Example: monitoring at former mining sites

Large quantities of residues possibly containing radionuclides remaining at or near the surface and mine workings that may remain open are typical of former mining sites. Potential contaminant sources that require monitoring include areas not remediated to free release, surface and underground workings, tailings ponds and waste rock piles.

Increased surface areas underground, the opening of airflow pathways and the lowering of the groundwater table may allow radon to migrate from radionuclide bearing rocks into buildings above the mine site, thus possibly creating a radiological problem. As long as the mine ventilation is operating, the concentrations are kept below levels of concern and the radon is vented in a way that avoids significant exposures. Without ventilation the radon concen-tration in dwellings on the surface may increase significantly. Radon levels may need to be monitored and appropriate management strategies introduced.

10.5.2. Monitoring at waste rock piles and tailings ponds

After remediation, monitoring of seepage water for aqueous contami-nants, air for radon and engineered structures, such as covers, for their stability will be required to prove the long term effectiveness of the remediation measures and to provide the necessary reassurance to the public [117, 118]. The duration of the performance verification monitoring phase is usually determined by the licensing authorities in consultation with the operators, taking into account the overall management plan. Inspections may be timed so as to efficiently capture any potential change and may be as far apart as several decades. The measurements mainly relate to the:

(a) Quality of seepage water and groundwater; the monitoring of the chemical composition may extend over considerable periods of time, possibly 20 years or more.

(b) Radon exhalation and the radon concentration of the air close to the ground over a sufficiently long time to gain confidence that stable conditions have been achieved; such measurements may need to be continued for a considerable number of years. Owing to changing seasonal exhalation conditions, two measurements per annum, one in winter and one in summer, are typically needed.

(c) Soil mechanical parameters of covers and other engineered structures in order to detect unfavourable changes in water content, porosity, density, soil fabric, etc.

Measurements are usually carried out by the operator or the site steward and are periodically reviewed by the regulatory authorities.

10.5.3. Monitoring at closed mines

Closed mines present a special category of objects requiring monitoring, particularly concerning the chemistry of any discharging mine water [119, 120].

Acid mine drainage is a common problem, which is exacerbated in some (uranium) mines by residual fluids from in situ leaching operations.

A reliable model based forecast of the mine water development can provide a good reference for the scope, frequency and likely duration of monitoring activities. The contaminants to be monitored depend very much on the specific situation, but commonly involve radioactive components, non-radioactive contaminants, such as arsenic and heavy metals, and major constit-uents. Comparison of measured concentrations with modelled forecasts gives an indication of how long any water treatment and monitoring may be needed.

In deep mines, depending on the mine geometry, the main processes that maintain concentration gradients are convection and diffusion. The water volumes to be treated under a stewardship programme depend on the respective recharge rates in the area and the ensuing water balance in the mine.

Mine water volume streams can be as high as 500–1000 m³/h.

Underground mines typically extend below the water table, and restoring the water table to pre-mining levels or another suitably defined operational level is part of a decommissioning and stewardship programme. The objectives of the flooding are to:

(a) Stop oxidation processes;

(b) Minimize water treatment costs and emissions and maximize the radiation protection of the workers by suitable controls on the flooding.

A stepwise flooding scheme, whereby the monitoring results provide data for corrective actions if the system does not behave as predicted or envisaged, is recommendable.

Safe mine closure requires a thorough understanding of the hydrogeology and hydraulics of the mine and the surrounding environment. Meaningful monitoring points are the basis for a model developed with this understanding, which is by no means trivial. A more detailed discussion of the respective requirements, however, is outside the scope of this report.

10.5.4. Scope of monitoring versus land use

Revegetation of covered waste rock piles is commonly allowed, or rather cannot easily be prevented in temperate or tropical climates. Other uses usually require a more involved permit procedure and appropriate monitoring. In order to determine the scope — from a radiological point of view — of potentially allowable site uses, expected exposures of critical groups or individuals are calculated for each use. The monitoring programmes are then designed to suit the site use chosen. Recreational uses with short term occupancy such as a golf course or an airfield for model aircraft, on waste rock piles, may be preferable, for instance, to industrial or residential developments.

10.6. RESEARCH NEEDS

Most monitoring programmes are labour intensive and, for large sites, the expenditure and effort required to operate monitoring programmes are signif-icant. The focus of research and development for this stewardship activity may,

therefore, be best oriented towards the development or improvement of monitoring techniques that reduce the effort and cost of operating the programmes over long periods of time:

(a) Improvement of in situ monitoring sensors with a view to increasing their stability and reliability over the longer term, thus reducing the need for and cost of maintenance;

(b) Improvement of multiparameter sensors for environmental monitoring of the various media used, including possibly the application of nanotech-nology;

(c) Improvement of reliable information networks (possibly utilizing wireless technologies) to make the data collection processes used more efficient;

(d) Development of improved image processing and pattern recognition capabilities to improve automated detection of change at a site, for example, by comparison of visual light or invisible light images in a time series [116];

(e) Improvements to security monitoring instrumentation and strategies, for example intrusion detection;

(f) Improvement of back-calibration methodologies that allow modelling to be used to extend the time frames for prediction;

(g) Further development of technologies for the communication of monitoring results to stakeholders, for example utilizing the Internet (webcams and display of data in real time);

(h) Further development of sensors and automation technologies and strategies in order to trigger alarms only when significant changes in measured variables occur.