8.2 Ongoing monitoring
There are several different areas where ongoing monitoring should be completed to prevent unexpected negative changes in mine drainage chemistry and subsequent negative impacts on streams. Monitoring of rock geochemistry is required from a resource development perspective as well as to ensure that waste-rock management strategies are appropriate. Monitoring of leachate from waste-rock piles or tailings piles, and treatment system discharge, is required to validate that management and treatment systems are working effectively. Monitoring of treatment systems may also be required to ensure the appropriate operation of the treatment system. Water quality and biological monitoring provide validation on a broader spatial scale that management and treatment systems are working effectively.
8.2.1 Monitoring of rock geochemistry
Rock geochemistry should be monitored throughout the operation of the mine to identify rocks that may affect mine drainage chemistry, and is used to determine appropriate ongoing management of waste rock, particularly if PAF rocks are present. Monitoring of rock geochemistry through acid-base accounting (ABA) for water quality purposes can be conducted alongside sample collection for exploration and resource development purposes. The specific requirements for effective monitoring of rock geochemistry in operational mines are difficult to generalise because they are site and deposit specific. This could mean that all or a selection of ABA tests are completed, or that field-portable tests such as paste pH are used as proxies for other ABA tests. The latter will have been determined through initial site investigations, and should be constantly updated as mining proceeds.
Sample types for rock geochemical monitoring can include core samples from resource development drilling, rock-chip samples from blast-hole drilling and mine-face samples or samples from waste-rock dumps from areas exposed by mining or development work. Appropriate strategies for monitoring waste rock during mining might require several samples for ABA per day or one sample per month or per 10 000 t waste rock depending on the variability of waste-rock geochemistry. If the geology and rock geochemistry is simple then less frequent monitoring is required than at complex sites.
If there are changes in the ABA characteristics of rocks collected during exploration and monitoring during mine operations, then additional rock samples should be collected and analysed. Sufficient additional samples should be collected to enable the geochemical variations to be defined. This information should be used to modify waste-rock management strategies accordingly.
Effective monitoring of rock geochemistry should mean that unexpected negative changes in mine drainage chemistry do not occur.
8.2.2 Leachate monitoring
Waste-rock piles and underground adits
Water quality monitoring of leachate from rock piles is also required to ensure that the selected management techniques are effective. As such, the water quality of leachate from waste-rock piles should be monitored on a regular basis. This may require that collection systems are put in place during waste-rock-pile construction. Samples of undiluted mine drainage should be collected as close as possible to the entrance to an adit for water quality monitoring (section 8.2.4).
Leachate from mine tailings inevitably discharges from beneath the tailings dam, and that water can be highly contaminated, especially with As and Sb at gold mines. Concentrations of these contaminants are generally lowered by adsorption to brown Fe oxyhydroxide precipitates, which form if pyrite is being oxidised in the tailings water pathway (see section 6.4.1). Formation of abundant Fe oxyhydroxide can take months or years after commissioning of the tailings facility. Hence, it is important to monitor water compositions regularly.
Water should be analysed monthly for pH and all consent elements, to low detection limits, at the point of discharge of water beneath the tailings. In addition, regular (at least bi-monthly) monitoring of waters up to 1 km downstream of the tailings facility is necessary to detect any seepage from the tailings into the groundwater system that bypasses the main water discharge point. The downstream monitoring can be in surface streams and/or groundwater wells drilled for the purpose.
During the mine operation, the tailings water is normally captured and returned to the tailings facility or a treatment plant. When mining ceases, seepage will continue for years or decades, so regular monitoring and appropriate water management and/or treatment is necessary as long as the seepage persists.
8.2.3 Treatment system monitoring
Once a treatment system is in place regular monitoring and maintenance is required to ensure that the system continues to operate as intended. At a minimum, this entails regular monitoring of the inlet and outlet water and maintenance of the system. Selection of biological monitoring sites should ensure that ongoing biological monitoring of ecosystems receiving treated water occurs (see sections 2.3.3 and 8.2.5).
Active treatment for AMD systems
For an active treatment system, both inlet and outlet water should be monitored regularly. If inlet water chemistry or flow rate changes, changes can be made to the system immediately to ensure that water continues to be treated adequately. For all active treatment systems, the following parameters should be measured on a regular basis from the inlet and outlet to the system:
- Flow rate;
- TSS concentration;
- pH, acidity, alkalinity;
- Dissolved oxygen; and
- Fe and Al concentrations, and any other metals or metalloids of concern in the mine discharge.
If possible, flow rate and pH should be monitored continuously. Continuous monitoring of pH is particularly essential to avoid problems associated with inefficient treatment or overtreatment, especially when using dry powder alkaline reagents (Waters et al. 2003). Frequency of analytical sampling should be based on the variability of the inlet water chemistry and the ability of the system to consistently treat to acceptable limits. Sampling is often conducted daily.
Depending on the chemicals used in active treatment, other water quality parameters should be measured to ensure they do not reach toxic levels. For example, regular monitoring of sodium is recommended for sodium-based systems, such as sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) (Waters et al. 2003). For treatment systems using ammonia, the concentrations of ammonia in the receiving water before treatment commences should be determined, and then regular monitoring of ammonia should be undertaken (Faulkner and Skousen 1991; Skousen et al. 2000).
Active treatment systems require frequent maintenance. For conventional systems each step in the ODAS (oxidation, dosing with alkali, sedimentation) process has specific operational and maintenance requirements. For example, in the oxidation step, if mechanical aeration is used, the stirring mechanisms require regular maintenance to ensure adequate operation. If chemical oxidation is used, regular sampling of the treated water is necessary to determine appropriate dosing rates, and the oxidant dispensing and mixing mechanisms require regular maintenance. The dosing-with-alkali step involves storage vessels, dispensing mechanisms, and mixing tanks and mechanisms, all of which have specific operational and maintenance requirements. Likewise, the sedimentation step involves the operation of clarifiers and addition of flocculants and coagulants with associated storage vessels, dispensing mechanisms, and agitation mechanisms, all of which have specific operational and maintenance requirements. Therefore, a detailed operation and maintenance manual should be prepared and tailored for each active treatment system.
Passive treatment systems for AMD
For passive treatment systems, the following parameters should be measured on a regular basis from the inlet and outlet to the system:
- Flow rate;
- Dissolved oxygen;
- Fe and Al concentrations (total and dissolved); and
- Any other metals or metalloids of concern in the mine discharge.
Monitoring of the inlet water is primarily of assistance in determining the cause of any change in outlet water chemistry. For example, this can identify whether deterioration in the quality of the outlet water is due to changes in the inlet water or due to a failure of the treatment system. Frequency of sampling should be determined based on site-specific variables. Variables that should be considered include: ability of system to continuously treat to acceptable limits, sensitivity of the receiving environment to changes in water chemistry, level of treatment by system, risk of failure of treatment system, site access, and ability of system to continue to treat the water under different flow rates and chemistries if the inlet chemistry and flow rate can vary significantly due to climatic variations. Samples can be collected daily, weekly, fortnightly or monthly.
For some passive treatment systems, additional water quality parameters should be added to the list of analytes for outlet water. For example, systems that rely on biological sulphate reduction for treatment, and systems that incorporate compost in their substrate such as vertical flow wetlands, anaerobic wetlands, and sulphate reducing bioreactors, should also include monitoring of the outlet water for: sulphides, dissolved organic carbon, total nitrogen, nitrate-N + nitrite-N, total phosphorus, and total biochemical oxygen demand. For systems that use steel slag as a neutralising agent (such as slag leach beds), it is possible that elements within the steel slag, such as barium, vanadium, manganese, chromium, As, silver and selenium may be released into the treated water upon dissolution (Simmons et al. 2002). Therefore, selected elements should be added to the list of analytes in the discharge from these systems. The full suite of metals should first be analysed to determine which metals have the potential to be released from the slag.
Passive treatment is typically considered low maintenance compared with active treatment. However, some maintenance is necessary to ensure continued adequate treatment of the mine drainage. Passive treatment systems have operational and maintenance requirements specific to each system. For example, open limestone channels may require regular scouring of the channel bed to dislodge and remove built-up precipitates, diversion wells require regular replenishment of limestone chips, and leach beds and vertical flow wetlands require regular flushing to remove precipitates that can clog passageways reducing permeability and treatment effectiveness. Operational and maintenance requirement details for each system are provided in Appendix F.
Sludge removal is often an important long-term maintenance issue for passive treatment systems (PIRAMID 2003). The removal of Fe-dominated sludge from settling ponds can be a significant long-term maintenance cost, unless ponds are massively oversized in terms of the metal load they receive. Sludge can either be removed from ponds in wet form using a vacuum truck or dry form using mechanical excavation. If there is insignificant leaching of trace elements, i.e. the sludge is stable, it may be able to be disposed of directly on site, or to landfill. If there is significant leaching of trace elements, stabilisation of the sludge may be required. If disposed of to landfill, dewatering of wet sludge, which typically contains between 1 and 5% solids, may be required although dewatering may not be required for on-site disposal. Dewatering will also reduce the volume and hence cost of disposal. Estimates of the volume of sludge that will be generated by a passive treatment system can be made using the computer program AMDTreat (Means et al. 2003) (see Appendix F.4).
Monitoring of As treatment systems
Active treatment systems
The same general monitoring requirements outlined above for both water and system monitoring apply. Regular sampling of water at various steps of the system is used to determine reagent addition rates and residence times, and outlet water quality. Water quality parameters that should be measured on a regular basis from the inlet and outlet to the system are:
- Flow rate;
- TSS concentration;
- pH, acidity, alkalinity;
- Dissolved oxygen; and
- As concentrations.
As outline above, each treatment step has specific operational and maintenance requirements. Therefore, a detailed operation and maintenance manual should be prepared and tailored for each active treatment system.
Passive treatment systems
The same general monitoring requirements outlined above for both water and system monitoring of passive treatment systems apply. The following parameters should be measured on a regular basis from the inlet and outlet to the system:
- Flow rate;
- pH, acidity, alkalinity;
- Dissolved oxygen; and
- As concentrations.
The detection of As in the outlet water can indicate when adsorption media should be replaced. Leach tests on the spent medium should be undertaken to ensure that there is no significant leaching of As or other trace elements after disposal.
8.2.4 Water quality
In this section, the focus is on water quality parameters that have been consented. These parameters should have been determined through initial site characterisations (see section 2.3) and should include all relevant parameters. The number of sites sampled should include those required for consent conditions as well as providing ongoing baseline data against which to measure any change. Water quality samples must be collected at the same sites and times as biological monitoring is undertaken, although water quality samples will also be collected on a more regular basis and potentially at additional locations.
Where to monitor
Site-specific factors are important in the selection of monitoring points for assessing the final downstream water quality, although important considerations include:
- Samples of undiluted mine drainage or leachate from tailings impoundments should be collected as close as possible to the source of the seep or entrance to an adit.
- The monitoring point should be at a point where mine drainage or tributaries are completely mixed with other catchment water. A general rule of thumb is that mixing occurs at a distance downstream of a tributary which is approximately ten times the width of the stream. However, care should be taken to validate that the sampling point is located after mixing is complete. This can be confirmed by measurement of physicochemical parameters across the stream from the sample collection point. If there is no change in physicochemical properties across the stream then the sampling point is acceptable.
- The monitoring point should be upstream as far as practical after mixing of mine drainage with other catchment water so that dilution effects do not mask changes in mine drainage quality. For example, if mine drainage contributes to a tributary to a major river, then the monitoring points should be on the tributary if possible rather than the major river.
- Monitoring points should be located where mine development will not occur. Ideally, these monitoring points are ones that have been used during baseline surveys (Chapter 2). Monitoring points are most valuable if used for assessing long-term water quality, and should be located where they will not have to be shifted. Collection of a large set of data from one monitoring location enables more subtle changes in water chemistry to be detected, compared with smaller datasets from multiple locations.
- The likely partitioning of trace elements between dissolved species and suspended particulate material at the dissolved concentrations of the trace element (see next section).
- The location of biological monitoring sites. Water quality and biological samples should be taken at the same site, and ideally, from the same sites used during baseline surveys (Chapter 2).
What to monitor
There are many alternative monitoring strategies to identify when chemical conditions within a stream depart from those expected or agreed during resource consent. However, costs can be unnecessarily high if complete chemical (all consented parameters) analyses are undertaken on each monitoring sample. Thus, a tiered approach can be applied to obtain the maximum useful information with minimal cost. An example of a tiered approach is provided below. The objective is to minimise the number of analysis parameters to reduce costs and provide maximum useful information.
Example three-tier monitoring system with rationale for sample types:
- Tier 1: The minimum useful analytical suite is turbidity, pH, EC and flow and potentially some specific trace elements. This approach assumes that pH and EC are suitable proxies to identify variations in stream chemistry caused by changes in flow volume or quality of mine drainage into the catchment. After mine operations commence, the relationships between pH, EC and trace elements of interest should be determined to establish whether pH and EC are suitable proxies. In some cases, where monitoring of specific contaminants is required, such as As, pH and EC might not be suitably sensitive proxies and other proxies might be identified if statistically valid relationships are established. If statistical relationships cannot be established then the specific trace elements should be added to tier 1.
- Tier 2: If pH departs from expected conditions by more than 0.5 units or EC departs from agreed conditions by more than 10 percent and flow is within 20 percent of background then sulphate concentrations should also be determined. If changes in mine drainage volume or quality cause the variations in pH or EC then it is likely that sulphate concentration will also change within the catchment. Increases in sulphate concentration of more than 10 percent could indicate that there has been a change in the volume or quality of mine drainage entering the catchment.
- Tier 3: If sulphate concentrations increase by more than 10 percent then a full analysis of all consented parameters should be conducted. Samples for tier 3 should be collected at the same time as tier 2 but only submitted for analysis if sulphate has increased by more than 10 percent.
Where specific trace elements are monitored, consideration should be given to whether it is appropriate to monitor the total or dissolved concentrations at a specific monitoring point. Partitioning between dissolved species and suspended particulate matter can be dynamic so that chemicals are associated with particulates in some parts of a catchment but are dissolved in other parts of a catchment. For example, many trace elements adsorb to suspended particulate material but desorb if chemical conditions become favourable. The relationship between suspended particulates and dissolved species is an active area of research and is likely to be site specific. Conditions that are identified in resource consent documents should be made with understanding of the partitioning of trace elements solid and dissolved species and might involve the expertise of an experienced geochemist or water quality scientist.
When to monitor
Monitoring should be undertaken at representative flow levels, which can be determined from regular flow monitoring. A tiered approach can be used for determining the frequency of monitoring.
- Continuous to daily to weekly monitoring of all tier 1 parameters is recommended at monitoring points at a specified time of day, as biological processes may result in diurnal variations in some parameters.
- Weekly to monthly monitoring of tier 3 parameters is recommended at monitoring points at a specified time of day.
- If check monitoring is carried out by regulators, it should be completed at the same time of sampling as routine monitoring.
There are many site-specific factors that might require changes to the monitoring strategies outlined above. These strategies should be considered a minimum or starting point for ongoing monitoring, and interpretation of results should include analysis of all changes in site conditions. In addition, broader environmental factors can also cause variations in the concentration of dissolved components in streams due to rainfall, drought, seasonal variability, snow melt or many other factors. Refinement of the strategies outlined above should be completed as analytical cost decreases and availability of portable analytical capability improves.
8.2.5 Biological monitoring
Ongoing monitoring should be undertaken to detect impacts occurring during and after active mining. As discussed in section 2.3.3, sampling should include both control or reference sites and potentially impacted sites to enable the detection and quantification of mining-induced change. If reference sites are not included, detection of changes relies on comparisons with pre-impact conditions, which is confounded by any other change over this time, such as large floods, droughts, vegetation regeneration or other factors, which might be unknown. Therefore data from multiple reference sites are essential for rigorous and meaningful consent monitoring.
The sampling of sites selected for ongoing consent monitoring should occur directly prior to any mining operations, and then at regular intervals afterwards. This ongoing monitoring is best conducted at least seasonally initially when rapid changes in systems may occur, and alongside water quality monitoring. However, if the intensity and type of mining activities remain constant over a long period then annual monitoring may be acceptable (e.g. spring or summer sampling). The duration of monitoring will be dictated by the conditions of resource consents, and will include both active mining and treatment phases. The continued monitoring of restoration activities is especially important, as it may take some time for fauna to recolonise habitats.
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