Shallow Water Table Fluctuations in Relation to Soil Penetration Resistance

Abstract

Hydrologic modeling of catchments is frequently hampered by lack of information on subsurface stratigraphy and zones of preferred flow. We evaluated the usefulness of soil penetration resistance, easily measured by a dynamic cone penetrometer, together with measurements of ground water level fluctuations, as a cost-effective means to infer subsurface flow patterns. At our field site at Sleepers River, Vermont, penetration resistance was lowest in the surficial 10 to 30 cm, then typically increased to a local maximum at 60 to 80 cm, which we interpreted as the soil/till interface. Below this depth usually lies a zone of decreased resistance in the till, giving way to either a gradual or abrupt increase in resistance toward the bedrock surface at 1 to 4.5 m depth. Penetration resistance had a weak but significant negative correlation with saturated hydraulic conductivity determined by bail tests (r 2 = 0.25, p < 0.05). At many wells, monthly ground water levels tended to cluster at or just above the resistant zone near the soil/till interface. Chemical and isotopic dynamics in nested wells finished above and below the resistant zone suggest that the zone may temporarily isolate the deeper ground water reservoir from meltwater inputs, which were clearly identified by low δ 18 O values. In ground water discharge zones, δ 18 O values tended to converge throughout the profile. In contrast, dissolved organic carbon (DOC) maintained a gradient of increasing concentration toward land surface, even in otherwise well-mixed waters, reflecting its rapid release from organic horizons. Understanding the effect of soil penetration resistance on ground water behavior may be useful in future catchment modeling efforts. 964Shallow Water Table Fluctuations conditions on these aquifer properties thus determines the position of the water table-and ultimately, streamflow. The position of the water table also plays a role in stream water chemistry. Infiltrating rain and snowmelt, modified by passage through the organic-rich surficial soil, recharge the ground water. The chemistry of this newly recharged shallow ground water may be markedly different than that of the deeper pre-existing ground water (Christopherson and Hooper 1992). The latter is generally dominated by solutes derived from weathering of minerals in the soil and bedrock. Freshly recharged ground water is generally dilute in weathering solutes but enriched in surficial soil leachates such as DOC, nitrate, and silica . It may take some time for these waters to mix, during which time the shallow ground water system remains chemically stratified. Younger ground water readily discharges to stream water because of higher hydraulic conductivities typically found in surficial soils (Bishop 1991;Kendall et al. 1999). The chemical and isotopic signature of this discharging shallow ground water will reflect its recent origin.In this study, conducted at Sleepers River, Vermont, we analyzed a 10-year record of monthly water table fluctuations and one snowmelt period of chemi...

Taking the pulse of snowmelt: in situ sensors reveal seasonal, event and diurnal patterns of nitrate and dissolved organic matter variability in an upland forest stream

“…1). On hillslopes, a dense basal till at 1-3 m depth is overlain by moderately to excessively well-drained inceptisols and spodosols, with histosols in wetlands (about 5% of the catchment area) and riparian areas (Shanley et al 2003). Stream discharge was calculated from a stage-discharge relationship, with stage measured every 5 min at a 120°v-notch weir instrumented with a float-driven potentiometer.…”

Carbon isotope fractionation of dissolved inorganic carbon (DIC) due to outgassing of carbon dioxide from a headwater stream

“…The calcareous rich units contain approximately 50% calcite (minor dolomite) and they make up slightly less than half the formation (Hall, 1959). Overlying the bedrock is a mantle of dense basal till derived from the bedrock host which varies in thickness from 1-4 m (Shanley et al, 2003). The till is depleted of carbonate in the upper surface as a result of weathering but retains carbonate in its lower horizons .…”

Variation in soil characteristics and hydrologic properties associated with historic land use near a recent landslide, Nagano Prefecture, Japan

“…Down to a depth of 1.3 m, the knocking cone penetrometer easily penetrated the soil (N 10 ≤ 7). Because of variable soil moisture conditions during the three different days in which this survey was conducted and the dependence of penetration resistance on soil moisture (Shanley et al, 2003), we could not assess the depths of the various soil horizons above bedrock. Only at point 5 the upper and lower bounds of the black layer were marked by sharp differences in N 10 values (Fig.…”