Microbial and environmental implications for use of monolayers to reduce evaporative loss from water storages

While the concept of applying surface layers to water bodies to reduce evaporative loss is an old one (La Mer, 1962), in practice, the economic and environmental costs of applying an artificial film thick enough to reduce evaporation has been limited to laboratory studies. The exception is with monolayers

Evaporation, seepage and water quality management in storage dams: a review of research methods

One of the most significant sources of water wastage in Australia is loss from small storage dams, either by seepage or evaporation. Over much of Australia, evaporative demand routinely exceeds precipitation. This paper outlines first, methodologies and measurement techniques to quantify the rate of evaporative loss from fresh water storages. These encompass high-accuracy water balance monitoring; determination of the validity of alternative estimation equations, in particular the FAO56 Penman- Monteith ETo methodology; and the commencement of CFD modeling to determine a 'dam factor' in relation to practical atmospheric measurement techniques. Second, because the application of chemical monolayers is the only feasible alternative to the high cost of physically covering the storages to retard evaporation, the use of cetyl alcohol-based monolayers is reviewed, and preliminary research on their degradation by photolytic action, by wind break-up and by microbial degradation reported. Similarly, preliminary research on monolayer visualisation techniques for field application is reported; and potential enhancement of monolayers by other chemicals and attendant water quality issues are considered

Impact of artificial monolayers on water quality, potable water treatment, human health and lake ecology

Water conservation strategies offer a feasible alternative to the unpopular commissioning of new reservoirs, and to the high infrastructure and running costs of desalination plants (McJannet et al., 2008). Increasing temperatures and decreasing rainfall constrain the feasibility and profitability of the provision of potable water and irrigated agriculture. Of the estimated 7,000 GL of water stored in a million or so small on-farm storages (< 10 ha capacity), up to 20% may be lost to evaporation (Craig et al., 2008). Strategies for reducing evaporative loss include physical floating covers, suspended physical covers, re-engineering the storage to reduce the surface to volume ratio, and applying an artificial monolayer. Of these options, only monolayer (a mono-molecular surface film) application is considered cost-effective for storages larger than 10 ha. The management of artificial monolayers to retard evaporative loss is an old concept (La Mer, 1962) that has not been adopted commercially due to extreme variability in field performance (Barnes, 2008). Recent research highlights deficiencies in the original products that in part account for variable field performance (Pittaway and van den Ancker, 2010b). The promise of improved field performance has encouraged managers of urban water utilities to consider monolayer application as part of their water conservation strategy (McJannet et al., 2008). In contrast to privately owned farm irrigation storages, the adverse effects of monolayer application on the ecology and water quality of the storage must be considered, as well as the potential for monolayer compounds to adversely affect potable water treatment processes. This literature review explores: the mechanisms by which artificial monolayers retard evaporative loss; how the application of an artificial monolayer may adversely affect the physical, chemical and biological processes that occur at the air/water interface; and the potential for monolayers to adversely affect aquatic food chains, potable water quality, and potable water treatment systems

Photodegradation of Australian freshwater microlayers and the implications for potable water management

Photodegradation has been known to break down toxic compounds in potable water storages as well as degrading pesticides and herbicides in agricultural water storages. In this study, the concentration and reactivity of humic substances (HS) present in natural microlayers on water storages in South East Queensland (SEQ) was investigated. Microlayer and subsurface samples were taken from eight water storages with dissolved organic carbon (DOC) used to quantify HS concentration. The E2/E3 ratio (ratio of absorbance at 250 nm to 365 nm) was used to indicate the molecular weight of DOC compounds, and absorbance at 253.7 nm and the permanganate index were used to compare the reactivity of humified DOC. The concentration of carbonyl compounds in the microlayer was also investigated as carbonyls are considered the most photoreactive functional group present in HS. Preliminary results indicate that the concentration of humic substances and their chemical reactivity in SEQ water storages are highly variable, reflecting the characteristics of the water catchments

Towards a biophysical understanding of observed performance of evaporation suppressant films applied to agricultural water storages - first analyses

The potential utility of monomolecular layers(‘monolayers’) and other surface film materials for the reduction of open water evaporation has long been argued. However, outside the laboratory, trials to quantify the effectiveness of artificial surface films have produced highly variable results after application to water surfaces, whether natural water bodies or managed farm storages. This paper briefly reviews the physical mechanisms involved in evaporation suppression and the biophysical literature on aquatic surface microlayers. The wide-ranging results from sixteen months of outdoor trough-scale and (simultaneous) replicated bucket-scale evaporation reduction trials are interpreted using biophysical measurements made on microlayer and immediate subsurface water samples taken from the experimental troughs. When the prevailing environmental conditions and other ancillary measurements are taken into account, plausible hypotheses arise to account for at least some of the observed trial-to-trial differences in evaporation reduction and surface film performance. These results have implications for both small-scale trailing of evaporation suppressants and the deployment and management of artificial surface film materials on agricultural water storages

Photodegradation of Australian freshwater microlayers

The composition of microlayers on freshwater storages in Northern Europe have been studied for decades. Research in Australia is more recent, highlighting differences in microlayer and subsurface water composition associated with vegetation and climate. Freshwater microlayers in Northern Europe are primarily formed on peat, whereas in Australia they are formed from bark and leaf litter. The hydrophobic, aromatic compounds concentrating within the microlayer strongly absorb ultraviolet light to produce photoreactive compounds. Rates of photodegradation are likely to be higher in subtropical Australia. South East Queensland (SEQ) experiences 290 clear days per year, in contrast to Sweden with only 165 clear days per year. Leaf fall in Europe occurs in Autumn prior to winter rain, whereas in SEQ leaf and bark fall predominantly in the dry winter, prior to summer storms. Furthermore, the hole in the ozone layer above Australia allows more UVB light to reach the surface. In this study, the concentration and reactivity of humic substances (HS) present in natural microlayers on water storages in SEQ was investigated. Microlayer and subsurface samples were taken from eight water storages with dissolved organic carbon (DOC) used to quantify HS concentration. The E2/E3 ratio (ratio of absorbance at 250 nm to 365 nm) was used to indicate the molecular weight of DOC compounds, and absorbance at 253.7 nm and the permanganate index were used to compare the reactivity of humified DOC. The concentration of carbonyl compounds in the microlayer was also investigated as carbonyls are considered the most photoreactive functional group present in HS. Significant regressions were obtained for the E2/E3 ratio and absorbance at 253.7 nm (r2 = 0.89), and the E2/E3 ratio and the permanganate index (r2 = 0.95). The regression for the permanganate index and UV absorbance was initially significant (r2 = 0.91), primarily reflecting differences in HS concentration. When data was standardised for DOC concentration, results for the eight storages tested clustered into four groups, reflecting the attributes of the water catchments. Results indicate that larger molecules more recently derived from wooded catchments absorb UV light more strongly, and are more chemically reactive (higher permanganate index). Smaller molecules derived from highly resilient carbon in the black vertisol soil of a cleared catchment absorbed less UV light, and were relatively unreactive (lower permanganate index). These preliminary results will be used to develop bioassays to compare the rate of photodegradation in the microlayers of freshwater storages in SEQ

Will artificial monolayers adversely affect water quality? Insights from twelve months of monitoring with no monolayer

Evaporative loss from water storages can be reduced by applying an artificial monolayer to the surface, but municipal water managers are concerned about the impact on potable water quality. A study has been undertaken over the last year on a 16 ha irrigation storage in the Lockyer Valley to characterise key water quality parameters prior to the application of an artificial monolayer. Analyses of water sampled every two weeks from the air/water interface and the subsurface have been interpreted in the context of climatic data recorded at the site. Preliminary results indicate that the natural storage dynamics associated with algal blooms, wind and rainfall may have more of an impact on water quality than the application of an artificial monolayer

Optimising the design, installation and operation of a monolayer application system on a farm dam via a 'Universal Design Framework'

Monolayer technologies can potentially provide a cost effective solution for reducing evaporation losses from farm dams. As every dam will have it's own set of environmental characteristics and user requirements, this needs to be considered holistically in order to determine a suitable monolayer material, application system and application strategy. Hence, a Universal Design Framework (UDF) was developed to enable this approach and to optimise the evaporation suppressing performance of monolayer. This paper details the UDF approach