Use of a Microelectrode System for the Determination of Redox Species and Leachate Tracers in the Slough at Norman Landfill, Oklahoma

WRD PROJECT #: MD157
PROJECT CHIEF: Lorah, Michelle
BEGIN DATE: 01-Apr-2001
END DATE: 30-Sep-2001

Customers currently supporting the project:

U.S. Geological Survey

Problem

Contamination is present in the porewater of a slough, a shallow intermittent stream and wetland, that overlies the leachate plume at a closed municipal landfill near Norman, Oklahoma, which was established in 1995 as a research site under the U.S. Geological Survey Toxic Substances Hydrology Program.

Temporary wells and permanent multi-level samplers have been sampled along a transect downgradient from the Norman Landfill and on either side of the slough to characterize the leachate plume in the alluvial aquifer. The anoxic leachate plume in the alluvial aquifer is characterized by elevated chloride, nonvolatile dissolved organic carbon, volatile organic compounds, pH, alkalinity, ferrous iron, ammonium, and methane, and by depletion of sulfate compared to background ground water. In addition, high concentrations of barium (up to 100 uM) were detected in the contaminant plume from barite dissolution where sulfate is depleted.

The slough, which lies roughly parallel to the landfill and about 100 m to the southwest, has ponded areas most of the year that are caused by beaver dams and is about 0.75 m deep. The slough flows southeast and discharges into a wastewater effluent stream from the City of Norman wastewater treatment plant. Above the wastewater effluent stream, the slough has a flow of less than 0.01 m 3/8 and has become dry during drought periods. Leachate-contaminated ground water from the shallow aquifer discharges to the slough along the northeast bank, whereas slough water recharges the aquifer along the southwest bank. This is based on the ground-water chemistry observed on either side of the slough along the transect and on the configuration of the potentiometric surface. The water table is shallow, rarely exceeding a depth of 2 m in the area of the leachate plume, and shows strong diurnal and seasonal fluctuations.

To estimate the sources of ground water discharging into the slough, Schlottmann measured chloride concentrations and enrichment in deuterium and 18 oxygen in 58 ground-water samples and 2 slough samples in three sampling events between 1995-97. The ground-water and slough samples were collected along the transect. These results suggested that uncontaminated native ground water is the dominant source of water to the slough. On two of the three sampling events, there was no apparent leachate discharge to the slough. On one sampling event, leachate discharge apparently occurred and was diluted approximately 70 percent by recent recharge before entering the slough.

Objectives

The objectives of this proposed study for FY01 are to (1) characterize the distribution of major redox species in shallow porewater of the slough during one sampling event, (2) identify potential tracer(s) of leachate contamination that could be measured with the microelectrode and test for these in porewater of the slough, and (3) adapt and test the microelectrode system for measurements in wetland and stream-bottom sediments.

Approach

Field measurements for redox species and potential leachate tracers will be conducted in July or August 2001 when surface-water levels are expected to be lower in the slough. This will allow easier access to sites and manipulation of the microelectrode during initial testing, and allow time for necessary laboratory preparation before field-testing. Laboratory preparation will include construction of gold amalgam microelectrode probes and develoment of calibration curves for each probe and for each analyte of interest. Redox species that will be analyzed include dissolved oxygen, manganese, ferrous iron, and sulfide. Barium could potentially be measured with the microelectrode system as a tracer of leachate-contaminated water discharging to the slough. Other potential tracers might be identified through assessing all analyses collected to date for metals. Calibration curves should be developed using shallow ground water collected from the site, providing calibration under similar ionic strength and pH conditions that will be encountered in the field and allowing testing and correction for possible interferences. Thus, a preliminary trip to the site will be necessary before actual field tests of the microelectrodes. The equipment we have used to vertically align the microelectrodes and push them into the sediment also will need to be adapted for stream-bottom measurements in 0.75 m deep surface water.

Redox and leachate discharge characterization of the slough porewater will be completed at two sites -one potentially leachate-contaminated site where the established transect of multilevel samplers borders the slough, and one background site upstream of the leachate plume. The microelectrode system, porous diffusion samplers (peepers), and temporary drive-point piezometers will be used to obtain redox measurements across the width of the slough and to a depth of about 2 m below land surface at each site (to the top of the alluvial aquifer). The microelectrode will be used to obtain detailed depth profiles at each site as deep as possible by direct push into the sediment (at least 50 cm based on our previous work). Deeper measurements will be obtained with the microelectrode by downhole measurements in existing multilevel samplers or newly installed temporary piezometers, and by direct measurements in the peeper water once they are removed from the sediment. The peepers that will be used are 60 and 120 cm long and contain 21 sample chambers, spaced about 2.5 and 5 cm apart, respectively. To reach greater depths in the wetland sediment and the top of the alluvial aquifer, we will use the 3 new multilevel samplers that were installed this summer in the slough along the established transect. At the background site, temporary PVC drive-point piezometers that have a 1.25 or 2.5 cm diameter will be manually pushed into the sediment.

Two microelectrode probes will be used to evaluate the ability to obtain in situ measurements of temporal variations from diurnal effects. One probe will be left in the wetland sediment and one in a piezometer for three to four days while periodic measurements of redox species are made. Sensitivity and calibration of the probes will be tested after this period. Currently, durability of the probes and precision of measurements under longer-term monitoring conditions is unknown.

Water samples will be collected from the peepers, multilevel samplers, and temporary piezometers for standard chemical analyses of redox and leachate tracer species for comparison to the microelectrode results. Water samples also will be analyzed for nonvolatile organic compounds (NVDOC) and methane to assist in characterization of the slough porewater. Concentrations of NVDOC will be determined using a carbon analyzer at the USGS, Reston, VA. Concentrations of methane will be determined by gas chromatography with a flame ionization detector in the USGS Maryland-Delaware-DC District.