The growing realization that remediation of many contaminated sites is technically infeasible has resulted in a shift in emphasis to containment as an alternative to remediation. Monitoring is required to demonstrate the effectiveness of engineered cover systems in minimizing infiltration into underlying waste. The purpose of this study is to evaluate a variety of monitoring technologies. Monitoring systems were installed in a resistive (GCL/asphalt) barrier at 1.3 m depth and a conductive (capillary) barrier at 2.0 m depth constructed near El Paso, Texas, in 1997.

Monitoring of most systems began in October 1997. The monitored components of the water balance include precipitation, surface runoff, lateral drainage from the asphalt layer, deep drainage, and water content. Evapotranspiration (ET) is calculated as the residual of the monitored components. Ground conductivity measurements using an EM38 (Geonics, Mississauga, Ontario) in the vertical and horizontal dipole modes were conducted concurrent with the neutron probe measurements to evaluate electromagnetic induction (EM) as a non-invasive technique for monitoring water content.

Results of the water balance monitoring for the last three months of 1997 and for the complete years 1998 through 2000 are presented in Table 1. Net infiltration was calculated as the sum of precipitation and irrigation minus runoff. The storage values shown are the results of the neutron probe surveys. The data indicate that both barrier designs have performed similarly. However, annual precipitation has averaged only 56% to 75% of the 30-year average (116% for 1998 including irrigation) with the result that neither barrier has been sufficiently stressed to induce drainage.

TDR has been of limited use for monitoring water content at our site. Periods of higher water content and/or higher soil temperature increased the sandy clay loam subsoil bulk conductivity to the point that TDR waveforms displayed little or no reflections. Probes installed in the topsoil were generally less affected. Water content values derived from analysis of the less attenuated waveforms were not in agreement with the neutron probe measurements despite calibration of the TDR probes using site soils. The TDR measurements also displayed seasonal changes in water content ranging from 0.05 to 0.10 m3/m3 at depths where no changes were indicated by the neutron probe measurements.

Electromagnetic induction has proved to be a rapid and accurate method for determining water storage at our site. The models predict water contents to within 0.007 m3/m3 of the measured averages. The GCL/asphalt barrier models had similar results. Using each of the individual location models to calibrate all of the remaining location data resulted in predicted water contents averaging within 0.015 m3/m3 of the measured average. These results demonstrate the usefulness of applying electromagnetic induction to obtain accurate measurements of water storage.

Heat dissipation sensors have been more reliable than thermocouple psychrometers for monitoring soil water potential at our site. The HDS instruments displayed rapid and stable responses to changes in water content and the data are in agreement with the neutron probe measurements. Duplicate HDS instruments generally displayed measurements within 10% of each other at potential values above -1.0 MPa. In contrast, most of the TCP data were not in agreement with the HDS or neutron probe data and duplicate TCP instruments rarely displayed similar responses. Heat dissipation sensors provide accurate measurements in the wet range (=-0.5 MPa) and have the additional advantage of measuring a wider range of potential energy values (-0.01 to -1.0 MPa) compared to thermocouple psychrometers (-0.5 to -8 MPa).

Reference
Reedy, R. C., and Scanlon, B. R., 2002, Long-term water balance monitoring of engineered covers for waste containment, in 2001 International Containment and Remediation Technology Conference, Orlando, Florida, Institute for International Cooperative Environmental Research, Florida State University, Paper ID. No. 073, http://www.iicer.fsu.edu, 3 p. [PDF]

February 2003