A Tracer Test for Detecting Cross Contamination Along a Monitoring Well Column

Abstract

A tracer test was used to evaluate whether cross contamination exists along a monitoring well completed through a shallow ground water system in fractured clay and screened in a sand and gravel aquifer. The fractured clay is separated from the sand and gravel deposit by a layer of highly plastic unfractured clay. A natural vertical downward hydraulic gradient of approximately 0.5 exists between the shallow system and the sand and gravel aquifer. Ground water contamination was detected in an adjacent monitoring well screened in the fractured clay and in the monitoring well screened in the sand and gravel deposit. No ground water contamination was apparent in an intermediate well screened in the unfractured clay layer. A tracer of sodium bromide was injected into a shallow boring near the monitoring wells. The tracer was detected in the monitoring well in the sand and gravel aquifer after three to seven days. The bromide concentration continued to increase in this well with time while the concentration in the shallow boring declined. This trend of tracer concentration indicates the tracer has in fact migrated downward and possibly traveled along the well column.

Guidelines and standard procedures for studies of ground-water quality; selection and installation of wells, and supporting documentation

“…Tests can be conducted between screens or sampler ports to evaluate the effectiveness of seals. Possible tests include pumping one screened interval and measuring the hydraulic response in another, or injecting a tracer in one screened interval and searching for the presence of that tracer in other intervals (Meiri, 1989;LeBlanc and others, 1991).…”

A New Method to Characterize Hydraulic Short‐Circuits in Defective Borehole Seals

“…Detection and analysis of hydraulic short‐circuits has been well documented in the literature. Meiri (1989), for example, showed that cross contamination between two aquifers may be detected by a simple tracer test. His detection method examines the chemical impact of a hydraulic short‐circuit but is only qualitative and does not determine the leakage rate.…”

“…For field conditions, detection and evaluation methods should be simple and should yield reasonably accurate material characteristics. Although Meiri (1989), Chapuis and Sabourin (1989), and Chapuis and Chenaf (1998) provide useful detection methods, there is as yet no known method to evaluate the hydraulic properties of a defect and to determine its flow rate.…”

Field Evaluation of Ultrasonic Method for Assessing Well Seals

“…When a riser (also commonly called a well casing) is placed in a borehole, an annular space is cre ated between the riser and the sur rounding soil. If not properly sealed, the annular space can be a potential pathway for transport of contami nants in the subsurface (Meiri 1989;Pekarun 1995;Lacombe et al 1995;Pankow and Cherry 1996). Con tamination can occur due to mixing of adjacent bodies of clean and con taminated ground water or from intrusion of contaminated surface water (Riewe 1996).…”