Period Covered by the Report: 2-1-00 to 12-21-00
Date of Report: 1-20-01
EPA Grant Number: R827015-01-0
Title: Demonstration of a Subsurface Drainage System for the Remediation of Brine-Impacted Soil
Investigators: Thomas M. Harris, John Veenstra
Institutions: University of Tulsa, Oklahoma State University
EPA Project Officer: Bala Krishnan
Project Period: February 1, 2000 to December 21, 2000 (N/C Ext. to April 22, 2001)
Project Amount: $99,769
Research Category: Brine spill remediation
Brine-impacted soil is the most common environmental problem associated with the onshore production of oil and gas. Salt causes the outright death of plants, and the consequent erosion of topsoil. The remediation of brine-impacted soil may be motivated by lease agreements, federal and state regulations, landowner claims, and the fear of long-term liability. At the present time, the most common remediation strategy applied to brine-impacted soil is in-situ chemical amendment (ISCA),. This treatment entails the application of gypsum, hay, etc. to the soil to restore its permeability and fertility. Since such treatments are designed to encourage the downward movement of salt through the soil profile, they will fail if there is an impediment (such as a low-permeability subsoil) to this downward movement. Such conditions are not uncommon in the oil-producing counties of northeastern Oklahoma, for example.
Subsurface drainage may be used to accelerate the remediation of brine-impacted soil by enhancing the lateral movement of salt through the contaminated topsoil. This project, which concerns the further development of this technology, has three objectives. The first is to evaluate innovative uses of limestone gravel in the drainage for the purpose of reducing the cost of installing the drainage. This material may also serve to enhance the permeability of the surrounding soil, by providing the calcium ions required to counteract the sodicity of the brine-impacted soil. In addition to the treatment of a contemporary brine spill, this strategy will be considered for the treatment a historical "brine scar", where topsoil applied to the site must be protected from the upward migration of salt during periods of dryness. The third objective is to demonstrate the use of a solar evaporation pond for collecting the salty leachate from a subsurface drainage system, and reducing its volume through evaporation in order to reduce the cost of disposal.
The Keefer lease, 1 mile east of Bartlesville, Oklahoma on Highway 60, has been chosen as the site for this demonstration. This lease was the site of a large waterflooding operation in the 1960's. It is currently operated by Marjo Oil Co. One area within this site (Area I) is contaminated with salt but has retained much of its topsoil. Another more extensive area (Area II) is deeply scarred by erosion, with absolutely no topsoil remaining.
The performance of the subsurface drainage systems is being assessed from the amount of salt removed as leachate by the system, as well as by the decrease in the salt content of the soil (and consequent revegetation) in the test plots. Due to the extremely hot and dry weather in Oklahoma during the late summer and fall little leachate has flowed up to this point. To date leachate samples have been collected only from the pond, and on only one date, 1/21/01.
The soil has been too hard to core so far; this was due to a lack of rainfall in the fall, and uncharacteristically cold temperatures in the winter. However, composite surface soil samples have been collected on two different dates, 9/24/00 and 1/21/01. These samples were collected with a shovel, forced into the soil to a depth of approximately 10 cm. Soil samples were extracted within two weeks of their arrival at the laboratory. Measurement of the conductivity and determination of the major anions and cations was then performed no more than three days after extraction. The leachate samples were analyzed at the same time as the soil extracts.
The conductivity of samples of leachate from the pond and of soil extracts was measured using a YSI conductivity bridge and dip probe. The concentrations of chloride and sulfate ion in the leachate sample and the soil extracts were determined by ion chromatography. The concentrations of the major cations (sodium, calcium and magnesium ion) were determined in both soil extracts and the leachate sample using inductively coupled plasma atomic emission spectrophotometry (ICP-AES).
The conductivity of the single evaporation pond sample, collected on 1/21/01, is 1000 mhos. A sample of standing water collected from test cell II-1 provided a conductivity of 1950 mhos. Thus, the leachate in the pond is more dilute than the standing water, which is presumably in equilibrium with the soil in that cell. It may be concluded that the pond has received run-off in addition to leachate from the subsurface drainage system. Remedial construction work on the area surrounding the pond will be performed to correct this problem.
Based on the analysis of the extracts of the soil samples, it may be concluded that the extent of brine contamination in Area I is far from being uniform. Nevertheless, since every test condition is being evaluated in at least different test cells, and the cells were assigned the test conditions randomly, we should still be able to formulate conclusions about efficacy of the various treatments. Also, there was a significant decrease in the contamination levels from the first sampling date (9/24/01) to the second sampling date (1/21/01). While it is tempting to attribute this decrease in contamination to the remediation system, it is more likely due to an artifact of the samples. All of the data in these figures correspond to surface samples. It has been observed in other studies that the concentrations of brine components will increase at the surface during periods of hot, dry weather. Such conditions did exist at the time of the first sampling; in fact, the first sampling had been preceded by a period of 3 months during which it rained only once, for a total of less than 1 cm of precipitation.
The conductivity and the chloride and sodium ion concentrations have not increased significantly in the test cells of Area II to which topsoil has been applied. In addition, test cells II-3 and II-4 show even less increase in these values than cell II-2. This observation is consistent with the gravel layer in cells II-3 and II-4 acting as an effective barrier to the upward movement of salt, thus ensuring that the topsoil applied to the site will not be contaminated itself.