399Altos de la Vanega

Examination of research into the overall environmental benefits of SuDS and PTS shows that work tends to be conducted at two contrasting scales: either the whole catchment scale where a wider area is monitored or at a site scale where a group or single system is monitored more rigorously. Work monitoring catchments provides an important part of our understanding of UDP because it allows a greater insight into how existing drainage and water systems interact and helps identify the most significant challenges with respect to water quality in different areas.

Bressy et al. (2014) compared a typical piped drainage catchment to three others that were served by a range of SuDS measures all located in Northern France. A detailed investigation of the catchments was undertaken, along with monitoring of rainfall and discharge. Water and soils were also sampled. Results showed that a reduction in pollutants correlated well to the reduction of mass discharges through loss to infiltration, but there was variation in results between sites, with some sites demonstrating greater reductions in contaminant mass than in water volume.

This study concluded that rather than focusing only on large events which lead to the greatest risk of flood damage there is an increased opportunity to reduce mass discharges and subsequently pollutant discharge from smaller more frequent rainfall events, through increased levels of infiltration. However it cannot be assumed that just because water is lost to infiltration that all pollutants are captured in soils and surface layers. Depending on the particle size of pollutants and whether they are held in solution, there is risk of contamination of ground water in sensitive areas. Indeed the study does identify that the reduction in pollutants is explained primarily through a reduction in runoff volumes, and does not demonstrate that results reveal any kind of “purifying effect” in the classical meaning (i.e. lower concentrations).

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It is also important to appreciate that the varying climatic conditions between countries means that actions seen as desirable in one country maybe inappropriate in another. Burns et al. (2012) detail a study from Melbourne where a comparison of the hydrologic effects of two alternative conventional approaches to urban stormwater management was made. These were a drainage-efficiency (no retention or treatment) focused and a pollutant-load-reduction (under drained bio filtration system) focused approach. The various disadvantages of these approaches were identified, and their hydrological outcomes contrasted with a more progressive method which focused on restoring a natural flow regime (combined rainwater harvesting and vegetated infiltration system). This study identified that both the conventional approaches failed to sufficiently retain storm water and disrupted river flows and highlights the need for water retention features to be used in conjunction with pollution control measures. Whist important, these findings are less relevant to those countries with a higher average rainfall, where stormwater runoff will form a smaller proportion of river base flow.

Other studies focus on the potential of the existing drainage system to contribute to pollutant loading of receiving waters. For example G. Kim et al. (2007) who completed a study in Daejeon City in South Korea. This examined the volumes and quality of the effluent discharging from CSOs in the city. The study monitored a single CSO discharge from a catchment area of 136 ha, over 5 rainfall events. Only 3 of these events were of sufficient size to cause CSO discharge which was sampled using auto sampling equipment, with further manual samples being collected during very high discharges. Results observed show that only an average of 10mm of rainfall was required for CSO discharge to occur, resulting in very poor quality discharge with high levels of solids, organics and nutrients observed in samples. It also indicated that by attenuating the equivalent of 5mm of rainfall the pollutant loading from the CSO to receiving water could be reduced by 80%.

While this study provides useful data into the quality of discharge effluent of CSOs in the region, it is likely that due to the limited number of storm events and monitoring points used that further data collection from other locations and from a greater number of storm events may result in an a different conclusion.

Some studies have investigated supplementing long term best practice approaches with sort term interventionist actions in order to quickly deliver water quality benefits. Özkundakci et al. (2010) reports the results of a 5 year project to restore Lake Okaro in New Zealand, which

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had become eutrophic, to return it to more oligotrophic conditions. To achieve this reversal a 2.3 ha purpose made wetland was construction and protection of riparian margins through measures such as livestock exclusion, fencing and planting of native plant species along the stream banks and lake margins. These measures to reduce external loading of Phosphorous was complimented by application of alum and modified zeolite chemicals to absorb Phosphate from the lake, to reduce internal loading.

As a result of these measures the total phosphorus concentration in the lake decreased by 41%. This is a good example and represents a more intensive approach addressing not only the external diffuse nutrient pollution inputs of a water body but treatment of that water body to address the existing pollution issues. The use of geoengineering techniques such as the dosing of the lake with Alum and zeolite is less common than the control of external loading with riparian management, etc. However the use of such a technique is less applicable to rivers due to the retention time of water. Also the study does not give specific details in relation to the application of the zeolite. Was it applied in a solid form meaning that subsequently it needed to be recovered once sufficient time had pass to allow absorption of phosphates?

A number of studies have also investigated the ‘first flush’ phenomenon, which is closely linked to understanding the importance of particle size of pollutants. J. H. Lee et al. (2002) undertook a study of 13 separate urban catchments in Chongju in South Korea, with the aim of observing the ‘first flush’ phenomenon. In this study the ‘first flush’ was defined as being “the occurrence of high concentrations of pollutants in the early stages of a storm event”. Thirty eight separate storm events were monitored and based on the ratio of runoff volume to pollutant mass over the duration of events it was considered that a ‘first flush’ was observed. The strength of the effect varied for different variables over different catchments with a variety of predominant land uses, for example TSS loading was high from residential areas and conversely COD was lower from catchments with greater industrial coverage. The ratio was also found to be influenced by the percentage of impervious surface area within a watershed and was observed to be more pronounced in smaller watersheds.

While the overall number of storm events monitored in the study was large, events were split over 13 separate watersheds meaning that only between 2 to 3 events were recorded at each. As observed in section 2.3.5 it also needs to be considered that there is no consensus in the

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literature in relation to a clear definition nor standard method to assess the strength of the ‘first flush’ (Deletic, 1998). J. H. Lee et al. (2002) further observed that through the use of different methods of analysis the strength of the observed ‘first flush’ varied.

It is important to consider the importance of particle size when considering the rate of nutrient wash off in urban areas. (Miguntanna et al., 2013) conducted a study in Southport Australia, which examined the importance of nutrient wash-off with respect stormwater runoff. Using small uniform 3m2 plots located in three different urban districts where the land use was predominately residential, industrial and commercial respectively. The available pollutant load at each site was determined by vacuuming a plot of equivalent size at each site so it could be factored into results. Six plots at each site were then subject to simulations of 6 different rainfall intensities of differing duration. Samples of runoff from plots were collected and tested for a range of Nitrogen and Phosphorous indicators as well as examined for particle size.

It is not clear from the paper how samples were collected, i.e. manually or using automation. It is also unclear what the antecedent period was prior to the rainfall simulation as this would likely have affected results. It was found that Nitrogen and Phosphorous displayed different behaviour in response to the simulated rainfall, it was the quantity of Nitrogen wash-off was limited to qualities in the initial pollutant load whereas wash-off quantities of Phosphorous was limited by the availability of runoff to transport it. Nitrogen was detected in higher dissolved quantities that Phosphorous, indicating that was more readily removed by lower intensity rainfall. In particulate form Nitrogen was predominately seen in the small fractions less than 150 microns, whereas Phosphorous was observed in similar quantities at particles sizes smaller and larger than 150 microns.

Chiew and McMahon (1999) used a straight forward modelling approach to estimate runoff and diffuse pollution loads for urban catchments in Australia. The study found that the key variable for estimating annual runoff was the fraction of effective impervious area within the catchment. Using a simple runoff rainfall plot, the angle of the slope of a best fit line gave a good approximation of the impervious area. As illustrated in the sample plot from the study, included as Figure 7.

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Figure 7 - Sample plot from Chiew and McMahon (1999)

To calculate the annual pollutant volumes the Event Mean Concentration (EMC) of known storm events was multiplied by runoff. In the absence of primary data on EMC’s for the studies’ catchments researchers used data from published literature. Using these inputs it was found the resulting model could predict with reasonable accuracy the expected long term runoff from a catchment. However in the absence of local data on water quality the estimations of pollutant load were less accurate. Liu et al. (2013) who conducted a similar study into the influence of rainfall and catchment characteristics on stormwater quality, reported similar findings. They found that complex response of urban storm water quality to rainfall may not be adequately represented by the limited number of factors used in modelling to make accurate site specific predictions.

When considering modelling work such as this it is essential to have accurate rainfall data in relation to studies of catchments but also to appreciate long term rainfall trends. Osborn et al.

(2000) analysed rainfall changes between 1961–1995 from 110 UK weather stations on a seasonal basis. Their observations identified that over this period, for most locations in the country, winter rainfall distributions had moved from a situation where there were higher contributions from lower and medium rainfall events to one where greater contributions came from heavy rainfall events. Conversely summer trends were the opposite, with a decrease in the importance of heavy events and greater contribution coming from low and medium events. Trends in spring and autumn from some regions show the same behaviour as the overall winter trend whereas others show the opposite. These observations of trends in rainfall demonstrate the need to ensure adequate retention and infiltration provision.

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