Physical controls on septic leachate movement in the vadose zone at the hillslope scale, Putnam County, New York, USA

Abstract

Abstract:The fate and transport of contaminants in the vicinity of septic fields remains poorly understood in many hydrogeomorphological environments. We report hydrometric data from an intensive hillslope-scale experiment conducted between 29 August and 11 November 1998 at a residential leach field in New York State. The objective of our study was to characterize water flux within the vadose zone, understand the physical controls on the flux, and predict how this ultimately will affect subsurface water quality. Soil-water flux was calculated using matric potential measurements from a network of 25 tensiometer nests, each nest consisting of three tensiometers installed to depths of 10, 50 and 130 cm. Unsaturated hydraulic conductivity curves were derived at each depth from field-determined time-domain reflectometry-tensiometry moisture-release curves and borehole permeametry measurements. Flownets indicated that a strong upward flux of soil water occurred between rainstorms. Following the onset of (typically convective) rainfall, low near-surface matric potentials were rapidly converted to near-saturated and saturated conditions, promoting steep vertical gradients through the near-surface horizons of the hillslope. Lateral hydraulic gradients were typically 10 times smaller than the vertical gradients. Resultant flow vectors showed that the flux was predominantly vertical through the vadose zone, and that the flux response to precipitation was short-lived. The flux response was controlled primarily by the shape of the unsaturated hydraulic conductivity curves, which indicated a rapid loss of conductivity below saturation. Thus, soil water had a very high residence time in the vadose zone. The absence of rapid wetting at 130 cm and the delayed and small phreatic zone response to rainfall indicated that water movement through macropores did not occur on this hillslope. These results are consistent with a Cl tracing experiment, which demonstrated that the tracer was retained in the vadose zone for several months after injection to the system.

A new tool for hillslope hydrologists: spatially distributed groundwater level and soilwater content measured using electromagnetic induction

“…The shallow piezometers were installed to about 1 m below the water table across the hillslope and screened over a 30 cm depth interval. Given that the EMI surveys were conducted during dry weather conditions, and that there were no hydrologically confining layers in either the lower vadose or upper phreatic zones (Sherlock et al, 2002), the piezometric head measurements should give a close estimate of d app .…”

“…Of the 116 EMI survey points, 32 were located within 1 m of a piezometer nest, and 19 were located within 1 m of a tensiometer nest. The tensiometer and piezometer networks were installed previously as part of the hydrometric investigation of the septic field (Sherlock et al, 2002). We use the EC data collected from these EMI survey points to compare with measurements of soil-moisture content and water-table depth derived from the co-located instrument clusters.…”

“…The study site was instrumented previously to identify the dominant flow pathways and physical controls on septic effluent migration at the hillslope scale (Sherlock et al, 2002). Hydrometric, water tracing and geophysical techniques were used to achieve this goal.…”

“…A 50 by 50 m plot was established around a residential septic field in Putnam County, New York ( Figure 1) as part of a larger investigation of groundwater flow and transport (see Sherlock et al, 2002). Depth to the water table below the leachate distribution lines ranges from 2 to 3 m, depending upon the time of year.…”

“…Average annual rainfall is 1160 mm. Rainstorms are often intense, resulting in rapid changes in near-surface soil moisture conditions (Sherlock et al, 2002).…”