Discussion Paper on Brackish Urban Lake Water Quality in South East Queensland Catalano, C.L. 1, Dennis, R.B. 2, Howard, A.F.3 Cardno Lawson Treloar12, Cardno3 Abstract Cardno has been involved in the design and monitoring of a number of urban lakes and canal systems within south east Queensland for over 30 years. There are now many urban lakes in South East Queensland and the majority have been designed on the turnover or lake flushing concept, whereby it is considered that, if the lake is flushed within a nominal timeframe, then there is a reasonable expectation that the lake will be of good health. The designs have predominately been based on a turnover or residence time of around 20-30 days and some of the lakes reviewed are now almost 30 years old. This paper reviews this methodology against collected water quality data to provide comment on the effectiveness of this method of design for brackish urban lakes in South East Queensland and also to indicate where computational modelling should be used instead of, or to assist with, this methodology. 1. Introduction Lakeside developments are very popular in South-East Queensland. The lake is generally artificial, created out of a modification of an existing watercourse or lowland area for a source of fill for the surrounding residential construction. They are used to provide visual and recreational amenity, sometimes including boat navigation and mooring areas, and can also serve as detention basins and water quality polishing devices. As with anypermanent water feature, they inevitability also become an aquatic habitat. There is now a much greater push for computational modelling to be used in the design of urban lake systems, however, historically this has not always been possible due to the computational resources required and the lack of water quality data available. Most urban lakes were, therefore, engineered based on the turnover or lake flushing concept, whereby it is considered that, if the lake is ‘flushed’ within a nominal timeframe, then there is a reasonable expectation that the lake will be of good health. In most studies this was complemented with a conceptual model based on mass balance of flows and pollutants to check the overall loads the lake would be receiving. Most urban lakes are freshwater and there has been mixed success with these systems. The reasons why freshwater lakes sometimes have difficulty in remaining healthy are relatively well documented, as are guidelines for their design. There appears to be very little discussion on brackish urban lakes and hence they tend to be designed using similar retention times as freshwater systems. On review of several brackish urban lakes in south east Queensland, it would appear that they have far more stable health than freshwater systems. The reasons are discussed further in this paper. 2. Turnover, Overturn, Residence Time and Stratification In common engineering parlance, lake turnover is a term that has been used interchangeably with lake retention time, residence time, flushing time or water age. More commonly, however, the term turnover is used to describe a process where colder atmospheric temperatures during autumn or winter cause the surface of a lake to cool, thereby becoming denser than underlying water and subsequently sinking. This displaces the lower layers of the lake and forces them to the surface. This rising mass of water is then also cooled by the atmosphere when it reaches the surface. In very cold climates, this process would normally continue, effectively freezing a lake solid from the bottom up and destroying life within it, if it were not for the unique property of water of becoming less dense than its liquid phase as it begins to freeze below 4 degrees Celsius. The result is a floating sheet of ice that thickens as more water underneath it is frozen until further heat loss from the lake is minimised and both the temperature and life are maintained underneath it at around 4 degrees C (Karl, 2008). During spring and autumn, the surface layer of the lake will reach a point where it is either warmed or cooled respectively (depending on the season) to be similar in temperature to the bottom layer. The lake is then considered isothermal and is more easily mixed by winds and currents. In summer time, the surface layer can heat to a temperature and buoyancy where external energy sources such as wind mixing cannot impart enough energy into the lake to keep it fully mixed. Three layers are formed in these circumstances, the epilimnion, or surface layer, hypolimnion, or bottom layer, and the metalimnion, in between. Both the epilimnion and hypolimnion are commonly thought of as well mixed bodies of water, with the metalimnion being a layer of strong gradient between the two. The metalimnion forms a barrier between the upper and lower layers, limiting transfer (most importantly of oxygen) from the epilimnion to the hypolimnion and the lake is considered stratified. A thermocline is a theoretical line in the metalimnion, where the strongest gradient or change in temperature occurs. In practice, the thermocline is the commonly used term to describe the theoretical point of separation between the epilimnion and hypolimnion and the metalimnion term is often omitted. Stratification can also occur due to density differences caused by dissolved substances. Salt concentration is a common reason for stratification in brackish urban lakes, where fresh water inflows float over denser saline water. In this case, a halocline is formed to separate the epilimnion and hypolimnion. A halocline can create a much greater density differential than a thermocline and, therefore, can be much more difficult to breakdown. For example, freshwater at 15 degrees C has a density of approximately 999 kg/m3 and, at 25 deg C, has a density of 997 kg/m3. In comparison, seawater at 15 degrees C has a density of around 1025 kg/m3, hence a 10 degree temperature differential between two water compartments results in a much smaller density difference than a freshwater compartment over seawater, even at the same temperature. Lake turnover and lake overturn have been used to mean the same thing, no doubt due to most considering the terms similar. A lake overturn, however, is also called a limnic eruption and is quite a different phenomenon, where carbon dioxide (CO2) builds up to a point of near saturation in the deep section of a lake and suddenly erupts to the surface. The volume released can be significant and, as CO2 is more dense than air, lake overturns have been known to cause deaths to humans and animals due to asphyxiation. The most recent occurrence was in Lake Nyos in Cameroon, Africa, in 1986, where 80 million cubic meters of CO2 was released, killing between 1,700 and 1,800 people. Fortunately, these natural disasters are rare, with only two known to occur in history and only three lakes known to have these levels of CO2 buildup. All of these lakes are in Africa and the buildup is considered to be from volcanic activity (Wikipedia, 2010c). To avoid confusion, the terms lake residence time and flushing time will be used to describe the age of water in the lake and lake turnover will be used to describe the stratification/destratifcation process discussed above. Lake overturn will not be discussed further in this paper. 2.1 How is Residence Time Calculated? There are several ways to conceptualise the retention time of a lake. The purpose of the term is mainly to understand how long a dissolved substance will remain in a lake but, in the practice of estimating retention time, it is simply the lake volume divided by the average flowrate into or out from the lake system. Obtaining these parameters, however, is not always simple. Average lake volume and flows into and out from a system are affected by catchment inflows and evaporation (which vary seasonally and possibly with climate change), seepage, tidal forcings (which vary with spring and neap cycles and possibly with climate change), overflows from floods, culverts, weirs, gates, pumps and other control structures. Furthermore, with the presence of stratification and any horizontal non-uniformity, mixing of the newer water may not be uniform in all parts of the lake, necessitating that residence time should be calculated for each of these separate lake sub-volumes, or as a vertical and horizontal distribution. Finally, if the flushing water is sourced from a body which itself has a limited volume and flushing rate , there is the possibility of short circuiting of the ‘flushed’ water back into the lake again, resulting in a water age older than simple calculations would indicate. Where lakes cannot be reduced to a simple idealised concept, the age of water within a lake is often best calculated with the assistance of computational modelling. Figure 1 below shows a cross-section of salinity concentrations, estimated using a 3- dimensional computational model, from two brackish lakes, connected by a small channel. As fresh water enters the smaller upstream lake on the left hand side of the plot, it is shown to float on the surface and progressively mix with the small lake. Due to the momentum of the inflow, a salt wedge is commencing to form, rather than a more horizontal halocline, restricting mixing in the downstream lake on the right hand side of the plot. Figure 1 – Cross-Section Salinity Plot of Two Connected Lake Systems (Cardno Lawson Treloar, 2008) 2.2 So What is an Appropriate Residence Time? Natural lakes around the world have residence times ranging from less than a day to many years, governed by the size of the lake relative to the inflow.