Drought, deluges and disinfection: Managing water quality in an environment of climate change
This story was developed by the AWA's Water Quality Monitoring and Analysis Specialist Network based on a panel discussion at Ozwater'19.
While it is obvious that stored water volumes are dwindling in many parts of Australia, the effects of climate change are manifesting in other ways that have direct and significant impacts on the urban water industry. With escalating concerns surrounding the effects of climate change and water management in Australia, most often the focus is placed on water quantity over water quality. However, changes to raw water quality leading to deterioration in drinking water quality and boil water alerts (BWAs) are being observed around the country, thus adaption is required to ensure that water that is available for drinking is clean and safe water into the future.
The generally accepted impacts of climate change include increased temperatures and more erratic weather conditions; the key aspect to how climate change is affecting the weather is the observation of increased frequency and severity of extreme weather events.
In New South Wales (NSW) during 2017 and 2018, a majority of the state experienced below average rainfall, and average temperatures exceeding those on record with 2019 being the driest and warmest on record.
This resulted in most of the state being categorised as being in severe or critical stage drought. As an example, in the Macquarie Valley system, over the last six years inflows were approximately 47% of the long-term average.
Similarly, the average inflow into Perth’s drinking water reservoirs since 2011 (~ 50 GL) is roughly half of the average of the period from 2002 to 2011 (~ 100 GL), which were half of the average of the period from 1974 to 2000 (~ 200 GL). Figure 1 shows the annual stream inflow to Perth drinking water reservoirs for the period 1911 to 2017.
With long periods of dry weather comes an elevated risk of severe bushfires. From 2001 to 2011 Australia experienced 20 major bushfires, including several in the main drinking water catchments of Sydney, Canberra and Melbourne; and more since, including the unprecedented bushfires recently experienced in NSW and Victoria. Increased bushfire prevalence may not be a key impact on water quality on its own, but when coupled with a rain event there is a high potential for changes to raw water quality.
Some may say that the only certainty of drought is a flood to end it. However, any extreme weather event during or soon after a long period of dry weather is likely to significantly impact water quality. The key aspect to note from these trends is that when extreme weather events happen in conjunction with each other, the impacts are compounded compared to when the events happen in isolation.
Below average rainfall causes low flows in waterways and leads to increases in water temperatures, which in turn causes the frequency and scale of algal blooms and associated anoxic (low dissolved oxygen) to increase.
This confluence of events has had devastating effects on water quality; impacting the quality of water for drinking and irrigation as well as the ability to sustain aquatic life. The large fish kills in the Menindee Lakes system in 2019 have been a key reminder of how important water quality is to the sustainability of ecosystems.
Three Australian case studies that were part of research funded by WaterRA and the Water Research Foundation documented some of the observed impacts of bushfires that occur in combination with rainfall events.
During bushfires there is a loss of infrastructure and vegetation in catchments. The damage to soils, which is less noticeable, results in the slow recovery of vegetation, meaning that any subsequent extreme weather event could have a serious impact.
For example, when a bushfire is followed by an extreme rain event, the loss of vegetation and the generation of ash and silt results in a range of compounds, such as sediments, natural organic matter, dead animals, faeces and nutrients being transferred into raw water storages, which affects the water quality through increased colour, turbidity, dissolved organic carbon, nutrients (increasing the risk of algal blooms), iron/manganese (leaching from the sediments under anoxic conditions), and pathogens.
Combining long periods of low rainfall with an extreme wet weather event has a unique impact compared to rainfall events under typical weather conditions. An example of such an event that occurred in Western Australia during a summer tropical “low” saw over 200 mm of rain fall in a couple of hours, causing rapid inundation of the flood plain and flows into the dam. This led to a spike in turbidity and an unexpected shift in algal and bacterial communities, which took about four weeks to re-establish pre-event baseline conditions.
The event showed an extreme change in turbidity, from around 20 Nephelometric Turbidity Units (NTU) pre-event to peaks of over 250 NTU. This declined on the recession-limb of the hydrograph, returning to pre-event turbidity after about four weeks.
The collection of water samples before, during and after the event allowed a unique understanding of the large variation in the algal and bacterial communities. The dam feeds a water treatment plant that includes multiple barriers to deal both with particulate and dissolved matter as well as microbes.
Using forward-flow dose control, the operators were able to maintain plant performance even under these extreme conditions. Were the treatment plant reliant on a single treatment barrier such as chlorination, there would have been a considerable risk that the treatment plant process would be challenged and unable to maintain the treated water at the necessary quality criteria.
Similarly, the events that led to the floods in January 2011 and ex-tropical cyclone Oswald in February 2013 in South-East Queensland resulted in the level in Lake Wivenhoe peaking well over 100 percent. As an effect, Seqwater observed very high peaks in turbidity as high as 4000 NTU) in the feed water to the Mt Crosby Water Treatment Plants; with the February 2013 event causing the plants to shut down through exceedance of the chemical dosing capacity and turbidity critical control point (CCP) of 2000 NTU.
Other documented Australian case studies of extreme rainfall events have shown similar outcomes, but in addition to the impacts on water quality extreme events have the capacity to create physical impacts to systems which can have an effect on the ability to supply safe drinking water to customers such as; disruption to power and communications and damage to pumping/treatment equipment.
A basic rule of disinfection chemistry is that as the temperature of water increases, the efficacy of chlorine, as a disinfectant, also increases. However, the higher the temperature, the faster the decay rate of residual chlorine. Thus, in locations where water is transported via above-ground pipelines over long distances, such as in the Goldfields and Agricultural Region (GAR) of Western Australia,
prolonged periods during the summer of high air temperature have the potential to reduce disinfection ability to levels that could present a possible health risk to consumers. Reduction in disinfection residuals and increases in temperature (> 38 °C, with temperatures up to 45 °C observed) creates ideal conditions for the presence of Naegleria fowleri, which is an amoeba and tolerant to relatively high temperatures but can infect humans through the nasal passages where they travel into the brain and destroy brain tissue (Trolio et al,2008).
Over the past 20 years, climate variation has influenced rainfall distribution and intensity across the south-western region of Western Australia (WA). As a result, some drinking surface water storage reservoirs are now at their lowest storage on record and show fluctuations in water quality.
As many country reservoirs are critical sources of supply, it is an imperative to understand the linkage between water quality, inflow and storage volume. Ideally, a forecasting capability is also required to allow timely operational response to maintain quality within prescribed limits.
Currently, operational responses have relied on comparison of individual sample results with defined trigger levels. The objective was to develop a numeric method that could support proactive operational decision-making for reservoirs and downstream drinking water supplies. Two reservoirs were selected in the south-west of WA that serve as drinking water sources, have extensive data records, and storage volume that responds to variations in rainfall. One reservoir supplies part of the metro area and the other is a sole source of supply to a large country water scheme.
In long water distribution systems, with respect to the impacts of solar radiation on water temperature, mitigation strategies are limited. However, employing heat shields on above-ground pipelines that can’t be buried is a strategy being implemented in WA and found to provide benefits in reducing water temperature. Additionally, closely monitoring temperature and chlorine residual, with the ability to top-up chlorine concentrations where required, is the key to maintaining conditions to mitigate the risk of Naegleria colonisation in reticulation networks during extended periods of above average temperature.
Managing the raw water source as the first barrier to contamination in a drinking water system has the advantage of minimising the risk of poor feed water quality, thus taking some of the pressure off treatment plants to meet quantity and quality targets. A highly efficient method for minimising anoxia in the hypolimnion of deep water bodies is oxygenation. Aeration and destratification systems are often sufficient to mitigate the effects of stratification and low DO levels but are often more suited to shallow water bodies and rivers. Oxygenation (especially if coupled with the dosing of sub-flocculation doses of metal salts) is an emerging technology that has been shown to reduce the levels of iron/manganese and nutrients in feed water, significantly reduce the incidence of algae blooms and associated algae-related issues.
The role of analysers, manual sampling and new analytical techniques and mathematical modelling are the key to monitoring conditions and facilitating the forecasting of future water quality conditions that are seen as a result of climate change. Real-time online monitoring of water quality allows more rapid response to incidents where previously weekly or monthly grab sampling at the treatment plant off-take gave minimal information about a parameter.
Next Generation Sequence (NGS) analysis is a DNA-based emerging technology that enables understanding of the organisms (bacteria, algae, invertebrates, fish, etc) that have been in contact with a water matrix. This analytical technique allows the observation of shifts in the types of organisms over a sampling period. Using NGS during extreme events can provide an indication of the pathogen/bacterial challenge on the downstream treatment system if sufficient baseline information is also obtained.
Through automation technology and machine-learning it is also possible to assist water treatment operations with such aspects as chemical dosing, based on real-time changes rather than daily jar testing. Advances in technology to analyse the strength of floc (the key factor in obtaining good solids and organics removal in water treatment) will assist in the optimisation of water treatment processes during changing water quality conditions.
Around Australia many utilities are heading into uncharted territory with respect to potential impact to water quality from extreme weather events. As runoff from raw drinking water catchments into groundwater, reservoirs and storages is decreasing, there is an increasing need to operate storages at reduced levels, sometimes even below the known lowest operating level. For many, the quality of the water at these levels is unknown, which presents the risk of not being able to produce safe drinking water with the existing treatment system.
The Australian water industry has been working toward the development of the next version of the Australian Drinking Water Guidelines. Some of the expected inclusions are health-based targets (HBTs), water safety planning requirements and changes to contaminant limits (e.g. disinfection by-product (DBP) compounds). Having to consider these additions whilst maintaining a safe and reliable water supply will put increased pressure on water quality managers and drinking water production personnel during a situation of worsening water quality due to climate change.
Some of the key questions that need answering, include:
- At very low storage levels, what is the influence of wind de-stratification, turbidity plumes, groundwater seepage, dissolved oxygen?
- How fast can concentrations and water quality conditions change?
- How could this be monitored meaningfully (especially in big catchments) and then how could the information gathered be used to inform operations?
- Do we invest in treatment for infrequent hazards?
- Can we plan for resilience to climate change when events are so extreme compared to average operation?
This paper was developed based on the outcomes of the Water Quality Monitoring and Analysis specialist network panel discussion entitled Drought, Deluges and Disinfection; Managing Water Quality in an Environment of Climate Change as part of the Ozwater'19 conference program. We would like to thank Andrew Bath, Principal, Water Quality, Water Corporation; Duncan Middleton, Principal, Water Quality, Seqwater; Fiona Smith, Executive Manager, Water and Catchment Protection, WaterNSW; Stuart Khan, Professor Civil and Environmental Engineering; and Sally Williamson, Senior Water Technologist, Aurecon.