On completion, bores should be developed to remove excess fine sediments in and surrounding the filter pack, to increase near-bore permeability and minimise the turbidity of future water samples.
New bores should not be developed within 24 hours of construction, to allow for concrete and bentonite seals to stabilise. Purging should continue until the purge water is visually clear or for a maximum period of two hours. Field water quality parameters should be measured and recorded to confirm removal of any added water.
Groundwater flow assessment:
Groundwater flow assessments apply to high and extreme risk proposals only. In order to gauge the direction and rate of flow of water in aquifers, the hydraulic conductivity of the aquifer and the slope of the watertable (hydraulic gradient) should be determined where practical.
To calculate the hydraulic gradient, measure the depth to groundwater and convert this to Australian Height Datum (AHD). Measurements can be made physically using a float, string and tape measure or with an electronic water level probe.
The hydraulic conductivity of the aquifers should be determined by constant-rate pumping tests, falling head (confined aquifers only) or rising head permeability test methods, depending on bore construction and aquifer permeability. If necessary, drilling logs and groundwater sampling measurements can supplement these calculated values.
The hydraulic gradient, hydraulic conductivity and estimated effective porosity can then be used to estimate groundwater flow velocities. These measurements will help to assess the potential lead time for contamination or impact on environmentally sensitive areas.
Monitoring program:
Groundwater samples should be collected from each monitoring bore on site to gain information on pre-construction groundwater conditions. Groundwater monitoring (sample collection, preservation and analysis) should be undertaken in accordance with the Australian Standards (AS/NZS 5667 ‘Water Quality Sampling’ series with particular reference to Part 11: Guidance on sampling of groundwater).
Baseline groundwater monitoring should be carried out to establish the natural variation in groundwater characteristics including:
- daily variation in groundwater level (e.g. due to gravitational or tidal influence)
- seasonal variation in groundwater level or quality
- response of groundwater to local or regional rainfall (recharge).
Groundwater sampling should not be carried out within one week of bore development, to allow for water levels and groundwater chemistry to stabilise. Prior to sampling, water levels in each bore should be measured over the shortest possible period of time, to minimise tidal and barometric variations. Water levels should be converted to AHD.
Bores should then be purged so that representative groundwater samples are obtained. To confirm the completion of purging and to obtain a preliminary assessment of groundwater quality, appropriate water quality parameters (e.g. pH, electrical conductivity, temperature) should be measured during purging operations.
Purging should continue until readings have stabilised (within 0.1 pH units, 5 per cent of reading and 0.2°C respectively). Purging data, including field parameters, general colour, turbidity and odour should be recorded.
As a guide, a minimum of three to five casing and filter pack volumes (casing volume + ~0.2 x filter pack volume) will need to be removed to adequately purge a bore using conventional sampling methods.
Groundwater monitoring can be carried out by farm employees with appropriate training. Equipment used to monitor groundwater needs to be properly maintained and calibrated.
If monitoring is undertaken by farm employees, it is strongly recommended that at least annually, the proponent engage a suitably qualified independent person to carry out the sampling to validate the data collection methodology.
Any significant inconsistencies between the routine sampling and the independent sampling should be investigated and appropriate corrective actions taken.
In addition to groundwater monitoring, it is strongly recommended that monitoring and recording of key environmental and operational conditions should be undertaken. Key parameters include rainfall, pond water levels and pond water quality (e.g. electrical conductivity). This data will assist in the detection or evaluation of allegations of environmental harm caused by containment structures.
Performance indicators:
It is beyond the scope of these guidelines to specify performance indicators that can be applied generally. Performance indicators will need to be determined on a site-specific basis according to the baseline monitoring, which will assist in establishing the natural variations in groundwater depth and quality. Performance indicators may also be defined by reference to control bores where these are available. Indicators for problems may include:
- A close link between pond status (filled/empty) and groundwater level and quality (e.g. electrical conductivity) that is not detectable in the control bores. However, hydrostatic mounding of groundwater does not, by itself, indicate a problem.
- A trend for increasing groundwater level and/or field parameters over time that is independent of regional recharge and is not detectable in the control bores.
Decommissioning of monitoring bores:
If monitoring bores need to be decommissioned (e.g. due to a modification to the farm layout), the bores need to be safely removed to ensure that the potential for future environmental harm is minimised. Decommissioning monitoring bores should comply with the Australian guidelines or equivalent standard specified in Minimum Construction Requirements for Water Bores in Australia, Agriculture and Resource Management Council of Australia and New Zealand.
Maintenance:
A program of routine maintenance of containment structures is important. Typical maintenance activities include:
- repairs to erosion and beaching of embankments
- repairs to pond floors where erosion and scour from aerators has occurred
- repairs to erosion and scour around inlet and outlet structures
- removal of woody vegetation (trees and shrubs) in pond embankments.
In addition to these routine maintenance activities, typical pond management disturbs a shallow layer on the pond bottom (0.05–0.10m) at the end of the production cycle when the pond floor is tilled and limed.
During all maintenance activities, it is critical to maintain the integrity of the engineering of the containment structures. Repeated disturbance may result in the integrity of the lining being compromised and result in subsequent problems. Therefore it is important that maintenance activities do not disturb the impermeable layer.
Where significant maintenance works or alteration of farm layout are undertaken, it is strongly recommended that the works be designed, supervised and certified by a suitably qualified person and undertaken in accordance with these guidelines.
Corrective actions:
Industry experience has shown that investigating allegations of unsatisfactory performance of containment structures and correcting these problems can be expensive. Costs may include engaging technical and legal professionals and undertaking detailed analysis. Significant cost savings can be made by following the critical steps in these guidelines.
- appropriate site selection
- appropriate design including the provision of adequate buffers
- construction to appropriate standards
- certification of construction
- baseline and routine monitoring
If performance indicators are not complied with, it is strongly recommended that a person investigates the reason for the non-compliance and determines appropriate actions. It is also strongly recommended that the relevant authorities are notified of problems with the containment structures, and are consulted about further investigations, and appropriate corrective actions.
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