la noche (Groffman et al 2009).…”